Chapter 16: Evolution of Populations

Chapter 16:Evolution of Populations

16.1 Genes and VariationVariation and Gene Pools• Gene pool – all genes in a population of

organisms• Allele frequency – the number of times the

allele occurs in a gene pool• In genetic terms, evolution is any change in

the frequency of alleles in a population.

Gene Pool for Fur Color in Mice

Sources of Genetic Variation• Mutations• Any change in a sequence of DNA

• Gene Shuffling• Crossing-over• Sexual reproduction• Random arrangement of chromosomes in

Metaphase I of meiosis

Single-Gene and Polygenic Traits• Single Gene Traits – traits controlled by one

gene that has two alleles• Example: Widow’s Peak –

AA or Aa - have widow’s peakaa - have no widow’s peak

• Polygenic Traits – traits controlled by two or more genes • Example: Height in humans

Bell shaped curve is normal distribution

16.2 Evolution as Genetic Change

• If an individual dies without reproducing, it does not contribute its alleles to population’s gene pool.

• If an individual produces many offspring, its alleles stay in the gene pool and may increase in frequency.

• Populations, not individual organisms, can evolve over time.

• Natural selection can affect the distributions of phenotypes in three ways:

1. Directional Selection• Phenotypes shift toward homozygous

dominant or homozygous recessive• Example: Darwin’s Finches• The finches had beaks of different sizes to

eat different food. What if the supply of small seeds disappeared only leaving the large, hard seeds? Those finches with big beaks would survive causing a shift to that phenotype

Small Medium LargeBeak Size

2. Stabilizing Selection• When intermediate forms (heterozygote) of

a trait are favored and alleles that specify extreme forms (homozygote) are eliminated from a population

• Example: Darwin’s Finches• What if the supply of seeds was mostly

medium size seeds?  

Beak Size

Num

ber

of B

irds

in P

opul

atio

n

Small Medium LargeBeak Size

3. Disruptive Selection• Forms at both extremes of the range of

variation• Extremes are favored and the

intermediate form is selected against• Example: Darwin’s Finches• What would happen if the supply of

medium seeds disappeared?

Example: Darwin’s finches• What would happen if the supply of

medium seeds disappeared?

Small Medium LargeBeak Size

• Genetic Drift• Genetic Drift – a random change in allele

frequencies over the generations• Genetic drift has a greater effect on small

populations.

1. Founder Effect • Occurs when allele frequencies in a group

of migrating individuals are by chance not the same as that of their original population

• Example: beetles• Two small groups of different beetles

leave the population.

• These two small groups start their own population.

Population A Population B

• The two new populations are genetically different from the original population.

2. Bottleneck • Occurs when the population undergoes a

dramatic decrease in size.• Causes:• Natural catastrophes• Predation• Disease

• Evolution Versus Genetic Equilibrium• 1908 Hardy and Weinberg independently

suggested a scheme whereby evolution could be viewed as changes in the frequency of alleles in a population of organisms

• Hardy-Weinberg – allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change.

• Genetic Equilibrium – When allele frequencies remain constant

• 5 conditions are required to maintain genetic equilibrium from generation to generation:1. There must be random mating2. Population must be very large3. There can be no movement of genes into or

out of the population 4. No mutations5. No natural selection – all genotypes must

have an equal rate of survival and reproduction

16.3 The Process of Speciation• Natural selection and chance events can

change the relative frequencies of alleles in a population and lead to speciation.

• Speciation – formation of a new species• Species – a group of organisms that breed

with one another and produce fertile offspring.

• Isolating Mechanisms• As new species evolve, populations

become reproductively isolated from each other.

• When the members of two populations cannot interbreed and produce fertile offspring, reproductive isolation has occurred.

3 Types of Reproductive Isolation1. Behavioral Isolation• Occurs when a species does not recognize

another species as a mating partner because it does not perform the correct courtship rituals, display the proper visual signals, sing the correct mating songs or release the proper chemicals

2. Geographic Isolation• Occurs when two populations are separated

by geographic barriers such as rivers or mountains.

3. Temporal Isolation• Occurs when two species mate or flower

during different seasons or at different times of the day

17.4 Patterns of Evolution• Macroevolution – large-scale evolutionary

patterns and processes that occur over long periods of time.

• Extinction• More than 99% of all species that have

ever lived are now extinct• What effects have mass extinctions had on

the history of life? Mass extinctions have:• Provided ecological opportunities for

organisms that survived• Resulted in bursts of evolution that

produced many new species

• Divergent Evolution• Two or more species that originate from a

common ancestor.• Adaptive radiation – a type of divergent

evolution – the process by which a species evolves into several different species

• The disappearance of dinosaurs then resulted in the adaptive radiation of mammals.

time A time B time C time D

parent species

time

• Convergent Evolution• Convergent evolution – the process by

which unrelated organisms come to resemble one another.

• Convergent evolution has resulted in sharks, dolphins, seals, and penguins.

• Coevolution• Coevolution – the process by which two

species evolve in response to changes in each other over time.

• Example: predator prey