Biology Reference
In-Depth Information
dying, surviving and reproducing; but the evolutionary consequence is that the
frequencies of
genes
in the population change. To clarify the link from gene to individual
it is necessary to distinguish the process of natural selection from the design or purpose
that this leads to in individuals. The process of natural selection is about gene dynamics
over time. Specifically, genes which increase the fitness of the individuals carrying them
by influencing the form of some heritable trait, such as a behaviour, will increase in
frequency over time. A consequence of this is that heritable traits which increase fitness
will accumulate over time, with natural selection leading to an increase in the mean
fitness of individuals in a population (note that conflicts between individuals mean that
this isn't the same as the overall population fitness). This will lead to organisms behaving
or appearing as if they were designed to maximize their fitness.
The gene and individual viewpoints are, therefore, not competing but just different
ways of looking at the same thing. If we were to ask 'what kind of organism would we
expect natural selection acting on genes to produce?', then we would answer 'organisms
that are designed to maximize their fitness', with the most general definition of fitness
being inclusive fitness (Box 15.1). The beauty of this is that it takes evolutionary theory
based on genes and translates it into a theory about how individuals behave, which is
what fieldworkers can go out and observe, and what theoreticians can use ESS theory to
model with ease. This also makes clear and justifies why intentional language is
sometimes used, such as altruism, spite or 'trying to maximize fitness' - the dynamics
of natural selection will lead to individuals behaving as if they have intention or
purpose.
However, although we can usually assume the equivalence of the genetic and the
individual approach, there are rare occasions when they will disagree. If a gene can
increase its own transmission, at a cost to the individual or other genes in the genome,
then it may be selected to do so. In Chapter 10 we discussed how sex ratio distorters
such as X drivers and male killers do this. In these cases, we have to take a gene's eye
view to explain the behaviours. Nonetheless, such 'ultra-selfish' genes are extremely
rare because the best way for genes to increase their transmission to the next generation
is usually by increasing the reproductive success of the individual carrying them. Ultra-
selfish genes tend to be associated with aspects of reproduction such as the sex ratio,
which can be distorted to their own good. Contrast this with behaviours such as foraging
or avoiding predators, where it is hard to imagine how a gene could increase its own
transmission, other than to increase the success of the individual.
There are two other reasons why genetic conflicts might have relatively rare
consequences for behaviour. Firstly, even in cases where genes can distort things to their
own favour, this can lead to the rest of the genome becoming united to suppress this
distortion, in what Egbert Leigh (1971) eloquently called the 'parliament of the genes'.
Secondly, the traits examined by behavioural ecologists tend to be relatively complex
adaptations, built by multiple genes. The evolution of such traits has relied upon
multiple genes all working in the same direction, in a way that would have been
prevented by genetic conflicts. Consequently, if we are concerned with explaining
adaptation then we are focusing on traits where genetic conflicts are likely to be
relatively unimportant.
To sum up, the gene and individual approach are equivalent as long as we assume
that genetic conflicts are relatively unimportant within individuals. This is expected on
theoretical grounds, but also has been empirically validated by the success of the ESS
Natural selection
on genes will lead
to individuals
which behave as
if they are trying
to, or were
designed to,
maximize their
fitness …
… and so the
genetic and
individual
viewpoints are
equivalent
Genetic conflicts
can occur …
… but are
relatively rare …
… and are
unlikely to lead to
complex
adaptations
