Northern elephant seals are remarkable animals. They spend most of their
lives at sea. While hunting fish and squid, the seals dive a mile deep and can
hold their breath for more than an hour. During the breeding season, males
have dramatic fights, and the most successful mate with dozens of females
(FIGURE 7.1).
Elephant seals are exceptional in yet another way: they are one of the least
genetically variable mammals known [4]. This is a recent state of affairs. When
European settlers arrived on the Pacific coast of North America in the 1700s, they
found elephant seals living from Alaska south to Mexico. The seals were easily
hunted for their blubber and fur, and by the mid-1800s the population had
been reduced to fewer than 40 individuals. Fortunately, hunting was banned
before the species was driven to extinction. Today there are more than 100,000
northern elephant seals, and the population continues to expand.
Comparison of specimens collected in the 1800s with living individuals shows
that elephant seals had much more genetic variation before their population
crashed [4, 13]. This loss of variation illustrates one of the effects of random
genetic drift, an evolutionary process we have mentioned but not yet discussed
in any detail. Drift explains features of the living world that natural selection can-
not. It also provides us with tools to estimate population sizes, phylogenies, and
other important features of nature.
This chapter opens by looking at drift from two perspectives: forward in time
and backward in time. It then turns to the factors that determine the strength of
drift, and how drift affects genetic variation. The next topic is how drift interacts7
These females of the northern elephant seal (Mirounga angustirostrus) have come
ashore to breed. A severe population crash in the nineteenth century caused
by hunting dramatically reduced genetic variation in this species, an example of
random genetic drift.Genetic Drift:
Evolution at
Random
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