Points to remember about Natural Selection • Populations evolve, but individuals do not. I.E., Natural selection acts on the level of the individual, but populations are the smallest unit that can evolve. • Natural selection only works on heritable variations, not acquired traits. • Natural selection can only work with what it’s given. Variations are produced by different genetic mechanisms. • Natural selection is situational to a given environment in a given time and place. • No ideal organism. • Fitness of a given trait changes.
Population Genetics Microevolution
Evolution is…. • For Darwin (1859): Evolution is gradual change of heritable traits in a population across generations, eventually generating species over time. • For the Modern Evolutionary Synthesis (early 20th c.): Evolution is a change in allele frequency over time
Population Genetics - Terms • Population:Localized group of individuals of the same species • Species:Group of populations whose individuals can potentially interbreed • Gene Pool:Total aggregate of genes in a population at one time • Allele:an alternative form of a gene • Homozygous:have identical alleles for a given trait (dominant or recessive) (e.g. AA or aa in a diploid) • Heterozygous:have >1 different alleles for a given trait (e.g. Aa or aA in a diploid) Why aren’t all individuals of one species identical?
Genetic Diversity is the product of… • Sexual reproduction • Genetic recombination • Mutation- key form of genetic variation in asexual reproduction, substrate for ‘new’ genetic material • Provides alternative alleles that may or may not be useful with changes in environment = raw material for evolution • R.A. Fisher's Fundamental Theorem of Natural Selection: the rate of natural selection is proportional to the degree of variation within a population
The Modern Evolutionary Synthesis in action… Hardy-Weinburg Equilibrium p2 + 2pq + q2 = 1
The Modern Evolutionary Synthesis in action… Hardy-Weinburg Equilibrium p2 + 2pq + q2 = 1 Assumptions 1. Large population size 2. No gene flow 3. No mutation 4. Random mating 5. No selection
The Modern Evolutionary Synthesis in action… • Microevolution (=shift in allele frequencies) happens when the H-W assumptions fail ! • Small population size –> Genetic Drift • Migration –> Gene Flow • Mutation • Natural Selection • Nonrandom mating
Genetic Drift: random changes in allele (and genotype) frequencies from generation to generation due to sampling error
Genetic Drift, part A: The bottleneck effect “Alleles” in original population “Alleles” remaining after bottleneck
Bottleneck effect Cheetah www.petsdoc.com/pics/funpages/ wildlifephotos/cheetah.jpg
Northern Elephant Seal Bottleneck Effect • Reduced to 20 individuals in 1896 • Now 30,000 individuals, with no detectable genetic diversity http://www.icsi.berkeley.edu/~dbailey/gallery/image/elephant.jpg
Genetic Drift, Part B: The founder effect • Change in allele frequencies when a new population arises from only a few individuals e.g., only a few fish are introduced into a lake e.g., only a few birds make it to an island
www.fishbase.org Scorpaenidae Lionfish Pterois volitans Founder Effect New Atlantic population, maybe from only 10 individuals
Gene Flow • addition or removal of alleles due to individuals entering or leaving a population from another population
On average, two humans differ by 0.1% (1 in 1000 bp). • Is race a genetic reality? • NO: ~90% of genetic variation occurs within any single "race". LOTS OF OVERLAP • YES: only 10% of the variation can be explained by Asian, African, or European origin.
Mutation • random, heritable changes in DNA that introduces new alleles into a gene pool
= random, heritable changes in DNA sequences that introduce new alleles into a gene pool Mutation members.tripod.com/~Alphacentaur/ X-men.JPG
Directional selection Beak size selection in a Galápagos population of the medium ground finch
Diversifying selection Small bill size for soft seeds, large bills for hard seeds; selection against intermediate sized bills which crack both types of seeds inefficiently
Stabilizing selection http://www.hummingbirds.net/gallery0.html
What maintains genetic variation? • Diploidy • Balancing selection • Heterozygote advantage • Frequency-dependent selection • Neutral variation
The two-fold disadvantage of sex BUT, genetic diversity allows adaptation to changing environments
Sexual Selection • non-random mating where individuals have sexually dimorphic traits that make them more attractive to mates Broad-billed hummingbirds http://www.hummingbirds.net/