Evolution of Populations Chapter AP Biology

Recap

Two Types of Evolution Macroevolution Evolution that takes place over LONG periods of time Change on a grand scale Mass extinctions Microevolution Change that occurs within a species Change in gene frequencies Focus of Darwin’s theory of evolution via natural selection

How can we study microevolution? Study population genetics to understand changes that occur within a species Hardy-Weinberg

There are 3 mechanisms for evolution Natural Selection Leads to an accumulation of favorable adaptations in a population Genetic Drift Chance events that alter allele frequencies Gene Flow The transfer of alleles between populations

23.3 Natural selection, genetic drift, and gene flow can alter allele frequencies in a population

Review: Natural Selection This is a change in gene frequencies due to differential reproductive success. Example: antibiotic resistance Variation already present Introduction of antibiotic selects for those with higher resistance Frequencies for the resistance gene increase over time

Genetic Drift Changes in the gene pool of a small population due to chance (random) Founder Effect or Bottleneck Effect

Genetic Drift: Founder Effect A few (random) individuals from a population start a new population with a different allele frequency than the original population

Genetic Drift: Bottleneck Effect The population experiences a huge decrease in size Result: severe reduction in diversity of the original gene pool because a small percentage of chance survivors remain Endangered species can experience this

Gene Flow Migration of fertile individuals, or the transfer of gametes, between populations Populations may gain or lose alleles Reduces differences between neighboring populations BB bb

23.4 Natural selection is the only mechanism that consistently causes adaptive radiation

Natural Selection Natural selection results in development of features which increase an organism’s likelihood for survival and reproduction—adaptations Therefore, Darwin’s explanation of evolution goes like this: Adaptation + natural selection  change within species Leads to adaptive evolution

Modes of Natural Selection A.Directional Selection – favors variants of one extreme B.Disruptive Selection – Opposite phenotypic extremes favored over intermediate C.Stabilizing Selection – favors intermediate variants by selecting against extreme phenotypes

Maintaining Variation Natural selection is the main mechanism of evolution For this to work, variation must be maintained Variation is good (why?)

Conserving Variation: Polymorphism The presence of two or more distinct phenotypes in a population Example: sexual dimorphism Leads to the enigma of sexual reproduction Evolutionary advantage outweighs cost of sex Costs: energy, resources, competition Advantage: pass on genes, variation of genes (2 parents vs. 1 parent, meiosis)

Conserving Variation: Diploidy The presence of homologous chromosomes (one from mom, one from dad) maintains variety in the population A dominant allele can cover up a harmful recessive one Example: cystic fibrosis

Conserving Variation: Heterozygote Advantage Heterozygotes more likely to survive than homozygotes Example: sickle-cell anemia and malaria resistance

Natural Selection isn’t perfect 1.Nature can only select for what traits are present. 2.Ancestry is a legacy that is modified; nothing is built from scratch. 3.Adaptations are often compromises. 4.Chance, natural selection and the environment interact.