Presentation on theme: "Effects of selection The reproductive success of an individual over its lifetime is known as its fitness. When individuals differ in their fitness selection."— Presentation transcript:
1Effects of selectionThe reproductive success of an individual over its lifetime is known as its fitness.When individuals differ in their fitness selection takes place.
2Measures of FitnessIn practice, fitness can be difficult to measure over an organisms lifetime.Instead other measures that correlate well with lifetime fitness are used to estimate fitness: e.g. survival to reproductive age or reproductive success in a single season.
3Measuring fitnessThe goal in studying selection is to relate variation in fitness to variation in phenotype.E.g. we can try to compare variation in fitness to an animal’s size or camouflage color or some other phenotypic measure.
4Measuring fitnessRemember, fitness is a result of the organisms entire phenotype.Population genetics, however, looks at the evolution of alleles at a single locus.
5Relative fitnessPopulation geneticists condense all the components of fitness (survival, mating success, etc.) into one value of fitness called w.
6Converting genotype fitness to allele fitness Evolution depends on changes in the gene pool so we need to consider how alleles affect fitness rather than how genotypes affect fitness.The general selection model (next slide) enables us to assess how individual alleles contribute to fitness.
7General selection model for diploid organisms Genotype A1A1 A1A2 A2A2 Initial freq p2 2pq q2 Fitness w11 w12 w22 Abundance In gen t+1 p2 X w11 2pq X w12 q2 X w22 Weighted freq. gen t+1 (p2 X w11)/w (2pq X w12)/w (q2 X w22)/w
8General selection model for diploid organisms The term “Abundance in gen t+1” tells us for each genotype its abundance relative to other genotypes in the next generation Abund. gen t+1 p2 X w11 2pq X w12 q2 X w22 To convert these to true frequencies we standardize them by dividing them by the average fitness of the population w.
9Formula for w (average fitness of population) for two alleles A1 and A2 w = p2 X w11 + 2pq X w12 + q2 X w22Note that the formula is the sum of the fitness values for each genotype multiplied by (i.e. weighted by) the genotype frequencies.
10General selection model for diploid organisms Normalized weighted freq. gen t+1(p2 X w11)/w (2pq X w12)/w (q2 X w22)/wThese are the frequencies of each genotype in generation t +1.
11General selection model for diploid organisms Using these weighted genotype frequencies we can calculate the allele frequencies in generation t+1.Need to sum alleles across genotypes.For the allele A1 it will be the frequency of the A1A1 homozygotes plus half the frequency of the heterozygotes.
12General selection model for diploid organisms Frequency of allele A1 [p(t+1)]P(t+1) = (p2 X w11 + pq X w12)/wFrequency of allele A2 [q(t+1)]q(t+1) = (q2 X w22 + pq X w12)/w
13Example of allele change under selection Starting allele frequencies: A1 = 0.8, A2 = 0.2Fitness w w12 w22w = p2 X w11 + 2pq X w12 + q2 X w22= (0.64 X 0.9) + (0.32 X 1) + (0.04 X 0.2)==
14Example of allele change under selection P(t+1) = (p2 X w11 + pq X w12)/wP(t+1) = 0.64 X X 1)/0.904= /0.904= 0.814Allele A1 has increased in abundance slightly. In this example the success of the alleles A1 and A2 is very sensitive to the frequency of A2.
15Example of allele change under selection In this example, heterozygotes have the highest fitness, but if A2 becomes too common A2A2 homozygotes begin to appear and these have very low fitness.At lower frequencies of A2 then A2A2 homozygotes will be rarer and the A2 allele will increase.In next slide we lower frequency of A2 to 0.1.
16Example of allele change under selection Allele frequencies: A1 = 0.9, A2 = 0.1Fitness w w12 w22w = p2 X w11 + 2pq X w12 + q2 X w22= (0.81 X 0.9) + (0.18 X 1) + (0.01 X 0.2)==
17Example of allele change under selection P(t+1) = (p2 X w11 + pq X w12)/wP(t+1) = (0.81 X X 1)/0.911= ( )/0.911= (allele A1has declined very slightly from frequency of 0.9 and allele A2 has increased to a frequency of 0.101
18Average excess of fitness There are other ways of computing the effects of selection on allele frequency.One approach uses something called the average excess of fitness.
19Average excess of fitness A relatively simple formula allows us to calculate the net fitness contribution of an allele, which is called the average excess of fitness.This is the difference between the average fitness of individuals with that allele and the average fitness of the entire population.
