Population Genetics HardyWeinberg Equilibrium Determination Which of these

Population Genetics

Hardy-Weinberg Equilibrium Determination Which of these populations are in Hardy. Weinberg equilibrium? a) b) c) d) A B both A and B neither A nor B

Question 6 – Chap. 23 • Researchers examining a particular gene in a fruit fly population discovered that the gene can have either of two slightly different sequences, designated A 1 and A 2. Further tests showed that 70% of the gametes produced in the population contained the A 1 sequence. If the population is at Hardy-Weinberg equilibrium, what proportion of flies carries both A 1 and A 2? • A 0. 7 B 0. 49 C 0. 21 D 0. 42 E 0. 09

Question from an earlier edition of Campbell • At a locus with a dominant and recessive allele in Hardy-Weinberg equilibrium, 16% of the individuals are homozygous for the recessive allele. What is the frequency of the dominant allele in the population? • A 0. 84 B 0. 36 C 0. 6 D 0. 4 E 0. 48

Hardy-Weinberg Equilibrium is based on: 1. A very large population where all genotypes are equally viable 2. Random mating (panmixia) 3. No mutations 4. No gene flow (dispersal of individuals and their genes) 5. No natural selection

Evolutionary Change • Evolution is a generation to generation change in a population’s frequencies of alleles – change in proportions of alleles in the gene pool is evolution at its smallest scale and is often referred to as microevolution • The two main causes of microevolution are genetic drift and natural selection

Natural Selection

Genetic Drift • Random changes in gene frequency in a population – this can lead to losses in genetic diversity – the population becomes more homozygous • this is most important in small populations

Genetic Drift CRCR CRCW CW CW CRCR CRCW Generation 1 p (frequency of CR) = 0. 7 q (frequency of CW) = 0. 3

Genetic Drift CRCR CRCW CW CW CRCR 5 plants leave offspring CW CW CRCR CRCW CRCR CRCW Generation 1 p (frequency of CR) = 0. 7 q (frequency of CW) = 0. 3 CW CW CRCR CRCW Generation 2 p = 0. 5 q = 0. 5

Genetic Drift CRCR CRCW CW CW CRCR 5 plants leave offspring CW CW CRCR CRCW CRCR CRCW Generation 1 p (frequency of CR) = 0. 7 q (frequency of CW) = 0. 3 CW CW CRCW 2 plants leave offspring CRCR CRCR CRCW Generation 2 p = 0. 5 q = 0. 5 CRCR Generation 3 p = 1. 0 q = 0. 0

Population Bottleneck

Original population

Original population Bottlenecking event

Original population Bottlenecking event Surviving population

Northern Elephant Seal

Northern Elephant Seal Population

Pre-bottleneck (Illinois, 1820) Post-bottleneck (Illinois, 1993) Greater prairie chicken Range of greater prairie chicken (a) Location Illinois 1930– 1960 s 1993 Population size Percentage Number of alleles of eggs per locus hatched 1, 000– 25, 000 <50 5. 2 3. 7 93 <50 Kansas, 1998 (no bottleneck) 750, 000 5. 8 99 Nebraska, 1998 (no bottleneck) 75, 000– 200, 000 5. 8 96 (b)

Pre-bottleneck (Illinois, 1820) Greater prairie chicken (a) Range of greater prairie chicken Post-bottleneck (Illinois, 1993)

Location Illinois 1930– 1960 s 1993 Population size Number Percentage of alleles of eggs per locus hatched 1, 000– 25, 000 <50 5. 2 3. 7 93 <50 Kansas, 1998 (no bottleneck) 750, 000 5. 8 99 Nebraska, 1998 (no bottleneck) 75, 000– 200, 000 5. 8 96 (b)

Founder effect – founder population and three possible new populations

Mal de Meleda – founder effect

Serial Founder Effect • Serial founder effects have occurred when populations migrate over long distances. Such long distance migrations typically involve relatively rapid movements followed by periods of settlement. The populations in each migration carry only a subset of the genetic diversity carried from previous migrations. As a result, genetic differentiation tends to increase with geographic distance.

Movement of mitochondrial genes out of Africa

‘Wisteria vine’ model of human genetic diversity

Gene Flow • Gene flow is the movement of alleles in and out of a population • Gene flow occurs because gametes or fertile individuals move from one population to another and take their genes with them

Gene flow

Gene Flow in Conifers

60 Survival rate (%) 50 Population in which the surviving females eventually bred Central Eastern Central population NORTH SEA Eastern population Vlieland, the Netherlands 40 2 km 30 20 10 0 Females born in central population Females born in eastern population Parus major

Non-Random Mating • Hardy-Weinberg assumes random mating – if mating is not random then the population may change in the short term – the most common form of non-random mating is in-breeding – the mating of closely related individuals • In fact inbreeding is very common – many mammals probably mate with first or second cousins in the wild; many plants self-pollinate – the ultimate form of inbreeding • Inbreeding tends to produce homozygous populations

Inbreeding and White Squirrels

Mutations • Mutations are the ultimate source of new genetic variations – a new mutation that is transmitted in gametes immediately changes the gene pool of a population by inserting a new allele into the gene pool