1. POPULATION GENETICS & SPECIATION
CHAPTER 16

2. 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

3. Causes of variation What causes genes to vary in a population???
mutation- random change in a gene that is passed on to offspring
Recombination- reshuffling of genes in a diploid individual (crossing over)
Random pairing of gametes- each organism produces large numbers of gametes

4. Gene pool
Gene pool – term used to describe the total genetic information available in a population.

5. Allele frequency – dividing the number of a certain allele by the total number of alleles of all types.
allele frequencies
200 rr
500 Rr
300 RR
= 400 r
= 500 r
= 500 R
= 600 R
allele
frequencies:
900/2000 = 0.45 r
1100/2000 = 0.55 R
total = 2000 alleles

6. Genotypic frequency is the frequency of a genotype — homozygous recessive, homozygous dominant, or heterozygous — in a population.
genotype frequencies
genotype
frequencies:
200 white
500 pink
300 red
200/1000 = 0.2 rr
500/1000 = 0.5 Rr
300/1000 = 0.3 RR
total = 1000 flowers

7. 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.

8. 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.

10. 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 .

11. Nonrandom mating – many species do not mate randomly
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

12. 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.

13. ISOLATION & SPECIATION How 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.

14. 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”