# Evolution of Populations

## Presentation on theme: "Evolution of Populations"— Presentation transcript:

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 p pq + 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

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

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

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”