Evolution__3rd_Edition

294 PART 3 / Adaptation and Natural Selection
We define the question of “what is
the unit of selection?”
The segregation distorter gene
breaks Mendel’s laws
organs, cells, proteins, and genes. (Though if we trace the effect down through the
nucleotides and their constituent atoms, it again disappears. A lion’s atoms survive just
as well whether it is alive and well fed or dead due to starvation.)
The levels of organization, from gene through individual lion to Felidae, are to a
large extent bound together in their evolutionary fate, and what benefits one level will
usually also benefit the others. However, this is not always so. Male lions can only join a
pride by forcibly evicting the incumbent males. In the fight, lions may get killed or
wounded, and in any case lions have a low rate of survival after they have been evicted
from a pride. These fights have losers as well as winners: here the benefit of winning is
confined to the individual (or male coalition) level and below. The lion species does not
benefit. The survival of the species may be little affected by the death of male lions,
because the mating system is polygynous and has plenty of males to spare; but the effect
is clearly not positive.
Different adaptations, therefore, have different consequences for different units in
nature. At one extreme, there may be adaptations a an improved DNA replication
mechanism perhaps a that could benefit all life, but most adaptations will benefit only
a smaller subsample of living things. Because the levels of living organization are bound
together, if natural selection produces an adaptation to benefit one level, many other
levels will benefit as a consequence. The question in this chapter is whether natural
selection really acts to produce adaptations to benefit one level, with benefits at other
levels being incidental consequences, or whether it acts to benefit all levels. And if it
benefits one level, which is it? In evolutionary biology, this question is expressed as
“What is the unit of selection?”
We shall seek to answer it by looking at a series of adaptations that appear to benefit
different “levels” in the hierarchy of biological organization. Some adaptations seem to
be in the interests of individual genes, at the organism’s expense, others benefit organisms
at the group’s expense, others may benefit higher levels. When we have seen the
example, we can discuss generally which of the types of adaptation we should expect to
see most often in nature.
11.2 Natural selection has produced adaptations that
benefit various levels of organization
11.2.1 Segregation distortion benefits one gene at the expense
of its allele
With normal Mendelian segregation at a genetic locus, on average half of an organism’s
offspring inherit one of the alleles and the other half the other allele. Mendelian segregation
is so to speak “fair” in its treatment of genes: genes emerge from Mendelian segregation
in the same proportions as they went in. There are, however, some curious cases in
which Mendel’s laws are broken in which one of the alleles, instead of being inherited
by 50% of a heterozygote’s offspring, is consistently overrepresented. The segregation
distorter gene of Drosophila melanogaster is an example of this phenomenon, which is
also called meiotic drive.
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