17.2 – Evolution as Genetic Change in Populations
Chapter 17 – Evolution of
Populations
17.2 - Evolution as Genetic Change in
Populations
17.2 – Evolution as Genetic Change in Populations
How Natural Selection Works – Review
In genetic terms, what does fitness mean?
-Passing on genes onto offspring
Adaptation: Any genetically controlled trait that
increases an individual’s ability to pass along its
alleles.
17.2 – Evolution as Genetic Change in Populations
Natural Selection on Single-Gene Traits
• Natural selection on a single-gene trait can lead
to changes in allele frequencies and thus to
differences in phenotype frequency.
17.2 – Evolution as Genetic Change in Populations
Natural Selection on Polygenic Traits
• Polygenic traits have a range of phenotypes that often form
a bell curve.
• The fitness of individuals may vary from one end of the
curve to the other.
• Natural selection can affect the relative fitness of a
phenotype and result in one of three types of selection.
• Direction Selection
• Stabilizing Selection
• Disruptive Selection
17.2 – Evolution as Genetic Change in Populations
Directional Selection
• When individuals at one end of the curve have higher
fitness than individuals in the middle or at the other end.
• The range of phenotypes shifts.
17.2 – Evolution as Genetic Change in Populations
Stabilizing Selection
• Individuals near the center of the curve have higher fitness
than individuals at either end.
• Center of the curve at its current position, narrows the
overall graph.
17.2 – Evolution as Genetic Change in Populations
Disruptive Selection
• When individuals at the upper and lower ends of the
curve have higher fitness than individuals near the
middle.
• Acts against individuals of an intermediate type
• Can create 2 distinct phenotypes
17.2 – Evolution as Genetic Change in Populations
Changes in Gene Pools Due to Chance
• Genetic Drift: a change in the gene pool (allele
frequency) of a population due to chance.
• Over time, a series of chance occurrences can cause an
allele to become more or less common in a population.
17.2 – Evolution as Genetic Change in Populations
Examples of Genetic Drift
• Bottleneck Effect - Chance decreases the size and genetic
variation of the population, decreases adaptability
• Ex. - when a population is reduced in size (due to a
disaster) and ultimately the gene pool of the
population is also reduced in size
17.2 – Evolution as Genetic Change in Populations
Examples of Genetic Drift
•Founder Effect - when few individuals colonize an isolated
environment, reducing the gene pool of the population
•Chance decreases genetic variation
•Change in the gene pool is determined by the founders
of the colony
17.2 – Evolution as Genetic Change in Populations
Other Mechanisms of Gene Pool Changes
• Gene Flow - the exchange of genes with another population
• Occurs when fertile individuals or their gametes migrate
between populations (ex: wind and pollen)
• Tends to reduce genetic differences between populations
17.2 – Evolution as Genetic Change in Populations
Natural Selection and Fitness
• Only natural selection leads to adaptation
• Genetic Drift, Gene Flow, and Mutations do not
necessarily lead to adaptation
• These do not tend to increase fitness within a
population
UseGenetic
the choices
on the
right to
17.2 – Warm-up:
Evolution as
Change
in Populations
describe the scenarios on the left
Scenario 1: 4 moose were taken from the Canadian
mainland to Newfoundland, in 1904. These 2
males and 2 females rapidly formed a large
population of moose that now flourishes in
Newfoundland.
Scenario 2: The fossil remains of pygmy (or dwarf)
mammoths (1.5 m to 2 m tall) have been found
on Santa Rosa and San Miguel Islands off the
coast of California. This population of pygmy
mammoths is descended from a population of
mammoths of normal size (4 m tall). Dwarfing is
common in island populations and is not the
result of chance events.
Scenario 3: Bearded Vultures in the Alps was nearly
hunted to extinction. The population is currently
small and lacks genetic variation.
Which mechanism is
most likely to have
contributed to the 3
scenarios to the left?
A. Bottleneck Effect
B. Founder effect
C. Natural Selection
17.2 – Evolution as Genetic Change in Populations
Evolution VS Genetic Equilibrium
• If a population is not evolving, allele frequencies in
its gene pool do NOT change this is genetic
equilibrium
• Sexual reproduction and allele frequency: A
population of sexually reproducing organisms could
remain in genetic equilibrium (random mating)
17.2 – Evolution as Genetic Change in Populations
Hardy-Weinberg Principle
• Hardy-Weinberg principle - states that allele
frequencies in a population should remain constant
unless one or more factors cause those frequencies
to change.
• 5 conditions that disturb genetic equilibrium and
cause evolution to occur:
17.2 – Evolution as Genetic Change in Populations
5 Conditions that Disturb Genetic
Equilibrium
1. Nonrandom mating
a. Sexual selection: When individuals of a species select
mates based on heritable traits (size, strength, color, etc)
2. Small population size (Genetic drift happens easily)
3. Immigration/Emigration (Introduction of new alleles to a gene
pool, or removing alleles from a gene pool)
4. Natural Selection (If different genotypes have different
fitness, evolution will occur)
5. Mutation