Population Evolution

Ch.16

(16-1) Population Genetics

• Study of evolution from a genetic point of view

• Population: individuals of the same species that interbreed

Populations

• Variations w/in a pop.– Bell shaped curve

How to get Variations

• Genetic factors– Mutations– Recombination (crossing-over & indep.

assortment)– Random fusion of gametes (fertilization)

• Environmental factors

Definitions

• Gene pool: total genetic info in a pop.

• Allele frequency: how often a certain allele occurs in the gene pool– # of certain alleles /

total # of alleles in pop.

Definitions (cont.)

• p = freq. of dominant allele

• q = freq. of recessive allele

• Phenotype frequency: # of individuals w/ a particular phenotype / total # of individuals

Hardy-Weinberg Equation

• p2 + 2pq + q2 = 1

• p2 = ho/go dominant

• 2pq = he/go

• q2 = ho/go recessive

Hardy-Weinberg Equilibrium

• Allele frequencies remain constant over time (no evolution)

• Pop. in H-W equil. when certain assumptions are held…

H-W Assumptions

1. Random mating

2. No selective advantage of genotypes (no natural selection)

3. No mutations

4. No migration of individuals

5. Large pop. size

How This Relates to Evolution?

• Evolution occurs when there’s

a disruption of equil.

(16-2) Causes of Equilibrium Disruptions

1. Mutation

2. Migration

3. Genetic drift

4. Nonrandom mating

5. Natural selection

Mutation

• Occur at low rates

• Mutagen: mutation-causing agent– Radiation– Chemicals

• Can produce new alleles in a pop.– Most are harmful

Migration

• Immigration: movement into a pop.

• Emigration: movement out of a pop.

• Gene flow: genes moving from 1

pop. to another

Genetic Drift

• Allele freq. in a pop. change as a result of random events or chance– Very significant in small pops.

• Ex: old-order Amish & genetic disorders

Nonrandom Mating

• Mate selection based upon:– Geographic area– Physical characteristics

• Assortive mating

Sexual Selection

• Choosing a mate based on certain traits– In order to leave offspring male must be

selected by female

• Genes of successful reproducers, rather than those of successful survivors are amplified by natural selection

Natural Selection

• 3 types:1. Stabilizing

2. Directional

3. Disruptive

Stabilizing Selection

• Individuals w/ the average form of a trait is the most fit

Directional Selection

• Individuals that display a more extreme form of a trait have the highest fitness

Disruptive Selection

• Individuals w/ either extreme variation of a trait have the greater fitness than the avg. of the trait

(16-3) Formation of Species

• Speciation: species formation

• Morphology: internal & external structure & appearance of an organism– Used for classification

Biological Species Concept

• A pop. of organisms can successfully interbreed but cannot breed w/ other groups

• Modern definition of species includes both morphology & biological species concept

Isolating Mechanisms

• Speciation begins w/ isolation

• 2 important types:– Geographic isolation– Reproductive “

Geographic Isolation

• Physical separation of members of a pop.– Ex: canyon develops through habitat

• Leads to allopatric speciation

Reproductive Isolation

• Results from barriers (not physical) to successful breeding b/w pop. groups in the same area

• 2 types:– Prezygotic: before fertilization

• Difference in mating times

– Postzygotic: after fertilization• Offspring may be unhealthy or infertile

Rates of Speciation

• Gradualism– Species develop by consistent & slow

evolution

• Punctuated equilibrium– Stops & starts in evolution in response to

dramatic environmental changes cause speciation

Gradualism

Punctuated Equilibrium