MECHANISMS FOR EVOLUTION

Objectives State the Hardy-Weinburg theorem Write the Hardy-Weinburg equation and be able to use it to calculate allele and genotype frequencies List the conditions that must be met to maintain Hardy Weinburg equilibrium

VOCABULARY POPULATION SPECIES GENE POOL GENE FLOW BOTTLENECK EFFECT FOUNDER EFFECT HETEROZYGOTE ADVANTAGE HYBRID VIGOR STABILIZING SELECTION DIRECTIONAL SELECTION DIVERSIFYING SELECTION SEXUAL DIMORPHISM

POPULATION SPECIES GENE POOL Localized group belonging to the same species SPECIES Naturally breeding group of organisms that produce fertile offspring GENE POOL Total aggregate of genes in a population at any one time Most species are not evenly distributed over a geographic range. Individuals are more likely to breed with others from their population center

HARDY – WEINBURG THEOREM Describes a NON-EVOLVING population In the absence of other factors the segregation and recombination of alleles during meiosis and fertilization will not alter the overall genetic make-up of a population The gene pool will remain constant unless acted upon by outside agents. No evolution!

Imagine an isolated wildflower population with the following characteristics Diploid with both pink and white flowers Pink is dominant A and white is recessive a There are 480 pink flowers and 20 white 320 are AA 160 are Aa (p + q)2 = 1 p + q = 1 p2 + 2pq +q2 = 1

p2 = frequency of AA 2pq = frequency of Aa q2 = frequency of aa Calculate q2 first There are 1000 alleles AA -- 320 x 2/plant = 640 Aa --160 x 1/plant = 160 800 aa -- 20 x 2/plant = 40 Aa – 160 x 1/plant =160 200 Frequency of A = 80% and a = 20%

Condition for Hardy-Weinburg Large population No net mutation Isolated population Random mating No natural selection

MICROEVOLUTION LEADS TO MACROEVOLUTION

CAUSES OF MICROEVOLUTION GENETIC DRIFT BOTTLENECK EFFECT FOUNDER EFFECT

Bottleneck Population bottlenecks occur when a population's size is reduced for at least one generation. Because genetic drift acts more quickly to reduce genetic variation in small populations, undergoing a bottleneck can reduce a population's genetic variation by a lot, even if the bottleneck doesn't last for very many generations. 

An example of a bottleneck Northern elephant seals have reduced genetic variation probably because of a population bottleneck humans inflicted on them in the 1890s. Hunting reduced their population size to as few as 20 individuals at the end of the 19th century. Their population has since rebounded to over 30,000 — but their genes still carry the marks of this bottleneck: they have much less genetic variation than a population of southern elephant seals that was not so intensely hunted.

Founders Founder effects A founder effect occurs when a new colony is started by a few members of the original population. This small population size means that the colony may have: reduced genetic variation from the original population. a non-random sample of the genes in the original population.

An example of Founders Effect  the Afrikaner population of Dutch settlers in South Africa is descended mainly from a few colonists. Today, the Afrikaner population has an unusually high frequency of the gene that causes Huntington's disease, because those original Dutch colonists just happened to carry that gene with unusually high frequency. This effect is easy to recognize in genetic diseases, but of course, the frequencies of all sorts of genes are affected by founder events.

GENE FLOW – genetic exchange between populations due to migration Mutation – a new mutation that is transmitted in a gamete can immediately change the gene pool NONRANDON MATING Breed with other members of the “neighborhood” promotes inbreeding Assortative mating – mate with others like themselves NATURAL SELECTION

Variation within Populations Most heritable variation is measured by Quantitative characters (vary along a continuum ie. Height) are polygenetic Discrete characters (pink or white) are located on a single gene Polymorphism – two or more forms of a discrete character are represented in a population

GEOGRAPHICAL VARIATION A cline is a graded change in some trait along a geographical axis.

MODES OF SELECTION