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 p 2 + 2pq + q 2 = 1 p 2 = ho/go dominant 2pq = he/go q 2 = 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