Lecture 2 A brief introduction to evolutionary thinking

1. Lecture 2A brief introduction to evolutionary thinking

2. Today: • brief history of evolutionary theory • natural selection • evolutionary thinking and some important evolutionary themes (following the paper by Stephen Stearns) • Adaptation, Constraints, Trade-offs, Conflict

7. A very brief history of evolution • Evolutionary ideas go back long before Darwin (Lamarck, Erasmus Darwin) • Darwin was the first to present an overwhelming case for “descent with modification” • Critically, he also articulated a mechanism for evolution: natural selection

8. “I have called this principle, by which each slight variation, if useful, is preserved, by the term Natural Selection”. Charles Darwin (1809-1882) Charles Darwin in 1854, five years before publishing The Origin of Species.

9. evolution by natural selection • Alfred Russel Wallace (while suffering from a bout of malaria) hit upon the same insight before Darwin had published • On the Tendency of Varieties to Depart Indefinitely From the Original Type

10. evolution by natural selection • Alfred Russel Wallace (while suffering from a bout of malaria) hit upon the same insight before Darwin had published • They co-published a paper in 1858 but it wasn’t until the publication of The Origin of Species (1859) that the idea caught on

11. evolution by natural selection • Natural selection was eclipsed for several decades because of misunderstandings about inheritance. • Darwin, unaware of Gregor Mendel’s discoveries about genetics, adopted incorrect ideas about genetics: pangenesis, blending inheritance • In fact, genetics depends on particulate inheritance

12. evolution by natural selection • In 1900, Mendel’s findings were rediscovered. • But instead of leading directly to a positive re-appraisal of Darwin’s ideas on natural selection, early geneticists were originally opposed • The main problem was their focus on mutations of large effect • JBS Haldane, RA Fisher, Sewall Wright were population geneticists who synthesized genetics and evolution: the modern synthesis

13. evolution by natural selection • Darwin started his argument for natural selection with insights from pigeon-breeding (he spent a lot of time drinking with animal breeders and became a pigeon fancier himself) • “But man can and does select the variations given to him by nature, and thus accumulate them in any desired manner. He thus adapts animals and plants for his own benefit or pleasure. He may do this methodically, or he may do it unconsciously by preserving the individuals most useful to him at the time, without any thought of altering the breed.”

14. evolution by natural selection • “It is certain that he can largely influence the character of a breed by selecting, in each successive generation, individual differences so slight as to be quite inappreciable by an uneducated eye. This process of selection has been the great agency in the production of the most distinct and useful domestic breeds. That many of the breeds produced by man have to a large extent the character of natural species, is shown by the inextricable doubts whether very many of them are varieties or aboriginal species.”

15. evolution by natural selection • A process much like artificial selection, used by breeders of domesticated plants and animals to select for desirable traits,also happens in nature: “Why, if man can by patience select variations most useful to himself, should nature fail in selecting variations useful, under changing conditions of life, to her living products?”

16. evolution by natural selection • A process much like artificial selection, used by breeders of domesticated plants and animals to select for desirable traits,also happens in nature: • Individuals within populations are variable • The variations among individuals are, at least in part, passed on from parents to offspring. • In every generation, some individuals are more successful at surviving and reproducing than others • The survival and reproduction of individuals are not random: those with the most favorable variations are naturally selected

17. evolution by natural selection • “That many and grave objections may be advanced against the theory of descent with modification through natural selection, I do not deny. I have endeavoured to give to them their full force. Nothing at first can appear more difficult to believe than that the more complex organs and instincts should have been perfected not by means superior to, though analogous with, human reason, but by the accumulation of innumerable slight variations, each good for the individual possessor.”

18. evolution by natural selection • “Nevertheless, this difficulty…cannot be considered real if we admit the following propositions, namely, -- that gradations in the perfection of any organ or instinct, which we may consider, either do now exist or could have existed, each good of its kind, -- that all organs and instincts are, in ever so slight a degree, variable, -- and, lastly, that there is a struggle for existence leading to the preservation of each profitable deviation of structure or instinct. The truth of these propositions cannot, I think, be disputed.

19. evolution by natural selection • Natural selection is a “blind watchmaker” • It’s what?

20. evolution by natural selection • William Paley’s argument for a designer: • “In crossing a heath, suppose I pitched my foot against a stone and were asked how the stone came to be there, I might possibly answer that for anything I knew to the contrary it had lain there forever; nor would it, perhaps, be very easy to show the absurdity of this answer.”

21. evolution by natural selection • William Paley: • “But suppose I had found a watch upon the ground, and it should be inquired how the watch happened to be in that place, I should hardly think of the answer which I had before given, that for anything I knew the watch might have always been there.”

