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bio 201 l7.doc

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University of Toronto St. George

LECTURE 7 Natural Selection Microevolution: Evolutionary Forces -five evolutionary forces significantly alter the genetic structure of a population 1. mutation 2. gene flow 3. genetic drift 4. nonrandom mating 5. selection Mutation • Source of new variation • Relatively rare • May be • bacteria • short generation time • mutations- important source of variation • beneficial mutations can increase in frequency very quickly - Created by binary fission - If a mutation is beneficial it would be really quickly passed on, if they are more beneificial they will reproduce more quickly Gene Flow • Migration: movement of individuals from one population to another, followed by breeding • – Immigration • – Emigration - Flow of genetic information or passing on genetic information. It is important because it is about reproduction. It is done by immigration and emigration. Genetic Drift - Random change in gene frequency in small populations over time due to chance: can occur in 3 situations: 1. Chance events 2. Bottleneck effect 3. Founder effect • Chance events – small populations are especially prone to loss of alleles though chance - Dealing with small population sizes. When you have small population sizes what happens with chance events bottle neck effect and founder effect Bottle Neck Effect • Population bottleneck – genetic change resulting from a dramatic reduction of population numbers - You could have extinction, If you have small population size you already have a small gene pool. If a predator takes away the only one type of gene pool then that gene will be lost. If there is a larger population of one gene then losing one will not affect the gene pool. - You start with a large population, but then as you move up the population you end up having a smaller population. - An example of a bottle nect affect is the elephant seal. Founder Effect • genetic differences resulting from a small number (few individuals) of breeding individuals • When few individuals colonize a region these individuals will not have all alleles of the source population - Similar to bottle neck but in reverse. A HUMAN EXAMPLE OF THE FOUNDER EFFECT •Ellis-van Creveld syndrome – allele for syndrome in population higher population •short arms and legs •extra fingers •heart defects •high mortality rate Nonrandom Mating  not by chance •Mating that occurs more or less frequently than expected by chance Darwin said not equal opportunity mating in population is non random event SEXUAL SELECTION sexual dimorphism  difference in male / female Conspicuous behavior Sage grouse (attract mates and predators) Conspicuous plumage Sexual Selection “… depends on the advantage which certain individuals have over other individuals of the same sex and species, in exclusive relation to reproduction.” – Charles Darwin Peacock Displays: Survival of the Fittest? + Mating Success - Survival (predation) Sexual Selection •often leads to exaggerated traits •Do females prefer exaggerated traits? Sexual Selection Testing female choice Sexually selected traits function solely to enhance Mating success •Mating success elongated > natural > shortened No significant results from different number of nest, normal size, reduce tails, lengthen tail Results: longer tails mated more Why Do Females Show Preferen
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