Evolution of
Evolvability
This paper shows how evolution tunes the content and frequency of genetic variation to enhance its evolvability. Genetic evolution is not random or entirely blind. Genetic systems are like nervous systems and brains—they have been structured and organised by evolution to enhance their ability to discover effective adaptations.
(For
a more general approach to the evolution of evolvability, see
Chapters
8 to 12 inclusive of the on-line book Evolution's Arrow. It examines in
detail how evolution itself has evolved. It shows how evolution has discovered
new and better evolutionary mechanisms (mechanisms that discover and perpetuate
adaptations). The book looks at the evolution of pre-genetic, genetic,
psychological, cultural, and supra-individual evolutionary
mechanisms.
It
also shows how evolution has progressively improved the evolvability of these
mechanisms, and how it can be expected to continue to do so in the future.
Chapter
8 is at:
http://www4.tpg.com.au/users/jes999/8.htm
)
(a final version of
this paper was published in the Journal of Social and Evolutionary Systems
(1997) 20: 53-73.)
1.
INTRODUCTION
The cognitive ability
of a genetic system is its capacity to discover and perpetuate beneficial
adaptations. In a typical genetic system, this will include the ability to
accurately reproduce the best genetic arrangements that have been discovered in
the past, and the ability to discover new and better genetic arrangements
through the testing of variants by trial and error.
Cognitive ability will
be influenced by genetic arrangements which affect the range and types of
variant genotypes which are produced and trialed within the genetic system. For
example, such a genetic arrangement might enhance the efficiency of the search
for beneficial adaptations by reducing the amount of trial and error necessary
to discover adaptations e.g. it may cause the production of variation which has
an increased probability of being beneficial because the content of the
variation is more likely to match likely environmental changes; or it may vary
the frequency of testing of variation across the genome in proportion to the
likelihood that variation in relation to each character will pay
off.
A genetic arrangement
which enhances cognitive ability will be selected within a population where the
arrangement captures sufficient of the fitness benefits which it produces
through the discovery of successful variants. Unlike other genetic arrangements,
the cognitive arrangements are not selected because they directly produce
phenotypic effects which increase the fitness of individuals which carry the
arrangements. Instead, cognitive arrangements succeed through their association
with successful variants which have been produced as a result of the effects of
the cognitive arrangements (e.g. through the hitch-hiking effect modelled by
Kojima and Schaffer, 1967).
However, as shall be
discussed in detail below, the evolution of cognitive arrangements faces a
fundamental difficulty. Arrangements which continually incur the cost of
searching for beneficial adaptation by trial and error can be favoured only
while there are beneficial adaptations to be discovered. During any period in
which beneficial adaptations cannot be discovered (e.g. because the population
is optimally adapted to a stable environment), such a cognitive arrangement will
be out-competed by arrangements which reproduce the optimum genotype without
testing variants.
Issues which will be
investigated in this article through the application of the cognitive
perspective to the evolution of the genetic system include: to what extent are
genetic cognitive arrangements able to overcome this difficulty and evolve? to
what extent have key features of the genetic system been shaped by selection for
enhanced cognitive ability? what genetic cognitive abilities can be expected to
have evolved? and, in particular, are there likely to be cognitive processes
within the genetic system which are predicted by the cognitive perspective but
which have not yet been recognised by current perspectives?
In addressing these
issues, the article focuses primarily on processes internal to the genetic
system which shape the production and testing of variation, rather than on
processes in which behaviour plays a significant role in doing so e.g. by its
impact on mate choice, the propensity to inbreed, and on population
structure.
We begin in Section 2
by identifying the impediments which are suggested by current theory to limit
the extent to which genetic arrangements will be shaped by selection for
cognitive ability. This is followed in Section 3 by consideration of the types
processes which might emerge in the genetic system to overcome these
impediments. Section 4 explores the consequences of this analysis for the
evolution of mutational systems, and Section 5 does the same for genetic systems
which include recombination. This is followed in Section 6 by consideration of
the extent to which selection which enhances cognitive ability will favour the
hierarchical organisation of the adaptive processes of the genetic system.
Section 7 applies the cognitive perspective developed in the article to the
explanation of the evolution and maintenance of sexual reproduction - an issue
which current evolutionary theory has had difficulty in resolving.
2.
IMPEDIMENTS TO THE EVOLUTION OF GENETIC COGNITION
Theory had little
difficulty in acknowledging and explaining the evolution of arrangements which
enhance genetic cognition while it was accepted that processes could be selected
for 'the good of the species'. The production of variation through mutation,
recombination, and sexual reproduction could be readily explained as enabling a
species to adapt to changing circumstances - species which failed to produce
this variation would be unable to adapt and would be out-competed by those which
could (e.g. see Mather, 1943; and Kimura, 1967).
However, this form of
explanation was largely discredited from the early 1960's through the work of
theorists such as Hamilton (e.g. 1964), Williams (e.g. 1966) and Maynard Smith
(e.g. 1971). These theorists emphasised that in most circumstances a character
would not prevail in a population unless it benefits the individuals exhibiting
the character, no matter how much the character benefits the population as a
whole. This observation pointed to particular difficulties in the evolution of
both cognitive arrangements and cooperation: although both cognition and
cooperation could clearly be beneficial to the population, in many circumstances
individuals which initiate cooperative behaviour or which produce the variation
which underpins cognition would not capture these benefits.
In the case of
cognition, this is because the need to adapt is not likely to be continuous, and
therefore will not provide continual benefits to individuals which initiate
cognitive processes. Over the longer term, enormous advantages may accrue to
populations which include cognitive mechanisms where, for example, occasional
environmental change provides strong selection in favour of the production of
variants. However, these benefits will not be captured by the individuals who
incur the costs of producing variants during periods when no change arises e.g. during periods in which the
population is well adapted to a stable environment. Under constant conditions,
arrangements which reproduce the optimal genotype without variation will be
favoured. The capacity to provide future cognitive benefits will not assist
shorter term competitive ability. Similarly, in the case of cooperation,
individuals which direct cooperation toward others without capturing any of the
benefits themselves (e.g. altruists) are likely to be out-competed no matter how
beneficial the cooperation is to the population as a
whole.
