Evolution of Evolution of PopulationsPopulations

Topics:Topics:

1.Types of Selection

2.Sources of Variation in a Population

• Causes of Microevolution

3.Sources of Stability in Populations

• Hardy-Weinberg Equilibrium

• One obstacle to understanding evolution is the common misconception that organisms evolve, in a Darwinian sense, in their lifetimes.• Natural selection does act on individuals by

impacting their chances of survival and their reproductive success.

• However, the evolutionary impact of natural selection is only apparent in tracking how a population of organisms changes over time.

It is the population, not It is the population, not its individuals, that its individuals, that

evolves.evolves.

IntroductionIntroduction

• Evolution on the scale of populations, called microevolutionmicroevolution, is defined as a change in the allele frequencies in a population.• Macroevolution leads to Speciation.

• For example, the bent grass (Argrostis tenuis) in this photo is growingon the tailings ofan abandoned mine,rich in toxicheavy metals.

• Individual plants do not evolve to become more metal-tolerant during their lifetimes.

• A population is a localized group of individuals that belong to the same species.

• One definition of a species (among others) is a group of populations whose individuals have the potential to interbreed and produce fertile offspring in a nature.

A population’s gene pool is defined by its allele

frequencies

• The total aggregate of genes in a population at any one time is called the population’s gene pool.

• It consists of all alleles at all gene loci in all individuals of a population.

Review of natural selection

Directional SelectionDirectional SelectionDiversifying Selection Diversifying Selection

((DisruptiveDisruptive))

Stabilizing SelectionStabilizing SelectionSexual SelectionSexual Selection

Types of selection

Animation

Directional Directional SelectionSelection Changing environmental Changing environmental

conditionsconditions give rise to give rise to directional selection, directional selection, where where one phenotype one phenotype replaces anotherreplaces another in the in the gene pool.gene pool. Can produce rapid shift in allelic Can produce rapid shift in allelic

frequencies.frequencies. Ex: Peppered mothEx: Peppered moth – – peppered moths, pesticide peppered moths, pesticide

resistance, antibiotic resistanceresistance, antibiotic resistance Occur in response to/when:Occur in response to/when:

* in response to directional change in the environment * in response to directional change in the environment * in response to one or more new environmental conditions* in response to one or more new environmental conditions* when mutation appears and proves to be adaptive* when mutation appears and proves to be adaptive

Diversifying Diversifying (Disruptive)(Disruptive) SelectionSelection Increases the extreme types in a Increases the extreme types in a

population at the expense of the population at the expense of the intermediate forms.intermediate forms.

One population divided into two.One population divided into two.

(bill size in seedcrackers)(bill size in seedcrackers)

• Diversifying selection can result in balanced polymorphism.• For example, two distinct bill types are present

in black-bellied seedcrackers in which larger-billed birds are more efficient when feeding on hard seeds and smaller-billed birds are more efficient when feeding on soft seeds.

Stabilizing SelectionStabilizing Selection Intermediate forms of a trait are favored Intermediate forms of a trait are favored

and alleles that specify extreme forms are and alleles that specify extreme forms are eliminated from a pop. eliminated from a pop.

Ex: Human birth weight stay between 6-8 Ex: Human birth weight stay between 6-8 lbs. Lower or higher has higher mortality.lbs. Lower or higher has higher mortality.

Class Quiz

Sexual SelectionSexual Selection

Sexual dimorphismSexual dimorphism – when sexually – when sexually reproducing species have a distinctly male reproducing species have a distinctly male or female phenotype (Ex. Peacocks)or female phenotype (Ex. Peacocks)

Sexual selection comes into play when Sexual selection comes into play when certain traits are advantageous simply certain traits are advantageous simply because males or females prefer them.because males or females prefer them. Coloration, strength- other “mate attractants”Coloration, strength- other “mate attractants” Females are the main agents of selection hereFemales are the main agents of selection here

• Four factors can alter the allele frequencies in a population:

1.genetic drift

2.natural selection

3.gene flow

4.mutation• All represent departures from the conditions

required for the Hardy-Weinberg equilibrium.

The two main causes of microevolution are genetic drift and natural selection

Overview Clip

Genetic DriftGenetic Drift EVOLUTION BY CHANCE

A random change in allele frequencies over generations, brought about by chance alone.

Impact minor in large pops, but significant in small pops.

In the absence of other forces, random change in allele frequencies leads to the homozygous condition and a loss of genetic diversity over the generations.(This happens in all pops; it just happens faster in small ones.)

