The Evolution of PopulationsThe Evolution of Populations

IntroductionIntroduction 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 its individuals, that evolves.It is the population, not its individuals, that evolves.

Shell diversity…Shell diversity…

Sexual Reproduction ReviewSexual Reproduction ReviewMutationMutation and Sexual recombination Sexual recombination generate

genetic variation◦MutationMutation

Only source of newnew genes (alleles) Point mutations◦Changes in one nucleotide base◦Can have significant impact (ie. hemoglobin)

Chromosome mutation◦Deletion, disruption, duplication, rearrangement◦Almost always harmful

Sexual Reproduction ReviewSexual Reproduction ReviewMost genetic variation are due to Sexual Sexual

recombination recombination ◦Rearranges alleles into new combinations◦Three mechanisms that “shuffle” alleles

Crossing-over – Prophase I meiosis Independent assortment during meiosis Fertilization

Population GeneticsPopulation GeneticsThe study of how populations change genetically over time.

Population GeneticsPopulation GeneticsThe biological sciences now generally define

evolution as being the sum total of the genetically inherited changes in the individuals who are the members of a population's gene pool.

Population◦A group of individuals of the same species that live in

the same area and interbreed.Gene Pool◦All of the alleles at all loci in all the members of a

population◦If all members of a population are homozygous for the

same allele, the allele is said to be fixed.

Hardy-WeinbergHardy-WeinbergThis definition of evolution was

developed largely as a result of independent work in the early 20th century by Godfrey Hardy, an English mathematician, and Wilhelm Weinberg, a German physician. 

Through mathematical modeling based on probability, they concluded in 1908 that gene pool frequencies are inherently stable but that evolution should be expected in all populations virtually all of the time. 

Hardy-Weinberg EquilibriumHardy-Weinberg EquilibriumThe H-W equation can be used to test whether

a population is evolvingThe Theorem is used to describe a population

that is not is not evolving.Under certain (unobtainable) conditions, allele

frequencies will remain constant over generations unless they are acted upon by forces other than other than Mendelian segregation and recombination

Hardy-Weinberg TheoremHardy-Weinberg Theorem

The fact is:◦allelic frequencies change.allelic frequencies change.◦Populations evolve.Populations evolve.

But notnot because of Mendelian genetics.The relative frequencies of alleles or genotypes

remain the same between one generation and the next.

5 conditions for Hardy-Weinberg5 conditions for Hardy-Weinberg

By applying the equation we can test this, but first, these conditions must be met◦No mutations◦Random mating◦No natural selection◦The population size must be extremely large. (no genetic

drift)◦No gene flow (emigration, immigration)

Hardy-Weinberg TheoremHardy-Weinberg Theorem

Obviously, the Hardy-Weinberg equilibrium cannot exist in real life. Some or all of these types of forces all act on living populations at various times and evolution at some level occurs in all living organisms.

There are two formulas that must be memorized: p2 + 2pq + q2 = 1p + q = 1

Hardy-Weinberg TheoremHardy-Weinberg Theoremfor a trait controlled by a pair of alleles (A and a)

◦ pp is defined as the frequency of the dominant allele ◦ qq as the frequency of the recessive allele.  

In other words, p equals all of the alleles in individuals who are homozygous dominant (AA) and half of the alleles in people who are heterozygous (Aa) for this trait in a population. 

In mathematical terms, p = AA + ½Aa

Hardy-Weinberg TheoremHardy-Weinberg Theorem

Likewise, q equals all of the alleles in individuals who are homozygous recessive (aa) and the other half of the alleles in people who are heterozygous (Aa).

q = aa + ½AaBecause there are only two alleles in this case, the

frequency of one plus the frequency of the other must equal 100%, which is to say

p + q = 1

Hardy-Weinberg TheoremHardy-Weinberg Theorem

All possible combinations of alleles occurring randomly (p + q)² = 1

or more simply

p² + 2pq + q² = 1p² + 2pq + q² = 1pp22 = percentage of homozygous dominant individuals

qq22 = percentage of homozygous recessive individuals2pq2pq = percentage of heterozygous individuals

Hardy-Weinberg TheoremHardy-Weinberg TheoremFrom observations of phenotypes, it is usually only possible to

know the frequency of homozygous recessive people, or q² in the equation, since they will not have the dominant trait. 

Those who express the trait in their phenotype could be either homozygous dominant (p²) or heterozygous (2pq). 

The Hardy-Weinberg equation allows us to predict which ones they are.  Since p = 1 - q and q is known, it is possible to calculate p as well. 

This then provides the predicted frequencies of all three genotypes for the selected trait within the population.

Try these problemsTry these problems

http://www.k-state.edu/parasitology/biology198/hardwein.html

MicroevolutionMicroevolution

A generation-to-generation change in a population’s frequencies of alleles.

Causes of Microevolution Causes of Microevolution in a populationin a population

Four factors can alter the allele frequencies in a population:• mutation (rare)• genetic drift• gene flow• natural selection

All represent departures from the conditions required for the Hardy-Weinberg equilibrium.Natural selection is the only factor that generally adapts a population

to its environment.Selection always favors the disproportionate propagation of

favorable traits.The other three may effect populations in positive, negative, or

neutral ways.

MutationMutationChange 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.

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

Causes of Microevolution Causes of Microevolution in a populationin a population

Genetic Drift: is the change in allele frequency of a small population, due to chance.

Unpredictable, random, nonadaptive The smaller the population, greater the

chance

Genetic DriftGenetic Drift

Genetic Drift: Two ExamplesGenetic Drift: Two ExamplesBottleneck = an

environmental crisis may reduce the size of the original population and the small surviving population may not be representative of the original population’s gene pool.