Since the 1960's, the
consequences of this individual-focused perspective have been worked out in
detail for the key processes of the genetic system which are involved in the
maintenance and production of variation. For example, in relation to the
accumulation and maintenance of variation within the gene pool, theory predicts
that under constant conditions, the optimal genotype will prevail and all other
variation will be removed (e.g. Wright 1935; and Lewontin, 1974); in relation to
the production of new variation by mutation or by recombination, theory predicts
that under constant conditions mutation and recombination rates will evolve to
zero across the genome (e.g. see Karlin and McGregor, 1974; Maynard Smith, 1978;
and Lieberman and Feldman, 1986); and if sexual reproduction is to be maintained
in a population, there must be a short term advantage to sex over asex (however,
this advantage must outweigh not only the cost of producing variation, but also
the additional ‘two fold’ cost of sex identified by Maynard Smith,
1971).
But these clear and
unequivocal conclusions have created other difficulties for theory: if cognitive
mechanisms are continuously costly but provide fitness benefits only
intermittently, what accounts for the evolution of the processes which maintain
and produce variation? In particular, why is sex almost ubiquitous, why is
recombination and mutation prevalent across the genome, and why is genetic
variation found almost wherever it has been looked for by artificial selection
experiments? These difficulties have been particularly resistant: Roughgarden (1991) describes the
difficulty in accounting for the evolution of sexual reproduction as the major
unsolved problem of biology today, and Barton and Turelli (1989) describe as a
central paradox the maintenance of abundant polygenic variation in the face of
stabilising selection which is expected to eliminate the
variation.
Numerous attempts have
been made to explain within the individual-focused approach the evolution of
these processes which appear at first glance to have evolved on account of their
capacity to enhance the ability of the genetic system to adaptively respond to
selection. From the cognitive perspective being developed here, it is useful to
divide models which attempt to overcome this difficulty into two categories:
those which accept that the variation which underpins the adaptive response is
generally sub-optimal, but which explain the maintenance of the variation as an
unavoidable side effect of some other process; and models which explain the
maintenance and production of variation on the basis that it enhances the
discovery of beneficial adaptation (these will be referred to here as
‘cognitive’ models).
Examples of 'side
effect' models are: the mutation/selection balance theory which suggests that
the accumulation and maintenance of variation found in most quantitative
characters is the result of a balance between the continual creation of
variation through mutation and the removal by selection of these mutations which
are in most cases sub-optimal and deleterious (e.g. Wright, 1935; and Lande,
1975); pleiotropy theory which suggests that much of the variation associated
with quantitative characters is due to the deleterious pleiotropic effects of
genes which are nonetheless maintained because they have other beneficial
effects on the organism, or are maintained for some other reason (e.g. Hill and
Keightley, 1988; and Barton, 1990); and theories which explain the continual
production of deleterious mutation across the genome on the basis that its
suppression would be too expensive (e.g. Leigh, 1973; and Kondrashov,
1988).
Wright’s ‘shifting
balance’ theory which is directed at explaining how the structure and dynamics
of populations may be able to enhance the capacity to discover adaptations is
probably also best categorised as a ‘side effect’ model - the particular
population structures and dynamics which enhance cognition are not themselves
selected because of their cognitive contribution (Wright, 1977; and Schull,
1990).
The main 'cognitive'
models usually rely on some form of environmental heterogeneity which prevents
any single genotype from being optimal through time in all circumstances. The
basis of the heterogeneity may be biotic or abiotic, spatial (e.g. Ghiselin,
1974; Williams, 1975; Maynard Smith 1978; Slatkin, 1987; and Gillespie and
Turelli, 1989) or temporal (e.g. Van Valen, 1971; Williams, 1975; Hamilton 1980;
and Hamilton et al., 1990), and may be frequency dependent (e.g. the
impact of the heterogeneity may be affected by the proportion of the population
with a genotype which best exploits a particular part of the environment
[Roughgarden, 1972]).
Some approaches have
both a 'side-effect' as well as a 'cognitive' component. For example, the
explanation of the maintenance of sex and recombination provided by Kondrashov
(1988) relies on unavoidable mutation, much of which is deleterious, which is
shed through the capacity of recombination to produce variants which do not
include the deleterious genes.
The central difficulty
encountered by all side effect models is the necessity to demonstrate that the
maintenance of the sub-optimal variation as a side effect is indeed unavoidable,
particularly given that selection would strongly favour the establishment of
arrangements which would remove deleterious side effects. Under the
Neo-Darwinian adaptationist program, any proposition that a significant
characteristic of many organisms is sub-optimal and maintained as an unavoidable
side effect of features that are advantageous will generally be viewed with
suspicion and seen as an absolute last resort for evolutionary theory. On this
basis, the mutation balance theory has a heavy onus to demonstrate that
selection is unable to establish arrangements which prevent the production of
deleterious mutations (proponents of the theory generally argue that this
mutation is unavoidable because its prevention would not be cost-effective); and
the pleiotropy theory must demonstrate that beneficial genotypes without
detrimental pleiotropic effects are unable to be established either directly or
by combinations of genes which suppress the side effects. No side effect model
has yet discharged this onus sufficiently to gain wide-spread acceptance as a
comprehensive explanation of the maintenance and production of
variation.
The two main
limitations of the cognitive models are: first, the selection they postulate to
favour the maintenance and production of variation seems unlikely to be
sufficiently general in its effects across the genome to maintain variation for
all characters in which variation is found; and second, selection will favour
the maintenance and production of the variation only while the circumstances
which produce the selection continues.
Of course, this second
difficulty dissolves if the selection for cognitive capacities is unending -
i.e. if cognition is always beneficial because the population is continually in
a state of dis-adaptation, ensuring that the production of a variety of
genotypes continually pays off. However, it has not yet been convincingly
demonstrated that this condition would hold for all populations of all organisms
that appear to have genetic systems which exhibit cognitive capacities,
particularly given that any relaxation of the condition would have to be limited
because it would immediately result in selection against the cognitive capacity.
These difficulties in
demonstrating how cognitive mechanisms could be established where they provide
only intermittent benefits appear to have been the major impediment to the
development and acceptance of a general cognitive approach to the evolution of
the genetic system.
3. OVERCOMING IMPEDIMENTS TO
THE EVOLUTION OF GENETIC COGNITION
To what extent have
the evolution of cognitive arrangements within the genetic system been impeded
by the difficulties identified by the individual-focused
perspective?