Genetic drift is pronounced when very few individuals rebuild a pop or found a new one – get a BOTTLENECK:(a severe reduction in pop size, due to intense selection pressure or natural calamity) see page 257 – founder effect is one type of bottleneck.

Inbreeding – form of genetic drift in a small population. (pg 257)

Genetic DriftCauses of microevolution

loss of genetic diversity

ANIMATION

Short Clip

• The bottleneck effectbottleneck effect occurs when the numbers of individuals in a larger population are drastically reduced by a disaster.disaster.

• By chance, some alleles may be overrepresented and others underrepresentedunderrepresented among the survivors.

• Some alleles may be eliminated eliminated altogether.

• Genetic drift will continue to impact the gene pool until the population is large enough to minimize the impact .

Genetic Drift

Causes of microevolution

BottleneckingBottlenecking is an important concept in conservation biology of endangered species.

• Populations that have suffered bottleneck incidents have lost at least some alleles from the gene pool.

• This reduces individual variation and adaptability.

• For example, the genetic variation in the three small surviving wild populations of cheetahs is very low when compared to other mammals.

• Their genetic variation is similar to highly inbred lab mice.

Genetic DriftCauses of microevolution

Causes of microevolution

An example of a bottleneck:

Northern elephant seals have reduced genetic

variation probably because of a population bottleneck

humans inflicted on them in the 1890s. Hunting reduced their population size to as

few as 20 individuals at the end of the 19th century.

Their population has since rebounded to over 30,000—but their genes still carry

the marks of this bottleneck: they have much less genetic variation than a

population of southern elephant seals that was not

so intensely hunted.

Genetic Drift

Causes of microevolution

• The founder effect occurs when a new new population is started by only a few population is started by only a few individualsindividuals that do notdo not represent the gene pool of the larger source population.

• At an extreme, 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.

• Genetic drift would continueGenetic drift would continue from generation to generation until the population grew large enough for sampling errors to be minimal.

• Founder effects have been demonstrated in human populations that started from a small group of colonists.

Genetic DriftCauses of microevolution

Gene Flow Gene flow is the physical flow of allelesphysical flow of alleles into

or out of a population.Immigration – alleles coming in

(added)Emigration – alleles moving out (lost)

Gene flow counteracts differences that arise through mutation, natural selection, and genetic drift.

Gene flow helps keep separated populations genetically similar.

Causes of microevolution Gene Flow

Animation

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.

Causes of microevolution Gene Flow

AA mutationmutation is a change in an organism’s is a change in an organism’s DNA. DNA.

A new mutation that is transmitted in A new mutation that is transmitted in gametes can immediately change the gametes can immediately change the gene pool of a population by substituting gene pool of a population by substituting the mutated allele for the older allele.the mutated allele for the older allele. For any single locus, mutation alone does not For any single locus, mutation alone does not

have much quantitative effect on a large have much quantitative effect on a large population in a single generation.population in a single generation.

An individual mutant allele may have greater An individual mutant allele may have greater impacts later through increases in its relative impacts later through increases in its relative frequencies as a result of natural selection or frequencies as a result of natural selection or genetic drift.genetic drift.

Causes of microevolution MutationMutationMutation

While mutations at an While mutations at an individual locus are a rare individual locus are a rare event, the cumulative impact event, the cumulative impact of mutations at of mutations at allall loci can be loci can be significant.significant. Each individual has thousands of Each individual has thousands of

genes, any one of which could genes, any one of which could experience a mutation.experience a mutation.

Populations are composed of Populations are composed of thousands or millions of thousands or millions of individuals that may have individuals that may have experienced mutations.experienced mutations.

Over the long term, mutation Over the long term, mutation is a very important to is a very important to evolution because it is the evolution because it is the original source of genetic original source of genetic variation that serves as the variation that serves as the raw material for natural raw material for natural selection.selection.

Causes of microevolution Mutation

Clip: Review of factors that contribute to gene pool

change. (warning…cheesy but good info

Frequency Dependent Frequency Dependent SelectionSelection Preserves VarietyPreserves Variety Also called “minority advantage”Also called “minority advantage” Acts to decrease the frequency of the more Acts to decrease the frequency of the more

common phenotype and increase frequency of common phenotype and increase frequency of the less common one.the less common one.

EX: Sometimes predators tend to concentrate EX: Sometimes predators tend to concentrate on common varieties of prey and overlook rare on common varieties of prey and overlook rare ones. Within prey species, this could result in ones. Within prey species, this could result in the fitness of each variety being inversely the fitness of each variety being inversely related to its frequency in the population. related to its frequency in the population.

could maintain polymorphismcould maintain polymorphism

Allows us to Allows us to calculate the calculate the

frequencies of alleles frequencies of alleles in a populationin a population

• The Hardy-Weinberg theorem describes the gene pool of a nonevolving population.

