The Evolution of Populations Once you understand Genetics… it all makes sense!

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 “Individuals are selected, but populations evolve.”

Hardy-Weinberg Theorem “The frequencies of alleles in a population will remain constant .” Serves as a model for the genetic structure of a non-evolving population (equilibrium) 5 conditions: 1- Very large population size 2- No migration 3- No net mutations 4- Random mating 5- No natural selection Five Finger Evolution!

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) And if (p+q)2 also =1.0, then…. p2 + 2pq + q2 = 1.0 P2=frequency of AA genotype 2pq=frequency of Aa plus aA genotype q2=frequency of aa genotype;

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) There are 2 main types of Genetic Drift: Founder & Bottleneck Effects

Genetic Drift: The Bottleneck Effect: a reduction in population (natural disaster) the surviving population is no longer genetically representative of the original population Kills “non-selectively”

Genetic Drift continued…. Founder Effect: a cause of genetic drift attributable to colonization by a limited number of individuals from a parent population Again, not likely to have allele frequencies of the original population Darwin’s Finches

Microevolution continued…. GENE FLOW: genetic exchange due to the migration of fertile individuals or gametes between populations reduces differences between populations. Can even merge populations

Microevolution continued… Mutations: a change in an organism’s DNA (gametes; many generations) Mutation rates are typically only 1:10,000 gametes so…. It would take 2000 generations just to change allele frequencies by 1%. Has very little effect over the short term, but… ..is the original source of genetic variation

Microevolution continued…. Nonrandom mating: inbreeding and assortive mating (both shift frequencies of different genotypes) ASSORTATIVE MATING: When individuals mate with those with similar phenotypes. INBREEDING: Individuals tend to mate with those in their own proximity. Changes genotypic frequencies (p2, 2pq,q2) ……but NOT allele frequencies! (total p & q)

Microevolution continued…. Natural Selection: differential success in reproduction; only form of microevolution that adapts a population to its environment “..if you accept microevolution, you get macroevolution for free.”

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 Generated by: Mutation- usually harmful but can be beneficial in a changing environment. Genetic Recombination- actually provides nearly all variation in most populations

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 3 types: A. Directional ex: European Pepper Moth youtube! B. Diversifying (also called “Disruptive”) example: Tadpole sizes C. Stabilizing ex: Human Birthweight youtube AGAIN!

Fitness: Measured by the relative contribution that an individual makes to the gene pool of the next generation. RELATIVE FITNESS: measured by the “selection coefficient” the average successful variety will have a coeffient of “1”; all others a decimal fraction of 1. the new generation will produce a multiple of its fitness

Sexual selection Sexual dimorphism: secondary sex characteristic distinction Sexual selection: selection towards secondary sex characteristics that leads to sexual dimorphism

Does Evolution Fashion Perfection? …of course not. WHY? Organisms are locked into historical constraints. Adaptations are often compromises. (seal bodies, mole appendages) Not all evolution is adaptive. (genetic drift, for example) Selection can only edit variations that already exist. (New genes are not formed “on demand”)