20Equation for average excess of fitness for allele A1 (aA1) For example, for the allele A1 the average excess of fitness isaA1= [p X (w11 – w)] + [q X (w12 – w)]Where w11 – w is the difference in fitness between A1A1 individuals and the mean fitness of the population w.W12 is fitness of A1A2 heterozygotes. W is mean fitness of populationP and q are allele frequenciesSee Box 6.5 in your text page 168 for derivation of this formula.
21Allele frequency change between generations The average excess of fitness can be used to calculate how much an allele frequency will change between generationsΔp = p X (aA1/w)Δp is change in allele frequency from one generation to the nextp is the frequency of the A1 alleleaA1 is the average excess of fitnessAverage fitness of the population is w
22Average excess of fitness If the average excess of fitness is positive then an allele will increase in frequency.If average excess of fitness is negative then the allele will decrease in frequency.
23Allele frequency change between generations Δp = p X (aA1/w)The equation tells us that how fast an allele increases or decreases depends on both the strength of selection (value of aA1) AND how common an allele is in the population (p).Note that for rare alleles even strong selection will not necessarily result in a rapid increase in an allele’s frequency.
24Allele frequency change between generations Alleles can differ greatly in their fitness. E.g. some alleles cause severe diseases and are strongly selected against.Many alleles however differ only slightly in their average excess of fitness, but because the effect of the fitness difference compounds over time (just like interest on money) even small differences can result in big changes.
25Allele frequency change between generations The compounding effect of natural selection is more effective in large populations than small ones.In small populations drift can easily eliminate beneficial mutations. In larger populations drift has less of an effect.
26Natural selection more powerful in large populations Effects of drift strong in small populations but weaker in large populationsSmall advantages in fitness can lead to large changes over the long term in large popultions.
27Relative fitnessRelative fitness can be expressed in different ways but often the genotype with the highest fitness is designated as having a relative fitness of w = 1.Genotypes with lower relative fitness then have values for w of between 0 and 1.
28Relative fitnessAnother way differences in relative fitness are sometimes expressed by using a parameter (s) called the selection coefficient to describe the reduction in fitness of one genotype vs the other.A genotype that has a 20% lower fitness than a competing one would have an s value of 0.2.
29Strength of selectionStrength of selection has a strong influence on how fast an allele spreads.In pocket mice coat color is affected by a gene with two alleles D and d. D allele is dominant.DD: dark phenotypeDd: dark phenotypeDd: light phenotypeOn dark backgrounds light phenotype will be selected against.
30Figure 7. 11 Pocket mice live in light and dark rock habitats Figure 7.11 Pocket mice live in light and dark rock habitats. (A) Light-colored rock habitat, and light- and dark-coated mice on light rock, (B) dark lava field habitat of the rock pocket mouse, and light- and dark-coated pocket mice on dark rock.
32Strength of selectionThe higher the value of s the more strongly natural selection acts.
33Figure 7.12 The consequences of natural selection favoring a dominant allele. Here we plot the trajectory—the path over time—of the frequency p of the dominant A1 allele for three different selection intensities. The horizontal axis indicates time in generations, and the vertical axis, ranging from 0 to 1, indicates the frequency of the A1 allele. The initial frequency of the A1 allele is 0.005, and this allele increases to near-fixation in all three cases albeit at different rates for our three values of s.
34Frequency independent selection The mouse coat color example is an example of frequency-independent selection. The fitness of a trait is not associated with how common the trait is.
35Directional selection The commonest form of frequency- independent selection is directional selection.Under directional selection one allele is consistently favored over the other allele so selection drives allele frequencies in only one direction towards a higher frequency of the favored allele.Eventually favored allele may replace other alleles and become fixed.
36Gene interactionsWhether an allele is dominant, recessive or has additive effects (is codominant) will have a strong influence on how fast it spreads in a population.
37Relationships among alleles at a locus Additive: allele yields twice the phenotypic effect when two copies presentDominance: dominant allele masks presence of recessive in heterozygoteRecessive: two copies of recessive allele need to be present for alleles effect to be felt.
38Effects of selection on different types of alleles
39Figure 7. 13 Directional selection at one locus with two alleles Figure 7.13 Directional selection at one locus with two alleles. (A) In directional selection, one allele A1 is favored over another, A2. This can occur in different ways: A1 can be dominant (red), A1 and A2 can be codominant (blue), or A1 can be recessive (orange). (B) The trajectories of p, the frequency of the A1 allele, are illustrated from a starting value of p =
40Empirical examples of allele frequency change under selection Clavener and Clegg’s work on Drosophila.Two alleles for ADH (alcohol dehydrogenase breaks down ethanol) ADHF and ADHS
41Empirical examples of allele frequency change under selection Two Drosophila populations maintained: one fed food spiked with ethanol, control fed unspiked food.Populations maintained for multiple generations.