Stewart (1995 and
1997) has identified arrangements which can comprehensively overcome this
cognitive limitation and the closely related limitation applying to the
evolution of cooperative organisation amongst individuals (the cooperative
limitation arises where the cooperator is unable to capture the benefits of its
cooperative actions, and may therefore be out-competed, just as a cognitive
arrangement may also be out-competed because its capacity to provide future
cognitive benefits will not assist its shorter term competitive ability). The
arrangements outlined by Stewart overcome these limitations by the formation of
higher level organisations. These organisations each comprise a group of
individuals which are managed and controlled by other individuals which are in
hierarchical relationship with the original group. The hierarchical individuals
manage the organisation by feeding back to members of the group the consequences
to the organisation of the cooperative actions of the members (thereby
overcoming the cooperative limitation), and by feeding back to members the
expected future benefits of their actions (including their cognitive actions),
thereby overcoming the cognitive limitation. As a result, cognitive arrangements
which produce only future benefits will also be competitive in the shorter term.
For example, a human government may intervene in an economic system to sustain
research which would not otherwise be undertaken because it will pay off only in
the long term.
The emergence of
hierarchical individuals which are able to manage a group in this way will be
favoured by selection because the individuals will be able to harvest some of
the additional benefits produced by their promotion of cooperation and effective
cognition, and will be able to use these resources for management and for their
own reproduction. Examples of higher level organisations which are formed in
this way are groups of molecular processes managed by RNA to form early cells,
and groups of humans managed by rulers or governments to form human
societies.
However, the task here
is to explore the evolution of cognitive capacities within the typical genetic
system, and most organisms are not organised into higher level organisations in
the way described by Stewart. We will therefore now turn to identifying
circumstances and arrangements which could go at least some way to overcoming
the cognitive limitation without the formation of higher level
organisations:
We begin by noting
that cognitive mechanisms which produce intermittent benefits will be
out-competed under constant conditions only to the extent that they incur a
cost. For example, during any period when the impact of a cognitive mechanism on
fitness is neutral because it does not produce variants, it will not be at a
competitive disadvantage to alternatives which merely reproduce the optimal
genotype. Furthermore, the less frequently that variants are produced by a
mechanism, the lower the competitive disadvantage over any given period, and the
more likely that it will encounter changing conditions which provide it with a
selective advantage before it is eliminated from the population due to
competition.
This is a critical
point in understanding the evolution of cognitive capacities in genetic systems:
selection will favour the evolution of cognitive processes which are able to
survive periods in which there is little benefit in the testing of alternative
variation. Processes will be able to achieve this where the rate of production
of variation, and therefore the rate at which the process is removed from the
population during these periods, is sufficiently low. Selection will favour
processes of this type which are best at cost-effectively discovering new
beneficial adaptations when there is benefit in doing so - e.g. when
circumstances change. Processes with the highest cognitive ability will
out-compete less effective cognitive arrangements, and, in suitable
circumstances, will out-compete alternative arrangements which merely reproduce
the genotype which is optimal under most conditions, without testing variants.
Are genetic
arrangements which have these features feasible? What kinds of cognitive
processes might arise in response to this selection?
4. THE EVOLUTION OF MUTATIONAL
SYSTEMS
4.1 What circumstances will
favour on-going mutation?
We will proceed by
considering a simple case: the production of genetic variation by mutation in a
population of haploid asexuals. Theoretical work on the evolution of such a
system predicts non-zero mutation rates for certain genes in an environment
which cycles between two states (e.g. Leigh, 1970; and Ishii et al.,
1989). Mutation is favoured in genes which are optimal in one state of the
environment, are able to mutate to a form which is optimal in the alternative
state of the fluctuating environment, and are able to be restored by a reverse
mutation. Leigh (1970) and Ishii et al., (1989) demonstrate that the
optimal mutation rate in such a system where selection is strong is 1/n, where n
is the average duration of each environmental state, expressed as the number of
generations. The optimal mutation rate maximises the capacity of a mutator to
produce the particular mutation(s) which will spread through the population when
the environment enters the state favourable to the
mutations.
This work is very
limited in its ability to account for the evolution of the observed pattern of
mutation in organisms. It can account only for the maintenance of mutation which
produces variants each of which is beneficial in a recurring state of the
environment. In contrast, the evidence suggests that the overwhelming majority
of mutations are deleterious under all environmental conditions. These models
would therefore not even be able to account for the more general pattern of
mutation which presumably would be necessary for the initial discovery of the
specific mutational systems which are favoured in a cycling environment - the
work can account only for the persistence of these specific mutational systems
once they have been discovered.
However, an
examination of the work from the perspective being developed here will assist
the development of a theory of the evolution of mutation which is more generally
applicable. A key point illustrated by the models is that a mutator can be
maintained in a population no matter how infrequently the environment favours
the mutant it produces, provided the mutation rate is sufficiently low - this is
because the rate at which the mutator is removed from the population during a
period in which the mutation is non-optimal depends on the rate at which it
produces mutations; if the mutation rate is sufficiently low, it will survive
until the environment changes to a state which favours its mutations, no matter
how long this period is. In this way, mutators are able to persist away from
equilibrium during constant conditions until conditions change and cognition is
beneficial.
These considerations
point to a solution to the more general problem of how selection might favour
mutators which produces a variety of mutations, the majority of which are
deleterious. The success of such a mutator within a population will be impaired
only when an instance of the mutator cause an unsuccessful mutation, and then
only if the mutator faces competition from an alternative which produces fewer
unsuccessful mutations. Such a mutator will persist provided it produces a
successful mutant before its production of deleterious mutants removes it from
the population. When it produces a favourable mutation, it will increase again
to fixation. If the probability that a favourable mutant will be produced
increases to unity with sufficient time, this condition will be met provided the
mutation rate is sufficiently low - with a sufficiently low mutation rate, the
mutator will maintain its presence in the population until a favourable mutant
is produced. As for the case studied by Leigh and by Ishii et al., the
optimal mutation rate will be the
rate which maximises the capacity of a mutator to produce the mutation(s) which
will spread through the population when favourable circumstance arise. And the
optimal rate will be lower the longer it takes for the probability of the
production of a favourable mutation to increase to unity.
But how plausible is
this requirement that the probability of the production of a favourable mutation
increases to unity with sufficient time? Will this requirement be sufficiently
widely met to explain the ubiquity of mutation?
