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Chapter 16 Chapter 1 said that evolution was change over time. Chapter 15 said that evolution was change of a GROUP over time. Chapter 16 says that evolution, at the genetic level, is any change in the relative frequency of alleles in a population. Traits are characteristics that are controlled by genes. The different types of a gene are called alleles. In the simplest form, some alleles are dominant (A) while others are recessive (a). The phenotype or physical appearance of something is determined by the living thing’s genotype. The genotype is the two allele code. There are three possible genotypes: ◦ Homogyzous dominant (AA) = looks dominant ◦ Heterozygous (Aa) = looks dominant ◦ Homozygous recessive (aa) = looks recessive Genetic variation describes the differences of genes within a group. For example, everyone in this class is a human, but none (or few) of us are identical. Genetic variation is caused by: ◦ Mutations: random change in genetic material ◦ Gene Shuffling: recombination of genes in sexual reproduction A gene pool describes all the possible alleles in a group. ◦ Example: Imagine that there is a swimming pool with a bunch of floating letters in it. Some are B’s and others are b’s. One day there is a change in the environment, and most of the b’s leave. What is left? Allele frequency aka relative frequency is the percentage of one allele in a gene pool. For example, 50% of the alleles might have been B’s, but after the change, it might have dropped to 10%. Recall that only GROUPS can evolve, not individuals. If this is true, then genetic evolution can only occur if there is a change in the allele frequency of the gene pool. A single-gene trait is a characteristic that is controlled only by one gene. Individuals will either look dominant OR recessive. Example: ◦ Some people have widow’s peaks and others do not. If natural selection favors a particular phenotype, then individuals with that phenotype will be able to have more offspring, thereby changing the allele frequency. Example: There are black mice and light brown in a field. A recent drought has killed the grass, turning it light brown. Which type of mouse would survive? Which type of mouse would be more likely to reproduce? Recall from the previous unit that some traits are controlled by multiple genes. This creates diverse, intermediate phenotypes. ◦ Example: Although adult humans can be any height, humans usually fall into a range of heights. When these intermediate phenotypes are graphed, it forms a bell curve because most individuals are usually within a broad range of phenotypes. Changes in the environment may favor one or more parts of the bell curve. There are three major types of such selection curves: ◦ Directional Selection ◦ Stabilizing Selection ◦ Disruptive Selection Directional Selection occurs when individuals at one end of the spectrum have higher fitness. As a result, the entire bell curve shifts. Stabilizing Selection occurs when individuals in the middle phenotypes have the most fitness. As a result, the entire bell curve heightens in the middle. Disruptive Selection occurs when individuals at upper and lower ends of the curve are the most fit. As a result, the entire bell has two peaks. Natural selection usually pressures the survival of a species. HOWEVER, sometimes there is a random change in the allele frequency called genetic drift. Example: The United States is approximately 72% white/Caucasian and 13% black / African American. In contrast, Thornwood High School is 90.5% black, 5.8% Hispanic, and 1.8% white. How would the allele frequency change if only Thornwood students were able to reproduce? How does this demonstrate genetic drift? According to the Hardy-Weinberg Principle, allele frequencies will stay constant unless there is pressure to change. If evolution describes genetic change, then this genetic equilibrium explains when allele frequencies remain constant. There are 5 requirements to maintain genetic equilibrium between generations: ◦ ◦ ◦ ◦ ◦ 1. mating is random 2. large population 3. no migration / immigration 4. no mutations 5. no natural selection (all phenotypes are equally likely to survive) A species is a group of living things that can reproduce and have fertile offspring. The process of making a new species over time is speciation. Speciation requires reproductive isolation that allows members from a species to evolve separately from each other so that they can longer reproduce with each other. Isolation Mechanisms ◦ Behavioral ◦ Geographic ◦ Temporal Even though individuals from the same group might have the ability to reproduce, they choose not to because they prefer different behaviors. After several generations of breeding based on these behaviors, they can become separate species. Sometimes a change in the environment can create a physical separation (e.g. river, mountain). Over time, the groups on either side of the divide will reproduce, thereby creating different species. For some species, individuals would mate, but the timing is off. This time separation creates new species as new generations are born. ◦ Example: If one bug reproduces in the spring and another type of bug only reproduces in the fall, then they cannot reproduce together. Darwin studied the myriad of finches in the Galapagos Islands. Here is a summary of how they evolved. ◦ 1. The “founding” finches arrived in the islands. ◦ 2. Some of the earliest finches were isolated from each other. ◦ 3. Due to natural selection pressures, there were changes in the gene pool. ◦ 4. As time passed, the new generations of finches were not able reproduce with each other (reproductive isolation). ◦ 5. The different species of finches eventually had to compete for resources. ◦ 6. Evolution continues.