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 p2 +2pq +q2 =

1 Ex 1000 /360 recessive Dominant?

Heterozygote? Always find q

q2 = 360/1000 =.36,q=.6p + q = 1 , p = 1- q =.4p2 = .16 = 1602pq = 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

Microevolutiongeneration-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 heterozygousEx: self pollinating plantsExplain 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 beakSmall beak

Stabilizing

Modes of selection

Which mode of selection might lead toward speciation?

Sexual Selection Natural selection for mating success Sexual dimorphismmales 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 populationVariation between populationsMutations/sexual recombinationBalanced polymorphismNo perfection

Variation Within a Population Morphs different

forms Polymorphism2 or

more distinct morphs

These butterflies aregenetically identical at the loci forcoloration, but they emerge atdifferent 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 muchEx 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