22. evolution by natural selection • William Paley: • “Yet why should not this answer serve for the watch as well as for the stone? Why is it not as admissible in the second case as in the first? For this reason, and for no other, namely, that when we come to inspect the watch, we perceive—what we could not discover in the stone—that its several parts are framed and put together for a purpose.”

23. evolution by natural selection • “Nothing at first can appear more difficult to believe than that the more complex organs and instincts should have been perfected not by means superior to, though analogous with, human reason, but by the accumulation of innumerable slight variations, each good for the individual possessor.” • “Let us hope that what Mr. Darwin says is not true; but, if it is true, let us hope that it will not become generally known."

24. Evolutionary thinking • The Stearns paper is the introduction to a book about evolutionary medicine, hence the medical focus • “Evolutionary biology is a rich collection of well-developed alternative approaches to the interpretation of biological diversity and organismal design.”

25. Evolutionary thinking • Evolutionary topics: • Adaptations • Relationships and history • Neutral versus selectively advantagous variation • The study of conflicts and cooperation • Maladaptation: Why am I not perfect?

26. Evolutionary thinking • Wikipedia: Evolution • The basic mechanisms that produce evolutionary change are natural selection (which includes ecological, sexual, and kin selection) and genetic drift; • these two mechanisms act on the genetic variation created by mutation, genetic recombination and gene flow. • Natural selection is the process by which individual organisms with favorable traits are more likely to survive and reproduce. If those traits are heritable, they are passed to succeeding generations, with the result that beneficial heritable traits become more common in the next generation. • Given enough time, this passive process can result in varied adaptations to changing environmental conditions.[6]

34. Evolutionary thinking • Different sorts of evolutionary biologists: • Population geneticists (genes, alleles, change or maintenance in variation) • Evolutionary ecologists (design of phenotypes for survival and reproduction, life history, sexual selection, behaviour)

35. Evolutionary thinking • Different sorts of evolutionary biologists: • Molecular evolutionists (history stamped into genomes, patterns in DNA and the processes underlying them) • Systematists (phylogenies, relationships among taxa • Paleontologists (fossils, deep time, major trends) • No clear boundaries, and many evolutionists where several hats. All try to observe patterns and infer process that underly evolution

36. Evolutionary thinking • Different evolutionary approaches: • Changes in gene frequencies (population and quantitative genetics) • Optimization approach • Game theory approach • Phylogenetic approach

37. natural selection and adaptation • In short: Heritable variation plus Differential survival and reproductive success Leads to Non-random survival and reproduction such that favorable variation is naturally selected

38. natural selection and adaptation • The ultimate source of variation is mutation (in DNA, it turns out) • Mutation is random • Selection is NOT! • Natural selection filters and preserves random mutation-derived variation and is the antithesis of randomness (it is, however, still blind)

39. natural selection and adaptation • “Selection extracts order from randomness” • “THERE IS GRANDEUR IN THIS VIEW OF LIFE” • 31 genes, each with 26 alleles • This is just one of the 2631 possible “haplotypes” • http://home.pacbell.net/s-max/scott/weasel.html

40. natural selection and adaptation • Adaptation is both a process and a state • The process of adaptation = what happens over successive generations of selection of heritable variation in reproductive success • The state of adaptation = a particular trait that does a job very well, just as though it were designed by an engineer • E.g. opposable thumb, acute hearing, vision, sex drive, camouflage, venom, crystallins in eyes

41. Constraints on adaptation • Natural selection does not produce perfection • Rather it is a “tinkerer” that produces “good-enough” solutions to context dependant problems. • Often this occurs through duplication and divergence

42. Constraints on adaptation • “As natural selection acts by competition, it adapts the inhabitants of each country only in relation to the degree of perfection of their associates; so that we need feel no surprise at the inhabitants of any one country, although on the ordinary view supposed to have been specially created and adapted for that country, being beaten and supplanted by the naturalised productions from another land.”

43. Constraints on adaptation • “Nor ought we to marvel if all the contrivances in nature be not, as far as we can judge, absolutely perfect; and if some of them be abhorrent to our ideas of fitness.”

44. Constraints on adaptation • Time is a major constraint on adaptation. It takes time to generate variation and select for it • Absorbtion of milk sugar (lactose) by human adults • Normally lactase is effective until weaning age (about 4). If you can’t digest lactose you suffer flatulence, intestinal cramps, diarrhea, nausea, vomitting • But in cultures where dairy has been used for thousands of years, >90% of adults can digest lactose, like giant, lumbering babies • Thousands of years, but probably not decades, are sufficient for this selection to shape human evolution

45. Constraints on adaptation • Trade-offs are major constraints too • E.g. sex is dangerous for fruit flies. There is a trade-off between survival and reproduction • Similarly, there is a trade-off between having a robust immune system and suffering from asthma, or lupus, or diabetes type I • Other trade-offs?

50. Trade-offs