Transcript mechanisms
Mechanisms of Evolution and their effect on populations Genetic variation Is needed for evolution to occur! Variations that occur are RANDOM Variation occurs through: sexual reproduction mutations important to know Individual organisms DO NOT evolve - Populations DO The gene pool of a population consists of all the alleles of a all the genes of each individual in that population The percentage of each allele of a any given gene present in the population determines the genetic characteristics of that population. How does Evolution occur? Small events accumulate to form a new species. Changing percentages / frequencies of alleles within a population leads to evolution in a population. FIVE common factors can change the allele frequency. 1. mutations Mutations change the DNA of an individual Since the DNA is heritable this can affect the gene pool The more genetic variation the greater the diversity and the greater chance of a selective advantage 2. GENE FLOW The movement of alleles from one population to another due to migration Ex: pollen being blown across into a new field Ex: a lone wolf travels long distance to find a new mate 3. NON-RANDOM MATING Mating based on selecting a mate that displays a particular phenotype. Prevents individuals with a certain phenotype from mating. Ex: Caribou females choose the male that wins a fight with another male Ex: Female sage grouse select males with colourful feathers 4. GENETIC DRIFT The change in allele frequency due to chance events. Occurs more frequently in small populations Can lead to the elimination of some alleles and thereby reducing genetic diversity GENETIC DRIFT GENETIC DRIFT - THE FOUNDER EFFECT A change in a gene pool that occurs when a few individuals start a new isolated population Ex: Afrikaner (Dutch) population in South Africa higher amounts of Huntington’s disease Ex: Amish population in Philadelphia - higher amounts of polydactylism (p. 355) GENETIC DRIFT - THE BOTTLENECK EFFECT Changes in gene distribution due to a rapid decrease in the size of the population Genetic diversity is reduced Ex: overhunting of cheetahs and elephant seals (very low genetic diversity) 5. NATURAL SELECTION Selective forces such as competition and predation affect populations. Some individuals are more likely to survive and reproduce than others. If having one allele provides an advantage, the frequency of that allele in the population will increase. NATURAL SELECTION STABILIZING SELECTION Natural selection that favours intermediate phenotypes and acts against extreme variants. This reduces variation and improves the adaptation of the population to aspects of the environment that remain fairly constant NATURAL SELECTION DIRECTIONAL SELECTION Natural selection that favours the phenotypes at one extreme. Results in a phenotype shift. Common when there is environmental change. NATURAL SELECTION DISRUPTIVE SELECTION Natural selection that favours the extremes of phenotypes. Can result in eliminating the intermediate. NATURAL SELECTION SEXUAL SELECTION Choices of females for their mates. Can result in SEXUAL DIMORPHISM when males and females species have different characteristics. Usually involves the competition between males (through combat or visual displays).