42Empirical examples of allele frequency change under selection Experimental population showed consistent long-term increase in frequency of ADHFFlies with ADHF allele have higher fitness when ethanol present.ADHF enzyme breaks down ethanol twice as fast as ADHS enzyme.
44Empirical examples of allele frequency change under selection: Jaeken syndrome Jaeken syndrome: patients severely disabled with skeletal deformities and inadequate liver function.
45Jaeken syndromeAutosomal recessive condition caused by loss-of-function mutation of gene PMM2 codes for enzyme phosphomannomutase.Patients unable to join carbohydrates and proteins to make glycoproteins at a high enough rate.Glycoproteins involved in movement of substances across cell membranes.
46Jaeken syndromeMany different loss-of-function mutations can cause Jaeken Syndrome.Team of researchers led by Jaak Jaeken investigated whether different mutations differed in their severity. Used Hardy-Weinberg equilibrium to do so.
47Jaeken syndromePeople with Jaeken syndrome are homozygous for the disease, but may be either homozygous or heterozygous for a given disease allele.Different disease alleles should be in Hardy-Weinberg equilibrium.
48Jaeken syndromeResearchers studied 54 patients and identified most common mutation as R141H.Dividing population into R141H and “other” alleles. Allele frequencies are:Other: 0.6 and R141H: 0.4.
49Jaeken syndromeIf disease alleles are in H-W equilibrium then we would predict genotype frequencies ofOther/other: 0.36Other/R141H: 0.48R141H/R141H: 0.16
50Jaeken syndrome Observed frequencies are: Other/Other: 0.2 Other/R141H: 0.8R141H/R141H : 0Clearly population not in H-W equilibrium.
51Jaeken syndromeResearchers concluded that R141H is an especially severe mutation and homozygotes die before or just after birth.Thus, there is selection so H-W assumption is violated.
52Testing predictions of population genetics theory If an allele has a positive average excess of fitness then the frequency of that allele should increase from one generation to the next.Obviously, the converse should be true and an allele with a negative average excess of fitness should decrease in frequency.
53Tests of theoryDawson (1970). Flour beetles. Two alleles at locus: + and l.+/+ and +/l phenotypically normal.l/l lethal.
54Dawson’s flour beetles Dawson founded two populations with heterozygotes (frequency of + and l alleles thus 0.5).Expected + allele to increase in frequency and l allele to decline over time.
55Dawson’s flour beetles Predicted frequencies based on average excess if fitness estimates and observed allele frequencies matched very closely.l allele declined rapidly at first, but rate of decline slowed.
57Dawson’s flour beetles Dawson’s results show that when the recessive allele is common, evolution by natural selection is rapid, but slows as the recessive allele becomes rarer.Hardy-Weinberg explains why.
58Dawson’s flour beetles When recessive allele (a) common e.g genotype frequencies are:AA (0.05)2 Aa (2 (0.05)(0.95) aa (0.95)20.0025AA Aa aaWith more than 90% of phenotypes being recessive, if aa is selected against expect rapid population change.
59Dawson’s flour beetles When recessive allele (a) rare [e.g. 0.05] genotype frequencies are:AA (0.95)2 Aa 2(0.95)(0.05) aa (0.05)20.9025AA Aa aaFewer than 0.25% of phenotypes are aa recessive. Most a alleles are hidden from selection as heterozygotes. Expect only slow change in frequency of a.
60Predicting allele frequencies under selection What is the predicted allele frequency after one generation for the + allele in Dawson’s beetle experiment?We can calculate the average excess of fitness and use our formula for Δp (change in p) to find out.
61Parameters for Dawson’s flour beetle experiment Fitness w w+l wllAllele frequencies + = 0.5, l = 0.5Genotype frequencies in initial generation++ = 0.25 (p2)+l = 0.5 (2pq)ll = 0.25 (q2)
62w (average fitness of population) for Dawson’s flour beetle experiment w = p2 X w++ + 2pq X w+l + q2 X wll= (0.25 X 1) + (0.5 X1) + (0.25 X 0)= 0.75
63Using average excess of fitness to calculate + allele after selection For the + allele the average excess of fitness isa+= [p X (w11 – w)] + [q X (w12 – w)]a+ = [0.5 ( ) + [0.5 X ( )]= 0.25Δp = p (a+ / w)= 0.5 (0.25/0.75) = 0.167P t+1 = P + Δp = = 0.667