The Evolution of Populations
Population genetics Population: a localized group of individuals belonging to the same species Species: a group of populations whose individuals have the potential to interbreed and produce fertile offspring Gene pool: the total aggregate of genes in a population at any one time Population genetics: the study of genetic changes in populations Modern synthesis/neo-darwinism Individuals are selected, but populations evolve.
Hardy-Weinberg Theorem Serves as a model for the genetic structure of a nonevolving population (equilibrium) Evolution = change in allele frequencies in a population hypothetical: what conditions would not cause allele frequencies to change? non-evolving population REMOVE all agents of evolutionary change 1. very large population size (no genetic drift) 2. no migration (no gene flow in or out) 3. no mutation (no genetic change) 4. random mating (no sexual selection) 5. no natural selection (everyone is equally fit)
Hardy-Weinberg Equation p=frequency of one allele (A) q=frequency of the other allele (a) p+q=1.0 (p=1-q & q=1-p) p 2 =frequency of AA genotype; 2pq=frequency of Aa genotype; q 2 =frequency of aa genotype; frequencies of all individuals must add to 1 (100%), so: p 2 + 2pq + q 2 = 1
Using Hardy-Weinberg equation population: 100 cats 84 black, 16 white How many of each genotype? q 2 (bb): 16/100 =.16 q (b):.16 = 0.4 p (B): 1-0.4 = 0.6 p 2 =.36 2pq=.48 q 2 =.16 BB Bb bb What Must assume are the population genotype frequencies? is in H-W equilibrium!
5 Agents of evolutionary change Genetic Drift Gene Flow Non-random mating Mutation Selection
Microevolution A change in the gene pool of a population over a succession of generations Genetic drift: changes in the gene pool of a small population due to chance (usually reduces genetic variability)
Microevolution- type of genetic drift The Bottleneck Effect: type of genetic drift resulting from a reduction in population (natural disaster) such that the surviving population is no longer genetically representative of the original population
Conservation issues Bottlenecking is an important concept in conservation biology of endangered species loss of alleles from gene pool reduces variation reduces adaptability Peregrine Falcon Breeding programs must consciously outcross Golden Lion Tamarin
Microevolution- type of genetic drift Founder Effect: a cause of genetic drift attributable to colonization by a limited number of individuals from a parent population just by chance some rare alleles may be at high frequency; others may be missing skew the gene pool of new population human populations that started from small group of colonists example: colonization of New World
Microevolution Gene Flow: genetic exchange due to the migration of fertile individuals or gametes between populations (reduces differences between populations) seed & pollen distribution by wind & insect migration of animals
Microevolution Mutations: Mutation creates variation a change in an organism s DNA (gametes; many generations); original source of genetic variation (raw material for natural selection)
Microevolution Nonrandom mating: Sexual selection inbreeding and assortive mating (both shift frequencies of different genotypes)
Sexual selection Sexual dimorphism: secondary sex characteristic distinction Sexual selection: selection towards secondary sex characteristics that leads to sexual dimorphism
Sexual selection QuickTime and a TIFF (Uncompressed) decompressor are needed to see this picture.
Microevolution NaturalSelection differential success in reproduction; climate change food source availability predators, parasites, diseases toxins only form of microevolution that adapts a population to its environment combinations of alleles that provide fitness increase in the population
Natural Selection Selection acts on any trait that affects survival or reproduction predation selection physiological selection sexual selection
Variation & natural selection Variation is the raw material for natural selection there have to be differences within population some individuals must be more fit than others
offspring (mm) Beak depth of Beak depth Where does Variation come from? Mutation random changes to DNA Sex errors in mitosis & meiosis environmental damage mixing of alleles recombination of alleles new arrangements in every offspring new combinations = new phenotypes spreads variation offspring inherit traits from parent 11 10 9 8 Wet year Dry year Dry year Dry year 1977 1980 1982 1984 Medium ground finch 8 9 10 11 Mean beak depth of parents (mm)
Population variation Polymorphism: coexistence of 2 or more distinct forms of individuals (morphs) within the same population Geographical variation: differences in genetic structure between populations (cline)
Variation preservation Prevention of natural selection s reduction of variation Diploidy 2nd set of chromosomes hides variation in the heterozygote Balanced polymorphism 1- heterozygote advantage (hybrid vigor; i.e., malaria/sickle-cell anemia); 2- frequency dependent selection (survival & reproduction of any 1 morph declines if it becomes too common; i.e., parasite/host)
Natural selection Fitness: contribution an individual makes to the gene pool of the next generation 3 types: A. Directional B. Diversifying C. Stabilizing
Effects of Selection Changes in the average trait of a population DIRECTIONAL SELECTION STABILIZING SELECTION DISRUPTIVE SELECTION giraffe neck horse size human birth weight rock pocket mice