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What is Evolution?

BIOL2007 Evolutionary Genetics course website: http://ucl.ac.uk/~ucbhdjm/courses/ (searching for "biol2007" on Google is easier!). What is Evolution?. Produces biological diversity - DNA sequence variation - Bacteria - Flowering plants - Sexual selection in birds

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What is Evolution?

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  1. BIOL2007 Evolutionary Genetics course website: http://ucl.ac.uk/~ucbhdjm/courses/ (searching for "biol2007" on Google is easier!) What is Evolution? Produces biological diversity - DNA sequence variation - Bacteria - Flowering plants - Sexual selection in birds - Human species Futuyma 2005: 14 copies in science library Barton 2007 6 copies Freeman & Herron 2004-7 18 copies

  2. DNA sequence variation 200 bp of the 18,000 bp of aligned mitochondrial DNA of great apes

  3. Evolution: definition Darwin: “descent with modification” A change in morphology, ecology, behaviour, physiology Change must be genetic Modern, genetic definition: “evolution is change in gene frequencies between generations” Evolutionary Genetics: mechanisms Science: understanding; predictions

  4. What causes evolution? • a) Natural selection • b) Mutation • c) Genetic drift, or neutral, • random evolution • e) Migration, or gene flow • This lecture: simple examples of evolution by natural selection

  5. What is natural selection? “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"!) However, many interesting types of evolution involve natural selection

  6. Selection and the single gene “Quantitative traits” e.g. behaviour, IQ, beak size usually multiple loci versus … Single-locus traits Evolution by natural selection can occur in both Many single-locus traits are involved in resistance to stress (often caused by humans)

  7. Examples of single-gene traits • Industrial melanismin moths (resistance to urban pollution) • Heavy metal tolerancein plants growing in mine tailings • Malaria resistancein humans (sickle-cell haemoglobin, etc.) • Drug/antibiotic resistancein bacteria, protozoan parasites • Human genetic diseaseslike cystic fibrosis, Huntington’s disease etc. • Pesticide resistance(mosquitoes, insects, weeds, fungi, warfarin resistance in rats) We used to do an essay on this for tutorial; there are many references on reserve, still; see eUCLid

  8. Cyclodiene (dieldrin, aldrin, endosulfan, -BHC) resistance: GABA-gated chloride channel insensitivity Insecticide resistance Ala302 Ser ….In all these species: Drosophila melanogaster Drosophila simulans Peach potato aphid Myzus persicae Coffee-berry borer Hypothenemus hampei Housefly Musca domestica Cockroach Blatella germanica Whitefly Bemisia tabaci Flour beetle Tribolium castaneum … creates increasing problems in agriculture and disease control (e.g. malaria)

  9. Cystic fibrosis in humans (a recessive): … a large diversity of “loss-of-function” alleles F508 Incidence: 1/2500 of births. So q = (1/2500) = 0.02. About 2-3 of you in this room will carry the allele ... hmm!

  10. How does evolution by natural selection work? Evolution by natural selection is an inevitable,mathematical process The frequency of an allele will change, and its rate of change depends on relative fitness. Mathematical evolutionary theory helps us understand. For example, given information aboutfitness, howfast isevolution? Useful:help us understand antibiotic resistance, or pest resistance, for instance Evolution is a predictive science!Useful, as well as fun!

  11. Differences between ecology and evolution Ecologists: dynamics of numbers of individuals (or species); generally ignore genetic variation. Evolutionists: changes within populations, & how might lead to speciation and macroevolution; ignore numbers of individuals. Ecology has Lotka-Volterra competition equations: concerned with numbers of individuals (…………v. difficult to solve!!)

