Measuring genetic variability Studies have shown that most natural populations have some amount of genetic diversity at most loci locus = physical site where a gene resides on a chromosome plural is loci For instance, you can measure the number of different alleles of a given protein using gel electrophoresis

Protein electrophoresis A gel is a porous slab of jello-like material, which large molecules like proteins can slowly move through By running a current across the gel, one end becomes positively charged Proteins with a negative charge will move towards the + end of the gel + -

Protein electrophoresis Why might proteins carry a negative charge? Some amino acids are negatively charged in solution (glutamate, aspartate) Some are positively charge (lysine, arginine, histidine) By adding up the total number of + and – charged amino acids, you can figure out the total (or net) charge of a protein The net charge of Lys-Asp-Asp-Ser-Thr-Arg-Glu-Glu + - -

The net charge of Lys-Asp-Asp-Ser-Thr-Arg-Glu-Glu but different alleles may change the total charge on a protein: Consider a different allele of this protein: Lys-Asp-Thr-Ser-Thr-Arg-Glu-Glu This allele has a net charge of -1 - Protein electrophoresis

Sampling variation in a population (1) Sample many individuals (2) Grind them up and load a drop of the mush onto each lane of a gel (3) Run a current across the gel (4) Stain the gel by adding a chemical that will turn dark when a particular enzyme reacts with it - - +

This individual was homozygous for the most common allele Different protein alleles that show up on such a gel are termed allozymes Variation in a population: Allozyme electrophoresis

- + Different protein alleles that show up on such a gel are termed allozymes This individual was heterozygous: - Variation in a population: Allozyme electrophoresis

- + What is the frequency of “a” allele in this population? AaaAaa Variation in a population: Allozyme electrophoresis

- + AaaAaa total # of a alleles: 10 total # of alleles: 12 frequency of a: 10/12 = 83.3% Variation in a population: Allozyme electrophoresis What is the frequency of “a” allele in this population?

Integrates Darwinian evolution by natural selection with Mendelian principles of inheritance Shows how changes in the frequency of alleles in a population affect the frequency of the traits they control Evolution: change in allele frequency over generations How do alleles behave in a sexually reproducing, diploid population? Population Genetics

In a population where adults mate randomly every generation, does the frequency of alleles change over time? Assume a population where there are two alleles of a gene, A and a - frequency of allele A in the gene pool is 60%, or in other words, 60% of sperm and 60% of eggs made by adults in this population carry the A allele - frequency of allele a in the gene pool is 40%, or 0.4 Population Genetics

What will be the frequencies of the various genotypes (AA, Aa, aa) after one round of random mating? odds that an A sperm will meet an A egg: (frequency of X (frequency of =frequency of A sperm) A egg)AA zygote 0.6 X 0.6 =0.36

EggSpermZygoteProbability A A AA0.6 x 0.6 = 0.36 A a Aa0.6 x 0.4 = 0.24 a A aA0.4 x 0.6 = 0.24 a a aa0.4 x 0.4 = 0.16 Aa

= 1 These probabilities are the genotype AA Aa aa frequencies of the next generation So what will the allele frequencies be after this generation reproduces? (will the frequencies change?) Calculate new gamete frequencies, as before: AA is 36% of the population (0.36), so 36% of gametes are A Aa is 48% of the population, so 24% of gametes are A and 24% are a aa is 16% of the population, so 16% of gametes are a New allele frequencies: A: = 0.6 a: = 0.4

The allele frequencies for the A and a alleles do not change from generation to generation - they are in equilibrium - hence, the population does not evolve This illustrates an example that is true in general: allele frequencies do not change from generation to generation The general case is stated algebraically as the Hardy-Weinberg equilibrium principle Hardy-Weinberg Equilibrium

The frequency of A in the population is called p The frequency of a in the population is called q When there are only 2 alleles, p + q = 1 What are the odds of each genotype after a round of mating? AAAaaa p x p(p x q) + (q x p)q x q p 2 2pqq 2 frequencies in the parental gametes Hardy-Weinberg Equilibrium

AAAaaa p x p(p x q) + (q x p)q x q p 2 2pqq 2 so, we’ve gone from allele frequencies in the parental gene pool to genotype frequencies among the offspring What happens when these offspring reproduce?… Calculate new gamete frequencies, as before: AA has frequency = p 2, so p 2 gametes will carry the A allele Aa has frequency = 2pq, so ½ (2pq) or pq gametes will carry A Hardy-Weinberg Equilibrium

AA has a frequency = p 2, so p 2 gametes will carry the A allele Aa has a frequency = 2pq, so ½ (2pq) or pq gametes will carry A New allele frequency: A: p 2 + pq The frequency of A can be re-stated as: p 2 + pq = p(p + q) Since p + q = 1 = p(1) = p Hardy-Weinberg Equilibrium

Thus, we can draw 2 conclusions from the Hardy-Weinberg equilibrium principle: #1) frequency of an allele stays the same over generations - it doesn’t matter what the particular allele frequencies are - it doesn’t matter how many alleles there are for a gene #2) when allele frequencies are given as p and q, the genotype frequencies will be: p 2 + 2pq + q 2 The Hardy-Weinberg principle predicts that evolution will not happen in a population -- unless one of the underlying 5 assumptions is violated Hardy-Weinberg Equilibrium

