MECHANISMS FOR EVOLUTION CHAPTER 20

MECHANISMS FOR EVOLUTION CHAPTER 20

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 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 A theory that describes a NON-EVOLVING population. This could be used to compare a population to at a later time to look for changes. In the absence of other factors the segregation and recombination of alleles during meiosis and fertilization will not alter the overall genetic makeup of a population. (i.e. A population will stay the same over time)

Conditions for Hardy-Weinburg In order for a non-evolving population to exist. All of the following conditions must be met. Large population No net mutation Isolated population Random mating No natural selection In order to detect changes we must know the current genetic make-up.

Let s test Imagine an isolated wildflower population with the following characteristics: Diploid flowers with a mix of pink and white flowers. Pink is dominant A and white is recessive a There are 480 pink flowers and 20 white. What are the possible alleles? A (pink allele) or a (white allele) What do we know about their genotypes?

Hardy-Weinburg Equation Convert all numbers to ratios and let: p = dominant allele & q = recessive allele this is the Allele ratio For all the potential genotypes: or

p 2 = frequency of AA 2pq = frequency of Aa q 2 = frequency of aa Go with what we know first: Calculate q 2 : We can see the recessive trait. q 2= 20/500 = 0.04 - or 4% of the population Use the q 2 to find q 0.04 = 0.2 So: q = 0.2 or (20%) Then use q to find p

The allele frequencies are: P = 1 q P = 1 0.2 P = 0.8 or 80%

Now to find out the genotype ratio s in this population: p 2 = frequency of AA 2pq = frequency of Aa q 2 = frequency of aa

MICROEVOLUTION LEADS TO MACROEVOLUTION

MECHANISMS FOR EVOLUTION DO POPULATIONS OR INDIVIDUALS EVOLVE? WHAT IS A GENE POOL HOW CAN THE GENE POOL CHANGE? MUTATION GENETIC DRIFT GENE FLOW NON-RANDOM MATING NATURAL SELECTION WOULD THESE THINGS EFFECT A LARGE POPULATION OR A SMALL POPULATION MORE? Life Sciences-HHMI Outreach. Copyright 2006 President and Fellows of Harvard College.

CAUSES OF MICROEVOLUTION MUTATIONS provide new alleles in a population and provide the variation for evolution to occur, should the mutation lead to some adaptive advantage. Mutation alone does not cause evolution, but provide a selective advantage that due to natural selection can lead to a shift in allele frequency.

CAUSES OF MICROEVOLUTION GENETIC DRIFT in small populations the frequencies of alleles can be drastically affected by chance events BOTTLENECK EFFECT if populations are driven to the point of extinction the remaining individuals do not carry a true representation of the original gene pool. FOUNDER EFFECT when a small number of individuals colonize a new area they only carry with them a small representation of the total number of the alleles from the gene pool.

GENE FLOW genetic exchange between populations due to migration NONRANDON MATING Breed with other members of the neighborhood promotes inbreeding Assortative mating mate with others like themselves. This is the premise behind artificial selection.

CAUSES OF MICROEVOLUTION NATURAL SELECTION any environmental factor that leads to a particular allele having some adaptive advantage. There are three ways that natural selection can affect the frequency of traits: Stabilizing selection Directional selection Disruptive selection Page 693

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