Evolution of Populations Chapter 16

Darwin and DNA In the 1930s, Mendel’s work was connected to Darwin’s work and scientists understood that genes control heritable traits In the 1950s, the Watson-Crick model of DNA allowed scientists to demonstrate the molecular nature of a mutation and genetic variation

Where does variation come from? Gene Shuffling- which is the extensive mixing of genes during MEIOSIS or sexual reproduction Mutations- or changes in the structure of DNA

How do scientists study variation and evolution? It’s studied in populations, or a group of individuals of the same species that interbreed Because they interbreed they share a common group of alleles called a gene pool In population on 100 mice… 40 Black Alleles (B) 60 Brown Alleles (b)

How do we know a population is evolving? When there are changes in the relative frequency of alleles in the gene pool How do the allele frequencies change? Natural Selection (directional, stabilizing, disruptive, or sexual selection) Random Change (mutations or genetic drift) Migration or gene flow

Natural Selection on Populations DIRECTIONAL SELECTION: Individuals at one end of curve have higher levels of fitness than the individuals at the other end of the curve Ex: Peppered Moths

Natural Selection on Populations STABILIZING SELECTION: Individuals in the center of the curve have higher levels of fitness than the individuals at either end Example: Birth weight in humans or marine creatures on ocean floor

Natural Selection on Populations DISRUPTIVE SELECTION: Individuals on the ends of the curve have higher fitness levels than individuals in the center of the curve Example: Beak size in Finches

Natural Selection on Populations

Mutations The frequency of a mutation in the population will depend on the effects of natural selection- will it make the individual more likely to survive and reproduce? If the mutation increases fitness, it will spread throughout the population!!

Genetic Drift The random change in the gene pool of a population All populations are subject to genetic drift, but small populations are impacted more Bottleneck Effect: Occurs when a natural disaster greatly reduces the size of the population and the gene pool (population loses variation) Founder Effect: Occurs when a few individuals from a population colonize an isolated island or new habitat (population loses genetic variation)

Hardy-Weinberg Principle Allele frequencies in a population will remain constant unless one or more factors causes them to change A population will NOT evolve if… 1. Random Mating 2. Large Populations 3. No Immigration or Emigration 4. No Mutations 5. No Natural Selection

Evolution of a Species Biological Species Concept: a species is a population that will interbreed and produce fertile offspring When a population evolves enough genetic change (either through natural selection or genetic drift) that it no longer interbreeds with the original population, it is considered a new species

Reproductive Isolation When a reproductive barrier keeps species from interbreeding. Barriers include: Temporal (timing)- different mating seasons Behavioral- different courtship or mating behaviors Geographic- populations divided by barriers like glaciers, valleys, rivers, etc.