The 5 assumptions: 1) There is no natural selection - all individuals survive and reproduce equally - if individuals of some genotypes survive and reproduce more than others, then allele frequencies may change from one generation to the next 2) There is no mutation - alleles don’t change to other existing alleles or new alleles Hardy-Weinberg Assumptions

The 5 assumptions: 3) There is no migration - no individuals moved into or out of the population - if individuals with certain genotypes leave the population, then the allele frequencies may change Hardy-Weinberg Assumptions

The 5 assumptions: 4) There were no chance events that caused some individuals to pass on more alleles to the next generation - this is termed genetic drift - commonly happens in small populations - genetic drift causes evolution by changing allele frequencies - Hardy-Weinberg Assumptions

The 5 assumptions: 5) Individuals mate at random - individuals are not more likely to mate with others of their own genotype examples: a) big individuals do not prefer big individuals b) habitat choice: mate where you like to hang When individuals mate non-randomly, genotype frequencies change over generations and the model is violated Hardy-Weinberg Assumptions

How do scientists use Hardy-Weinberg equilibrium theory? - useful as a null model-- something to be disproven - Go out into the field, sample allele and genotype frequencies - if allele frequencies change over time, or if genotype frequencies cannot be predicted from allele frequencies, then the null model (H-W equilibrium) is not correct Use of Hardy-Weinberg

If null model is wrong, one of the assumptions is being violated - indicates that selection, mutation, or other force is acting on a population - functions as a spotlight, drawing attention to potential cases where a population may be evolving You can’t easily go out into the field and tell if a population is under natural selection or experiencing high migration.... Use of Hardy-Weinberg

Happens when individuals with certain phenotypes survive or reproduce at higher rates than others - bottom line: differential reproductive success - when phenotype is derived largely from genotype, evolution can happen selection on phenotype Selection

When selection increases the reproductive success of certain genotypes, do allele frequencies change over generations? Take the earlier example: frequency of allele A was 60%, and the frequency of allele a was 40%, in a population that makes 1,000 zygotes AAAaaa (actual # of individuals) Selection and Allele Frequencies

When selection increases the reproductive success of certain genotypes, do allele frequencies change over generations? Take the earlier example: frequency of allele A was 60%, and the frequency of allele a was 40% AAAaaa Now assume that genotypes differ in their rates of survival (due to effects on phenotype) - all AA survive - only 75% of Aa survive - 50% of aa survive Selection and Allele Frequencies

AAAaaa x 100%x 75%x 50% = 800 survivors When this generation makes gametes, what will the allele frequencies be? First calculate genotype frequencies… Total individuals of each genotype / total # of individuals AA360/800 = 0.45 Aa360/800 = 0.45 aa80/800 = 0.1 Selection and Allele Frequencies

Total individuals of each genotype / total # of individuals AA360/800 = 0.45 Aa360/800 = 0.45 aa80/800 = 0.1 Calculate allele frequencies, when these individuals make gametes Frequency of A allele = frequency of AA + ½ frequency of Aa = (0.45) + ½ (0.45) = = Frequency of allele A was originally selection changed allele frequencies - thus, the population evolved in response to selection Selection and Allele Frequencies

Laboratory experiments using fruit fly Drosophila melanogaster have shown that many forms of “artificial” selection cause rapid evolution by changing allele frequencies in experimental populations - scientists change the conditions of experimental populations - after many generations, check to see if allele frequencies have changed, relative to control populations Artificial selection experiments

Change conditions of 2 experimental populations: add ethanol After 50 generations, allele frequencies had changed relative to control populations flies fed ethanol- spiked food

In the previous case, selection changed allele frequencies Can selection change genotype frequencies, instead? Consider this population: Frequency of A = 0.5 after one round a = 0.5 of random mating: Genotype:AAAaaa frequency: (# out of 1,000 individuals) Selection and Genotype Frequencies

Genotype:AAAaaa frequency: Now introduce selection: only 50% of homozygotes survive AAAaaa # of adults: (750 survivors) New genotype frequencies: (total # of each genotype / total # of individuals in population) AA125 / 750 = Aa500 / 750 = aa125 / 750 = Selection and Genotype Frequencies

New genotype frequencies: (total # of each genotype / total individuals in the population) AA125 / 750 = Aa500 / 750 = aa125 / 750 = New allele frequencies, when these individuals produce gametes … Frequency of A allele = frequency of AA + ½ frequency of Aa = (0.167) + ½ (0.667) = = 0.5 This was the initial frequency of the A allele! Selection and Genotype Frequencies

Despite strong selection against homozygotes, allele frequencies didn’t change population did not evolve Conclusion #1 of Hardy-Weinberg wasn’t violated by selection.... what about conclusion #2: can you still predict genotypes from the new allele frequencies? Selection and Genotype Frequencies

Can you still predict genotypes from the new allele frequencies? New allele frequencies:A = 0.5a = 0.5 New genotype frequencies: AA125 / 750 = Aa500 / 750 = aa125 / 750 = Frequency of the A allele: 0.5 Predicted frequency of the AA genotype: (0.5) 2 = 0.25 Actual frequency of the AA genotype: 125/750 = Selection and Genotype Frequencies

Frequency of the A allele: 0.5 Predicted frequency of the AA genotype: (0.5) 2 = 0.25 Actual frequency of the AA genotype: 125/750 = Selection took the population out of Hardy-Weinberg equilibrium Conclusion #2 (genotypes are predicted from allele frequencies) is violated  Selection and Genotype Frequencies