Population Genetics

The “Modern Synthesis” of evolution is Darwinismenlightened by the understanding of molecular genetics which has been gained since Darwin.

The key to understanding how evolution occurs isa move from viewing genetics in terms of individualsand their alleles to --

the frequencies of those alleles among the genes ofall individuals comprising a population.

We know about genes and particulate inheritance.Darwin did not. He was neither the first not thelast to accept blended inheritance. He wrotebefore Mendel had described recessive traits. Toexplain evolution, he fell back into a second error:the inheritance of acquired traits.

Most phenotypes, resulting from the influence ofmany genes, do seem to be inherited as if blended.Without a mechanism for particulate inheritance,it was hard to establish the concept.

Mendel’s genetics disappeared into the literature until the beginning of the 20th century.

The rediscovery of Mendelian genetics led to anumber of leading biologists claiming that evolutionresulted from inheritance of mutations. Evolution, inthis view, moved rapidly and by jumps, rather thangradually, as Darwin had believed.

Failures to accept “the modern synthesis” ofMendelian genetics and Darwinian evolution persisted into and after WWII - e.g. Lysenko.

To understand the modern synthesis, we need toconsider the genetics of populations, rather thanindividuals.Consider a Punnett square for a single trait cross:

(male) ½A ½a

½A AA Aa (female)

½a Aa aa

in describing this cross, we have shown the effectsof meiosis: 1/2 the sperm carry A, 1/2 a, and similarly for the eggs.

Now recognize that the fractions of these two allelesin the population may not be equal, and there may bemore than two alleles. The sum of all alleles for a trait in the population is the gene pool for that trait.

We measure fraction p of the genes in this genepool are of type A, and fraction q are type a (assuming males and females are genetically similar). Now the Punnett square looks like this:

male pA qa

pA p2AA pqAa female

qa pqAa q2aa

A mating like this does not change gene frequencies.Evolution is a change in the composition of the gene pool. Gradual change is called microevolution.

In a population that had those allele frequencies,they would remain unchanged indefinitely if theconditions for Hardy-Weinberg equilibrium held.

What are the conditions?

1. Large population size2. No migration (gene flow occurring through

immigration or emigration)3. No mutation4. Random mating (no assortative mating)5. No natural selection

Do the conditions often apply (or apply for long)?

The Hardy-Weinberg Law is a null hypothesis.It holds (as what is called the Hardy-Weinbergequilibrium) when things don’t change, i.e.

1. In large populations there is no genetic drift. In small populations random events (mortality of a single individual) may materially affect gene frequency. This happens in small island popula- tions or populations of endo (internal) parasites.

2. There is no movement between populations, that would be gene flow. The genes moved would change the frequencies in both source and recipient populations.

3. There is no mutation. If one A mutated to a per 100 alleles, then what was 50% A in the starting population would become 49%A after mutation. Actual mutation rates are about 1/106 per gene, but that translates to about 1 mutation per gamete for us. We are, thus, each unique.

4. Mating (fertilization) occurs randomly. If blondes would only marry blondes (real ones) (blond hair being recessive), there would be a much higher frequency of the blond phenotype. Let’s look at an example of this:

We can figure out gene frequencies in a population if we know the frequency of the recessive phenotype. For these individuals, knowing the phenotype frequency we also know the genotype and gene frequencies. The frequency of the recessive phenotype is q2. That is also the frequency of the homozygous recessive genotype. Then the frequency of the recessive gene is the square root of q2 q.

Now for the example:

We start with 100 people (50% male). 1 out of 10is a natural blond. That means q2 = .1, and q=.316.

p = 1 – q = .684. Those would be the values indefinitely if mating were random, but…

If blondes only mate with blondes, then the 5 blondmales mate with the 5 blond females, and produce10 blond children in the next generation. As to the other 90 (or 180 genes): p2 = (.684)2 (·100) 46 are homozygous for dark hair (or 92 dark-haired genes), and2pq = 2(.684)(.316) (100) 44 are heterozygous (another 44 genes for dark hair)

The overall frequency for the dark hair gene among the mating population of dark-haired individuals is:

136/180 = .755…Assuming that the dark haired individuals mate randomly: male gametes

.75B .25b.75B .5625 BB .1875 Bb

female gametes .25b .1875 Bb .0625 bb

BB and Bb have the dark hair phenotype. Take these fractions and use them to correct to total 90 individuals to keep the population constant in size 51BB + 34Bb are dark haired, 5bb are blonds

Add these to the 10 blonds from assortative mating, and now there are 15 blonds instead of 10 out of 100,and 85 instead of 90 with dark hair.