The probability that a
random mutation in an organism will be favourable is likely to increase
significantly with time. This is because the probability that a random change
will be beneficial will increase as the adaptedness of the organism to its
environment decreases. And given sufficient time, the adaptedness of a
population can be expected to decline as changes in the biotic and abiotic
environment accumulate. The probability that a mutation will be successful will
also increase with time because new opportunities for adaptation will open up
due to internal changes in the genetic system itself. For example, there will be
advantage in masking sub-optimal mutation, and in contributing to optimising
other successful genetic change in the population. On a long enough time scale,
increasing change is inevitable, and increasing maladaptation is also therefore
inevitable. Opportunities for successful mutation will therefore continue to
increase through time. This is a general consideration which should apply
ubiquitously. Consequently, if a mutator causes a sufficiently low mutation
rate, it can survive until the rising level of maladaptation sufficiently
increases the probability that mutation will be successful. Because there will
be differences between characters in the rate at which environmental and other
changes produce maladaptation, the optimal mutation rate will vary across the
genome.
Other factors will
also contribute to this evolution of non-zero mutation rates. For example: the
optimal mutation rate will generally be greater than otherwise if the
suppression of mutation incurs a fitness cost which increases as the magnitude
of the suppression increases (Ishii, 1989); and at lower rates of mutation,
mutators will experience longer periods during which they do not encounter
competition from alleles which further suppress or prevent mutation (this is
because these alleles can be generated only by mutations, and the lower the
mutation rate, the lower the rate at which such alleles will be discovered -
cognition is essential to the discovery of adaptations which reduce cognitive
ability).
4.2 Enhancement of cognitive
ability
The selection which
arises under the circumstances which have been described will not only favour
non-zero mutation rates, it will also favour mutators which produce a pattern of
mutations which have a higher probability of including beneficial mutations. All
other things being equal, mutators which produce such a pattern of mutations
will out-compete mutators with a lower probability of producing beneficial
mutations. Alternative mutators will therefore compete on the basis of the
content of the pattern of mutations they produce, as well as on the frequency of
the production of mutations. This competition will produce intense selection - a
mutator which fails to discover adaptations because alternative mutators do so
first will be removed from the population - it will not be able to recoup the
cost of producing variation.
For example,
selection acting on this basis would favour a variant mutator which generates a
higher proportion of mutations which have a greater chance of success because
they are phenotypically meaningful e.g. because they produce phenotypic changes
which are at least consistent with the functional organisation of the organism,
and are not lethal during development. In this way, selection will increase the
probability of successful mutation by tuning the manner in which genotypic
variation maps onto phenotypic variation (this relationship is termed the
genotype-phenotype map by Wagner and Altenberg, 1996). By way of further
example, selection can also be expected to tune the pattern of variation so that
it is more likely to include mutation which will be successful given recurring
environmental fluctuations. The specialised mutation systems analysed by Ishii
et al. are an example of such highly targeted mutation: each mutator
produces a single mutation which is beneficial under particular recurring
conditions - no mutation is produced which is unconditionally
deleterious.
The evolutionary
improvement of cognitive ability within and between populations will create
conditions which will favour the further evolution of cognitive arrangements: as
evolution produces mutators which are able to discover beneficial mutation with
less trial and error, higher mutation rates will be favoured because mutation is
likely to be successful at lower levels of maladaptation; as a population adapts
more closely to prevailing environmental conditions, the rate at which the
population becomes maladapted due to environmental changes will increase; the
higher the rate of successful mutation in one part of the genome, the greater
the opportunities for complementary adaptation in other parts; and species will
be maladapted sooner as their environment changes more rapidly due to the more
frequent adaptation of other species (Glesener and Tilman, 1978 make a similar
point in relation to the evolution of sexual
reproduction).
If selection favouring
enhanced cognitive ability is to be effective, there has to be heritable
variation in the cognitive ability of mutational systems. At least some
variation of this kind is likely to arise readily. For example, mutators which
cause mutations in different sub-sets of the genome (e.g. in different regions
of a particular chromosome) are likely to differ in this respect, if only
because the content of their mutational patterns will differ by chance.
Selection will operate on any bias which arises in the content of the pattern of
mutation, whatever its cause or origin. To the extent that suitable variation
arises, selection will establish a pattern of mutation across the genome whose
frequency and content is non-random relative to the past selective pressures
encountered by the population. And it will be non-random relative to future
pressures to the extent that future environmental conditions (including changes
in conditions) are similar to past conditions (and past changes). In this
respect, the evolved pattern of mutation will be like any other adaptation which
is established through natural selection - the adaptations which exist at any
time are those which are selected by past circumstance, and they will be
relevant to future conditions only to the extent that the future resembles the
past.
4.3 Tracking an environmental
variable
How effective is such
a mutational system in a population of haploid asexuals likely to be at
responding to a typical adaptive challenge encountered by the population? A
common adaptive challenge would be to adapt the population to an environmental
variable which is relatively constant on the time scale of the life of the
organism, but which varies over a wide range on longer time scales. How
effectively would a mutational system adjust a phenotypic character which has
the potential to adapt the population if the character is adjusted appropriately
as the variable changes?
The genetic system
would adjust the character through the production of mutations. Mutations would
be selected where they better adapt the character to the state of the variable
which prevails at the time. As the environmental variable changes, successful
mutation will adjust the character by moving the genetic system from one genetic
arrangement to another. Given the nature of genetic action, it is highly
unlikely that these various genetic arrangements would be able to be achieved
entirely by a sequence of mutations of one gene which determines the character.
Instead, tracking of the environmental variable is likely to also necessitate
changes to other genes so that they modify the effect of the original gene, with
each new modifier contributing to moving the character from one adaptive state
to another. When the variable changes, the character will respond as a result of
changes to genes which already affect the character and/or as a result of the
establishment of new modifiers through changes to other genes. As more genes
affecting the character are accumulated, it would become more probable that a
given change in the character could be achieved by a mutation which changes an
existing modifier gene (e.g. by disabling the gene or by reversing the
disablement), rather than by the establishment of a new modifier gene (disabling
or re-enabling an existing modifier is likely to move the character in a
direction which has previously proved to be beneficial). In principle, the
variety of genetic responses which are necessary to match the variety of the
environmental variable could be produced by appropriate combinations of
mutations which switch genes on or off, switch genes between other useful
states, and produce new modifier genes. It is important to note that as these
arrangements evolve, there are likely to be many different ways in which a
particular state of the phenotypic character can be produced genetically. For
example, various combinations of active and disabled modifiers will produce the
same phenotypic effects. Selection will not distinguish between or favour these
alternatives. For this reason, in a large population the genetic arrangements
which determine a character are likely to differ significantly across the
population even though the arrangements produce the same phenotypic effect, and
once lineages diverge in this way, they are unlikely to converge again.
