Biology Chapter 16 Evolution of Populations Part II • MICROEVOLUTION: Evolution is any change in the relative frequency of alleles in a population. Microevolution refers to small scale change in allele frequency over time. III. Sources of Genetic Variation A. Two sources of genetic variation 1. Mutation a. Ultimate source of variation. b. Any change in a sequence of DNA c. Most mutations are bad. Example: UV, radiation, toxins d. Mutations that produce changes in an organism’s phenotype and increase an organism’s fitness, or its ability to reproduce in its environment, will be passed on. 2. Genetic shuffling that results from sexual reproduction. a. Independent assortment during meiosis produces 8.4 million possible combinations. b. Crossing-over. 16-2 Evolution as Genetic Change I. Natural Selection on Single-Gene Traits A. Reminder: Evolution is any change over time in the relative frequencies of alleles in a population. Populations, not individual organisms, evolve over time. B. Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution. Effect of Color Mutations on Lizard Survival (Figure 16-5): 1. Organisms of one color may produce fewer offspring than organisms of other colors. Example: Red lizards are more visible to predators and therefore, may be more likely to be eaten and not pass on that red gene. II. Natural Selection on Polygenic Traits Natural selection can affect the distribution of phenotypes in any of three ways: (1) directional selection (2) stabilizing selection (3) disruptive selection. A. Directional Selection 1. One of the two possible extremes is favored. Example: Darkcolored peppered moths in regions of England with industrial pollution. Section 16-2 Directional Selection Figure 16–6 Key Directional Selection Low mortality, high fitness Food becomes scarce. High mortality, low fitness B. Stabilizing Selection 1. Intermediate characteristics are favored. Examples: Human babies with very high or very low birth weights have lower survival than babies with intermediate weights. Stabilizing Selection Figure 16–7 Section 16-2 Stabilizing Selection Key Low mortality, high fitness High mortality, low fitness Birth Weight Selection against both extremes keep curve narrow and in same place. C. Disruptive Selection 1. Natural selection moves characteristics toward both extremes, and intermediate phenotypes become rarest. Example: Populations of West African birds with either large or small, but not intermediate size beaks. Section 16-2 Disruptive Selection Figure 16–8 Disruptive Selection Low mortality, high fitness High mortality, low fitness Population splits into two subgroups specializing in different seeds. Beak Size Number of Birds in Population Key Number of Birds in Population Largest and smallest seeds become more common. Beak Size Genetic Bottleneck 2. Founder effect: A population can become limited in genetic variability if it’s founded by a small number of individuals. Example: Polydactyly in Amish. Figure 16-9: Founder Effect Sample of Original Population Descendants Founding Population A Founding Population B IV. Hardy-Weinberg and Genetic Equilibrium A. What would be necessary for no change to take place? 1. Hardy-Weinberg principle states that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change. 2. If allele frequencies remained constant then it there would be genetic equilibrium. 5. Five conditions necessary for HardyWeinberg Equilibrium NOTE: Hardy-Weinberg equilibrium rarely exists in natural populations but understanding the assumptions behind it gives us a basis for understanding how populations evolve. Conditions necessary for Hardy-Weinberg Equilibrium a. The population is very large. b. The population is isolated (no migration of individuals, or alleles, into or out of the population). c. Mutations do not alter the gene pool. d. Mating is random. e. All individuals are equal in reproductive success (no natural selection). Large Ground Finch Small Tree Finch Woodpecker Finch 16-3 The Process of Speciation I. How do we get new species? A. What is a Species? 1. Species: a group of interbreeding organisms that breed with one another and produce fertile offspring. This means that the individuals of the same species share a common gene pool. Diversity in Humans 2. If a beneficial genetic change occurs in one individual, then that gene can be spread through the population as that individual and its offspring reproduce. B. Isolating Mechanisms (Leads to a new species!) Reproductive Isolation – members of two populations cannot interbreed and produce fertile offspring. PRE-Mating Reproductive Isolation – involves mechanisms which do not allow mating to occur in the first place. 1. Behavioral Isolation: Members of two populations are capable of interbreeding but have differences in mating displays or courtship rituals. a. specific scents (pheromones of insects). b. color patterns/strutting. c. specific sounds or calls. Courtship Dance Different Mating Songs 2. Geographic/Ecological Isolation: Two populations are separated by geographic barriers such as rivers, mountains, or bodies of water. When has speciation occurred? 3. Temporal Isolation: Two or more species live in the same habitat but have different mating/reproductive seasons. a. Brown trout and Rainbow trout are found in the same streams but Rainbow trout spawn in the Spring and Brown trout spawn in the Fall. b. Three similar species of orchid living in the same tropical habitat each release pollen on different days; therefore, they cannot pollinate one another. NOTE: Several isolating mechanisms can compound one another to insure mating doesn’t occur. This permits two species to occupy the same valuable habitat and prevents wastage of valuable gametes. POST-Mating Reproductive Isolation – (fertilization occurred and zygote formed) 1. Hybrid inviability: hybrid zygotes fail to develop or fail to reach sexual maturity. 2. Hybrid sterility: Hybrids fail to produce functional gametes. Example: horse x donkey => mule (sterile). Hybrid Sterility + Donkey Horse = Mule (sterile)