prinCiples of evolution 441What causes evolution?- Evolution results from changes in the genetic makeup of
populations of organisms. - Members of a population share many traits overall, but in
most populations there also is a great deal of underlying
genetic variation.
taKe-Home messagegene pool The total
number and kinds of
genes present in a given
population.
macroevolution Large-
scale patterns, trends, and
rates of evolutionary change
among groups of organisms.
microevolution Genetic
changes that may give rise
to new species.
population A group of indi-
viduals of the same species
living in a given area.a Key evolutionary idea: individuals vary
makeup of whole populations of
organisms. In biology, a population
is a group of individuals of the same
species occupying a given area. As
you know from your own expe-
rience with other people, there is
plenty of genetic variation within
and among populations of the same
species (Figure 23.2).
Members of a population have
similar traits—that is, phenotypes.
They have the same general form
and appearance (morphological traits),
their body structures function
in the same way (physiological traits), and they respond
the same way to certain basic stimuli (behavioral traits).
However, the details of traits vary quite a bit. For instance,
individual humans vary in the color of their body hair, as
well as in its texture, amount, and distribution—an exam-
ple that only hints at the great genetic variation in human
populations. Populations of most other species show the
same kinds of variation.Genetic differences produce variation
In theory, the members of a population have inherited the
same number and kinds of genes. These genes make up
the population’s gene pool. Remember, though, that each
kind of gene in the pool may have slightly different forms,
called alleles. Variations in traits in a population—skin or
hair color, say—result when individuals inherit different
combinations of alleles. Whether your hair is black, brown,
red, or blond depends on which alleles of certain genes you
inherited from your mother and father.
If you go through a bag of chocolate candies that have
different-colored sugar coatings, you’ll see that some colors
turn up more or less often than others do. The same is true
for the gene alleles in a population. Some are much more
common than others. The manufacturer can adjust the
number of “reds” or “blues” in the overall mix of candy
pieces, but where genes are concerned, changes come about
by mutation, natural selection, and other processes. Those
changes can lead to microevolution, which we consider next.Figure 23.2 The traits of individuals in a
population vary. These photos show a small
sample of the outward variation in our own
species, Homo sapiens.All images © Jupiterimages, except bottom right: © Roderick Hulsbergen/www.photography23.2
n Evolution occurs in populations of organisms. It begins
when the genetic makeup of a population changes.
n Links to Concepts of heredity 19.1, Genes 20.1The history of life on Earth spans nearly 4 billion years. It is
a story of how species originated, survived or went extinct,
and stayed put or spread into new environments. The over-
all “plot” of the story is evolution, genetic change in lines of
descent over time. Microevolution is the name for cumu-
lative genetic changes that may give rise to new species.
Macroevolution is the name for the large-scale patterns,
trends, and rates of change among groups of species. Later
on you’ll get a fuller picture of these two patterns. Here, we
begin with a fundamental prin ciple of evolution: The traits
of individuals in a population vary.Individuals don’t evolve—populations do
An individual fish, flower, or person doesn’t evolve. Evo-
lution occurs only when there is change in the geneticCopyright 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).