In this way the
characteristic genetic architecture of characters which are polygenically
determined is likely to arise. The architecture arises through the accumulation
of genes and mutators which are each selected because they contribute to the
tracking of an environmental variable. The emergence of the architecture
improves the capacity of mutation to adjust the relevant phenotypic character to
track the variable. The evolution of these cooperative and coadapted assemblages
of genes is not impeded by any cooperative limitation - linkage ensures that
individual genes capture the full benefit of any effects they have on other
genes which increase the success of the assemblage as a
whole.
The efficiency of such
a system in tracking the environmental variable will depend in large part on the
extent of trial and error needed to discover the mutations which adjust the
character so that it continues to be optimal. The extent of trial and error
would be substantially reduced where the only mutations which are produced are
ones which move the character between states which are all optimal for some
particular value of the environmental variable. This might be achieved within
such a polygenic system by, for example, mutators which produce only mutations
which disable or reverse the disablement of modifier genes, or which switch the
directional effect of modifiers on the character (the simple two state mutation
system examined by Leigh and Ishii et al. is a single-locus counterpart
of such a polygenic arrangement). As the environmental variable changes,
selection would operate on the variation produced by such a targeted mutational
system, selecting the particular variants most adapted to the prevailing value
of the variable.
Selection would favour
mutators which produce patterns of mutations which improve the efficiency of the
system in this way. However, the discovery of mutators with such narrow and
precise effects would be likely to necessitate considerable trial and error
which itself would be costly and prolonged. This process of discovery would need
to be repeated for each and every character of the organism, and would also need
to be repeated whenever there was a change in the range over which an
environmental variable typically moved.
Arrangements which are
able to produce such a beneficial pattern of variation for all characters
immediately, without the need for the costly process of discovery, would have a
substantial cognitive advantage, and be strongly favoured by
selection.
5. EVOLUTION OF GENETIC SYSTEMS
WHICH INCLUDE RECOMBINATION
Such a pattern of
targeted variation would tend to be produced as a matter of course where genetic
material is exchanged between homologous chromosomes which include polygenic
arrangements and originate from different individuals in the population (e.g.
through the process of recombination associated with sexual reproduction or
related arrangements such as conjugation). If the corresponding parts of
homologous chromosomes which are exchanged contain different genetic
arrangements, the resulting recombinant chromosomes will constitute variant
genotypes. Importantly, this exchange process will produce variants even where
selection has imposed a single optimal phenotype throughout the population. As
we have noted, such a population is likely to be genotypically varied even
though it is phenotypically uniform, because there are likely to be many
different ways in which a particular state of a phenotypic character can be
produced genetically by polygenic arrangements. Selection will not distinguish
between or favour these alternatives, and a population is likely to accumulate
greater genetic diversity of this type through time. Recombination between these
phenotypically equivalent but genotypically diverse chromosomes has the
potential to produce new chromosomes which are both phenotypically and
genotypically variant. The rate at which this variation is produced within the
population will depend on the frequency of recombination. When there is no
recombination, the genotypic diversity will be retained without cost to fitness.
The capacity of such a system to produce variation will not diminish as the
optimal phenotype changes through time because recombination is likely to
continue to create a diversity of polygenic arrangements which produce the
particular phenotype which is favoured at any particular time. And stabilising
selection will tend to maintain the linkage disequilibrium which is necessary if
recombination is to continue to produce variation (Wimsatt, 1981 discusses the
capacity of linkage and selection to maintain this linkage
disequilibrium).
Returning to the
example of adaptation tracking a changing environmental variable, it is
significant that the variants produced through recombination will comprise
different combinations of the accumulated modifier genes which each tend to move
the state of the character in a direction which previously has proved
beneficial. The presence or absence of a particular gene in a combination will
have a similar effect to a mutation which enables or disables the gene - the
state of the character will be moved to a state which has previously been
optimal. The variants will therefore tend to produce states of the character
which are all optimal for some particular value of the environmental variable.
In contrast, mutational systems which are not able to precisely target mutations
in the way discussed above will tend to produce mutations which lie outside this
range of states and have little chance of producing beneficial
adaptation.
This points to the key
cognitive advantage of recombinational over mutational systems: recombination
operating on polygenic arrangements generates variation in a way which increases
the chances that the variation will include beneficial variants. It does this by
combining in different ways genetic elements which have proven beneficial under
particular conditions in the past. This will ensure, as we have seen, that the
variation is more likely to move characters to states which have been beneficial
in the past and which may recur in the future. In the same way, it will produce
variation which is more likely to map into meaningful phenotypic change - this
is because the variation will comprise various combinations of genes which
selection has accumulated over time and which are therefore likely to have
proved to be phenotypically meaningful in the past. Thus, for example, if the
accumulated genes tend to be modular in their phenotypic effects, variants
produced by recombination will also tend to exhibit modularity of genetic
effects (modularity is used here in the sense developed by Wagner and Altenberg,
1996 which refers to a genotype-phenotype map in which there are few pleiotropic
effects amongst characters which serve different functions - pleiotropic effects
fall mainly among characters which are part of a single functional complex). In
contrast, un-targeted mutational systems acting on the same genetic arrangements
will not exhibit any such fundamental tendency to produce variation which
exhibits modularity, and modularity will be maintained only by selection.
The capacity of a
system which generates variation by recombination to persist through time and to
be selected on the basis of its cognitive ability will encounter the impediments
identified by the individual-focused perspective - individuals which carry the
arrangements will be selected against during constant conditions, no matter how
beneficial recombination is over the longer term. But these impediments will be
overcome in the same way as for mutational systems: selection will favour the
evolution of arrangements which produce variation at a rate which is
sufficiently low to enable the arrangements to survive periods of constant
conditions, and which are best at discovering new adaptations when there is
benefit in doing so. As we have seen, the production of variants can be
reasonably expected to pay off eventually for all characters as the adaptiveness
of characters diminishes over time due to environmental change and due to
changes within the genetic system itself (including due to the production of
deleterious variation [see Kondrashov, 1988]). And no matter how long it
typically takes for the adaptiveness of a character to diminish sufficiently,
the optimal rate for the production of variation in the character will be
non-zero. The capacity of recombination to produce better-targeted variation
means that these optimal variation rates will be higher than otherwise, and the
magnitude of maladaptation needed to ensure that the variation will be
successful will be lower.