The phenotypic and genotypic frequencies have changed; microevolution has occurred. But, how often does assortative mating of the sort presented in this example occur in nature?

5. No natural selection occurs. When natural selection occurs the survival and reproduction of different phenotypes differs. Some have higher survival and/or reproduction; they leave behind a larger fraction of the offspring that form the next generation (differential reproductive success). Their genes represent a greater fraction of the gene pool in the next generation.

A numerical example: selection against the sickle cell gene. We will conveniently forget the advantageous effects of being heterozygous.

An example of selection: Sickle cell anemia

Begin with 50% of the genes S and 50% s.

The initial, randomly mated cross is:

.5S .5s

.5S .25 SS .25 Ss

.5s .25 Ss .25 ss

We will assume the .25ss die without reproducing.Now calculate new gene frequencies. The 75% ofthe population of offspring surviving to reproduceare the ‘whole’ population. Now 66% of the genesare S and .33 are s

The cross in the 2nd generation is:

.66S .33s

.66S .44SS .22Ss

.33s .22Ss .11ss

Natural selection against the homozygous recessiveshas reduced the fraction from 25% to 11% in onegeneration. It would further reduce the fraction eachgeneration, but since there are fewer of them, fewerwould be selected against, as well.

N.B. natural selection - acts on phenotypes- selects only among variants present

Natural selection acts on phenotypic variation.Where does the variation come from?

Ultimately, all genetic variation in living organismsoriginates as mutations.

The variation we observe in a population is alsodetermined by: 1) recombination (sexual reproduction) 2) the spread of variants in a population due to drift,and 3) the effects of environmental variation on the relative success of different phenotypes.

One view of the amount of genetic variation in a species is the fraction of its genes that are hetero-zygous. That fraction in part indicates the amountof outcrossing (breeding with unrelated members ofthe species) and in part reflects the history of the species.

Cheetahs went through a severe bottleneckwithin the last 10,000 years; only 0.07% of theirgenes are heterozygous.

Humans have not gone through a bottleneck like that;7% of our genes are heterozygous.

Why are so many genes not heterozygous?- because altered alleles are not as ‘good’ as the ones that persist. Others have been removed by selection.

While examples indicate how gene frequencies can change, the most common cause of genetic change (microevolution) in natural populations is natural selection…

Natural selection can occur in different ways. We categorize the basic types of natural selection into three forms: stabilizing selection, directional selection, and diversifying (or disruptive) selection.

Modes of Natural Selection

1) Stabilizing selection -- acts against extreme forms, favors intermediates- one example: human birth weights

2) diversifying (or disruptive) selection - acts against intermediates, favors extremes - example - selection of different coloration

patterns in Papilio to resemble noxious but unrelated butterflies

3) directional selection - favors one extreme, selects against the opposite extreme - shifts the phenotype distribution curve in one direction. Numerous examples:

industrial melanismpesticide or drug resistance

There is another form of selection:4) sexual selection - leads to evolution of secondary sexual characters - results in sexual dimorphism - usually males evolve showy characters, e.g.:

a) tails of peacocks; peahens are drably colored b) antlers of deer or caribou - females lack

antlers c) colors of male mallards at breeding time,...

- Why? usually females choose mates, showiest or most dominant male gets a large harem, others remain generally unmated

So, to take a human view, imagine John Travolta inSaturday Night Fever, or...

(Sorry, I couldn’t find a good copy of the classicpose in a white polyester suit, strutting his stuff)

These are Wodaabe men from Niger in a pose off, where the women select the most beautiful men. They are wearing lipstick and other makeup, where the males of many animal species are naturally decorated (e.g. cardinals, peacocks, birds of paradise).

Questions about selection: 1) Are most genes subject to the intense natural

of (most) of these examples?

No! These extreme examples make evolutionmore apparent, and occurring more rapidly.

2) Are some genes strongly conserved through the varieties of living things?

Yes! For example, there have been only a handfulof changes in the base sequence of cytochrome Cfrom bacteria to man.

3) Is all genetic variation adaptive?No! Much of the variation is neutral. None of thevariants confers a selective advantage.

Does natural selection “perfect” organisms? No! Why? 1. Organisms are locked into historical constraints. 2. Adaptations are compromises. 3. Not all evolution is adaptive. Chance frequently

plays a large role. 4. Selection can only act on (and edit) variations

(phenotypes) that exist.