Genetic Drift: Two ExamplesGenetic Drift: Two ExamplesBottleneck - The few

survivors that “pass through the bottleneck” may have a gene pool that no longer reflects the original population gene pool.

The California condor was reduced to nine individuals.

Genetic Drift: Two ExamplesGenetic Drift: Two Examples

The cheetah has reached the brink of extinction twice◦Ice age 10, 000

years◦19th century

hunting

Genetic Drift: Two ExamplesGenetic Drift: Two Examples• Founder Effect

• This occurs when a few members of a population colonize an isolated island, lake , or some other new habitat.

The allele frequency may not represent the gene pool of the larger population they left.

Thus diseases of recessive genes, which require two copies of the gene to cause the disease, will show up more frequently than they would if the population married outside the group.

Genetic Drift: Two ExamplesGenetic Drift: Two Examples

Ellis-Van Creveld SyndromeAMISH

• relatively high frequency of certain inherited disorders among human populations established by a small number of colonists.

Gene FlowGene FlowA population gains or looses alleles by genetic

additions to and/or subtractions from the population.

Members of a population are far more likely to breed with members of the same population than with members of other populations.

Gene FlowGene FlowGene Flow tends to reduce the genetic

differences between populations

Individuals near the population’s center are, on average, more closely related to one another than to members of other populations

Natural SelectionNatural SelectionRelative fitness refers to the

contribution an organism makes to the gene pool of the next generation relative to the contributions of other members.

It is measured by reproductive success

Acts more directly on the phenotype, changing the allele frequency of the population in three ways

Natural SelectionNatural Selection

1) artificial selection (insecticide resistance)

2) Large black bears survive periods of extreme cold better than small ones so become more common during glacial periods

Directional Selection Directional Selection shifts the overall makeup of the population by favoring variants of one extreme over another

Directional SelectionDirectional Selection

Directional SelectionDirectional Selection

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

Natural SelectionNatural Selection

African fire-bellied seed cracker finch

Disruptive Selection Disruptive Selection occurs when conditions favor individuals on both extremes of a phenotypic range rather than individuals with intermediate phenotypes

Natural SelectionNatural SelectionStabilizing Selection Stabilizing Selection acts against both extreme

phenotypes and favors intermediate variants Siberian Husky

◦ Medium dog, males weighing 16-27kg (35-60lbs).

◦ Strong pectoral and leg muscles, allowing it to move through dense snow.

◦ If heavier muscles, it would sink deeper into the snow. They would move slower or would sink and get stuck.

◦ If lighter muscles, it would not be strong enough to pull sleds and equipment.

◦ So, stabilizing selection has chosen a norm for the size of the Siberian Husky.

In this case, darker mice are favored because they live among darkrocks and a darker fur color conceals Them from predators.

These mice have colonized a patchy habitat made up of light and dark rocks, with the result that mice of an intermediate color are at a disadvantage.

If the environment consists of rocks of an intermediate color, both light and dark mice will be selected against.

Phenotypes (fur color)

Original population

Originalpopulation

Evolvedpopulation

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of of SelectioSelectionn

Sexual Selection Sexual Selection leads to sexual dimorphismsleads to sexual dimorphisms

Two varieties of sexual selection: Intrasexual selection ◦ competition between the same sex- usually male

vs. male◦ Example: deer or rams butting heads, antlers,

horns, large stature or musculature Intersexual selection◦ female mate choice based on appearance or

behavior of males◦ Example: peacock plumage, elaborate mating

behaviors

Sexual selection and the evolution of male appearance

DiploidyDiploidypreserves variation in eukaryotes◦Two copies of every gene◦Prevents the elimination of recessive alleles via

selection because they do not impact the phenotype in heterozygotes.

◦Even recessive alleles that are unfavorable can persist in a population through their propagation by heterozygous individuals.

Both quantitativequantitative and discretediscrete characters contribute to variation within a population.Quantitative characters are those that vary along

a continuum within a population.For example, plant height in our wildflower population

includes short and tall plants and everything in between.

Quantitative variation is usually due to polygenic inheritance in which the additive effects of two or more genes influence a single phenotypic character.

Discrete characters, such as flower color, are usually determined by a single locus with different alleles with distinct impacts on the phenotype.

PolymorphismsPolymorphismsTwo or more distinct forms of a discrete

character in a population. (dominant and recessive alleles)

Allows for natural selection to act on phenotypes changing the allele frequencies of the gene pool.

It gives the heterozygous individual the ability to have a better reproductive success. Called the Heterozygote advantage.

This maintains both alleles in the population.

Heterozygote AdvantageHeterozygote Advantage

A situation in which a single disadvantageous allele is not selected out of a population, because, when a person is heterozygous for that allele (the person has one disadvantageous allele and one normal allele), the person gains some sort of local advantage by having the disadvantageous allele.

Heterozygote AdvantageHeterozygote Advantage

AA = No sickle (Dead - malaria)

Aa = sickle trait

aa = sickle disease (Dead)

Plasmodium falciparum

Most Species Exhibit Geographic Most Species Exhibit Geographic VariationsVariationsVariations in gene pools between populationsCline = a gradual change in a trait corresponding

to a graded change in some geographic axis. Some of the variation has a genetic basis.

yarrow plants in the Sierra Nevada Mountains decrease in size with increasing elevation.

Why doesn’t natural selection produce Why doesn’t natural selection produce perfect organisms?perfect organisms?

Selection can only edit existing variations.Evolution is limited by historical constraints.Adaptations are often compromises.Chance, natural selection, and the environment

interact.