Lesson Overview
17.1 Genes and Variation
Lesson Overview
Genes and Variation
Genetics Joins Evolutionary Theory
How is evolution defined in genetic terms?
In genetic terms, evolution is any change in the relative frequency of alleles in the
gene pool of a population over time.
Lesson Overview
Genes and Variation
Genotype and Phenotype in Evolution
Natural selection acts directly on phenotype, not genotype.
Some individuals have phenotypes that are better suited to their environment
than others. These individuals produce more offspring and pass on more copies of
their genes to the next generation.
Lesson Overview
Genes and Variation
Populations and Gene Pools
A gene pool consists of all the genes, including all the different alleles for each
gene that are present in a population.
Lesson Overview
Genes and Variation
Populations and Gene Pools
Natural selection operates on individuals, but resulting changes in allele
frequencies show up in populations. Populations, rather than individuals, evolve.
Lesson Overview
Genes and Variation
Sources of Genetic Variation
What are the sources of genetic variation?
Three sources of genetic variation are mutation, genetic recombination during
sexual reproduction, and lateral gene transfer.
Lesson Overview
Genes and Variation
Mutations
Mutations that produce changes in phenotype may or may not affect fitness.
Some mutations may be lethal or may lower fitness; others may be beneficial.
Lesson Overview
Genes and Variation
Lesson Overview
17.2 Evolution as Genetic
Change in Populations
Lesson Overview
Genes and Variation
How Natural Selection Works
How does natural selection affect single-gene and polygenic traits?
Natural selection on polygenic traits can affect the distributions of phenotypes in
three ways: directional selection, stabilizing selection, or disruptive selection.
Lesson Overview
Genes and Variation
Directional Selection
Directional selection occurs when individuals at one end of the curve have higher
fitness than individuals in the middle or at the other end.
For example, if only large seeds were available, birds with larger beaks would have
an easier time feeding and would be more successful in surviving and passing on
genes.
Lesson Overview
Genes and Variation
Stabilizing Selection
Stabilizing selection occurs when individuals near the center of the curve have
higher fitness than individuals at either end.
For example, very small and very large babies are less likely to survive than
average-sized individuals. The fitness of these smaller or larger babies is therefore
lower than that of more average-sized individuals.
Lesson Overview
Genes and Variation
Disruptive Selection
Disruptive selection occurs when individuals at the upper and lower ends of the
curve have higher fitness than individuals near the middle.
For example, in an area where medium-sized seeds are less common, birds with
unusually small or large beaks would have higher fitness. Therefore, the
population might split into two groups—one with smaller beaks and one with
larger beaks.
Lesson Overview
Genes and Variation
Lesson Overview
17.3 The Process
of Speciation
Lesson Overview
Genes and Variation
Isolating Mechanisms
Reproductive isolation occurs when a population splits into two groups and the
two populations no longer interbreed.
Lesson Overview
Genes and Variation
Behavioral Isolation
Behavioral isolation occurs when two populations that are capable of
interbreeding develop differences in courtship rituals or other behaviors.
Lesson Overview
Genes and Variation
Geographic Isolation
Geographic isolation occurs when two populations are separated by geographic
barriers such as rivers, mountains, or bodies of water.
For example, the Kaibab squirrel is a subspecies of the Abert’s squirrel that formed
when a small population became isolated on the north rim of the Grand Canyon.
Separate gene pools formed, and genetic changes in one group were not passed
on to the other.
Lesson Overview
Genes and Variation
Temporal Isolation
Temporal isolation happens when two or more species reproduce at different
times.
For example, three species of orchid live in the same rain forest. Each species has
flowers that last only one day and must be pollinated on that day to produce
seeds. Because the species bloom on different days, they cannot pollinate each
other.
Lesson Overview
Genes and Variation
Founders Arrive
Many years ago, a few finches from
South America—species M—arrived on
one of the Galápagos islands, as shown
in the figure.
Because of the founder effect, the allele
frequencies of this founding finch
population could have differed from
those in the South American population.
Lesson Overview
Genes and Variation
Geographic Isolation
Many years ago, a few finches from
South America—species M—arrived on
one of the Galápagos islands, as shown
in the figure.
Because of the founder effect, the allele
frequencies of this founding finch
population could have differed from
those in the South American population.
Lesson Overview
Genes and Variation
Changes in Gene Pools
Over time, populations on each island
adapted to local environments.
Natural selection could have caused two
distinct populations to evolve (A and B),
each characterized by a new phenotype.
Lesson Overview
Genes and Variation
Behavioral Isolation
Over time, populations on each island
adapted to local environments.
Natural selection could have caused two
distinct populations to evolve (A and B),
each characterized by a new phenotype.
Lesson Overview
Genes and Variation
Competition and Continued Evolution
Birds that are most different from each
other have the highest fitness. More
specialized birds have less competition
for food. Over time, species evolve in a
way that increases the differences
between them, and new species may
evolve (C, D, and E).
Lesson Overview
Genes and Variation
Lesson Overview
17.4 Molecular Evolution
Lesson Overview
Genes and Variation
Gene Duplication
Where do new genes come from?
One way in which new genes evolve is through the duplication, and then
modification, of existing genes.
Lesson Overview
Genes and Variation
Molecular Clocks
What are molecular clocks?
A molecular clock uses mutation rates in DNA to estimate the time that two
species have been evolving independently.