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Transcript
The “Theory” of Evolution can explain both short -term and long term adaptation, including speciation and the radiation of species to produce all of the biological diversity we see today. Curiosity about the origins of present biological diversity is ancient. Attempts to explain the biological diversity we see around us have ranged from the mythological and religious to the scientific. Attempts to use a scientific approach were instigated by observations of (among others): • The fossil record. • Comparative anatomy. • Comparative embryology and developmental studies. • Geological discoveries that indicated the earth was much older than various creation stories allowed for. Figure 13.00a Figure 13.11 Figure 13.9 Figure 13.12 Jean-Baptist Lamark An accomplished French biologist - one of the first to use the term. Contributed to the cell theory; remembered mainly for his “theory of acquired characters,”or transmutation, that included a complexifying force that drove organisms to evolve into more complicated forms, and in which the use or disuse of characteristics led to differences in their inheritance (i.e. giraffes evolved long necks because they needed to, not because of the natural selection of some mutations over others). Lamark’s ideas pre-dated those of Darwin. Thomas Malthus British mathematician and economist who in “An Essay on the Principle of Population” (1798) theorized that human population growth would one day outstrip the food supply. Darwin’s more generalized version of this train of thought states that resources in nature are always limited and therefore must be competed for by members of a population (and among populations of different species as well) creating a selection pressure that will favor the survival of some individuals over others. Because his work on the topic was so comprehensive, Charles Darwin is given most of the credit (or blame) for originating the theory of evolution. Darwin’s curiosity about nature, his ability to observe clearly and think creatively, and the experiences he had as part of the HMS Beagle’s expedition to S. America allowed him to create a theory of natural selection, or descent with modification, that has stood the test of time. A Theory of Evolution “timeline” showing the relationship of Darwin’s work to that of other influential thinkers of the time, and other world events. The Voyage of the H.M.S.Beagle (to map the coast of S. America to produce better charts for Her Majesty’s Navy ) Darwin’s finches Finches on the Galapagos islands had diverged from each other recently enough that their “relatedness” was more-or-less obvious. Life on the Galapagos was “recent” in the sense that the islands had only been thrust up out of the sea for a few million years, which allowed evolution in progress to be seen more clearly. Organisms like the Galapagos marine iguana are found nowhere else on earth Alfred Wallace, who as a young man wrote a letter to Darwin that prodded him into finally publishing his theory of evolution. Wallace was studying Beetles in Malaysia and came to essentially the same conclusions about natural selection as Darwin had. In order to keep his “priority” on the subject yet not slight Wallace, Darwin published a joint paper on the topic with Wallace before publishing his full length book. Wallace went on to become a distinguished entomologist and naturalist in his own right. Title page of the evil book that shook Victorian society to its core, and is still provocative today. As he expected, Darwin was vilified for his views by Victorian society which was (superficially at least) very religious in orientation. The Evolution - Creation debate persists to this day with Darwin still being viewed as an evil person by many of his Creationist and “Creation Science” minded detractors. A Rogues Gallery of “evolutionist free thinkers” and their adversaries (all except Lyell were portrayed in the film seen in class) Capt. Robert Fitzroy Geologist Charles Lyell Lord Bishop Samuel Wilberforce Anatomist Richard Owen Naturalist & Professor Thomas Huxley (“Darwin’s Bulldog”) Charles Lyell and the Question of Time Charels Lyell, although older, was a historical contemporary and friend of Darwin who was not featured in the “Dangerous Idea” film. Lyell is given most of the credit for the theory of uniformitarianism, the understanding that the geologic features of the earth, as we now see it, are the result of slow natural forces acting over vast periods of time, and that these forces probably operated in ancient times the same way as they do now. Accordingly, the earth must necessarily be much older than any interpretation of the Biblical creation story would lead one to believe. This understanding has since received massive amounts of support from modern techniques, radioisotope dating, evidence for plate tectonics and so on. Lyell was receptive to Darwin’s ideas on evolution and encouraged him to publish them, although initially he seems to have not fully appreciated how much Darwin’s ideas differed from those of Lamark, he eventally supported the theory of Natural Selection in full. So the deposition of all of these sedimentary layers of rock took millions of years, not to mention the time it took the Colorado River to erode its way down through them. Similar empirical observations and reasoning used by Lyell as he studied the structure of Mt. Etna on Sicily led to his original theory of uniformitarianism and its implication that the earth must be, at minimum, millions of years old. Because these processes probably worked at the same rate historically as they do now, the earth must be VERY old. Plate Tectonics and Paleogeography Modern geology has allowed us to reconstruct what the earth was like in ages past. From: http://www2.nau.edu/rcb7/globehighres.html Plate Tectonics and Paleogeography - 2 This picture shows the location of PA in the Devonian period, 417 to 354 Million Years Ago. Devonian age strata are exposed throughout eastern PA. You are here equator From: http://www.geo.ucalgary.ca/~macrae/timescale/timescale.html So how does evolution work? Darwin’s genius does not reside merely in his understanding that evolution occurred, others before him had come to the same conclusion. His seminal contribution was the mechanism that he proposed to be responsible for it: Natural Selection Only ”survivors” are able to pass on their genes (which differ due to mutational events during DNA replication) to their offspring; so eventually all the members of a population have the altered genes, and we say the population has adapted. Many adaptations added over long periods of time accumulate within an isolated population to produce reproductive isolation and speciation. The type of mutations necessary for this scenario to play out must involve prior gene duplication to allow for new traits to be created without disabling essential old traits. This is seen also in that, as species have evolved over time, the trend has been toward more complex organisms with more DNA (on average ) per