Evolution of Populations Population = A localized group of organisms which belong to the same species. Species = Actual or potentially interbreeding natural populations which are reproductively isolated from other groups. Gene Pool = The total aggregate of genes in a population
Hardy-Weinberg Theorem In the absence of other factors, the segregation and recombination of alleles during meiosis and fertilization will not alter the genetic makeup of a population. Describes a non-evolving population p2 + 2pq + q2 = 1 Where p = frequency of A allele q = frequency of a allele
The Hardy-Weinberg theorem
The Hardy-Weinberg theorem
Question 2 b) If the frequency of the homozygous dominant genotype is 0.49, what is the frequency of the dominant allele. Let the dominant genotype be represented by AA…. AA = 0.49 … (A2 = 0.49) same as (p2 = 0.49) So what is the frequency of A (p) ?? A (p) = 0.7
Question 2 c) If the frequency of the homozygous recessive genotype is 0.36, what is the frequency of the dominant allele. Let the recessive genotype be represented by aa…. aa = 0.36 … (a2 = 0.36) So …a = 0.6 Then what is A ? A = 0.4
Question 2 d) If the frequency of the homozygous dominant genotype is 0.49, what is the frequency of the homozygous recessive genotype. Let the dominant genotype be represented by AA…. AA = 0.49 … (A2 = 0.49) Then A = 0.7 so…a must = 0.3 Therefore aa = 0.3x0.3 = 0.09
Hardy-Weinberg Theorem Conditions needed for H-W to work Very large population size Isolation from other populations No mutations Random mating No natural selection Hardy-Weinberg animation
Microevolution
Microevolution MICROEVOLUTION = Small scale evolutionary change represented by a generation-to-generation change in a population’s allele or genotype frequencies
Microevolution Genetic drift Gene flow Mutation Nonrandom mating Natural selection Natural selection is adaptive. It leads to accumulation of favorable adaptations in a population
Genetic Drift Changes in the gene pool of a small population due to chance The larger the population…the less important is genetic drift Most (but not all) natural populations are so large that the effect of genetic drift is negligible Reduces overall genetic variability
Genetic drift
Genetic Drift - Bottleneck Effect Drastic reduction in population size by some natural disaster which kills organisms nonselectively The smaller population is now unlikely to represent the genetic makeup (diversity) of the original population Some alleles will be overrepresented, others will be absent. Example: hunting to near extinction
The bottleneck effect: an analogy
Cheetahs, the bottleneck effect (Very low genetic diversity)
Founder Effect When a few individuals colonize a new habitat. The smaller the population size … the less likely the genetic makeup of the colonists will represent the gene pool of the large population that they left
Gene Flow Migration of fertile individuals or transfer of gametes (wind blown pollen for example) between populations Extensive gene flow can eventually group neighboring populations into a single population
Gene flow and human evolution
Mutations Have very little immediate effect on a large population Important to evolution since it is the original source of genetic variation which is the raw material for natural selection
Nonrandom Mating Inbreeding Assortive mating. Individuals mate with partners that are like themselves in phenotypic characters
Natural Selection In any sexually reproducing population, variation among individuals exists (genetic variation) and some variants leave more offspring than others Natural selection is the differential success in reproduction Natural selection is the only cause of microevolution that is ADAPTIVE, since it accumulates and maintains favorable genotypes
Review of Microevolutionary changes Animation Link
Genetic Variation Review of genetic variation from sexual reproduction
Genetic Variation Polygenic characters Discrete characters Multiple loci involved-vary quantitatively (many intermediate phenotypes) within a population. Example = height Discrete characters Determined by a single locus Polymorphism - when 2 or more forms of a discrete character are well represented in a population
Polymorphism in garter snakes
Genetic Variation Geographic variation This variation in alleles exists among populations of most species Cline – a type of geographical variation that shows a graded change in some trait along a geographical feature (such as elevation)
Clinal variation in a plant
Genetic Variation Generation of variation Mutation Recombination Point mutation – involves a single base pair in DNA Chromosomal mutation – usually effect many gene locus and are almost always deleterious Recombination Nearly all genetic variation in a population results from new combinations of already existing alleles !
Genetic Variation Maintenance of variation (how is it preserved?) Diploid state hides some genetic variation from selection by the presence of recessive alleles in heterozygotes
Genetic Variation Maintenance of variation (how is it preserved?) Balanced Polymorphism = the ability of natural selection to maintain diversity in a population Heterozygote advantage Frequency dependent selection
Frequency-dependent selection in a host-parasite relationship
Frequency-dependent selection Polymorphism in sneetches (star-bellied and plain bellied forms) - a classic tale of frequency-dependent selection
Genetic Variation Neutral Variation Variations that confer no selective advantage/disadvantage
Natural Selection is Adaptive Fitness Measured by the relative contribution an individual makes to the gene pool of the next generation Selection acts on phenotypes and can only act indirectly on genotypes
Modes of selection
Modes of Natural Selection Stabilizing selection Favors intermediate variants by selecting against extreme phenotypes Directional selection Favors variants to one extreme Diversifying selection Opposite phenotypic extremes are favored over intermediate phenotypes
Modes of selection
Sexual Dimorphism (Male peacock) Distinction between secondary sexual characteristics of males and females
Sexual selection and the evolution of male appearance
Evolution does not fashion perfect organisms Adaptations are often compromises. An organism must be versatile enough to do many things. Not all evolution is adaptive (example genetic drift in small populations) Selection can only EDIT variations that exist. New alleles/genes are not formed by “mutation on demand”