Biology: A Global Approach
Chapter 23 Microevolution 2
Concept 23.1 Genetic variation makes evolution possible 2
Concept 23.2 The Hardy-Weinberg equation can be used to test whether a population is
evolving. 3
Concept 23.3 Natural selection, genetic drift, and gene flow can alter allele frequencies in
a population 5
Concept 23.4 Natural selection is the only mechanism that consistently causes adaptive
evolution 6
1
, Chapter 23 Microevolution
A common misconception about evolution is that individual organisms evolve. It is true that
natural selection acts on individuals. But the impact of natural selection only becomes
apparent in how a population of organisms changes over time.
Focusing on evolutionary change in populations, we can define evolution on its smallest
scale, called microevolution, as a change in allele frequencies in a population over
generations. There are three main mechanisms that can cause such changes in allele
frequencies: natural selection, genetic drift (change events that alter allele frequencies), and
gene flow (the transfer of alleles between populations). Each of these mechanisms affects
the generic composition of populations, but only natural selection consistently improves the
degree to which organisms are well suited for life in their environment.
Concept 23.1 Genetic variation makes evolution possible
Genetic Variation
Individuals within all species vary in their phenotypic traits.
Such phenotypic variations often reflect genetic variation, differences among individuals in
the composition of their genes of other DNA sequences. Some heritable phenotypic
differences occur on an ‘either-or’ basis. Characters that vary in this way are typically
determined by a single gene locus, with different alleles producing distinct phenotypes. In
contrast, other phenotypic differences vary in gradation along a continuum. Such variation
usually results from the influence of two or more genes on a single phenotypic by multiple
genes.
Genetic variation at the whole-gene level can be quantified as the average percentage of loci
that are heterozygous.
Considerable genetic variation can also be measured at the molecular level of DNA. But little
of this variation results in phenotypic variation. Many nucleotide variations occur within
introns, noncoding segments of DNA lying between exons, the regions retained in mRNA
after RNA processing. And of the variations that occur within exons, most do not cause a
change in the amino acid sequence comparison.
It is important to bear in mind that some phenotypic variation does not result from genetic
differences among individuals. Phenotype is the product of an inherited genotype and many
environmental influences. In general, only the genetically determined part of phenotypic
variation can have evolutionary consequences.
Sources of Genetic Variation
Genetic variation originates when mutation, gene duplication, or other processes produce
new alleles and new genes. Genetic variants can be produced rapidly in organisms with
short generation times. Sexual reproduction can also result in genetic variation as existing
genes are arranged in new ways.
Mutations can be caused by factors such as errors in DNA replication, exposure to UV light
and other high-energy forms of radiation, and exposure to certain chemicals. A change of as
little as one base in a gene, a point mutation, can have a significant impact on phenotype.
In some cases, natural selection quickly removes such harmful alleles. In diploid organisms,
however, harmful alleles that are recessive can be hidden from selection. Indeed, a harmful
recessive allele can persist for generations by propagation in heterozygous individuals.
2
Chapter 23 Microevolution 2
Concept 23.1 Genetic variation makes evolution possible 2
Concept 23.2 The Hardy-Weinberg equation can be used to test whether a population is
evolving. 3
Concept 23.3 Natural selection, genetic drift, and gene flow can alter allele frequencies in
a population 5
Concept 23.4 Natural selection is the only mechanism that consistently causes adaptive
evolution 6
1
, Chapter 23 Microevolution
A common misconception about evolution is that individual organisms evolve. It is true that
natural selection acts on individuals. But the impact of natural selection only becomes
apparent in how a population of organisms changes over time.
Focusing on evolutionary change in populations, we can define evolution on its smallest
scale, called microevolution, as a change in allele frequencies in a population over
generations. There are three main mechanisms that can cause such changes in allele
frequencies: natural selection, genetic drift (change events that alter allele frequencies), and
gene flow (the transfer of alleles between populations). Each of these mechanisms affects
the generic composition of populations, but only natural selection consistently improves the
degree to which organisms are well suited for life in their environment.
Concept 23.1 Genetic variation makes evolution possible
Genetic Variation
Individuals within all species vary in their phenotypic traits.
Such phenotypic variations often reflect genetic variation, differences among individuals in
the composition of their genes of other DNA sequences. Some heritable phenotypic
differences occur on an ‘either-or’ basis. Characters that vary in this way are typically
determined by a single gene locus, with different alleles producing distinct phenotypes. In
contrast, other phenotypic differences vary in gradation along a continuum. Such variation
usually results from the influence of two or more genes on a single phenotypic by multiple
genes.
Genetic variation at the whole-gene level can be quantified as the average percentage of loci
that are heterozygous.
Considerable genetic variation can also be measured at the molecular level of DNA. But little
of this variation results in phenotypic variation. Many nucleotide variations occur within
introns, noncoding segments of DNA lying between exons, the regions retained in mRNA
after RNA processing. And of the variations that occur within exons, most do not cause a
change in the amino acid sequence comparison.
It is important to bear in mind that some phenotypic variation does not result from genetic
differences among individuals. Phenotype is the product of an inherited genotype and many
environmental influences. In general, only the genetically determined part of phenotypic
variation can have evolutionary consequences.
Sources of Genetic Variation
Genetic variation originates when mutation, gene duplication, or other processes produce
new alleles and new genes. Genetic variants can be produced rapidly in organisms with
short generation times. Sexual reproduction can also result in genetic variation as existing
genes are arranged in new ways.
Mutations can be caused by factors such as errors in DNA replication, exposure to UV light
and other high-energy forms of radiation, and exposure to certain chemicals. A change of as
little as one base in a gene, a point mutation, can have a significant impact on phenotype.
In some cases, natural selection quickly removes such harmful alleles. In diploid organisms,
however, harmful alleles that are recessive can be hidden from selection. Indeed, a harmful
recessive allele can persist for generations by propagation in heterozygous individuals.
2
