Chapter 16 Evolution and Populations Variation and Gene Pools Variations are differences between members of a population. A population is a group of individuals of the same species that interbreed. Because members of a population interbreed they share a common group of genes called a gene pool. Gene pool: all of the genes including all of the different alleles in a population. Relative frequency: the number of times that the allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur. How does variation and gene pools apply to evolution? Evolution involves changes in populations over time. Evolution is any change in the relative frequency of alleles in a popultion. What causes genetic variations Mutations: a change in the sequence of DNA which would alter the protein or trait that is produced. Mutations are random. Sometimes they are harmful but sometimes they are helpful. The helpful ones are passed on to offspring. Recombination (Crossing-Over): occurs during meiosis where gene pairs cross over during Prophase I. Single-Gene and Polygenic Traits Single-Gene traits: controlled by a single gene that has 2 alleles. Ex. Widow’s peak in humans. Polygenic traits: controlled by 2 or more genes. Ex. Height in humans Usually a bell curve or normal distribution of individuals in a population occurs in polygenic traits (Most people are average height; neither very tall or very short). Most human traits are polygenic. Widow’s Peak Bell-Curve Natural Selection on Single-Gene Traits Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution. Ex. Black versus red lizards If red lizards are more visible to predators then they will most likely not live long enough to reproduce. Black lizards might absorb more sun and warm up faster on cold days which would allow them to move faster to feed and avoid predators. The allele for black color might increase in frequency. Natural Selection on Polygenic Traits Natural selection can effect Polygenic traits in 3 ways: Directional Selection Stabilizing Selection Disruptive Selection Directional Selection When individuals at one end of the curve have higher fitness than individuals in the middle or at the other end, directional selection takes place. The range of phenotypes will shift as some individuals fail to survive and reproduce while others succeed. Example of Directional Selection Beak size in Darwin’s finches. Birds with bigger beaks can feed more easily on larger, harder, thicker-shelled seeds. Suppose a food shortage causes the supply of small and medium sized seeds to run low, leaving only larger seeds. Birds with larger beaks would have better access to food therefore the average beak size of the population would increase. Stabilizing Selection Occurs when individuals near the center of the curve have higher fitness than individuals at either end of the curve. The situation keeps the center of the curve in its current position. Example of Stabilizing Selection Ex: Infant birth weight. Human babies born smaller than average are likely to less healthy and less likely to survive. Babies that are much larger than average have more difficulty being born. The fitness of these larger or smaller individuals is lower than that of more average-sized babies. Disruptive Selection Occurs when individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle. Selection acts against those individuals of intermediate type. Can cause a single curve to split in two or create 2 distinct phenotypes. Genetic Drift Random change in allele frequency in a population. In small populations, individuals that carry a particular allele may leave more descendents than other individuals, just by chance. Over time, a series of chance occurrences of this type can cause an allele to become common in a population. Founder effect Genetic drift may occur when a small group of individuals colonize a new habitat. These individuals may carry alleles in different relative frequencies than did the larger, original population. If so, the population the “found” will be genetically different from the parent (original) population. Darwin’s Finches Hardy-Weinberg Principle States that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change. The situation where allele frequencies remain constant is called genetic equilibrium. If the allele frequencies do not change the population will not evolve. 5 Conditions are Required to Maintain Genetic Equilibrium Random mating Large population No movement into or out of the population No mutations No natural selection Speciation The process by which new species are produced. New species are produced by reproductive isolation, where members of 2 populations cannot interbreed and produce fertile offspring. 3 causes of reproductive isolation: Behavioral isolation Geographic Isolation Temporal Isolation Behavioral Isolation Occurs when two populations are capable of interbreeding but have differences in courtship rituals or other reproductive strategies that involve behavior. Ex. Eastern and Western Meadowlarks: use different songs to attract mates. Eastern and Western Meadowlark Geographic Isolation 2 populations are separated by geographic barriers such as rivers, mountains, or bodies of water. 2 separate gene pools form where genetic changes that appeared in one group are not passed to the other. Natural selection worked separately on each group forming distinct subspecies. Ex. Brown Bear and Polar bear Brown Bear versus Polar Bear Temporal Isolation 2 or more species reproduce at different times therefore they cannot breed with one another. Ex. Common in plant species where pollination occurs only at specific times for each species…preventing cross-pollination or interbreeding.