27Genetics and Evolution
CHAPTER 2
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What Is the Molecular Basis of Evolution?
Scientists began to understand the mechanics of heredity and
how evolution works in populations long before molecular
biologists identified the genetic basis of evolutionary change. With the discovery of DNA (de-
oxyribonucleic acid) molecules in 1953, scientists came to understand how genetic information
is stored in the chromosomes of a cell. Genes, specific portions of DNA molecules, direct the
synthesis of the protein molecules upon which all living organisms depend. Through the process
of biological reproduction, each of us inherits a combination of genes from our biological parents
that creates a unique new individual.
What Is Evolution?
Although all living creatures ultimately share a common an-
cestry, they have come to differ from one another through
the process of evolution. Biological evolution refers to genetic
change over successive generations. The process of change is
characterized by descent with modification, as descendant
populations diverge from ancestral ones. As a population’s
genetic variation changes from one generation to another,
genetic change is reflected in visible differences between or-
ganisms. With sufficient genetic change, a new species can
appear. Thus, the process of evolution provides a mechanism
to account for the diversity of life on earth.
What Are the Forces
Responsible for Evolution?
Four evolutionary forces—mutation, genetic drift, gene flow,
and natural selection—account for change in the genetic
composition of populations. Random mutations introduce
new genetic variation into individual organisms. Gene flow
(the introduction of new gene variants from other popu-
lations), genetic drift (random changes in frequencies of
gene variants in a population), and natural selection shape
genetic variation at the population level. Natural selection is
the mechanism of evolution that results in adaptive change,
favoring individuals with genetic variants relatively better
adapted to the conditions of local environments.