POPULATION GENETICS

& SPECIATION

CHAPTER 16

POPULATION GENETICS

• Population genetics is the study of evolution from a genetic point of view

• Evolution at the genetic level is also called microevolution (a change in the collective genetic material of a population)

• Population – consists of a group of individuals of the same species that can interbreed.

• A population is the smallest unit in which evolution can occur

Causes of variation

• What causes genes to vary in a population???

1. mutation- random change in a gene that is passed on to offspring

2. Recombination- reshuffling of genes in a diploid individual (crossing over)

3. Random pairing of gametes- each organism produces large numbers of gametes

Gene pool• Gene pool – term used to

describe the total genetic information available in a population.

200 rr

500 Rr

300 RR

allele frequencies

900/2000 = 0.45 r

1100/2000 = 0.55 R

total = 2000 alleles

allelefrequencies:

= 400 r

= 500 r= 500 R

= 600 R

Allele frequency – dividing the number of a certain allele by the total number of alleles of all types.

200 white

500 pink

300 red

• genotype frequencies

200/1000 = 0.2 rr

500/1000 = 0.5 Rr

300/1000 = 0.3 RR

total = 1000 flowers

genotypefrequencies:

Genotypic frequency is the frequency of a genotype — homozygous recessive, homozygous dominant, or heterozygous — in a population.

HARDY-WEINBERG EQUILIBRIUM

• allows scientists to determine whether evolution has occurred. The law essentially states that if no evolution is occurring, then an equilibrium of allele frequencies will remain the same from generation to generation, and it is based on the following conditions:

• No mutations must occur (alleles remain the same)• No gene flow can occur (i.e. no migration of individuals into, or

out of, the population). • Random mating must occur • The population must be large• No selection can occur so that certain alleles are not selected for,

or against.

DISRUPTANCE OF GENETIC EQUILIBRIUM• Mutation - random change in a gene that is

passed on to offspring. Spontaneous mutations occur constantly, at very low rates under normal conditions.

• Gene flow- process of genes moving from one population to another.

- Immigration: the movement of individuals into a population.

- Emmigration: the movement of individuals out of a population.

• Genetic drift- In each generation, some individuals may, just by chance, leave behind a few more descendents than other individuals. The genes of the next generation will be the genes of the “lucky” individuals, not necessarily the healthier or “better” individuals.

• Natural selection- Stabilizing selection: individuals with avg. form of a

trait have the highest fitness- Disruptive selection- individuals with either extreme

variation of a trait have greater fitness .

• Nonrandom mating – many species do not mate randomly. Mate selection is often influenced by geographic proximity, or mates that have similar traits (assortative mating).

• Sexual selection- selection in which a mate is chosen on the basis of a particular trait or traits

http://www.youtube.com/watch?v=WhFKPaRnTdQ

FORMATION OF SPECIES• Speciation: the process of species formation• Morphology – internal and external structure

and appearance of an organism (scientists used the morphological species concept)

• Biological species concept- a species is a population of organisms that can successfully interbreed but cannot breed with other groups.

ISOLATION & SPECIATIONHow do species give rise to other different species?

• Geographic isolation – the physical separation of members of a population. (ex. A deep canyon could develop, or a river could change course forcing the population to separate and become isolated)

• Allopatric Speciation- happens when species arise as a result of geographic Isolation, more likely to occur in small populations.

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

• Sympatric Isolation- occurs when two subpopulations become reproductively isolated within the same geographic area.

• Rates of Speciation – - Gradualism: the idea that speciation occurs at a

regular, gradual rate- Punctuated Equilibrium: the idea that speciation

happens in “bursts” - http://www.youtube.com/watch?v=2oKlKmrbLoU- http://www.youtube.com/watch?v=rlfNvoyijmo