HUMAN BIOLOGY

442 Chapter 23

different versions of a trait. The “fittest” traits—and the

gene alleles that govern them—are more likely to be

“selected” for survival.

5. A population is evolving when some forms of a
   trait are becoming more or less common relative
   to the other forms. The shifts are evidence that the
   corresponding versions of genes are becoming more or
   less common.


6. Over time, shifts in the makeup of gene pools have
   been responsible for the amazing diversity of life forms
   on Earth.
   As natural selection occurs over time, organisms come to
   have characteristics that suit them to the conditions in a
   particular environment. We call this trend adaptation.
   Recall from Section  1.6 that the accumulated evidence
   of evolution and natural selection has elevated both ideas
   to the status of fundamental principles of the living world.
   Later we will consider a few examples of this evidence.

Chance can also change a gene pool

Natural selection is not the only process that can adjust the

relative numbers of different alleles in a gene pool. Chance

can also play a major role. This kind of gene pool tweak-

ing is called genetic drift. Often the change is most rapid

in small populations. In one

type of genetic drift, called

the founder effect, a few indi-

viduals leave a population

and establish a new one. By

chance, the relative numbers

of various alleles in the new

population probably will

differ from those in the old

group. For example, geneticists have strong evidence that

ethnic Finns are descended from a small band of people

who settled in what is now Finland about 4,000 years ago.

Today, blond hair and blue eyes are distinctive Finnish fea-

tures. In addition, at least thirty genetic disorders that are

rare elsewhere are common in Finland.

The makeup of a gene pool also can change as indi-

viduals migrate into or out of a population. This physical

movement of alleles, or gene flow, helps keep neighboring

populations genetically similar. Over time it tends to coun-

ter the differences between populations that arise through

mutation, genetic drift, and natural selection. Now that

international travel is common, gene flow among human

populations has increased dramatically.

In Finland, there has historically been little gene flow.

Climate and geography have isolated Finns from the rest

of Europe for centuries. Even now, Finns have much less

genetic variation than other Europeans.

Finnish boy

Layne Kennedy/Documentary Value/Corbis

n From the perspective of evolution, genetic mutations are

important because they are the source of alleles—the

alternative forms of genes.

n Links to Gene mutations 20.8, Changes in chromosomes 20.9

Mutation produces new forms of genes

A mutation is harmful when it alters a trait such that an

affected individual can’t survive or reproduce as well as

other individuals. For example, for us humans, small cuts

are common. But before modern medical treatments were

available, people with hemophilia,

whose blood does not clot properly,

could die young from such minor

injuries. As a result, the various

hemophilias were rare, because

affected people rarely lived long

enough to pass on the faulty genes.

By contrast, a beneficial trait improves

some aspect of an individual’s func-

tioning in the environment and so

improves chances of surviving and

reproducing. A neutral trait, such as

attached earlobes in humans, nei-

ther helps nor hurts survival.

natural selection can reshape the genetic

makeup of a population

Changes in genes are the raw material of microevolution,

the process that leads to new species. Actually, several

processes are included in this category, but the one that

probably accounts for most changes in the mix of alleles

in a gene pool is natural selection. As Section  23.1 noted,

Darwin formulated his theory of evolution by natural

selection by correlating his understanding of inheritance

with certain features of populations. In 1859 he published

his ideas in a classic book, On the Origin of Species. We can

express the main points of Darwin’s insight as follows:

1. The individuals of a population vary in their body
   form, functioning, and behavior.


2. Many variations can be passed from generation to
   generation. This simply means that different versions
   of genes—alleles—can pass from parents to offspring.


3. In every set of circumstances, some versions of a
   trait are more advantageous than others. That is,
   some traits impart a better chance of surviving and
   reproducing. The expression “survival of the fittest”
   is verbal shorthand for this advantage.


4. Natural selection is the difference in survival and
   reproduction that we observe in individuals who have

23.3 microevolution: How new species arise

adaptation Accumulation
of adaptive traits.

gene flow Physical move-
ment of gene alleles into or
out of a population’s gene
pool.

genetic drift Chance
change in a gene pool.

natural selection Process
of evolution in which indi-
viduals of a species are
more or less likely to survive
and reproduce, depending
on the details of their inher-
ited traits.

Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).