Evolution, 4th Edition

564 CHAPTER 21

There are now more than 7 billion humans on Earth, but our numbers were

very much smaller just a few dozen generations ago. The human population living

in Africa 100 Kya had an effective population size of only about 10,000 individuals,

so few that we would now consider them an endangered species. Genetic drift was

intense. As a result, the large majority of amino acid substitutions in the proteins

of the human lineage were fixed by drift, not by positive selection [35]. And many

of them were deleterious.

The effects of drift were even more severe for the humans who left Africa. Recall

from Chapter 7 that the effective size of a population can be estimated from the

level of heterozygosity (genetic variation). DNA sequence variation in human

populations outside Africa suggests that the original exodus may have involved

only about 2000 people. Each time humans spread to an even more remote part of

the planet, a small number of bold colonists set out. This caused a series of genetic

bottlenecks that increased in number the farther from Africa they went. Population

genetic theory shows that bottlenecks cause heterozygosity to be lost, so we expect

variation to decline with distance from Africa. That is exactly the pattern seen in

native populations around Earth today (see F i g u r e 7.7). But the smaller a popula-

tion is, the less effective natural selection becomes (see Chapter 7). Those bouts of

intense drift caused by repeated episodes of colonization fixed many deleterious

mutations. Again, the pattern seen in modern humans matches what theory pre-

dicts (FIGURE 21.20). Africans have the fewest deleterious mutations, while Euro-

peans and Asians have more. People whose ancestors managed to spread all the

way across Asia, the Bering Strait, and finally into the New World, are burdened

with even more deleterious mutations.

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_21.20.ai Date 01-18-2017

San Mbuti

Mozabite

Pathan

Cambodian

Yakut Maya

San

Mbuti

Mozabite

Pathan

Cambodian

Yakut

Maya

12,850

12,950

13,000

12,900

Number of deleterious mutations

12,750

12,700

12,800

(A)

(B)

FIGURE 21.20 The number of deleterious mutations

in the human genome increases with the distance of a

population from southern Africa. (A) Samples from seven

populations were analyzed from the points shown on

the map. Pathways by which ancient humans may have

colonized Earth are shown by the arrows. (B) The y-axis

shows the number of mutations in samples of people

from each population that alter protein structure and are

estimated to have negative fitness effects. The colors for

each population correspond to the points on the map.

The populations are arranged from left to right with

increasing distance from Africa along the hypothesized

migration routes. For each population, the horizontal bar

shows the median, the box shows the middle 50 percent

of the distribution, and the whiskers show the full range

of values. (After [27].)

21_EVOL4E_CH21.indd 564 3/22/17 1:51 PM