Evolution, 4th Edition

170 CHAPTER 7

College Station, Texas, and genotyped them at several loci [35]. FIGURE 7.4 shows

the data from one locus. Allele frequencies differ on the two sides of the street. The

street acts as a strong barrier to dispersal between the two blocks, and the allele

frequency differences between them almost certainly accumulated by drift.

The genealogy of genes

For a modest cost, you and your parents can have your genomes analyzed. With

those data, you can determine which gene copy at a locus you inherited from your

mother and which from your father. With the genomes of your ancestors going

back further in time, you could trace the evolutionary history of every gene in your

genome. Now imagine we did that for every person now alive. The result would

reveal a remarkable fact: for any given locus in the human genome, there was a

copy of that gene at some time in the past that was the ancestor of all copies of the

gene now carried by all living humans.

To make these ideas more clear, consider a second simulation experiment

(FIGURE 7.5). Here we follow the genealogy of the 10 gene copies at a locus in a

population of 5 individuals. The gene copies may or may not have the same DNA

sequence, but we again ensure there is no selection—all individuals have equal

chances of surviving and reproducing. In the first generation of the simulation,

some gene copies are lost when the individuals carrying them die by random

events, while others are lost when by chance they fail to be passed to a gamete

during meiosis. Looking at Figure 7.5B, we see that only six of the ten copies of the

gene present in generation 0 left descendants to generation 1. By chance alone, four

gene copies present in generation 0 will never be represented in future generations.

When generation 2 is produced by generation 1, accidents of survival and repro-

duction eliminate one more of the original 10 lineages (see Figure 7.5C). Once a

lineage goes extinct, it can never come back from the dead, so the total number of

lineages descending from the original population can only decrease in time.

By the time we reach generation 7, all of the gene copies are the descendants

of just one copy that was present in generation 0 (see Figure 7.5D). Looking from

generation 7 backward in time, we can trace the ancestry of all the gene copies.

That genealogy, shown by the red lines, is called the gene tree.

What happens if we run the simulation again? Genetic drift is random, and so

different copies of the gene will become the lucky winners that become ancestors

of all the future copies. Furthermore, the number of generations that it takes for Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_07.04.ai Date 11-14-2016 01-18-17

Cornu aspersum

0 2010

Meters

FIGURE 7.4 Allele frequency differences 10050

among populations of the garden snail

Cornu aspersum. The pie diagrams show the

frequencies of two alleles at the GOT-1 locus

in groups of snails found on two city blocks.

The sizes of the circles are proportional to

the numbers of individuals in each group.

The “50” allele (shown in blue) is quite

common in the left block but almost absent

from the right block. That difference almost

certainly evolved by drift. (After [35].)

07_EVOL4E_CH07.indd 170 3/23/17 9:09 AM