cell. “Descent with modification” has therefore favored the development of more complex organisms over time. Individual genetic variation is the result of mutation and provides the spectrum of traits that can be selected among by natural selection. More intra-species variation Examples of short-term adaptation produced by natural selection are easy to find, especially with species like insects and bacteria that have short generation times. Selection and adaptation down on the farm Adaptive camouflage in different mantis species from different locations also shows natural selection at work. Geographic isolation allows populations to evolve that occupy the same niches found elsewhere but in different “forms” = species. Marsupials were able to persist in Australia long after they had been out-competed by placental mammals elsewhere. The result of accumulated adaptations is speciation = microevolution Speciation Mechanisms Changes in individuals within a population that prevent successful hybridization with individuals of similar but different populations result in new species. Usually population isolation over long periods is necessary for these to occur. Populations that can no longer inter-breed are by definition different species. Given vast tracts of time this process of cumulative adaptation -> > -> speciation leads to macroevolution with a common ancestor producing many species that in turn arise and become extinct over time. Modern gene (DNA) sequencing techniques clearly show similarities in gene structure that are greater between species that one would judge to be more closely related based on visual clues. Evidently what enables the roughly 2% difference between chimp and human DNA to have such a significant effect is that much of the difference is in genes that control development - genes that turn other genes on and off as an individual fetus develops before and after birth. THE “C VALUE PARADOX” In general, more complex organisms have more DNA per cell because of the accumulation over time of gene duplications and new genes that have resulted from recombination of fragments of some of these extra genes. However, exceptions to this general trend exist. This is termed THE “C VALUE PARADOX” How can the evolution of increasing complexity be explained? If a needed gene were to somehow mutate into another that produced a new protein with a valuable function, it would be of Zero value if the first needed gene product were no longer being produced as a result. The organism would probably die (and that’s not evolving). What is necessary is prior gene duplication. This sort of process makes “extra” gene copies available that mother nature can ”play with” through mutations that recombine gene pieces into new constructs. DNA analysis (sequencing) clearly demonstrates that this, in fact, is how evolution has proceeded at the molecular level. This is a slow process and wouldn’t necessarily - in itself -lead to evolution. However combined with the process of natural selection working over vast reaches of time it has created the history of organismal diversity that we see all around us today. We are no different A computer-generated “average” human being as she would appear if the various human facial feature differences could be somehow “blended” The picture represents, in essence, a sort of computer-generated counter evolution. Odds & ends The mechanisms that have contributed to the tremendous natural diversity we see today go beyond the simple form of natural selection that we have explained to this point, and its fair to say that we don’t totally understand them all. A sampling follows: Types of natural selection results The benefit, or lack thereof conferred by a gene can depend on the environment and the genetic background within which it occurs. Even genes that seem to represent harmful mutations can be maintained in populations if they confer an advantage in certain environments. The gene for altered hemoglobin that results in sickle cell disease also confers resistance to infection by the malaria parasite. So the presence of the parasite creates a positive selection pressure for maintaining what would otherwise be a harmful gene within populations. Genetic Drift: Chance alone causes allele frequency in a small population to change over time. This is one of several mechanisms that favor gene pool changes over time that are not powered by natural selection The Bottleneck effect causes changes in allele frequency; in effect creating a new gene pool from a selected sub-set of the parent population alleles. The Founder effect works in a similar way; here a random sub-sample is split off to found a new population. Gene pool allele frequencies resist change over time. Hardy was a pure mathematician who formulated the “Law” after being asked by Punnett why it was that dominant genes wouldn’t increase in a population over time. He evidently held applied math in some contempt which comes through in the wording of his 1908 paper (Mendel was “rediscovered” in 1900) on the subject: To the Editor of Science: I am reluctant to intrude in a discussion concerning matters of which I have no expert knowledge, and I should have expected the very simple point which I wish to make to have been familiar to biologists. However, some remarks of Mr. Udny Yule, to which Mr. R. C. Punnett has called my attention, suggest that it may still be worth making... Suppose that Aa is a pair of Mendelian characters, A being dominant, and that in any given generation the number of pure dominants (AA), heterozygotes (Aa), and pure recessives (aa) are as p:2q:r. Finally, suppose that the numbers are fairly large, so that mating may be regarded as random, that the sexes are evenly distributed among the three varieties, and that all are equally fertile. A little mathematics of the multiplication-table type is enough to show that in the next generation the numbers will be as (p+q)2:2(p+q)(q+r):(q+r)2, or as p1:2q1:r1, say. The interesting question is — in what circumstances will this distribution be the same as that in the generation before? It is easy to see that the condition for this is q 2 = pr. And since q12 = p1r1, whatever the values of p, q, and r may be, the distribution will in any case continue unchanged after the second generation (. . . Yeah, right) Hardy and Weinberg never worked together W. Weinberg G. Hardy Wilhelm Weinburg was a German physician who also had an academic interest in genetics. Ironically he had lectured on his “principle of genetic equilibrium” in 1908, the same year that Hardy’s paper was published. The coincidence, however, wasn’t discovered until 1943. Eventually both men were given credit for the concept. Hardy-Weinberg equlibrium conditions The allele frequency equilibrium described by Hardy and Weinberg only holds when: • The population is extremely large • Mating is random • Mutation is absent • Natural selection is not at work • Immigration and emigration are non-existent (no gene flow) Figure 23.3a The Hardy-Weinberg theorem Figure 23.3b The Hardy-Weinberg theorem Extra Photo 13.03x2