Presentation on theme: "1 Population Genetics and Natural Selection Chapter 8 Microsatellite alleles for three loci depict genetic variation within a Great Tit population."— Presentation transcript:
1 Population Genetics and Natural Selection Chapter 8 Microsatellite alleles for three loci depict genetic variation within a Great Tit population.
2 Definitions Evolution – a change in gene frequencies in a population; may lead to speciation. Natural Selection – differential reproduction and survival of individuals in a pop. due the environmental influences on the pop.; the environment affects fitness. Offspring like parents Variation among individuals is heritable. More offspring than environment can support Some individuals have higher fitness under particular environmental conditions.
3 Gene – a discrete unit of hereditary information consisting of a specific nucleotide sequence in DNA Alleles – alternative forms of a gene Genotype – the genetic makeup of an individual Phenotype – the physical and physiological traits of an organism Campbell and Reese 2002
4 Variation Within Populations Phenotype is environmental and genetic Variation in Plant Populations Many plant species differ dramatically in form from one elevation to another (e.g. ) Potentilla glandulosa. Distinctive ecotypes locally adapted and genetically distinctive populations within a species. Cross breeds produce viable seeds that are reproductive – not separate species.
5 Hypothesis: Morphology differences partly due to genetics. Null Hypothesis: No genetic difference; all environmental.
6 Hardy Weinberg Hardy Weinberg principle states that in a population mating at random in the absence of evolutionary forces (natural selection), allele frequencies will remain constant. The H-W Model allows for predicting genetic change due to environmental forces. Hypothesis: environmental factor influences change in allele frequency change. Null Hypothesis: allele frequency remains unchanged. 1 = (p+q) 2 = p 2 +2pq+q 2
7 Conditions Necessary for Hardy Weinberg Random Mating No Mutations Large Population Size No Immigration Equitable Fitness Between All Genotypes Likely, at least one of these will not be met and allele frequencies will change. Potential for evolutionary change in natural populations is very great.
8 Natural Selection Some individuals in a population, because of their phenotypic characteristics, produce more offspring that themselves live to reproduce. Natural selection can favor, disfavor, or conserve the genetic make-up of a population.
9 Stabilizing Selection Stabilizing selection acts to impede changes in a population by acting against extreme phenotypes and favoring average phenotypes.
10 Directional Selection Directional selection leads to changes in phenotypes by favoring an extreme phenotype over other phenotypes in the population.
11 Disruptive Selection Disruptive selection creates bimodal distributions by favoring two or more extreme phenotypes over the average phenotype in a population.
12 Change Due To Chance Random processes can change gene frequencies in populations, especially in small populations. Called genetic drift. Major concern of habitat fragmentation is reducing habitat availability to the point where genetic drift will reduce genetic diversity within natural populations.
13 Genetic Variation In Island Populations In general, genetic variation is lower in isolated and generally smaller, island populations. Reduced genetic variation indicates a lower potential for a population to evolve.