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• • • • • Chapter 16 Section Assessments: Due Fri. 5/2 Chapter 16.1 SA: p. 396 (1-5) Chapter 16.2 SA: p. 402 (1-5) Chapter 16.3 SA: p. 410 (1-2) Chapter 16 Assessment: p. 413 (1-10, 12, 17, 18, 19) • • • • • • • • • Gene pool Relative Frequency Single-gene trait Polygenic trait Directional selection Stabilizing selection Disruptive selection Genetic drift Founder effect • Hardy-Weinberg principle • Genetic equilibrium • Speciation • Reproductive isolation • Behavioral isolation • Geographic isolation • Temporal isolation • A weakness in Darwin’s theory stemmed from his lack of understanding in genetic variation. • In the 1930s, evolutionary biologists combined Mendel’s work with that of Darwin’s to support evolutionary change, specifically the process of natural selection. • How do you think Mendel would have felt about that? • Population: a group of individuals of the same species that interbreed. • Gene pool: consists of all genes, including all the different alleles, that are present 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. (Fig. 16-2) • Key Concept: In genetic terms, evolution is any change in the relative frequency of alleles in a population. • Key Concept: The two main sources of genetic variation are mutations and the genetic shuffling that results from sexual reproduction. • Remember mutations are changes in the genetic code. Some are expressed, some remain silent. Some mutations can affect an organism’s fitness, while others have not effect. • Crossing over and independent assortment during gamete formation leads to genetic variation. (23 pairs of chromosomes can have 8.4 million different combinations of genes!) • Key Concept: Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution. (Fig. 16-5) • Remember: Evolution is change over time in the relative frequencies of alleles in a population, therefore it is populations that can evolve, not individual organisms. • Key Concept: Natural selection can affect the distributions of phenotypes in any of three ways: • Directional Selection: when individuals at one end of the curve have higher fitness than individuals in the middle or at the other end. (Fig. 16-6) • Stabilizing Selection: center curve has higher fitness (Fig. 16-7) • Disruptive Selection: upper and lower ends have high fitness (Fig. 16-8) Average favored # of birds # of birds One extreme favored Birth Mass Beak Size # of birds Both extremes favored Beak Size • Key Concept: In small populations, individuals that carry a particular allele may leave more descendants that 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. (Fig. 16-9) • Genetic Drift: Random change in allele frequencies that occurs in small populations • Founder Effect: A situation in which allele frequencies change as a result of the migration of a small subgroup of a population • Genetic Equilibrium: A situation in which allele frequencies remain constant. • Key Concept: Five conditions are required to maintain genetic equilibrium from generation to generation: 1. 2. 3. 4. 5. Random mating Large population size No movement into or out of the population No mutations No natural selection • p+q=1 • p = dominant allele frequency (A) • q = recessive allele frequency (a) • p2 + 2pq + q2 = 1 • • • • p2: frequency of AA homozygous 2pq: frequency of Aa heterozygous q2: frequency of aa homozygous 1: sum of frequencies for all genotypes (100%) • Extra Credit Assignment!! Due Fri. 5/2 • Speciation: formation of new species • Key Concept: A new species evolve, populations become reproductively isolated from each other. • Reproductive Isolation: When members of two populations cannot interbreed and produce fertile offspring. They now have separate gene pools. • Causes of reproductive isolation: •Behavioral Isolation: capable of interbreeding, but have different courtship rituals or other reproductive behaviors. •Geographic Isolation: separated by physical barriers (i.e. rivers, mountains, etc…) • Temporal Isolation: reproduce at different times. Is the definition of species constant? “Can vs. Will” • Peter and Rosemary Grant have spent more than 20 years observing collecting data on Galapagos finches. • They found variation in physical characteristics. • They observed natural selection occurring during drought seasons. • Key Concept: Speciation in the Galapagos finches occurred by founding of a new population, geographic isolation, changes in the new population’s gene pool, reproductive isolation, and ecological competition. • “New data from genetics, physics, and biochemistry could have proved him wrong on many counts. They didn’t.” • “…while the Grants observed changes in the size of the finches’ beaks, they did not observe the formation of a new species.” • Read: Unanswered Questions (p. 410) • Why is understanding evolution important? After Break...Origins of the Earth!