Presentation on theme: "Population Genetics and Natural Selection"— Presentation transcript:
1Population Genetics and Natural Selection Chapter 8Microsatellite alleles for three loci depict genetic variation within a Great Tit population.
2DefinitionsEvolution – 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 parentsVariation among individuals is heritable.More offspring than environment can supportSome individuals have higher fitness under particular environmental conditions.
3Alleles – alternative forms of a gene Gene – a discrete unit of hereditary information consisting of a specific nucleotide sequence in DNAAlleles – alternative forms of a geneGenotype – the genetic makeup of an individualPhenotype – the physical and physiological traits of an organismCampbell and Reese 2002
4Variation Within Populations Phenotype is environmental and geneticVariation in Plant PopulationsMany plant species differ dramatically in form from one elevation to another (e.g. ) Potentilla glandulosa.Distinctive ecotypeslocally adapted and genetically distinctive populations within a species.Cross breeds produce viable seeds that are reproductive – not separate species.
5Hypothesis: Morphology differences partly due to genetics. Null Hypothesis: No genetic difference; all environmental.
6Hardy WeinbergHardy 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 = p2+2pq+q2
7Conditions Necessary for Hardy Weinberg Random MatingNo MutationsLarge Population SizeNo ImmigrationEquitable Fitness Between All GenotypesLikely, at least one of these will not be met and allele frequencies will change.Potential for evolutionary change in natural populations is very great.
8Natural SelectionSome 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.
9Stabilizing Selection Stabilizing selection acts to impede changes in a population by acting against extreme phenotypes and favoring average phenotypes.
10Directional Selection Directional selection leads to changes in phenotypes by favoring an extreme phenotype over other phenotypes in the population.
11Disruptive SelectionDisruptive selection creates bimodal distributions by favoring two or more extreme phenotypes over the average phenotype in a population.
12Change Due To ChanceRandom 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.
13Genetic 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.