Reviving group selection
"Group selection" refers to differential reproductive success
It is the idea that groups of organisms vary naturally - and
exhibit differential reproductive success (in terms of
spawning new groups) on the basis of the traits of the
Group selection - problems
Habitat-specific selection is of some interest since it
can help make models of group selection more viable.
Differential reproductive success of groups consisting
of sexually-reproducing individuals depends on the
groups maintaining their own identity in the face of
sexual recombination constantly stirring groups together.
Partial group isolation and population viscosity have been
invoked to explain how groups could maintain their
individuality in the past.
However, theoretical considerations have indicated that the
rate of between-group migration would have to be very low -
or - equivalently - population viscocity would have to be
very high in order to make the effect work - and
observations of actual groups of individuals suggest such
partial-isolation conditions are rarely met in practice.
Group selection - solutions
A simple observation puts quite a spanner in this
theoretical argument. The above argument is essentially
asserting that sexual groups don't vary from place to place
- because the stirring effect of sexual recombination is too
great for this to happen.
However human races present a fairly clear counter-example
to this - where there exists very significant geographical
variation in the human population.
Where did the analysis go wrong?
Several effects make group selection more viable than the
analasis above might suggest.
Among them are:
These effects act together synergetically:
- Habitat-specific selection;
- Divergent selection;
To consider the effects in turn:
- Habitat-specific selection acts to generate
- Xenophobic tendencies act to amplify existing variability
between groups, and to reinforce group boundaries.
- Divergent selection arises when hybrid offspring between
members of the two groups have reduced viability.
Habitat-specific selection arises when a single population is
exposed to a range of different environments - and is subjected
to different selection pressures in the different locations.
Selection on the basis of environmental properties is likely
to result in adaptations to local environments.
This effect can be seen clearly in the case of the manner in
which human skin colour is associated with lattitude.
It has been previously suggested (by Pagel and Mace) that
xenophobia is an adaptation to facilitate the existence of
cooperative groups. Distrust of outsiders has the effect of biasing
favourable interactions towards previous givers of altruistic events -
and results in a healthy distrust of outsiders.
There may also have been other reasons to distrust outsiders
historically. For one thing an outsider is normally less
likely to be a cousin than someone in an agent's existing
Divergent selection arises when hybrid offspring between
members of the two groups have reduced viability. That
could happen if the offspring contained a mixture of traits
- and winds up not being well adapted to the habitat of
either of its parents.
Such selection pressure typically causes females of one
group to reject the advances of males not of their own
Divergent selection is normally a form of sexual selection.
It can happen due to females rejecting the advances males
from other groups - in order to avoid having to invest in
"half-breed" offspring - that do not fit into either group -
or who may have reduced fertility for other reasons.
Divergent selection reduces the effect of sex in stirring
the population by reducing gene flow across group boundaries.
It invalidates the assumption that group membership does not
influence breeding prospects.
Causes of habitat-specific selection
Different habitats may contain:
Such factors can create significantly selection pressures in
- Different sources of food;
- Different predators;
- Different competitors;
- Different pathogens;
- Different temperature, humidity, illumation - and other physical factors;
For example, an organism that eats slow moving sedentary
creatures on one continent - but is forced to chase fast-
moving creatures on another one may develop different
adaptations in the two environments do deal with the
different problems - despite the two populations remaning
genetically linked by gene flow between them - with
the differences being continually reinforced by selection.
Note that groups adapted to particular prey can be
obliterated when their prey goes extinct.
Similarly, groups which have lacked recent exposure to a
particular predator may be obliterated if the predator
invades their environment.
Habitat-specific selection is made possible because of the
effect of environmental inheritance.
As well as inheriting genes from their parents, organisms
also inherit their environment.
The environment contains a lot of information - far
more than is in any organism's genome.
However the environment is not usually static. Mobile
organisms can uften easily move to new locations -
and when that can happen the significance of environmental
inheritance is reduced.
Environmental inheritance can expected to be powerful when
movement to new environments is more difficult.
For example, if the organisms are land-dwellers living on an
island, the chances that they will migrate to a different
environment from the one their parents inhabited decrease.
There's a similar effect when environments are large,
compared to the distance organisms can travel. Latitude is
often strongly inherited for this reason, along with its
associated weather patterns.
Consider a model where habitat-specific selection acts on
one gene. Different alleles can be expected to be favoured
in their respective environments - and will be found
preferentially in them.
However other loci are likely to be less strongly affected.
The effect on them will be due to linkage with the affected
genes. Genes on other chromosomes are likely to escape any
effect - while genes nearby on the same chromosome are
likely to be most affected.
In practice, traits are often complex - and are associated
with more than one locus. Environmental differences may
create a whole suite of associated adaptations - with a
large number of genes being involved. The genes involved
may well be numerous - and spread across many chromosomes.
In that way, selection for a particular suite of traits by
the environment may create selection pressure that affects
practically every locus - via linkage.
I don't expect these considerations such as these to lead to
a radical revival of group selection. However, group
selection does seem to me to have more going for it than
many suspect, and does seem to have suffered from some
unfair public relations in the past.
- Steve Connor - Racism and xenophobia linked to biological fear of outsiders in Stone Age