Evolutionary Mechanisms

1. Types of Selection Directional selection Stabilizing selection Disruptive selection Sexual selection Artificial selection

a) Directional Selection Selection that favors an increase or decrease in the value of a trait from the current population average. If a population of hummingbirds enters a habitat with longer flowers, longer bills will be selected for. Common in artificial breeding Strawberries are selected for bigger, sweeter fruit.

b) Stabilizing Selection Selection against individuals exhibiting traits that deviate from the current population average. If hummingbirds live in a habitat of medium length flowers, then medium length bills will be selected for.

c) Disruptive Selection Selection that favors two or more variations of a trait that differ from the current population average. Suppose only short and very long flowers are available to the hummingbirds….the medium billed hummingbirds would be selected against.

d) Sexual Selection The favoring of any trait that enhances the mating success of an individual. In many species, females choose males based on physical traits. Bright colouration Specific behaviours Larger body size Number of blue trinkets in their nest! (bowerbirds)

e) Artificial Selection The selection by humans of animals, plants, or other organisms to breed together. Artificial selection is also known as “selective breeding” Breeders use artificial selection to ensure the continuation of desirable traits and to develop new varieties.

2. Evolutionary Change without Selection Genetic drift Bottlenecks and the Founder Effect The Hardy-Weinberg Principle Biotechnology

a) Genetic Drift Changes to allele frequencies due to chance. More pronounced in small populations. Seneca army base deer

b) Bottlenecks Genetic bottleneck – a dramatic reduction in population size resulting in genetic drift. If 10 000 hummingbirds are reduced to 50 hummingbirds they are unlikely to contain all the alleles found in the larger population.

c) Founder Effect When a small number of individuals establishes a new population. A small number of finches from South America established a founding population on the Galapagos Islands.

d) The Hardy-Weinberg Principle In large populations, in which only random chance is at work, allele frequencies are expected to remain constant from generation to generation.

e) Biotechnology Humans use technology to change the genetics of organisms in order to produce a desired trait.