Evolution, 4th Edition

418 CHAPTER 16

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_16.17.ai Date 01-24-2017

K/Pg Boundary

Human Chimpanzee Gorilla Orangutan Baboon Macaque Vervet Squirrel

monkey

Marmoset Titi

monkey

Mouse

lemur

Lemur Galago

77.5 (97.7–67.12)

57.1 (71.4–49.4)

42.9 (52.4–37.3)

30.5 (36.4–26.9)

18.3 (20.8–16.3)

8.6 (9.2–7.7)

9.9 (11.8–8.9)

6.6*

(8.0–6.0)

6.6*

(7.0–6.0)

20.8 (24.9–18.2)

17.1 (20.5–15.0)

40.9 (51.0–35.3)

Cretaceous

Miocene

Oligocene

Pliocene

Pleistocene

Eocene

Paleocene

80

0

10

20

30

40

50

60

70

Time (Mya)

FIGURE 16.17 Estimated divergence times

for some lineages of primates, based on

maximum likelihood estimates of branch

lengths and calibration of the rate of

sequence evolution using several fossils.

Asterisks denote calibrated nodes (branch-

ing points). The best estimates of diver-

gence time are in boldface. The values in

parentheses show the error, or range of

likely divergence times. (After [33].)

immediately favorable, but become advantageous and sweep through the popula-

tion later, for example when selection pressures change. These mutations affect the

gene trees on a smaller region of the chromosome (compare Figures 5.15 and 5.17)

[29]. So clues about whether adaptation resulted from new mutations versus stand-

ing genetic variation can be gleaned from the gene trees along a chromosome.

The three-spined stickleback (Gasterosteus aculeatus) has independently invaded

thousands of rivers and streams from marine populations in the northern Pacific

and Atlantic oceans. In most of the freshwater populations, bony armor plates on

the side of the body have been greatly reduced, a change that has been traced to

the Ectodysplasin (Eda) locus (FIGURE 16.18). In samples from diverse populations,

most genes have gene trees that cluster by geographic region, as we would expect.

But the Eda sequences in almost all fish with reduced (low) plates, whether they

are from Atlantic or Pacific coastal regions, form a single branch on the gene tree.

This adaptation, then, is based on an allele that has been present in both Atlantic

and Pacific marine populations, and has increased in frequency in many different

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