Mechanisms of Evolution Chapter 17 How Does Evolution Work? Individual organisms cannot evolve. Each individual’s traits are determined by its genes. Natural selection acts on the range of phenotypes in a population. Evolution occurs as the frequency of genes in a population changes. Evolution-What Happens? See page 404, Figure 5. Snapdragon Flowers Evolution occurs when there is a change in the gene pool-specifically a change in allele frequency. Definitions Gene pool: All alleles of the population’s genes. Allelic frequency: % of a specific allele in the gene pool. Genetic Equilibrium: This exists when the frequency of alleles remains the same over generations. The population is not evolving. When Evolution Occurs Evolution results when there are Forces that change allelic frequencies and disrupts genetic equilibrium. Forces that cause Evolution: 1. Gene flow: Transport of genes by migrating individuals. 2. Nonrandom Mating: Mating based on preferences example: a female may choose a mate based on male size, color, or ability to gather food Forces of Evolution Continued 3. Mutation: Change in DNA 4. Genetic Drift: chance event changes allelic frequencies – Greatly affect small populations such as the animals of the Galapagos Islands or Amish. Causes of Genetic Drift Random mating over a long time period No immigration of males No emigration of females Forces of Evolution Continued 5. The most significant factor causing changes in the gene pool (evolution) is natural selection. Overproduction of offspring Some variations increase or decrease chances of survival. These variations can be inherited. So, frequency of alleles changes over generations due to natural selection. Adaptation: over time, those traits that improve survival and reproduction become more common. 3 Types of Natural Selection Stabilizing selection – favors average individuals Directional selection – favors one of the extreme variations of a trait Disruptive selection – favors individuals with either extreme of a trait (eliminates intermediate phenotypes) Stabilizing selection favors average individuals Directional selection favors one of the extreme variations Disruptive selection or Diversifying selection favors both extreme variations of a trait What is a Species? A population or group of populations whose members have the ability to breed with one another and produce fertile offspring Evolution of Species (Speciation) Significant changes in the gene pool can lead to evolution of a new species over time. Speciation occurs when members of similar populations no longer interbreed to produce fertile offspring within their natural environment. Why Don’t the Populations Interbreed? Geographic isolation – physical barrier divides a population. Reproductive isolation – formerly interbreeding organisms can no longer mate to produce offspring.. Polyploidy – results in immediate reproductive isolation. Very common in plants. Speciation can occur quickly or slowly Gradualism – idea that species originate through a gradual accumulation of adaptations. Punctuated equilibrium – hypothesis that speciation occurs relatively quickly, in rapid bursts, with long periods of genetic equilibrium in between. Patterns of Evolution Adaptive Radiation – ancestral species evolves into an array of species to fit diverse habitats. This is a type of divergent evolution where species diverge or become less and less alike as they adapt to different environments. Convergent Evolution – Unrelated species occupy similar environments in different parts of the world. Similar pressures of natural selection lead to similar adaptations.