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What Causes Evolution? What Is Natural Selection?

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What Causes Evolution? What Is Natural Selection? BIOL2007 Evolutionary Genetics course website: http://ucl.ac.uk/~ucbhdjm/courses/ (searching for “BIOL2007 timetable” on Google is easier!) What causes evolution? What is Evolution? a) Natural selection Darwin: “descent with modification” b) Mutation c) Genetic drift, or neutral, A change in morphology, ecology, behaviour, random evolution physiology e) Migration, or gene flow Change must be genetic This lecture: simple examples of evolution by natural selection Modern, genetic definition: “evolution is change in gene frequencies between generations” What is natural selection? The peppered moth Biston betularia “a consistent bias in survival or fertility between genotypes within generations” Selection often causes evolution, but may also prevent evolution (e.g. stable polymorphism) Evolution does not require selection (e.g. drift -- important: > 95% of genome maybe "junk"!) Left: form typica (left, and However, many interesting types of evolution carbonaria (right) on lichen-covered involve natural selection trunk in Dorset. Right: on soot-covered tree near Birmingham A flow diagram for evolution by ns Selection against recessive allele Selection AGAINST recessive allele (= selection FOR dominant allele) Random mating Suppose there is “viability selection” (i.e. survival affected) so that … Offspring genotypes in Hardy-Weinberg ratios Genotypes AA Aa aa Total Natural Relative fitness, W 111-s - selection Offspring after selection Genotype frequencies before selection p2 2pq q2 1 (Hardy-Weinberg law) So now you can write an evolution computer program! Rel. frequencies p2 2pq q2(1-s)<1 after selection Numerical vs. analytical theory in this simple model, s is the “selection coefficient”(≈ fraction dying) 1 BIOL2007 – SELECTION AND THE SINGLE GENE SELECTION AGAINST RECESSIVE ALLELE (EQUIVALENT TO SELECTION FOR DOMINANT ALLELE) Suppose there is viability selection so that … Genotypes AA Aa aa Total Relative fitness, W 1 1 1-s Frequencies before selection p2 2pq q2 1 (Hardy-Weinberg law) Relative genotype frequencies p2 2pq q2(1-s) ≠1 after selection Frequencies should sum to 1! Therefore, need to divide by “mean fitness,” W= p2 +2 pq + q 2 (1 − s ) = 1 − sq 2 Genotype frequencies p 2 2 pq q 2 (1-s) after selection 1 1− sq 2 1− sq 2 1− sq 2 WHAT IS THE NEW FREQUENCY OF THE A ALLELE (p’)? p’ = new frequency of AA + ½ new frequency of Aa p 2 1 2 pq p2 + 1 2 pq p2 + pq p() p+ q p p' = + = 2 = = = 1 − sq22 ()1− sq 2 1− sq2 1− sq21− sq 2 1 − sq 2 WHAT IS THE RATE OF EVOLUTION PER GENERATION? We need to know the CHANGE OF GENE FREQUENCY, ∆p (obtained by subtracting old gene frequency from the new gene frequency). p p− p(1 − sq2 ) spq2 ∆p = p'- p = −p = = + 1 − sq2 (1− sq2 ) 1 − sq2 This is the basic equation for all of evolution by natural selection! The basic equation for evolution Dominance vs. recessives We can now answer the question: How fast do populations respond Natural selection at a dominant gene to natural selection? spq 2 2 Answer: ∆p = (p is frequency of A, q is freq. a) spq 2 ∆p = p'- p = + ≈ spq 2 1− sq 2 2 sp − If p is small, ~0.01 or less, q → 1; q → 1: ∆ p ≈ , i.e. RAPID 1 sq 1− s (if s is small) sq 2 If p is large, so that q ≈ 0.01 or less,p→1: ∆ p ≈ , i.e. very SLOW 1 In words: (q2 is a square of a very small number is itself even smaller!) The change in gene frequency per RESULT: ∝ generation is proportional to spq2 Selection for/against a DOMINANT gene at low frequency is RAPID ( p) Selection for/against a RECESSIVE gene at low frequency is SLOW ((∝ q2) …. many new single genes for resistance (melanism, insecticide resistance and so on) are dominant! The speed of evolution (the rate of gene frequency change per unit time) More generally … Complications – many! Overlapping generations Many different kinds of selection Dominance not complete p - fertility selection AA Aa aa - sexual selection 11–hs 1–s Non-random mating Multiple genes … -inbreeding time (generations) - mate choice &c &c…. rare gene recessive rare gene dominant But the basic principle remains the same! (from a programme written by a former B242 student, Wei-Chung Liu, available from the B242 website) Take-home points Further reading Evolution to a geneticist: a change in gene frequencies. FUTUYMA, DJ 2005. Evolution. Natural selection: a consistent bias favouring some genotypes over others. Chapter 12:270-280. Evolution can occur in the absence of natural selection, For readings on examples, see: Science Library: View BIOL2007 or B242 via genetic drift or neutral evolution. Teaching Collection by going to eUCLid; use Keyword, Basic Search, All Fields: B242. Natural selection can stabilize the status quo; zero evolution. Evolution at a single dominant gene: rate can be predicted If selected, dominant alleles evolve quickly when rare, slowly when common; recessive alleles evolve slowly when rare, quickly when common. We can estimate selection coefficients (s), fitnesses (W=1-s) and predict rates of evolution from data on survival or fecundity. Mathematical theory makes evolution a predictive science 2 ESTIMATING SELECTION 1) Change of gene frequencies per generation; result of selection, estimate ∆p; e.g. peppered moth; JBS Haldane estimated s = 0.5. 2) Distortion of Hardy-Weinberg ratios - problems? see next lecture 3) Comparison of birth or death rates between individuals W = RELATIVE fitness MOST DIRECT METHOD USING METHOD 3 TO ESTIMATE SELECTION IN PEPPERED MOTH e.g. survival in a field experiment on the peppered moth A) Central Birmingham number number proportion relative (W, the other released recaptured recaptured fitness, W way round) typica 144 18 0.125 0.43 1.00 carbonaria 486 140 0.288 1.00 2.30 B) Dorset wood number number proportion relative released recaptured recaptured fitness typica 163 67 0.411 1.82 carbonaria 142 32 0.225 1.00 SUMMARY OF FITNESSES: (Wc = 1 - sc) typica carbonaria selection coefficient against c Wcc WCc WCC sc City 0.43 1 1 +0.57 Wood 1.82 1 1 -0.82 HOW FAST WILL CARBONARIA INCREASE IN FREQUENCY in a city? ∆p = spq2/(1-sq2); suppose p = 0.5 to start with: = 0.57 x 0.5 x 0.52 / (1 - 0.57x0.52) = 0.08, or 8% per generation. .
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