A number of factors in
a recombination system will influence the rate at which variation is produced
for a particular character which is polygenically determined. These include: the
frequency and timing of sexual reproduction or conjugation; the rate at which
particular segments of chromosomes are exchanged; the location on the
chromosomes of these exchanges; and the genetic constitution of the segments
which are exchanged (e.g. if the segments which are exchanged have identical
phenotypic effects and epistatic effects are not significant, the exchange will
not produce variation, no matter how frequently it occurs). Selection can be
expected to tune these factors to optimise variation rates for each character.
Furthermore, these factors could themselves be determined and adapted by
polygenic arrangements, with the result that variation rates would respond
readily and effectively to selection, and the production of variation in
particular characters would increase or decrease as circumstances demand.
Observed patterns of recombination rates are consistent with these expectations,
varying widely across the genome, and responding readily to artificial selection
(e.g. see Brooks, 1988; and Lichten and Goldman, 1995 for
reviews).
Selection could be
expected to tune not only the frequency of variation, but also its content - it
will favour the further enhancement of the cognitive ability of a recombination
system to generate patterns of variation which are more likely to produce
beneficial adaptation. For example, selection could be expected to tune factors
such as the location of crossing over and the genetic constitution of the
segments which are exchanged. These can influence the content of the variation
which is produced by, for example, affecting the extent to which variants are
likely to deviate significantly from the parental genotypes. Furthermore,
polygenic arrangements which track environmental variables are likely to be
subject to selection which favours the production of patterns of variation which
are centred on the mid-range of the variable and which provide for flexibility
about the mid-range.
Together, these
various selective pressures will result in the differentiation of variation
across the genome in relation to its frequency and its content. This selection
favouring differentiation will in turn favour the production of variation whose
phenotypic effects impact only on the particular adaptations for which it is
specialised - in this way, modularity of effects will be further
favoured.
As for mutational
systems, the pattern of variation produced across the genome by a recombination
system can be expected to be highly non-random relative to the selective history
of the population, and highly adaptive to the extent that future environmental
and other changes are correlated with past changes.
Although cognitively
superior in a number of respects, recombination systems cannot fully replace
mutational systems. A recombination system needs to be complemented by
mutational arrangements which will coevolve with it. Selection will favour
mutational systems which establish new genes which might enhance the capacity of
the recombination system to produce adaptation to both recurring and new
environmental challenges, and which replace genes which are lost from the system
where, for example, the flexibility of the recombination system is overtaxed by
environmental change.
The analysis presented
here arrives at a similar picture of the functioning of the typical recombining
eukaryote genome to that presented by Mather (1943). Mather developed a
comprehensive understanding of how recombination operating on polygenic
arrangements can produce patterns of variation across the genome which enable
populations to respond quickly and effectively to selection. His account
provides a more detailed exposition than that presented here of the way in which
such a system will produce and structure variation. However, widespread
acceptance of Mather's scheme has been hindered because he relied primarily on
selection operating at the level of the species to account for the continual
production of variation. Unwillingness to seriously consider his proposals has
been particularly marked since the rise of the individual-focused perspective.
The analysis presented here rescues Mather's conclusions from their suspect
evolutionary basis and integrates them with current mainstream evolutionary
theory by accounting for the continual maintenance and production of variation
on the basis of selection operating at the level of the
individual.
6. THE EVOLUTION OF ADAPTIVE
HIERARCHIES WITHIN GENETIC SYSTEMS
The conclusion that
fundamental aspects of the genetic system are shaped by selection favouring
cognitive ability suggests that it may be worthwhile to examine whether key
features of other cognitive systems are also found in genetic systems. A prime
candidate for this type of analysis is the hierarchical organisation of
adaptations which is characteristic of cognitive systems which accumulate
adaptations by trial and error in a dynamic environment. We will proceed by
briefly examining three examples of such adaptive systems:
Bateson (1963) and
Slobodkin and Rapoport (1974) have described this hierarchical form of
organisation and considered its cognitive significance in relation to
physiological adaptation. Bateson notes that when an environmental challenge is
encountered, the features of the physiological system which can adapt the
organism to the challenge typically respond on different time scales. For
example, an immediate response in humans to altitude is panting and a racing
heart, but in the longer term, adaptation will be achieved by the deeper and
more complex physiological changes associated with acclimation. If acclimation
does not adequately provide adaptation, further adaptation resulting from
longer-term genetic changes in the population is likely to follow. Slobodkin and
Rapoport explain the evolution of this hierarchical structure on the basis that
it achieves adaptation more economically - environmental perturbations are first
met by short-term responses involving relatively minor commitments on the part
of the organism which, if successful, make the longer-term, more major
commitments unnecessary.
Rappaport (1979)
suggests that the belief systems which underpin the adaptations of human
organisations such as the New Guinea tribes he studied are also ordered
hierarchically. Lower level beliefs change rapidly and continuously in response
to environmental challenges, and relate to goals and concerns which are highly
specific and concrete. In contrast, higher order beliefs change slowly in
response to environmental perturbations (sanctification preserves the highest
level beliefs from change), and relate to more general and less concrete goals.
The result is that when a tribe is confronted with environmental challenges,
changes in lower order beliefs and behaviour patterns are tested first, and only
where these fail to adequately deal with the challenge are higher order changes
tested.
The learning
classifier system of Holland et al. (1986) is a machine learning system
that uses a genetic algorithm to search for sets of rules which provide
solutions to particular problem situations. The sets of rules discovered by the
system are often found to be organised as a default hierarchy. These are rule
sets which allow the errors of imperfect default rules (rules that are correct
for some situations, but incorrect for others) to be handled by exception rules.
The exception rules produce a correct solution by shielding the default from
errors. An exception rule can be imperfect as well, and multiple layers of
exceptions can be used to further refine performance. This hierarchical
structure allows for more parsimonious rule sets, expansion of the solution
space, and graceful refinement of rule sets through incremental modification
e.g. by the layered addition of exception rules (e.g. see Holland et al.,
1986; Goldberg, 1989; and Smith and Goldberg, 1992).
The hierarchical
structure of adaptive processes which is illustrated in various forms by these
three adaptive systems has the advantage of enabling adaptation to be achieved
more efficiently. This advantage is most clearly demonstrated by considering an
ideal adaptive system in which both the adaptations and the rate of trialing of
alternative adaptations are capable of evolving. As we shall see below, such an
ideal system will adapt efficiently by reducing the extent of trial and error
needed to discover new beneficial adaptations, and by reducing the extent to
which the adaptations accumulated by the system over time are changed in
discovering new adaptations. This is the case both for adaptation to changing
circumstances and for better adaptation to existing
circumstances.