  12. Evolutionists study changes in gene frequency…. We’d like to know: How fast is evolution by natural selection? Selection against recessive allele Selection AGAINST recessive allele (= selection FOR dominant allele) Genotypes AA Aa aa Total Relative fitness,W1 1 1-s - in this simple model,sis the “selection coefficient” ( fraction dying) [NB: p+q=1, therefore (p+q)2 = p2+2pq+q2=1] Genotype frequencies (Hardy-Weinberg law) p2 2pqq2 1 Relative frequencies p2.1 2pq.1 q2(1-s) < 1 after selection

  13. Selection against recessive contd.

  14. Selection against recessive contd.

  15. Selection against recessive contd.

  16. A flow diagram for evolution by ns Random mating Offspring genotypes in Hardy-Weinberg ratios Natural selection Offspring after selection So now you can write an evolution computer program! Numericalvs.analyticaltheory

  17. The basic equation for evolution Natural selection at a dominant gene (if s is small) (p is the frequency of the dominant allele) In words: The change in gene frequency per generation is proportional to spq2

  18. Dominance vs. recessives How fast do populations respond to natural selection? Answer: (p is frequency of A, q is freq. a) If pis small, ~0.01 or less, , i.e. RAPID If p is large, so that q 0.01 or less, , i.e. SLOW (q2 is a square of a very small number  is itself even smaller!) RESULT: Selection for/against a DOMINANT allele at low frequency is RAPID (p) Selection for/against a RECESSIVE allele at low frequency is SLOW ((q2) …. many new single genes for resistance (melanism, insecticide resistance and so on) are dominant! Why?

  19. The speed of evolution (the rate of gene frequency change per unit time) p time (generations) advantageous recessive advantageous dominant (from a programme written by a former BIOL2007 student, Wei-Chung Liu, available from the BIOL2007 website)

  20. The peppered moth Biston betularia Left: form typica (left, and carbonaria (right) on lichen-covered trunk in my parents’ garden in Kent Right: on soot-covered tree near Birmingham in the 1960s

  21. Estimating selection • Change of gene frequencies per generation(e.g. peppered moth in 19th C; Haldane estimated s  0.5) • 2)Deviation from Hardy-Weinberg ratios(next lecture) • 3)Direct comparison of birth or death rates • We will use this method here using survival data in the peppered moth

  22. Survival in field experiments on the peppered moth • Central Birmingham • number number percent relative WC- the “other • released recaptured recaptured fitness, Wccway round” • typica, cc 144 18 12.5% 0.43 1.00 • carbonaria, Cc &CC 486 140 28.8% 1.00 2.30 • B) Dorset wood • number number percent relative • released recaptured recaptured fitness • typica, cc 163 67 41.1% 1.82 • carbonaria, Cc & CC 142 32 22.5% 1.00 • SUMMARY OF FITNESSES: (note: W = 1 - s) • typica carbonariaselection coefficient against carbonaria • WccWCcWCCscc • City 0.43 1 1 +0.57 • Wood 1.82 1 1 -0.82 Estimating selection in peppered moth

  23. HOW FAST would carbonaria increase in frequency in a 1950s 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. The speed of evolution by nat. sel.

  24. More generally … Complications – many! Many differentkinds of selection - fertility selection - sexual selection Non-random mating - inbreeding - mate choice Overlapping generations Dominance not complete AA Aa aa 1 1–hs 1–s Multiple genes … &c &c…. But the basic principle remains the same!

  25. Evolution, a fact? You can be a creationist and still take this course, but you do have to learn evolutionary biology to get a good grade! Evolution is a fact, and it’s hard to ignore … but, theory and fact: not so different Science:prediction, rather than “absolute truth” Religion:truth, belief is by faith. Very different. Karl Popper:science is falsifiable. Falsehoods disprovable; scientific truth cannot be proved!

  26. Take-home points Evolution to a geneticist:a change in gene frequencies. Natural selection:a consistent bias favouring some genotypes. Evolution can occur in the absence of natural selection. Natural selection can stabilize the status quo; zero evolution. Evolution occurs at predictable rates. 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

  27. Further reading FUTUYMA, DJ 2005. Evolution. Chapters 9 (p. 195), 11 (all) and 12 (pp. 270-285). FREEMAN, S, and HERRON, JC 2004. Evolutionary analysis. 3rd Ed. Chapters 3 and 5. References on natural selection : Science Library: View BIOL2007 Teaching Collection by going to eUCLid; use Keyword, Basic Search, All Fields: BIOL2007 or B242 (old number)

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