gENETIC dRIfT: EvolUTIoN AT RANdoM 173
ancestor is 2Ne generations. For Drosophila melanogaster, with an effective popula-
tion size of about 1 million, the ancestor of two gene copies typically existed about
2 million generations ago. For chimpanzees, with an effective population size of
about 20,000, the coalescence of two genes typically occurred about 40,000 gen-
erations ago [7]. Because of the random nature of drift, however, different pairs of
gene copies will show substantial variation around these averages.Populations that change in size
The evolutionary history of many species includes times when the population size
was reduced to small numbers. We began this chapter with the northern elephant
seal, whose population crashed in the 1800s and is now rebounding. This is a
dramatic example of a population bottleneck, the situation in which a population
is reduced to a small size for a small number of generations. The term “bottle-
neck” is a metaphor: we visualize a few individuals squeezing through a period of
reduced population size. A population bottleneck causes intense genetic drift for
a brief time.
In the northern elephant seal, the bottleneck involved a drastic reduction of the
entire species. Reduced population size also occurs during a founder event, when
a new population is begun from a small number of individuals. Even if that popu-
lation then grows to a large size, looking backward in time we will see a period
in which the ancestors of the present population were few in number. A founder
event, like a bottleneck, reduces genetic variation.
The zebra finch (Taeniopygia guttata) lives in Australia and the nearby Lesser
Sunda Islands, which they colonized from Australia roughly 1 Mya. The birds on
the Lesser Sundas show the effects of that founder event: they have much less
genetic variation than do Australian birds. This difference can be quan-
tified as the heterozygosity (also called the “nucleotide diversity”), sym-
bolized by π. This is the chance that two chromosomes in a population
have different nucleotides at a given site in their DNA sequence. Het-
erozygosity in zebra finches on the Lesser Sundas is only 20 percent
what it is in Australian birds (π = 0.002 vs. π = 0.010). The reduced het-
erozygosity on the Lesser Sundas suggests that population was founded
by as few as 9 birds (FIGURE 7.6) [2].
Founder events have also shaped genetic variation in human popula-
tions. Pennsylvania Amish are a religious community that is closed to
intermarrying with people from the outside. They now number about
30,000 individuals, but most are descendants of only a few hundred
individuals who arrived in the United States from Europe in the 1700s
(that is, about 12 generations ago). One result of a bottleneck or founder
event is to cause a random set of rare alleles to become more common.
A recessive mutation in the EVC locus causes polydactyly, a condition
in which individuals have six fingers or toes. The mutation is at a fre-
quency of about 7 percent in Amish populations in the United States,
hundreds of times higher than it is elsewhere in Europe or North Amer-
ica [26]. Genealogies show that all copies of the mutation in the Amish
descend from a single copy carried either by Mr. or Mrs. Samuel King,
who immigrated to America in 1744. By chance, one of those two was
carrying the rare polydactyly mutation. Had that person accidentally
missed the boat to America, the mutation would be absent from the U.S.
Amish population today. Chance is always a factor in evolution.
Modern humans expanded out of Africa starting about 60,000 years
ago. Small numbers of individuals colonized new regions and then
expanded still further until the entire planet was inhabited. NativeFutuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_07.06.ai Date 11-10-2016 01-18-19Zebra nch, Australia~918,7607,000,000 26,750Zebra nch,
Lesser Sundas2 Mya1 MyaTimeFIGURE 7.6 Estimates of the effective population
sizes for the zebra finch (Taeniopygia guttata) in Aus-
tralia and the nearby Lesser Sunda Islands through
time. Analysis of genetic variation suggests that the
Australian finches are descendants of a population
of roughly 19,000 birds that lived some 2 Mya. The
much smaller and less genetically diverse Lesser
Sunda population was founded by a small number
of colonists (perhaps only 9 individuals) that arrived
from Australia about 1 Mya. (Based on data from [2].)07_EVOL4E_CH07.indd 173 3/23/17 9:09 AM