Ten key ideas today:
#1) There several other causes of evolutionary change, in addition to natural selection, "survival of the fittest"
#2) One additional cause is sexual selection, mostly by females of which males they will breed with!
#3) Another cause of genetic change, not realized until ~1930s is (Random) Genetic Drift.
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.
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! 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!
#6) Hybrid corn and other crops usually yield more food, but are not superior in leaving offspring.
#7) There are many more different species near the equator;
#8) Numbers of species per island,
The Biology Dept seminar this past Monday afternoon was about mechanisms of speciation Some quotes from this seminar:
(regarding speciation)
"It is awash with unsound and often ??/ speculation" "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:
#10) A long standing paradox, in terms of Mendelian genes,
For example, if Aa were less fit that 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, 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? *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? 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?
|