Evolution of Populations

Evolution and Genetics Variations are inherited from one generation to the next leading to natural selection Differences that help organisms survive and reproduce become more common and differences that are not beneficial become less common It changes the genes!!!

Darwin and Mendel? Darwin did not know about how things are inherited yet (Mendel). We can use our knowledge from Mendel’s study of heredity and combine it with Darwin’s study of evolution to explain how inheritable variation appears and how natural selection effects that variation.

Genes and Variation Biologists studying evolution focus on a population = collection of individuals of the same species in a given area A population shares a common group of genes (genetic information) = gene pool Contain two or more alleles (forms of a gene) for each inheritable trait Relative frequency (%) = the number of times an allele occurs in a gene pool compared to the number of times other alleles occur

Sources of Genetic Variation Mutations = any change in a sequence of DNA Remember: mutations result as a mistake during replication or toxin (chemicals/radiation) Some mutations effect phenotypes (physical characteristics), which can effect an organism’s fitness (ability to survive) Gene shuffling = different gene combinations inherited during gamete production creating different genotypes (genetic makeup), different phenotypes and more variation Crossing over increases number of different genotypes Does not change the relative frequency of alleles in a population Think of a deck of cards – there are many possible combinations, but frequency remains same

Evolution as Genetic Change Natural selection acts on phenotypes, survival and reproduction determine which alleles are inherited, changing relative frequencies of alleles in a population over time. Thus evolution is any change in the relative frequencies of alleles in a population’s gene pool and acts on populations, not individuals.

Evolution of Polygenic Traits Natural selection can affect the distributions of phenotypes in any of three ways: Directional selection: When the entire bell moves left/right because there’s a higher fitness and increase in the number of individuals with the trait at one end of the curve Stabilizing selection: When the bell becomes more narrow, because there’s a higher fitness and increase in the number of individuals with the trait in the center of the curve Disruptive selection: The bell can split into two, because there’s a higher fitness and increase in the number of individuals at both ends of the curve

COPY THIS Graph of Directional Selection Section 16-2 Key Directional Selection Low mortality, high fitness High mortality, low fitness Food becomes scarce.

COPY THIS Graph of Stabilizing Selection Section 16-2 Stabilizing Selection Key Low mortality, high fitness High mortality, low fitness Selection against both extremes keep curve narrow and in same place. Percentage of Population Birth Weight

Copy this Graph of Disruptive Selection Section 16-2 Disruptive Selection Largest and smallest seeds become more common. Key Low mortality, high fitness Population splits into two subgroups specializing in different seeds. Number of Birds in Population Number of Birds in Population High mortality, low fitness Beak Size Beak Size

Genetic Drift Genetic drift = random change in allele frequencies In small populations, individuals that carry a particular allele may leave more descendants than other individual, just by chance. Over time, a series of chance occurrences can cause an allele to become common in a population. Genetic drift can happen when a small group of individuals colonize a new habitat carrying different relative frequencies that the larger population. Founder effect = allele frequencies change as a result of the migration of a small subgroup of a population

Speciation Speciation = formation of new species Species = group of organisms that breed with one another and produce fertile offspring (share a common gene pool) As new species evolve, populations become reproductively isolated from each other: When 2 populations can’t breed and produce fertile offspring, resulting in separate gene pools Behavioral isolation (sympatric): Capable of breeding, but have different courtship rituals or behaviors Geographic isolation (allopatric): Separate by geographic barriers Temporal isolation (sympatric): Reproduce at different times