Chapter 16Population Genetics and

Speciation

Section 1: Genetic Equilibrium

Section 2: Disruption of Genetic Equilibrium

Section 3: Formation of Species

Leopard Seal, Antarctica

Population Genetics(Microevolution)

Study of evolution from a genetic point of view1. Every population has some genetic variation that

influences fitness- Evolution is potentially a continuing process in all populations

2. Changes in selective factors in the environment will almost always be met by evolutionary responses

- leads to shifts in the frequencies of genotypes in a population

3. Rapid changes will often exceed the capacity of a population to respond by evolution

- decline could lead to extinction

Section 1: Genetic Equilibrium

Darwin’s finches

• Peter and Rosemary Grant• The Galapagos Islands: normally dry• El Nino: increase rainfall, vegetation flourishes• La Nina: periods of drought• Reproductive success and survival of

individuals differed between El Nino and La Nina years

• Caused dramatic evolutionary change

• Medium ground finch• Seeds, cracks with beak

• La Nina (drought): – amount of seeds dropped, seeds became tougher– population dropped

• 1400 in 1975 to 200 at end of 1977

– larger beaks could crack the larger seeds and survived better than those with smaller beaks

– Average beak size increased

Within a populationindividuals vary inobservable traits.

Traits cover a rangethat can be represented by a bell curve

• El Nino (wet) 1983– Small seeds in abundance– Those with smaller beaks handled the

smaller seeds better, able to survive, produce more offspring than those with larger beaks

– Average beak size returned to a lower value

Population genetics studies the ways in which populations respond to such selective pressures with changes in allele frequencies

Genetic Variation

1. Mutation: random change in a gene, passed on to offspring

2. Recombination: reshuffling of genes Independent assortment Crossing-over

3. Random Pairing of gametes

Gene Pool: total genetic information available in a population

When mate at random: all combinations of different alleles are possible

Peccaries are small, tough relatives of the modern pig, whose lineage diverged about 40 million years ago. They

live in southern Texas, Arizona, and New Mexico.

The forces of evolution shape and change the composition of this gene pool and thus the nature of the population.

Allele Frequency = # of a certain allele / total number of alleles

B: Long bristles on the bodies

b: short bristles

15 Individual peccaries in the population 30 alleles

If 6 alleles in this population are b, and 24 are B

Then the frequencies of these alleles are:

6/30 of the genes in the gene pool are b – a frequency of 0.2

24/30 of the gene in the gene pool are B – a frequency of 0.8

Phenotype Frequency = # of individuals with a particular phenotype / total number of individuals in the population

Hardy-Weinberg Equilibrium

Set of Assumptions1. No net mutations occur2. Individuals neither enter nor leave

the population3. The population is large4. Individuals mate randomly5. Selection does not occur*any exception to these can result in

evolution

No Net Mutations Occur• Spontaneous mutations occur constantly at very low rates• If exposed to mutagens, rates can increase significantly• Mutations can produce totally new alleles for a trait

Section 2: Disruption of Genetic Equilibrium

To be atEquilibrium

Individuals can neither enter nor leave a population

• Size of the population must remain constant

• If individuals move, genes move too

Immigration: movement into a population

Emigration: movement out of a population

Gene flow: process of genes moving from one population to another

To be atEquilibrium

Large Population

Genetic Drift: phenomenon by which allele frequencies in a population change as a result of random events, or chance

To be atEquilibrium

Individuals Mate RandomlyNonrandom Mating• Influenced by geographic proximity• Select a mate with similar traits:Assortative Mating• Sexual selection: females tend to choose males based on

certain traits (Planet Earth: Jungles, Birds of Paradise)

To be atEquilibrium

Birds of Paradise-Planet Earth

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No Natural SelectionOngoing process in nature

Some members of a population are more likely than others to survive and reproduce and thus contribute their genes to the next generation

To be atEquilibrium

Stabilizing Selection: individuals with the average form of a trait have the highest fitness

Disruptive Selection: individuals with either extreme variation of a trait have greater fitness than those with the average form of that trait

Directional Selection: individuals that display a more extreme form of a trait have a greater fitness than those with an average form

Black dots represent individuals that die before passing on their genes.

Hardy-Weinberg Conditions Animationshttp://nhscience.lonestar.edu/biol/hwe.html

http://www.accessexcellence.org/AE/AEPC/WWC/1995/hardyweinberg.php

Hardy-Weinberg EquationUsed to discover the probable genotype frequencies in a population

and to track their changes from one generation to the nextp2 + 2pq + q2 = 1 p + q = 1

p = frequency of the dominant allele (A)q = frequency of the recessive allele(a)

p2 is the predicted frequency of homozygous dominant people (AA)2pq is the predicted frequency of heterozygous people (Aa)q2 is the predicted frequency of homozygous recessive people (aa)

Speciation: process of species formation

Species: a single kind of organism.

-Members are morphologically similar (external structure and appearance)

-Can interbreed to produce fertile offspring. (biological species concept)

Section 3: Formation of Species

How do species give rise to other ones?

1. Geographic isolation: physical separation of members of a population.

Allopatric Speciation happens when species arise as a result of geographic isolation“different homeland”

No longer experience gene flow, so the gene pools of each separatepopulation may begin to differ due to genetic drift, mutations and natural selection

- come in contact with different environmental pressures

Reproductive Isolation: results from barriers to successful breeding between population groups in the same area.

• Pre-zygotic: occurs before fertilization

- active at different seasons or times of day

• Post-zygotic: occurs after fertilization

- zygote dies- F1 hybrids have reduced

fertility or sterility- Example: mule from horse

and donkey

Sympatric Speciation: when two subpopulations become reproductively isolated within the same geographic area

Rates of Speciation

Happens at a regular, gradual rate Sudden, rapid change followed by little to no change (few million yrs) (few thousand years)