Ten key ideas today: #1) There several other causes of evolutionary change, in addition to natural selection, "survival of the fittest" (which is a quote from Herbert Spencer, not Darwin!) #2) One additional cause is sexual selection, mostly by females of which males they will breed with! Examples: Peacocks; Sea Lions (why mostly females??) #3) Another cause of genetic change, not realized until ~1930s is (Random) Genetic Drift. (this is more subtle that just random events; the key point is that when populations are small, then some genes can go extinct even if they lack harmful effects. A far-fetched analogy is to imagine that if a flipped coin came up heads 10 times out of 10, ever, then the tail surface would permanently cease to exist! And could never come up again. Or if it came up tails 10 out of 10, then no more "heads"! Obviously, that couldn't happen; But in a small population, with genotypes AA, Aa and aa, it could happen that no individuals had ANY copies of "a" in a certain generation! Then the "a" allele would be LOST! The same could just as well happen to the A allele. Genetic drift becomes much, MUCH larger in small populations. #4) Geographical isolation is often what initiates speciation. Islands; Mountains, etc. #5) Once sub-species have evolved enough that hybrids are less fit, that creates a selection pressure against cross-breeding. (so that they become even more separated)<BR> This is probably why closely-related birds, toads, etc. tend to have the MOST different calls! (because there is a danger of hybridizing) #6) Hybrid corn and other crops usually yield more food, but are not superior in leaving offspring. And mules are sterile, etc. (different chromosome numbers) An interesting case: Swordtail x Platyfish hybrids develop cancer #7) There are many more different species near the equator; and a very consistent gradient of species number with latitude! True for animals, plants, fish, insects, mammals, birds, everything #8) Numbers of species per island, graphed as log of species versus logarithm of island area give very linear graphs, and experts argue about WHY! The Biology Dept seminar this past Monday afternoon was about mechanisms of speciation (Daven Presgrave) "Trying out" for a job as Assistant Professor of Biology at UNC Some quotes from this seminar: (regarding speciation) "we know virtually nothing about the general mechanisms of how this occurs" Lewontin "It is awash with unsound and often ??/ speculation"       quote from somebody else "Since 1989: a consensus has emerged" The speaker has been doing a genetic analysis, and following that up by molecular studies. #9) There are two forms of separation: "pre-zygotic" (mechanisms that avoid cross breeding)      versus "post-zygotic" (mechanisms that make hybrids infertile) #10) A long standing paradox, in terms of Mendelian genes, that hybrids can be less fit than either homozygous form. For example, if Aa were less fit that AA, then shouldn't aa be even worse than Aa? The great evolutionary biologist, Theodosius Dobzhansky famously proposed a likely explanation, by means of two pairs of alternative alleles: not just AA, Aa & aa, but also BB, Bb & bb For example, in two temporarily isolated populations, one might get to be AA BB and the other aa bb. and then Aa Bb might be infertile (or less fertile) This job candidate is concentrating on finding exactly which genes cause the infertility of hybrids between 2 species of Drosophila: D. melanogaster and D. similis (both African) These separated 2.5 million years ago.   Questions that you should now be able to answer & discuss a) Is survival of the fittest the only cause of evolutionary change? (Did Darwin himself even think it was?) (hint: no) *b) Did Darwin propose the phrase "survival of the fittest"? c) What are two other causes of evolution? (that really occur) d) In sexual selection, members of which sex do most of the choosing? e) And what are some ways that the anatomy of the other sex gets changed as a result of this form of evolution? f) Can small populations cause faster changes in gene frequencies? g) Can you explain this phenomenon? Why does it occur? h) What is meant by a hybrid? (offspring of cross between 2 species; or two subspecies; or sometimes used to refer to offspring of cross between genotypes) i) Are hybrids sometimes stronger or bigger than their parents? (yes) j) For speciation to occur, how should the fertility of hybrids differ from the fertility of their parents? k) What effect would it have on speciation if hybrids were more fertile, or otherwise left more offspring? l) If hybrids are less fertile, will evolution favor genes for instincts that reduce mating with members of the other kind? m) What is the surprising result of crossing the species of tropical fish called "Platys" with those called "Swordtails". n) Would you expect that such hybrids would occur in the wild? o) The number of different species of animals and plants varies with geography according to what general rules, if any. *p) For how long have the mechanisms of speciation been well understood and agreed upon by scientists who specialize in it? q) What is the distinction between "prezygotic" as opposed to "postzygotic" categories of mechanisms for minimizing interbreeding and gene transfer between potential species?   Evolution survey Understand Evolution? 9 yes 1 not completely Understand what is the evidence 8 yes 1 "not too convinced" 1 blank Most convincing: genomes confirm family tree 5 anatomy 1 Least convincing: computer simulations 4 Don't understand: How to breed differences Why does it prove anything to get same ages Geology Do NOT find convincing: Antibodies Did you change your mind: everybody says "No" Everybody says Evolution should not be excluded from High Schools and college courses.   Subject: Annual Biology and Philosophy Conference Duke Center for Philosophy of Biology is hosting its second annual conference on recent work in Biology and Philosophy. The conference will include contributed papers about topics related to adaptationism. If you would like to submit a contributed presentation, please send a 300 words abstract with your name and affiliation, to Frederic Bouchard by email at fb3@duke.edu before March 18th 2003 (extended deadline). Second Annual Conference on recent work in Biology and Philosophy "25 years of Panglossian Spandrels: A Conference on Adaptationism" April 11-13 2003 www.duke.edu/philosophy/bio/conference.html Speakers and Panelists will include: Andre Ariew (URI), Robert Brandon (Duke), Brian Hall (Dalhousie), Tim Lewens (Cambridge), Mohan Matthen (UBC), Gerd Muller (KLI), Frederik Nijhout (Duke), Mark Rausher (Duke), Leigh Van Valen (Chicago), Denis Walsh (Edinburgh) and more tba No registration fee. Please email if you have any questions: Frederic Bouchard fb3@duke.edu p.s. Last year's conference included Gunter Wagner, Brian Hall but also many Duke students including Matthew W. Hahn, Jason E. Stajich, and Gregory A. Wray, Andrew Yang and Jeff Marcus.

 

back to syllabus