The Evolution of Populations

Organisms do not evolve. – Common misconception. – An organism’s genes are fixed at birth. Populations are the smallest unit of evolution. Microevolution- the change in the genetic makeup of a population over time. Population genetics –the study of microevolution.

Vocabulary Population – a group of the same species in an area. Species – organisms capable of breeding and producing fertile offspring. Gene pool – the total collection of genes in a population

Allele Frequency (example population) Population: 500 snapdragons – 2 alleles for color R = red and R’ = white – Remember codominance: RR = red, R’R’ = white, and RR’= pink 320 red plants 160 pink plants 20 white plants

320 red, 160 pink, 20 white How many total alleles in this population? – 1000 (2 for each plant) What is the frequency of red alleles? – 0.8 There are 320 red flowers with 2 R genes each (640) There are 160 pink flowers with 1 R gene each (160) = 800 R genes out of 1000 total genes 800/1000 = 0.8

320 red, 160 pink, 20 white What is the frequency of white alleles? – 0.2 Use the same logic as above or… 1 – 0.8 = 0.2 If these allele frequencies remain constant over time, the population is not evolving for that trait. If these allele frequencies change, something is causing evolution of the population.

Hardy Weinberg Theorem Named after two scientists who independently came up with the idea. Describes a population that is not evolving – “Hardy Weinberg Equilibrium” – No change in allele frequencies over time Used as a benchmark to track the evolution of a population’s genetic structure.

Hardy Weinberg Theorem In order for a population to be in Hardy Weinberg equilibrium, the following conditions must be true: 1.Very large population size 2.No gene flow (immigration/emigration) 3.No mutation 4.Random mating 5.No natural selection

Hardy Weinberg Theorem These conditions are never fully met. Populations that are changing slowly appear to be close to equilibrium. For populations close to equilibrium, we can use the Hardy Weinberg theorem to calculate approximate allele frequencies

Math For a trait that has only 2 alleles p = the frequency of one allele (“A”) q = the frequency of the other allele (“a”) p + q = 1 (p + q) x (p + q) = p 2 +2pq +q 2 p 2 +2pq +q 2 = 1

Math (continued) p 2 = the frequency of individuals that are homozygous for the “A” allele (AA). q 2 = the frequency of individuals that are homozygous for the “a” allele (aa). 2pq = the frequency of individuals that are heterozygous (Aa).

Example 1 in every 10,000 babies are born with PKU in the USA. What is the percentage of people in the USA that are carriers of PKU (assuming that the population is in Hardy Weinberg equilibrium?

Answer Approximately 2% 1/10,000 = = aa (PKU is recessive) q 2 = aa q 2 = q = 0.01 P = 1 – 0.01 = 0.99 Aa = 2pq = 2(0.01)(0.99) = %

If a population is not in Hardy Weinberg equilibrium, then: The population size is small. Mutations are occurring. Natural selection is occurring. Gene flow is occurring. Nonrandom mating is occurring.

Small Population Size The smaller the sample size, the better the chance that statistical predictions will not come true. If a population is small enough, there may be a shift in allele frequency due to random chance (called genetic drift).

Small population size The bottleneck effect – environmental conditions (natural disaster etc.) causes the survivors’ allele frequencies to be different than the original population.

Small population size The founder effect – a small portion of a population becomes isolated from a larger population to make a new population. – The gene pool of the new population is not the same as the original (by chance). – Can occur when a new area is colonized.

The Founder Effect Old Order Amish populations are derived from a few dozen colonists who escaped religious persecution in Germany in 1719 to settle in Pennsylvania. The community is closed. Allele and genetic disease frequencies in Amish are significantly different from the German ancestral and the surrounding local populations.

Gene Flow When members of a population move to a different population. Can cause 2 populations to become one.

Nonrandom mating Random mating may occur for a particular allele (ex. mates are not often selected based on blood type). Most mating is not random, especially in animals.

Sexual reproduction promotes nonrandom mating. Sexual selection – natural selection for mating success Sexual dimorphism – differences between body plans of male and female. Intersexual selection – individuals of one sex choose their mate from the other sex.