The way in which this
is achieved can be demonstrated by examining how such an ideal adaptive system
which combines the most effective features of the three examples would adapt
through time in the face of environmental challenges. When environmental change
is encountered, tactical changes in more specific and specialised adaptations
would be trialed first. If these do not adapt the system, or if they do so only
with significant stress, strategic changes in more general features of the
system would be trialed. These more general features would usually be of larger
scale within the system, and create the systemic context within which the more
specific and specialised adaptations are optimised. The trialing of change would
be repeated at progressively higher hierarchical levels until the environmental
challenge is adequately met. Successful meeting of the challenge at a particular
level would remove stress from lower levels and restore their adaptive
flexibility.
In this way, costly
trialing of change in higher level default adaptations which have stood the
tests of time and which are also likely to be appropriate in the changed
environment would be undertaken only if adaptation of more specific features at
lower levels fails. The default hierarchy efficiently prioritises the order in
which adaptations are reconsidered and trialed for change when environmental
challenges are encountered. As well as minimising costly trial and error, this
ensures that general defaults which have provided a good strategic base for
lower level adaptations would be changed only as a last resort. Adaptation at
lower levels shields the defaults from change by modifying the effects of the
defaults to meet whatever circumstances arise (although this runs the risk of
protecting inappropriate higher level adaptations from reconsideration). In
contrast, an adaptive system which is not organised hierarchically in this way
would test alternatives in all accumulated adaptations at once. This would be
comparatively wasteful, and might result in inappropriate change to accumulated
strategic adaptations in circumstances where effective adaptation might have
been better achieved through tactical changes.
To what extent are the
adaptive processes of the genetic system likely to be organised hierarchically
in response to selection for enhanced cognitive ability?
A number of features
of the evolution and the organisation of organisms are likely to encourage the
emergence of hierarchical structure: As demonstrated by Holland’s classifier
system, the first adaptations which are discovered by a trial and error
evolutionary system are likely to be simple, general rather than specialised,
and of larger scale within the system of adaptive processes. As organisms
evolve, further adaptation can occur by changes to these existing adaptations,
or through the addition of new adaptations which build on and modify the
existing adaptations by refining them or by adapting them to new circumstances.
The later adaptations which modify and build on the earlier adaptations in this
way will generally be relatively more specialised and of smaller scale - a
number of specific adaptations will usually modify each earlier adaptation. The
likelihood that the effect of an adaptation will be changed by new modifying
adaptations rather than by innovation in the adaptation itself will increase
over time, as pointed out by Wimsatt (1986). This is because the more an
adaptation is entrenched by being built on and modified by later adaptations,
the more likely that trial and error change in the earlier adaptation will be
deleterious because it disrupts the beneficial effects of later adaptation. This
achievement of adaptation by changes to later adaptations or by new modifying
adaptations will also tend to shield earlier adaptations from pressure to
change.
This hierarchic
differentiation of adaptive processes and their adaptations will be further
enhanced by a complementary hierarchic differentiation of the rates at which the
genetic system trials variation. As we have seen, the optimal rate of variation
in a character depends on how long it will be before it becomes highly probable
that variation in the character will be beneficial. Optimal variation rates can
therefore be expected to be lower the more entrenched an adaptation, and the
more it is shielded by later adaptation. This hierarchic differentiation of
variation rates can also be expected to be reflected in a similar
differentiation in the rates at which variation in variation rates are trialed
and respond to selection - this is because it will be optimal for metavariation
to be trialed at a lower rate than the variation it controls (this form of
relationship between variation and metavariation will itself result in some
hierarchic differentiation within a genetic system).
The way in which
organisms are organised functionally will also contribute to the emergence of
hierarchies in the genetic system. Functionally, organisms are typically
organised hierarchically (e.g. organ systems, which are comprised of organs,
which are in turn made up of tissues, which are in turn comprised of cells).
This form of functional organisation is favoured by selection in part because it
enhances the ability of the individual organism to adapt effectively during its
life (Stewart, 1995). The critical point here is that the relationship between
the larger scale, higher level adaptations and the lower level, more specific
adaptations in these functional hierarchies is likely to produce the same
selective effects as the relationship between entrenched adaptations and the
later adaptations which modify them in the evolution of organisms. This is
because the higher an adaptation in the functional hierarchy, the more its
effects will be modified by lower level adaptations, and the less likely that
trial and error change in the higher level adaptation will be beneficial because
it is likely to disrupt the beneficial effects of lower level adaptation. Again,
adaptation at lower levels will shield higher levels from change, and the rates
at which variation in underlying genetic arrangements is trialed will also form
a complementary hierarchy.
On this basis, many of
the adaptive processes which comprise the genetic system of a population can be
expected to be organised hierarchically. This hierarchic organisation would
minimise the costly testing of variation by prioritising the rate at which
variation in adaptations is trialed, and the rate at which changes in these
rates are trialed. This prioritisation would complement and reinforce the
tendency of the hierarchic organisation of adaptive processes to preserve larger
scale, less specialised adaptations which are more likely to be beneficial
across a wider range of environmental circumstances, and which would provide a
useful fall back position when the population encounters new circumstances
- change to these more strategic
default adaptations would be likely only where tactical change to more
specialised adaptations fails.
This cognitive
perspective adds to the explanation provided by Wimsatt (1986) of the relative
stability over long evolutionary time scales of the fundamental body plans of
the main lineages of animals: the cognitive perspective suggests that the
fundamental body plans represent strategic adaptations of wide scale in the
organisation which are trialed for change on a longer time scale than later,
more specialised adaptations; and, in most circumstances, these adaptations at
lower levels of the default hierarchy will change to meet changing conditions,
preserving the strategic adaptations of the fundamental body plan (Stewart,
1995). The relative stability of adaptations at higher levels in the hierarchy
also explains the observation of Simpson (1953) and Valentine (1969) that taxa
of high rank tend to originate at earlier times in the fossil record than do
taxa of lower rank - a constant proportion of new lower taxa will not be
distinctive enough to from new higher taxa because it is more likely that they
will differ in relation to adaptations at lower levels of their adaptive
hierarchies, rather than at higher levels.
7.
THE EVOLUTION OF SEXUAL REPRODUCTION
Accounting for the
evolution and maintenance of sexual reproduction has proved to be a particularly
difficult challenge for evolutionary theory (e.g. see Maynard Smith, 1971 and
1978; Williams, 1975; and Bell, 1982).
