Evolution__3rd_Edition

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Summary
1 Adaptations evolve by means of natural selection.
When natural selection acts, it alters the frequencies of
entities at many levels in the hierarchy of biological
levels of organization. It also produces adaptations
that benefit entities at many levels.
2 The discussion of units of selection aims to find out
which level natural selection directly acts on, and
which ones it affects only incidentally.
3 Evolutionary biologists are interested in what the
unit of selection is both in order to understand why
adaptations evolve and also in order that, when they
study adaptations, they can concentrate on theoretically
sensible hypotheses.
4 We can find out which level of organization shows
adaptations by considering a series of adaptations at
genic, cellular, organismic, and group levels and asking
which evolves most often.
5 Segregation distortion is an adaptation of a gene
against its allelic alternatives. Examples of this kind are
rare.
6 In Weismannist organisms, with separate germ and
somatic cell lines, selection between cell lines is a
weak force. But many species do not have separate
germ lines and in these we expect cell lines to evolve
adaptations enabling them to proliferate at the
expense of other cell lines. No clear examples are
known, but Buss has suggested that the embryology of
modern Weismannist species can be explained by a
history of cell selection.
CHAPTER 11 / The Units of Selection 311
between interactors and replicators, and Dawkins (1982) between vehicles and replicators.
It is most important, however, to realize that there are two distinct issues and to
understand the arguments used in the two cases.
Adaptations evolve because the genes encoding them out-reproduce the alternative
genes. In this sense, adaptations can only evolve if they benefit replicators. Genes do
not, however, exist nakedly in the world, and the kinds of adaptations that evolutionary
biologists seek to understand, such as social behavior, beak shape, or flower coloration,
are not simple properties of genes. They are phenotypic properties of higher level
entities (whole organisms, or societies). We therefore also have to ask which higher
level entities should benefit from the natural selection of replicating genes. The answer
is usually organisms, but in some cases it is a family of genetically related organisms.
7 Adaptations are common at the level of organisms.
When genetic relatives interact, adaptations may
evolve for the benefit of kin groups (kin selection).
8 Group selection, in which selection produces
adaptations for the benefit of groups of unrelated
individuals, is thought to be a weak force.
9 Adaptations are possessed by the levels in the hierarchy
of life that show heritability, in the sense that
genetic changes are inherited by the progeny at that
level. Group selection is weak because of the low
genetic correlation (heritability) between succeeding
generations of groups.
10 Natural selection only adjusts the frequencies
of entities that are sufficiently permanent over evolutionary
time. It therefore fundamentally adjusts the
frequency of small genetic units. This small genetic
unit is called the replicator. The gene can be so defined
to be the unit of selection; but it is then not necessarily
always a cistron in length.
11 Adaptations evolve because they increase the
replication of genes. The replication of genes, in the
real world, is enhanced by adaptations that benefit
entities that show heritability.
12 The question of whether natural selection adjusts
the frequencies of genes or of organisms is distinct
from the question of the relative power of individual,
kin, and group selection.