Evolution And History

Evolutionary Forces 43

In such cases, it is unlikely that the

gene frequencies of the smaller

population will be

representative of

those of the larger

one. Isolated island

populations may

possess limited

variability due to

founder effects.

An interesting

example can be seen

on the Pacific Ocean

island of Pingelap in

Micronesia, where 5

percent of the popu-

lation is completely

color-blind, a condi-

tion known as ach-

romotopsia. This is

not the “normal” red-green color blindness that affects 8 to

20 percent of males in most populations but rather a com-

plete inability to see color. The high frequency of achro-

motopsia occurred sometime around 1775 after a typhoon

swept through the island, reducing its total population to

only twenty individuals. Among the survivors was a single

individual who was heterozygous for this condition. Af-

ter a few generations, this gene became fully embedded in

the expanding population. Today a full 30 percent of the

island’s inhabitants are carriers compared to a mere .003

percent seen in the United States.^8

Genetic drift is likely to have been an important factor

in human evolution, because until 10,000 years ago all hu-

mans were food foragers generally living in relatively small

communities. Whenever biological variation is observed,

whether it is the distant past or the present, it is always pos-

sible that chance events of genetic drift are responsible for it.

Gene Flow

Another factor that brings change to the gene pool of

a population is gene flow, or the introduction of new

alleles from nearby populations. Interbreeding allows

antibiotics, and chemicals used in the preservation of food.
Radiation, whether industrial or solar, represents another
important cause of mutation. There is even evidence that
stress can raise mutation rates, increasing the diversity nec-
essary for selection if successful adaptation is to occur.^7
In humans, as in all multicellular animals, the very na-
ture of genetic material ensures that mutations will occur.
For instance, the fact that a gene can be split by stretches
of DNA that are not part of that gene increases the chances
that a mistake in the process of copying DNA will cause
mutations. To cite one example, no fewer than fifty such
segments of DNA fragment the gene for collagen—the
main structural protein of the skin, bones, and cartilage.
One possible benefit of this seemingly inefficient situation
is that it allows the gene segments themselves to be shuf-
fled like a deck of cards, sometimes creating new proteins
with new functions. So although individuals may suffer as
a result, mutations also confer versatility at the population
level, making it possible for an evolving species to adapt
more quickly to environmental changes. Remember, how-
ever, that mutations occur randomly and thus do not arise
out of need for some new adaptation.

Genetic Drift

Another evolutionary force is genetic drift, or the chance
fluctuations of allele frequencies in the gene pool of a pop-
ulation. These changes at the population level come about
due to random events at the individual level. Over the
course of its lifetime, each individual is subject to a num-
ber of random events affecting its survival. For example, an
individual squirrel in good health and possessed of a num-
ber of advantageous traits may be killed in a chance forest
fire; a genetically well-adapted baby cougar may not live
longer than a day if its mother gets caught in an avalanche,
whereas the weaker offspring of a mother that does not die
may survive. In a large population, such accidents of nature
are unimportant; the accidents that preserve individuals
with certain alleles will be balanced out by the accidents
that destroy them. However, in small populations such av-
eraging out may not be possible. Some alleles may become
overrepresented in a population due to chance events.
Because today human populations are large, we might
suppose that human beings are unaffected by genetic drift.
But a chance event, like a rock slide that kills five people
from a small town, say a population of 1,000, could signifi-
cantly alter the frequencies of alleles in the local gene pool.
A particular kind of genetic drift, known as founder
effects, may occur when an existing population splits up
into two or more new ones, especially if one of these new
populations is founded by a small number of individuals.

genetic drift Chance fluctuations of allele frequencies in the

gene pool of a population.

founder effects A particular form of genetic drift deriving

from a small founding population not possessing all the alleles

present in the original population.

gene flow The introduction of alleles from the gene pool of

one population into that of another.

North Pacific

Ocean

South Pacific

Ocean

SOLOMON

ISLANDS

TRUK

ISLANDS

(CHUUK)

PINGELAP

POHNPEI

MICRONESIA

PAPUA

NEW GUINEA

MARSHALL

ISLANDS

MICRONESIAPINGELAP,

GUAM

(U.S.)

NORTHERN

MARIANA

ISLANDS

Equator

(^8) Sacks, O. (1998). Island of the colorblind. New York: Knopf.
(^7) Chicurel, M. (2001). Can organisms speed their own evolution? Science
292 , 1824–1827.