The Evolution of Populations

Individuals do not evolve, populations do…Darwin

Lynn English High School ScienceBiology/Ms. Mezzetti

POPULATION GENETICS AND SPECIATION

Genetic EquilibriumGenetic 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 VariationVariations in genotype arise by mutation,

recombination, and the random pairing of gametes.

Mutations Random pairingRecombination

The Gene Pool

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

Frequency of allelesAllele 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 occur2. Individuals do not enter or leave3. The population is large4. Individuals mate randomly5. Selection does not occur

Disruption of Genetic EquilibriumEvolution may take place when populations are subject

to genetic mutations, gene flow, genetic drift, nonrandom mating, or natural selection.

Mutations are changes in the DNAGene 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 speciesMorphological 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 mechanismsGeographic 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 IsolationBarriers 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.

ReviewWhat 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.