BIOL 1010 Introduction to Biology: The Evolution and Diversity of Life. Spring 2011 Sections A & B

Transcription

1 BIOL 1010 Introduction to Biology: The Evolution and Diversity of Life. Spring 2011 Sections A & B Steve Thompson: stthompson@valdosta.edu 1

2 Natural Selection and Variation through Mutation Or survival of the fittest. But what does that really mean? Certainly not the biggest, baddest ass around. Actually to be the fittest in this context is to pass on more of your genes to the next generation than your peers, i.e. to have higher reproductive success. And to start out with a bit of humor: The Banana an atheist nightmare! Yeah, right. 2

3 Once more, what s evolution? Evolution is a change in the genetics of populations over time. It occurs in a population when some alleles become more common, and other less common, from one generation to the next. A population is a group of interbreeding organisms, and an allele is a version of a gene. All populations have many alleles. Many explanations have been proposed for life s diversity. Evolution explains it all! 3

4 A history of evolutionary thought Buffon one of first to openly suggest that closely related species arose from a common ancestor and were changing. Hutton uniformitarianism the processes of erosion and sedimentation that act in modern times also occurred in the past. 4

5 History, cont. Cuvier catastrophism brief geological catastrophes are responsible for most geological formations. Principle of superposition lower layers of rocks (and fossils) are older than those above them. Fossils represented extinctions, but the organisms are replaced by those from surrounding areas. Lamarck inheritance of acquired characteristics. He was the first to suggest that animals could change or become extinct in response to interactions with their environment. Lyell renewed uniformitarianism natural processes are slow and steady. 5

6 This can be seen in places like Arizona s Grand Canyon Also see: and 6

7 And then along came Darwin... Charles Darwin five year voyage as naturalist on HMS Beagle. Read Lyell and trusted uniformitarianism. Tried to reconstruct the past from contemporary observations. Noted similarities and differences among organisms. Month in Galapagos Islands was the seed of theory of evolution by natural selection. from win-on-the-evolution-trail.html 7

8 One particularly striking observation was convergent evolution. This is the idea that many organisms have similar characteristics, because they evolved in similar environmental conditions, even though they are not closely related. 8

9 He returned to England and started thinking... The finches were especially interesting. There were 14 different types, yet all must have originated from one ancestor that made it the islands a very long time ago, i.e. descent with modification. 9

10 He combined ideas from Malthus Essay on the Principle of Population with this and what he had seen in artificial selection (selective breeding)... 10

11 To come up with the concept of natural selection: Which is the differential reproductive success of individuals with particular genotypes (in modern lingo). Or as he stated preservation of favourable variations and the rejection of injurious variations. See Natural_selection 11

12 Darwin described his theory of evolution by natural selection in a brief sketch in 1842, and in a longer analysis two years later, but he did not publish either. He submitted his paper with one from Wallace to the Linnaean Society in And then finally he published On the Origin of Species by Means of Natural Selections, or Preservation of Favored Races in the Struggle for Life in

13 Other beautiful cases of diversification by natural selection can be seen in Madagascar and Lake Victoria. Diversity and evolution of cichlid fishes LEMURS!! LEMURS!! Diversity of cichlids (Family Cichlidae) At least 100 species of lemurs in Madagascar. Around 250 different species of Cichlid fishes in Lake Victoria! And, of course, the well known antibiotic resistance in bacteria phenomenon. 13

14 Natural selection is but one mechanism involved in evolution, but it s certainly a powerful one, particularly in microevolution, i.e. relatively short-term changes in allele frequencies within a population or species. But where does this necessary variation come from? Primarily from mutation. This is the random occurrence of changes in an organism s DNA, and in sexual organisms this has to be in the germ line (egg or sperm) in order to be inherited. 14

15 Other sources of variation include... migration between populations (gene flow), shuffling of genes through sex (independent reassortment and recombination), and... horizontal gene transfer and hybridization. 15

16 Natural selection eliminates certain phenotypes. Mendel s work although contemporaneous with Darwin s, was not well known, so the mechanisms of how all this worked remained obscure. Now we now know that... Gene pool entire collection of genes and their alleles in a population. Proportion of alleles for each gene determines characteristics for that population. A change in allele frequency in a population is evolution. Poorly adapted phenotypes are weeded out. Adaptive phenotype in one set of of circumstances may be a liability in others. Constantly changing conditions means evolution never stops. 16

17 Natural selection does not have a goal! It does not lead to more perfect organisms it can t look ahead, nor predict what will work. Every genome has a limited potential. No gene pool contains every allele needed to confront every possible change in the environment. Disasters can indiscriminately wipe out the best allele combinations just by chance extinction is the rule, not the exception. Some harmful genetic traits are out of natural selection s reach (e.g. if they appear after reproductive age). 17

18 Modes of natural selection: Directional selection one extreme phenotype is fittest, others are selected against. This is a type of positive selection. Disruptive selection two or more extreme phenotypes are fitter than the intermediate phenotype, a.k.a. diversifying selection. This is also a type of positive selection. Stabilizing selection extreme phenotypes are less fit than the optimal intermediate phenotype most common in stable, unchanging environments, a.k.a. purifying or negative selection. Results in conservation of features. 18

19 These can be illustrated thus... 19

20 And a biggy balancing selection. This is also known as balanced polymorphism and is often maintained through heterozygote advantage, sometimes called overdominance. It can also occur through frequencydependent selection where the fitness of one phenotype depends on frequency of other phenotypes in the population. 20

21 The best known case is sickle cell anemia. Multiple alleles of a gene persist indefinitely in the population in balanced polymorphism. Why do seemingly harmful alleles persist? Often it s heterozygote advantage: The heterozygote has greater fitness than either homozygote. Sickle cell disease and malaria those with both sickle cell alleles die, but those with only one, i.e. are heterozygotes, are resistant to malaria! 21

22 The distributions are congruent: 22

23 Several human diseases are maintained in the population because of heterozygote advantage: 23

24 That s enough on natural selection for now. But we re not done for the day. We still need to go over population genetics and the neutral theory, so we ll do that right now. 24