However, the cognitive
advantages of recombination which have been identified above go a long way
towards accounting for the superiority of sex over asex in most circumstances.
When confronted with a series of environmental challenges, a sexual population
can be expected to be far superior to an asexual population at discovering
appropriate adaptations.
This cognitive
advantage will not, however, be sufficient to ensure that sex always beats asex.
This is because asex has up to a two fold advantage in fitness over asex in
general circumstances (Maynard Smith, 1971). The two-fold cost of sex arises
where half of the reproductive effort of sexuals goes into the production of
males which do not themselves contribute anything other than genetic material to
the next generation.
In what sort of
circumstance might the two-fold advantage outweigh the benefits of greater
cognitive ability? In addressing this issue it is useful to note that the
two-fold advantage results in the production of greater numbers of female
offspring, while improved cognitive ability has a greater capacity to produce
higher quality young. As indicated by Stewart (1993), this is significant
because during periods of ‘r selection’, fitness is likely to be influenced more
by the number of female young produced than by their quality - improvements in
quality achieved by superior cognition are unlikely to outweigh the advantage
achieved by producing twice as many female offspring. In contrast, during
periods of ‘k selection’, the best adapted individuals are likely to prevail -
there is little benefit in producing greater numbers of lower quality females
which will be out-competed.
On this basis, it can
be expected that asex is more likely to be favoured over sex in populations
which are continuously ‘r selected’. The distribution of sex largely accords
with this prediction (e.g. see Bell, 1982; and Bierzychudek,
1985).
In populations which
cyclically alternate between periods of ‘r’ and ‘k’ selection, the two-fold cost
of sex is more likely to outweigh the cognitive superiority of sex during ‘r
selection’. However, sexual reproduction can overcome this disadvantage in a
manner which is similar to the way in which other cognitive arrangements survive
periods in which cognition does not pay sufficiently: during periods of ‘r
selection’, individuals would avoid
the two fold cost by reproducing asexually; and during periods of ‘k selection’
or in other circumstances where the quality of offspring is more important than
the number, individuals would reproduce sexually. The ecological evidence seems
consistent with this prediction e.g see Bell (1982).
However, a general
difficulty remains: even under circumstances of ‘k selection’, sex would tend to
be ousted by asex at any time that the cognitive superiority of sex does not
provide sufficient fitness benefits to outweigh the two-fold cost. Would a
population at k continuously encounter circumstances which provide sufficient
benefits to cognition?
One response to this
issue is to attempt to identify environmental circumstances which may
continuously vary in ways which would provide continuous adaptive challenges for
the population (e.g. the parasite/host coevolution theory of Jaenike, 1978;
Hamilton, 1980; and Hamilton et al., 1990). The weakness of such
approaches is the implausibility that the continuous environmental variation on
which they rely is as continuous and ubiquitous as sex.
The cognitive
perspective developed here points to an alternative basis on which to suggest
that populations are likely to be continually adapting, and therefore
continually providing benefits to sexual reproduction. As we have briefly noted
above, as cognition improves, the circumstances in which cognition is exercised
and is likely to be beneficial will increase. This is because the population
will be able to discover beneficial adaptation to environmental changes to which
it previously did not adapt. Furthermore, the more closely the population adapts
to circumstances as they arise, the more likely it is that there will be benefit
in adapting as these circumstances change - members of populations with greater
cognitive abilities will tend to be relatively more specialised to prevailing
circumstances, and this relative specialisation will be maintained by further
adaptation as circumstances change.
These same influences
appear to result in the more or less continual adaptation to external and
internal change exhibited by advanced physiological systems. It seems possible
that these influences would produce the same outcome in genetic systems which
have sufficient cognitive ability. The difference is that genetic adaptation
responds to changes occurring on longer time scales (e.g. across generations) while physiological
adaptation responds to changes occuring on shorter time scales within the life
of individuals. The capacity of the physiological system to adapt individuals
would not remove the selection pressures favouring adaptation by the genetic
system to longer term changes. This is because selection operating on the
genetic system will tune and specialise physiological adaptive mechanisms so
that they deal optimally with the particular range of adaptive problems recently
encountered by the population. On longer time scales this range will change,
providing advantage to further genetic adaptation.
However, it is not
possible without further evidence to determine whether these evolutionary
processes will ensure that populations are continually adapting, providing
continual advantages to sexual reproduction.
But there is a further
mechanism which will overcome this difficulty and provide an advantage to sex in
widely applicable circumstances as a result of the cognitive superiority of sex
(Stewart, 1993). This advantage
arises because the emergence of an asexual clone from a sexual population will
cause environmental changes due to the clone’s significantly higher per capita
birth rate of females (the ‘two fold’ advantage). If the clone is to out-compete
the sexual population, the impact of these changes must be sufficient to prevent
the sexual population from reproducing through time. However, the cognitive
superiority of the sexual population gives it a greater potential to adapt to
the changes and increase its fitness relative to the clone. The strength of the
selection favouring the maintenance of sex should be stronger for specialised
members of saturated stable communities where the emergence of a clone is likely
to cause greater disruption to the adaptedness of itself and of the sexual
population. This is consistent with the ecology of sex. Because this mechanism
will operate whenever a sexual population is threatened by a clone, it does not
need to rely on some independent source of incessant change which would
continuously provide an advantage to sex.
8.
CONCLUSION
Natural selection
acting on a genetic system will not only discover adaptations, but will also
discover more effective ways to discover adaptations. The result will be a
genetic system which produces a pattern of variation which is highly
differentiated and specialised across the genome. The variation will be
hierarchically organised and will be tuned in relation to both frequency and
content to enhance its ability to discover beneficial variants. The pattern of
variation can be viewed as a set of hypotheses based on past experience about
the likelihood that future change in particular characters will be worthwhile
and about the type of changes which are likely to be beneficial. On a longer
time scale, the pattern of variation will change dynamically, with the content
and the rate of variation changing for particular characters in the light of
experience.
This cognitive
perspective provides a framework which appears to have the potential to resolve
some of the most intractable problems confronting evolutionary theory: the
maintenance of variation, the complex and prolonged response of populations to
artificial selection, the ubiquity of sex, and the highly differentiated
structure of mutation rates and recombination rates across the genome. In addition, it predicts phenomena not
previously clearly recognised: the hierarchic organisation of the adaptive
processes of the genetic system.
ACKNOWLEDGMENTS
I gratefully
acknowledge the benefit of useful comments from Jeremy Evans, David Richards and
Wilson Kenell.
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