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Evolution of Populations CHAPTER 16 Chapter 16 Intro • Darwin didn't know how heredity worked! How do you think heredity relates to evolution? Gene Pools • Gene pool-Combined genetic information of all member s of a particular population • Relative frequency- number of times that allele occur in a gene pool Gene Pool Relative frequencies of alleles Number of times alleles show up in gene pool Sources of Genetic Variation • Mutations-any change in DNA sequence • Gene shuffling- Occurs during production of Gametes • Gene pools are important to evolutionary theory, because evolution involves changes in populations over time. In genetic terms, evolution is any change in the relative frequency of alleles in a population Single gene trait • Single gene trait- controlled by a single gene that has two alleles • widow's peak—is a single-gene trait. Single gene trait • The allele for a widow's peak is dominant over the allele for a hairline with no peak. • As a result, variation in this gene leads to only two distinct phenotypes, Polygenic trait • Polygenic trait has two or more genes – Each gene of a polygenic trait often has two or more alleles. • One polygenic trait can have many possible genotypes and phenotypes. • Ex.) Human Height – The symmetrical bell shape of this curve is typical of polygenic traits. • A bell-shaped curve is also called a normal distribution. Natural Selection on Single Gene Traits • Natural selection on single gene traits can lead to a changes in allele frequencies and thus to evolution Natural Selection on Polygenic Traits • Directional Selection • Stabilizing selection • Disruptive Selection Natural Selection on Polygenic Traits • Directional Selection-one end has higher fitness Natural Selection on Polygenic Traits Stabilizing selectionmiddle has higher fitness Natural Selection on Polygenic Traits • Disruptive Selection-both ends have higher fitness Natural Selection on Polygenic traits • Directional Selection- Individuals at one end of the graph have higher fitness than individuals in the middle or at other end • Stabilizing Selection-Individuals near the center of the curve have higher fitness than individuals at either end of the curves • Disruptive Selection-Individuals at upper and lower ends have higher fitness than individuals near the middle Which type of selection? Which Type of Selection? Which Type of Selection? Genetic Drift • Genetic Drift- random change in allele frequency • In small populations, individuals that carry a particular allele may leave more descendants than other individuals do, just by chance. Over time, a series of chance occurrences of this type can cause an allele to become common Genetic Drift • Founder effectallele frequencies change as a result of migration of small subgroups of population Evolution vs. Genetic Equilibrium • To clarify how evolutionary change operates, scientists often find it helpful to determine what happens when no change takes place. • Biologists ask: – Are there any conditions under which evolution will not occur? – Is there any way to recognize when that is the case? Evolution vs. Genetic Equilibrium • The answers to those questions are provided by the Hardy-Weinberg principle, named after two researchers who independently proposed it in 1908. Hardy-Weinberg Equilibrium • Random Mating • Large Population • No Migration in or out of Population • No Mutations • No Natural Selection THIS NEVER HAPPENS!!! The End! Speciation • Formation of new species is called speciation Isolating Mechanisms • As new species evolve, populations become reproductively isolated from each other. • When the members of two populations cannot interbreed and produce fertile offspring, reproductive isolation has occurred. Reproduction Isolation • Reproductive isolation can develop in a variety of ways, including – behavioral isolation – geographic isolation – temporal isolation. Behavioral Isolation – Behavorial isolation-two populations are capable of interbreeding but have differences in courtship rituals or other reproductive strategies that involve behavior. – For example, the eastern and western meadowlarks are very similar birds whose habitats overlap in the center of the United States. Behavioral Isolation – Members of the two species will not mate with each other, however, partly because they use different songs to attract mates. – Eastern meadowlarks will not respond to western meadowlark songs, and vice versa. Behavioral Isolation Geographic Isolation • Geographic isolation - two populations are separated by geographic barriers such as rivers, mountains, or bodies of water. • The Abert squirrel lives in the Southwest. About 10,000 years ago, the Colorado River split the species into two separate populations. Geographic Isolation • Two separate gene pools formed. Genetic changes that appeared in one group were not passed to the other. • Led to the formation of a distinct subspecies, the Kaibab squirrel. Albert Squirrel Kaibab Squirrel Geographic Isolation Temporal isolation • A third isolating mechanism is temporal isolation, in which two or more species reproduce at different times. • For example, three similar species of orchid all live in the same rain forest. Each species releases pollen only on a single day. Because the three species release pollen on different days, they cannot pollinate one another. Temporal isolation