Evolution
Change in allele frequency in a population
over time.
Population
• A localized group of individuals of the
same species.
Importance of Hardy-Weinberg
• Way to calculate allele frequencies
through time.
• Yardstick to measure rates of
evolution.
Hardy-Weinberg Assumptions
1. Large Population
2. Isolation
3. No Net Mutations
4. Random Mating
5. No Natural Selection
If H-W assumptions hold true:
• The allele frequencies will not change
over time.
• Evolution will not occur.
Causes of Evolution
1. Genetic Drift
2. Gene Flow
3. Mutations
4. Nonrandom Mating
5. Natural Selection
Genetic Drift
• Changes in the gene pool a population
due to random chance
• Types:
1. Plain Genetic Drift
2. Bottleneck Effect
3. Founder's Effect
By Chance
Bottleneck Effect
• Loss of most of the population by
disasters.
• Surviving population may have a
different gene pool than the original
population.
Result
• Some alleles lost.
• Other alleles are over-represented.
• Genetic variation usually lost.
Importance
• Reduction of population size may reduce
gene pool for evolution to work with.
• Ex: Cheetahs
Founder's Effect
• The reduced genetic diversity that results
when a population is descended from a small
number of colonizing ancestors.
• Ex: Old-Order Amish
Result
• Genetic variation reduced.
• Some alleles increase in frequency while
others are lost (as compared to the
parent population).
Genetic Drift has a larger impact on small
populations
Gene Flow
• Movement of genes in/out of a
population.
• Ex:
• Immigration
• Emigration
Result
• Changes in gene frequencies within a
population.
• Immigration often brings new alleles into
populations increasing genetic diversity.
Mutations
• May change allele frequencies (small
population).
• Source of new alleles for selection.
• Often lost by genetic drift.
Nonrandom Mating
• Sexual-Selection
Natural Selection
• Differential success in survival and
reproduction.
Fitness - Darwinian
• The relative contribution an individual
makes to the gene pool of the next
generation.
Chapter 24
The Origin of Species
Question?
• What is a species?
• Comment - Evolution theory must
also explain how species originate.
Question?
• How many species of African Violets are
here?
Two Concepts of Species
1. Morphological
2. Biological
Morpological Species Concept
• Organisms with very similar
morphology (physical form) are the
same species
Problem
• Where does extensive phenotype
variation fit?
Biological Species Concept
• A group of organisms that could
interbreed in nature and produce
fertile offspring.
Problems
• Can’t apply BSC to:
• Asexual organisms
• Extinct species
Speciation (new species arising)
Requires:
1. Variation in the population.
2. Selection/Genetic Drift
3. Reproductive Isolation.
Speciation in lizards
https://www.youtube.com/watch?v=a
DIQFQOCGaI
Reproductive Barriers
• Serve to isolate a populations from
other gene pools.
• Create and maintain “species”.
Main Types of Barriers
Prezygotic - Prevent mating or
fertilization.
Postzygotic - Prevent viable, fertile
offspring.
Prezygotic - Types
1. Habitat Isolation
2. Temporal Isolation
3. Behavioral Isolation
4. Mechanical Isolation
5. Gametic Isolation
Habitat Isolation
• Populations live in different parts of
the environment
• Ex – mountains vs lowlands.
Temporal Isolation
• Breeding
seasons or time
of day different.
• Ex – flowers
open in morning
or evening.
Behavioral Isolation
• Mating or courtship
behaviors different.
• Different sexual
attractions operating.
• Ex – songs and
dances in birds.
Mechanical Isolation
• Structural differences that prevent
gamete transfer.
• Ex – anthers not positioned to put pollen
on a bee, but will put pollen on a bird.
Gametic Isolation
• Gametes fail to attract each other
and fuse.
• Ex – chemical markers on egg and
sperm fail to match.
Postzygotic Types
1. Reduced Hybrid Viability
2. Reduced Hybrid Fertility
Reduced Hybrid Viability
• Zygote fails to develop or mature.
• Ex – when different species of
frogs or salamanders hybridize.
Reduced Hybrid Fertility
• Hybrids are viable, but can't
reproduce sexually.
• Chromosome count often “odd” so
meiosis won’t work.
• Ex – donkeys and horses produce
mules
How do species occur?
1. Allopatric Speciation
2. Sympatric Speciation
Both work through a block of gene
flow between two populations.
Allopatric Speciation
• Allopatric = other homeland
• Ancestral population split by a
geographical feature.
Sympatric Speciation
• Sympatric = same homeland
• New species arise within the range
of parent populations.
What is the Speed of Speciation?
• Gradualism
• Punctuated Equilibrium
Gradualism Predicts:
• Long periods of time are needed
for steady, continuous evolution
• Should see plenty of “transition
species”
• Problem: not many “transition
species” fossils found
Punctuated Equilibrium
• Evolution has two speeds of
change:
• Gradualism or slow change
• Rapid bursts of speciation
Predictions
• Speciation can occur over short
periods of time in a changing
environment (natural disasters)
• Fossils of transition species are
rare
Summary
• Be able to discuss the main
theories of what is a “species”.
• Know various reproductive barriers
and examples.
Summary
• Know allopatric and sympatric
speciation.
• Know adaptive radiation.
• Be able to discuss gradualism and
punctuated equilibrium theories.