The Evolution of Populations Chapter 23

Topics I. Hardy Weinberg Theorem  Introduction  The theorem  Computing allelic frequencies  Microevolution II. Genetic Variation  Variation within a population  Variation between populations  Mutations/sexual recombination  Balanced polymorphism  No perfection

“Populations evolve, individuals do not”

Variation in a Natural Population

Introduction Gene Pool  all the alleles of all the individuals of a population Genetic Structure  the allelic composition of the population Fixed Alleles  same allele for everyone, no variation in a population Population Genetics  how populations change over time

Modern Synthesis Theory Darwin  found a mechanism of change Mendel  a model  particulate hypothesis Modern Synthesis Theory  brought all these together  evolution, ecology, population genetics,& taxonomy It emphasizes:  the importance of populations as the units of evolution,  the central role of natural selection as the most important mechanism of evolution, and  the idea of gradualism to explain how large changes can evolve as an accumulation of small changes over long periods of time Hardy & Weinberg  two mathematicians developed theorems separately at the same time

Hardy Weinberg Theorem The frequencies of alleles in a population’s gene pool remain constant over the generations (unless acted upon by agents other than sexual recombination) Describes a non-evolving population Makes 5 assumptions

Assumptions No mutations No emigration or immigration Large breeding population Random mating All alleles are equally viable (no selection

Computing Allelic Frequencies Population geneticists use p to represent the frequency of one allele and q to represent the frequency of the other allele p  frequency of the dominant allele q  frequency of the recessive allele The combined frequencies must add to 100%; therefore p + q = 1 If p + q = 1, then p = 1 - q and q = 1 - p.

In situations in which there are three or more alleles  AA, Aa and aa p= frequency of A q = frequency of a pq = frequency of Aa p + q = 1 Square the equation  p 2 +2pq +q 2 = 1 Ex 1000 /360 recessive  Dominant? Heterozygote? Always find q

q 2 = 360/1000 =.36,q=.6 p + q = 1, p = 1- q =.4 p 2 =.16 = 160 2pq = 2(.4)(.6) =.48 =480

Microevolution The Hardy-Weinberg theory provides a baseline against which we can compare the allele and genotype frequencies of an evolving population Microevolution  generation-to-generation change in a population’s frequencies of alleles Causes of Microevolution  Mutation  Gene Flow  Genetic Drift Founder Effect Bottleneck Effect  Non random mating  Natural selection

Mutation A change in an organism’s DNA A new mutation that is transmitted in gametes can immediately change the gene pool of a population by substituting the mutated allele for the older allele. Is vital to evolution because it is the only force that generates new alleles Is the original source of genetic variation that serves as the raw material for natural selection

Gene Flow A genetic exchange due to migration of fertile individuals or gametes between populations Population may lose or gain alleles  For example, a wildflower population consisted entirely of white flowers, its pollen (r alleles only) could be carried into our target population  This would increase the frequency of r alleles in the target population in the next generation Gene flow tends to reduce differences between populations  If extensive enough, gene flow can amalgamate neighboring populations into a single population with a common genetic structure.  The migration of people throughout the world is transferring alleles between populations that were once isolated, increasing gene flow.

Genetic Drift When changes in gene frequencies from one generation to another occur because of chance events that occur when populations are finite in size Natural disasters  sharp reductions due to randomness nothing to do with genes Genetic drift at small population sizes often occurs as a result of  The bottleneck effect  The founder effect

Genetic Drift wildflower population stable, only ten plants  some alleles can be completely eliminated

The Bottleneck Effect When the numbers of individuals in a larger population are drastically reduced by a disaster (fire, flood) Survivors pass through a restrictive bottleneck  gene pool no longer reflective of original population Reduces genetic variation and adaptability  Ex: Cheetah population, elephant seals in CA

The Bottleneck Effect: an analogy

Figure 23.5x Cheetahs, the bottleneck effect

The Founder Effect A new population is started by only a few individuals that do not represent the gene pool of the larger source population A population could be started by single pregnant female or single seed with only a tiny fraction of the genetic variation of the source population Have been demonstrated in human populations that started from a small group of colonists

Non Random Mating Selection preferences Inbreeding  get more homozygous and less heterozygous  Ex: self pollinating plants  Explain more

Natural Selection A violation of the conditions necessary for the Hardy-Weinberg equilibrium HW  all individuals in a population have equal ability to survive and produce viable, fertile offspring Natural Selection in a population with variable individuals, will lead some individuals to leave more offspring than others Selection  in some alleles being passed along to the next generation in numbers disproportionate to their frequencies in the present generation (wildflower example) Natural selection accumulates and maintains favorable genotypes in a population.

Modes of Natural Selection Directional Disruptive/Diversifying Stabilizing

Directional

Directional selection for beak size in a Galápagos population of the medium ground finch

Diversifying

Diversifying selection in a finch population Large beak Small beak

Stabilizing

Modes of selection Which mode of selection might lead toward speciation?

Sexual Selection Natural selection for mating success Sexual dimorphism  males and females look very different  birds Intrasexual selection  direct competition of one sex for mates of the opposite sex Intersexual selection  females are choosy about their mates, choice depends on the showiness of the male

Genetic Variation Each individual is unique  can observe phenotypic variations Not all phenotypic variation is heritable Types of Variation  Variation within a population  Variation between populations  Mutations/sexual recombination  Balanced polymorphism  No perfection

Variation Within a Population Morphs  different forms Polymorphism  2 or more distinct morphs These butterflies are genetically identical at the loci for coloration, but they emerge at different seasons Emerged in Summer Emerged in Spring

Variation Between Populations Geographical variation in the form of graded change in a trait along a geographic axis is called a cline Plant at a higher altitude are shorter

Mutations/Sexual Recombination New alleles originate only by mutation Mutations are changes in the nucleotide sequence of DNA Mutations of individual genes are rare and random Mutations in somatic cells are lost when the individual dies Only mutations in cell lines that produce gametes can be passed along to offspring Diploidy  hides a lot of variation

Sexual selection and the evolution of male appearance SEXUAL DIMORPHISM

Male peacock

Balanced Polymorphism Heterozygote Advantage  sickle cell, increases diversity, keeps both alleles in the population Neutral Variation  not helpful for success, variation doesn’t mean much  Ex fingerprints  no evolutionary advantage

Heterozygote advantage Heterozygotes at a particular locus have greater survivorship and reproductive success than homozygotes In these cases, multiple alleles will be maintained at that locus by natural selection.

No Perfection Evolution is imperfect Every species constrained to historical descent Evolution is an editing process, not a creation process Not all evolution is adaptive  bottle neck and founder effects

The two-fold disadvantage of sex