The Evolution of Populations Individuals do not evolve, populations do…Darwin Lynn English High School Science Biology/Ms. Mezzetti POPULATION GENETICS AND SPECIATION

Genetic Equilibrium Genetic Variation Population genetics is the study of evolution from a genetic point of view In the early 1900’s after Darwin’s death and the rediscovery of Mendel’s work, scientists started studying what caused variations in populations When measuring traits in a population a bell curve shows that most have average traits and few with extreme traits.

Bell Curve A bell curve shows the average of a population for certain traits Scientists wanted to look at what caused these variations

Causes of Variation Variations in genotype arise by mutation, recombination, and the random pairing of gametes. MutationsRandom pairingRecombination

The Gene Pool The total genetic information available in a population is called the gene pool.

Frequency of alleles Allele frequency is determined by dividing the total number of a certain allele by the total number of alleles of all types in the population.

The Hardy-Weinberg Genetic Equilibrium Allele frequencies in the gene pool do not change unless acted upon by certain forces. Hardy-Weinberg genetic equilibrium is a theoretical model of a population in which no evolution occurs and the gene pool of the population is stable. 1. No mutations occur 2. Individuals do not enter or leave 3. The population is large 4. Individuals mate randomly 5. Selection does not occur

Disruption of Genetic Equilibrium Evolution may take place when populations are subject to genetic mutations, gene flow, genetic drift, nonrandom mating, or natural selection. Mutations are changes in the DNA Gene Flow: Emigration and immigration cause gene flow between populations and can thus affect gene frequencies. Genetic drift is a change in allele frequencies due to random events which greatly affects small pop. Nonrandom mating : mating is nonrandom whenever individuals may choose partners, affected by sexual selection Natural selection can influence evolution in one of three general patterns.

Three kinds of Natural Selection Stabilizing selection favors the formation of average traits. Disruptive selection favors extreme traits rather than average traits. Directional selection favors the formation of more-extreme traits.

Formation of species Morphological Concept of Species: scientists classified species according to their structure and appearance- Problem: did not account for phenotypic differences Biological Species Concept: s species of a population of organisms that can successfully interbreed-Problem: does not account for extinct organisms

Results: Speciation One specie may evolve into two or more species Changes take place over a very long period of time.

Isolating mechanisms Geographic isolation-physical separation of a population Spotted owl subspecies living in different geographic locations show some genetic and morphological differences. This observation is consistent with the idea that new species form through geographic isolation. What separates them?

Geographic Isolation These two types of squirrels live on opposite sides of the canyon.

Reproductive Isolation Barriers to successful breeding between population groups in the same area Reproductive Isolation Animation

Rates of Speciation In the gradual model of speciation (gradualism), species undergo small changes at a constant rate. Under punctuated equilibrium, new species arise abruptly, differ greatly from their ancestors, and then change little over long periods.

Review What causes variations in populations? What disrupts genetic equilibrium? What do you need to be in Hardy Weinberg? Name two types of isolation. What happens when isolation takes place? Name 3 types of selection.