Evolution__3rd_Edition

306 PART 3 / Adaptation and Natural Selection
Heritability is the key to which
levels of organization show
adaptations
Organisms show heritability in this sense. A finch with an improved beak shape,
caused by a genetic change, will on average produce offspring with the improved beak
shape. Natural selection can work on individual finches.
But groups do not show this sort of heritability when group and individual advantage
are in conflict. 1 A genetic variant that increases a group’s chance of success tends
not to be inherited by future groups. Immigration contaminates the group’s genetic
composition, such that heritability from generation to generation is low. Thus altruistic
groups do not exclusively generate descendant altruistic groups, and selfish groups
generate selfish groups. Migration from selfish groups causes altruistic groups to
become selfish. Thus a group in one generation will only be genetically correlated with
the group of its offspring in the next generation when there is practically no migration;
then group selection works.
The same point can be made about kin selection and selection among cell lines.
Kin selection operates because an “offspring” kin group genetically resembles the
“parental” kin group. Cell selection, in Weismannist species, tends not to operate
because somatic cells, although they are inherited down a cell line during one organism’s
brief life, are not passed on from an organism to its offspring; but when they are
(in non-Weismannist species), cell selection and the evolution of cell line adaptations
becomes theoretically more plausible. For genic adaptations such as segregation distortion,
the same basic argument applies.
In summary, we should expect to find adaptations existing for the benefit of those
units in nature that show heritability. Adaptations will, therefore, usually benefit
organisms. The cases of adaptations that benefit higher or lower levels of organization
can be understood in the same general terms, because they only evolve in circumstances
when groups, or parts, of organisms show heritability from one generation to
the next. We can now give our first answer to the question of what is the unit of selection.
The general answer is “that entity that shows heritability”; more specifically, it is
usually the organism, with some interesting exceptions. This first answer specifies the
units in nature that should possess adaptations.
11.3 Another sense of “unit of selection” is the entity whose
frequency is adjusted directly by natural selection
Natural selection over the generations adjusts the frequencies of entities at all levels. We
have implicitly seen this in the example of the lion hunt. If the lions of one pride
become more efficient at hunting, perhaps because of some new behavioral trick, natural
selection will favor them. If the trick is inherited, that type of lion will increase in
frequency relative to other types of lion. All things associated with the trick will increase
in frequency too. The type of lion, its type of neurons and proteins, and their encoding
genes would all increase in frequency relative to their alternatives. When the hunting
1 Species-level heritability does exist with species selection, because individual and species advantage are not
in conflict (Section 23.6.3, p. 665). That is why species selection is more widely accepted, in theory, than group
selection.
..