456 Chapter 23
About 50,000 years ago humans began
domesticating wild dogs. By about 14,000 years
ago, people started to favor new breeds of dogs using
artificial selection. Dogs having desired forms of traits
were selected from each litter and later encouraged
to breed. Those with undesired forms of traits were
passed over.
This process has produced scores of domestic
dog breeds, including sheep-herding Border Collies,
sled-pulling Huskies, and dogs as strikingly different as
Great Danes and Chihuahuas (Figure 23.24).
With a little bit of sleuthing on the Web or in a library,
you should be able to discover numerous other exam-
ples of how humans have used artificial selection to
develop desired animal breeds or plant varieties.
How has artificial selection affected aspects of your
own life, such as pets, foods you eat, and ornamental
garden plants?
Figure 23.24 The Great Dane and the Chihuahua are both “designer”
dog breeds.summary
section 23.1 Evolution modifies
existing species. Therefore, broadly speaking,
all species, past and present, share a common
ancestry. The naturalist Charles Darwin
proposed that evolution could occur by way
of a process he named natural selection.
section 23.2 In biology, a population
is a group of individuals of the same species
occupying a given area. The totality of genes
in a population make up its gene pool. Each
kind of gene may have different alleles, and
these genetic variations produce variations
in traits.
The relative numbers of different alleles—that is, the
different versions of genes—can change as a result of four
processes of microevolution: mutation, genetic drift, gene flow,
and natural selection (Table 23.2). The large-scale patterns,
trends, and rates of change among groups of species over time
are called macroevolution.
section 23.3 The theory of evolution
by natural selection holds that there may be
a difference in the survival and reproduction
among members of a population that vary in
one or more traits. That is, under prevailing
conditions, one form of a trait may be
favored because individuals that have it
tend to survive and therefore to reproduce
more often, so it becomes more common than other forms of
the trait.
Organisms tend to come to have characteristics that suit
them to the conditions in a particular environment. This trend
is called adaptation.
A species is a genetic unit consisting of populations of
organisms that closely resemble each other and that can inter-
breed and produce fertile offspring under natural conditions.
New species come into being when the differences between
isolated populations become so great that their members are
not able to interbreed successfully in nature.Mutation A heritable change in DNA
Genetic drift Random fluctuation in allele frequencies
over time, due to chance occurrences
alone
Gene flow Movement of genes (alleles) among
populations as individuals migrate
natural Change or stabilization of allele frequencies
selection due to differences in survival and repro-
duction among variant members of a
populationTable 23.2 Major Processes of Microevolutionsections 23.4–23.6 Evidence of evolu-
tionary relationships comes in part from fossils
and studies of biogeography. Fossils are dated
using radiometric dating.
Comparative morphology often reveals
similarities in development that indicate an
evolutionary relationship. Similarities may reveal homologous
structures, shared as a result of descent from a common ancestor.expLoreon your oWn
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