gENETIC dRIfT: EvolUTIoN AT RANdoM 171
one copy to become the ancestor of all living copies will vary randomly between
the simulations.
We are now discussing the genealogy of genes: the paths of their inheritance
across generations. Looking backward in time, when the lineages of two gene cop-
ies merge we say that they coalesce. These events appear in Figure 7.5 as branch
points (nodes) on the gene tree that represent the most recent common ancestor of
the gene copies. As we have seen, if we go back far enough, we are certain to arrive
at the most recent common ancestor of all copies of the gene now present in a spe-
cies^1. That typically happens in the recent past when the population size is small,
and in the remote past when it is very large.
Figure 7.5 illustrates another key point. Although all the gene copies in genera-
tion 7 descended from a single copy in generation 0, that does not mean there was(^1) The picture is a bit more complicated for loci that recombine. Any given DNA base in a locus
has a gene tree of the sort we just described. But recombination between different parts of a lo-
cus causes them to have different gene trees. For the purposes of our discussion here, however,
we will ignore this complexity because the central principles remain intact. In particular, for
any locus there is a most recent common ancestor for all copies of the locus carried by living
individuals.
Futuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_07.05.ai Date 11-14-2016 01-18-17
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FIGURE 7.5 Evolution of a gene tree. In this example, there
is no selection, and differences in the number of descen-
dants left by different copies of the gene are entirely the
result of random genetic drift. (A) The population begins at
generation 0 with five diploid individuals, so there are a total
ten copies of the gene. The shapes and colors are to help
distinguish the different copies, which may or may not differ
in their DNA sequences. (D) By generation 7, all of the gene
copies are descendants from a single copy in generation
- Their gene tree is shown by the red lines. Coalescence
events are shown by gene copies with bold outlines. The
most recent common ancestor of all gene copies in genera-
tion 7 is in bold at the top of the gene tree in generation 0.
07_EVOL4E_CH07.indd 171 3/23/17 9:09 AM