Evolution of Populations

Evolution of Populations Genes and Variation Phenotype is a result of genotype and environment. Natural Selection works on Phenotypes. Ex: Snowshoe Hares don't always turn white in warmer climates, but would if further north. Gene Pool All the genes, including all the alleles for each gene present in a population. Allele Frequency The number of times an allele occurs in the gene pool, compared to the total number of alleles in that pool for that gene. Ex: 20 people on an island. 17 brown eyes (BB) 3 blue eyes (bb) What is the allele frequency of b? 6/40 = 0.15 Genetic Variation Where does it come from? 1. Mutations 2. Genetic Recombination during Sexual Reproduction 3. Lateral Gene Transfer Mutations Any change in the DNA of an organism. YOU were born with approximately 300 mutations. Most mutations do nothing (neutral mutations) Some may affect fitness. Only those affecting sperm or egg cells are passed to the next generation. Genetic Recombination Crossing over during meiosis produces extra variability. There are 8.4 million possible gene combinations in humans, which is why you don't look exactly like your brothers and sisters. Lateral Gene Transfer Bacteria will transfer portions of DNA between eachother, increasing genetic diversity. Single gene traits A trait controlled by only one gene. Results in 1-3 phenotypes Ex: Snail shell banding. Polygenic Traits Multiple genes control a single trait. Results in a wide range of phenotypes for a trait. Graphed, this shows a bellshaped curve. Ex: Height in humans Beak size in finches How Natural Selection Works Selection pressures: Organisms have multiple phenotypes for certain traits. If anything in the environment favors one phenotype over another, it will be selected for over time. Directional Selection When individuals at one end of the curve have higher fitness than those at the other end of the curve, the average of the population changes. Stabilizing Selection When individuals near the center of the curve have higher fitness. Disruptive Selection When individuals at the higher end have higher fitness. This type of selection can result in two separate groups within the same population, leading to speciation. Genetic Drift Random change in allele frequency over time. Usually seen in small populations. Example: 10 people start a hippie commune and 7 have widow's peaks. This trait will show up in higher numbers on the commune than in the general population. Genetic Bottlenecks A type of genetic drift that occurs after a population's numbers decline quickly. The Founder Effect The type of genetic drift that happens when a small group of individuals starts a new population. Often seen on islands. Genetic Equilibrium If there are no selection pressures for or against a trait, the frequency of dominant (A) and recessive (a) alleles for a trait will be constant, and you can predict the genotypes of the next generation. A deviation from the expected genotypic ratio in the next generation shows that evolution has occurred. What makes a population evolve? The following factors can cause a change in the next generation's genotypic ratios: Nonrandom mating Small population size Immigration or Emigration Mutations Natural Selection Speciation For one species to split into two, it takes more than just a change in allele frequencies. Isolation is the key! Reproductive isolation, where populations get split and can no longer interbreed Geographic Isolation Two populations are separated by geographic barriers such as rivers, mountains, or bodies of water. Given enough time, they adapt to their habitats differently and can no longer interbreed. Behavioral Isolation If a population becomes reproductively isolated because of a difference in courtship or other behavior. Example: Western and Eastern Meadowlarks Different song = no nookie Temporal Isolation Reproductive isolation resulting from a difference in the timing of reproduction. Example: Orchids with short, specific pollination dates