• This theorem states that the frequencies of alleles and genotypes in a population’s gene pool will remain constant over generations unless acted upon by agents other than Mendelian segregation and recombination of alleles.

The Hardy-Weinberg Theorem describes a nonevolving

population

Allele Allele FrequenciesFrequencies

Allele frequencies:Allele frequencies: the abundance the abundance of each kind of allele in a of each kind of allele in a population.population.

To compare, use Hardy-Weinberg To compare, use Hardy-Weinberg equilibrium:equilibrium:

pp22= frequency of homo dom = frequency of homo dom 2pq = frequency of hetero2pq = frequency of heteroqq22 = frequency of homo = frequency of homo

recessrecess

p2 + 2pq + q2 = 1

•Consider a population whose gene pool contains the alleles A and a.

•Hardy and Weinberg assigned the letter p to the frequency of the dominant allele A and the letter q to the frequency of the recessive allele a.

•Since the sum of all the alleles must equal 100%, then p + q = 1.

p2 + 2pq + q2 = 1p + q = 1

•They then reasoned that all the random possible combinations of

the members of a population would equal

•(p+q)2

•or p2 + 2pq + q2

p2 + 2pq + q2 = 1p + q = 1

The frequencies of A & a will remain unchanged generation after generation if the following conditions are met:

1. Large population.1. Large population.

-The population must be large to minimize random sampling errors.

2. Random mating.2. Random mating.

-There is no mating preference. For example an AA male does not prefer an aa female. (NO SEXUAL SELECTION)

3. No mutation. 3. No mutation. -The alleles must not change.

4. No migration. 4. No migration.

-Exchange of genes between the population and another population must not occur.

5. No natural selection.5. No natural selection.

-Natural selection must not favor any particular individual.

Example problem:

1 in 1700 US Caucasian newborns have cystic fibrous. (C for normal is dominant over c for cystic fibrous). 1-What percent of the population have cystic fibrous.

2-What percent of the population are carriers? •HWE= p + q = 1 or p2 + 2pq + q2 = 1•Since we know cccc = q2, calculate q2

•q2 =cccc= 1/1700= 0.00059= 0.059% (Answer to #1)

•cc =√.00059 =0.024= 2.4%•So, the cc allele frequency is 2.4%•Now, Calculate CC allele•HWE= p + q= 1•So, p= 1-q•p= 1- .024•p= 0.976 or 97.6 %

We still haven’t answered question #2

• pp = 0.976 or 97.6 %

• qq = 0.024 or 2.4%

• HWE= p2 + 2pq + q2 = 1

• 2pq= Heterozygous condition (carriers)

• 2pq= 2 (0.976 x 0.024)

• 2pq= 0.0468 or 4.68% of US Caucasians are carriers. (Answer to #2)

Example problem:

1 in 1700 US Caucasian newborns have cystic fibrous. (C for normal is dominant over c for cystic fibrous).

1-What percent of the population have cystic fibrous. (0.059%) 2-What percent of the population are carriers?

(4.68%)

Practice Problem:• If 9 percent of the population has blue

eyes, what percent is hybrid for brown eyes? Homozygous for brown eyes?

1. q2 = 9% = 0.09 (convert to decimal)

2. Solve for q: q = 0.3

3. Solve for p:

• p + q = 1 ; p = 1-0.3 ; p= 0.7

4. Solve for 2pq; 2(0.7 x 0.3)= 42% are hybrid

5. Solve for p2:

• p2 = (.7)2 =49% are homozygous dominant.

Practice Problem:• This is a classic data set on wing coloration in the scarlet

tiger moth (Panaxia dominula). Coloration in this species had been previously shown to behave as a single-locus, two-allele system with incomplete dominance. Data for 1612 individuals are given below:

• White-spotted (AA) =1469 Intermediate (Aa) = 138 Little spotting (aa) =5

• What are the frequencies of the A & a alleles?

A = (2 * (1469) + (138))/(2 * (1469 + 138 + 5)) = .954 or 95.4%

a = 1 - .954 = .046 or 4.6%

AA = (.954) 2 = .910 or 91% Aa = 2 (.954)(.046) = .087 or 8.7% aa = (.046) 2 = .002 or .2%

It is the changes in gene

frequencies over time that result in evolution.