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Influence of geological thinking on Darwin By time of “The Beagle” voyage idea that Earth was young was being challenged. Opposition based on principle of Uniformitarianism. Idea that geological processes happening today are the same as have operated in the past. Influence of geological thinking on Darwin Uniformitarianism contrasted with Catastrophism which proposed that current geological formations had resulted from catastrophic events (such as biblical flood) which occurred on scale unknown today. Uniformitarianism first proposed by James Hutton and championed by Charles Lyell. Influence of geological thinking on Darwin Hutton and Lyell inferred Earth must be very old based on measurements of rate of ongoing rock forming processes (e.g. deposition of mud and sand). These developments in geology focused Darwin on the potential importance of gradual change in shaping structures. Darwinian Natural Selection Artificial Selection Artificial Selection. Humans have selectively bred for desirable traits in domestic animals and plants for millenia. Process has produced our crop plants, garden plants, pets, and domestic animals. Darwin closely studied pigeon breeding. Artificial Selection Cauliflower, broccoli, kale, brussels sprouts all descended from wild cabbage. All these crops can be crossed and produce fertile offspring. Cauliflower: edible bit is the inflorescence or flower stalk. Artificial Selection Cauliflower has large dense infloresence. This results from mutant ‘loss of function’ alleles of two genes that affect flower structure and infloresence density. Artificial Selection Early farmers choosing among their crops selected those with largest infloresences. Process has resulted in cauliflowers that are homozygous for both loss of function alleles. Evolution by Natural Selection Darwin envisaged process similar to artificial selection that had produced organisms we see today. He called it Natural Selection. Evolution by Natural Selection Darwin proposed evolution the inevitable outcome of 4 postulates: 1. There is variation in populations. Individuals within populations differ. 2. Variation is heritable. Evolution by Natural Selection 3. In every generation some organisms are more successful at surviving and reproducing than other. Differential reproductive success. 4. Survival and reproduction are not random, but are related to variation among individuals. Organisms with best characteristics are ‘naturally selected.’ Evolution by Natural Selection If 4 postulates are true then the population will change from one generation to the next. Evolution will occur. Evolution by Natural Selection Darwinian fitness: ability of an organism to survive and reproduce in its environment. Fitness measured relative to others of its species Evolution by Natural Selection Adaptation is a characteristic or trait of an organism that increases its fitness relative to individuals that do not possess it. Testing the postulates Evolution of beak shape in Darwin’s Finches. Peter and Rosemary Grant’s (and colleagues) work on Medium Ground Finches Geospiza fortis On Daphne Major since 1973. Evolution of beak shape in Darwin’s Finches. Postulate 1. Is the population variable? Finches vary in beak length, beak depth, beak width, wing length and tail length. Evolution of beak shape in Darwin’s Finches. Postulate 2: Is variation among individuals heritable? Variation can be a result of environmental effects. Heritability: proportion of the variation in a trait in a population that is due to variation in genes. Evolution of beak shape in Darwin’s Finches. Peter Boag compared average beak depth of parents with that of their adult offspring. Strong relationship between offspring and parent beak depths. FIG 3.7 Evolution of beak shape in Darwin’s Finches. Postulate 3: Do individuals differ in their success at survival and reproduction? 1977 drought 84% of G. fortis individuals died, most from starvation. In two other droughts 19% and 25% of the population died. Evolution of beak shape in Darwin’s Finches. Seed densities declined rapidly during drought and the small soft seeds were consumed first. Average size and hardness of remaining seeds increased over the course of the drought. FIG 3.8b FIG 3.8A Fig 3.8c Evolution of beak shape in Darwin’s Finches. Postulate 4: Are survival and reproduction nonrandom? Do those who survive and reproduce have different characteristics than those that don’t? Evolution of beak shape in Darwin’s Finches. As drought progressed small soft seeds disappeared and large, hard Tribulus seeds became a key food item. Only birds with deep, narrow beaks could open them. Evolution of beak shape in Darwin’s Finches. At end of the 1977 drought the average survivor had a deeper beak than the average non-survivor and also a larger body size. FIG 3.9 Did the population evolve? Chicks hatched in 1978 had deeper beaks on average than those hatched in 1976. Population evolved. Fig 3.10 Evolution of beak shape in Darwin’s Finches. Variation in weather from year to year on Daphne Major over 30 years has led to variation in the traits that are favored by selection. Population has evolved over time. Fig 3.11 A Over the course of 30 years (1970 to 2000) beak size evolved. Rose sharply during drought (red line) then declined to pre-drought dimensions. Fig 3.11 B Over same 30-year period birds evolved more pointed beaks and (next slide) significantly smaller body size. Fig 3.11 C The nature of Natural Selection Many misconceptions about how selection operates and evolution occurs. Points to remember about natural selection Natural selection acts on individuals, but its effects accumulate in populations Individual finches live or die during a drought (the selection event). But change occurs in the characteristics of the population, not in individuals. Natural selection acts on individuals, but its effects accumulate in populations During drought individual finch’s beaks did not change, but average beak dimensions changed because more small-beaked birds died than largebeaked birds. Evolution causes changes in allele frequencies Evolution only occurs when traits have a genetic basis. If beak dimensions were environmentally induced, no evolution could take place. After drought, frequencies of phenotypes in next generation might have been the same as before. Natural selection does not plan ahead. Each generation is result of selection by environmental conditions of the previous generation. Evolution always one generation behind environmental changes. New traits evolve even though selection acts on existing traits. This occurs because: 1. mutation produces new alleles. 2. In sexually reproducing organisms meiosis and fertilization recombine existing alleles to produce new genotypes. New traits evolve even though selection acts on existing traits. Artificial selection for oil content in corn. After 60 generations oil levels were well above starting values. Fig 3.12 New traits evolve even though selection acts on existing traits. Natural selection can also modify existing features over time for a new purpose e.g. Panda’s thumb. Trait used in novel way and eventually developed into a new structure referred to as a preadaptation. This does NOT mean there is pre-planning by natural selection. Natural selection does not produce ‘perfect’ solutions Panda’s thumb not a perfect solution. Natural selection does not produce ‘perfect’ solutions On Daphne Major during drought finches with narrow beaks survived better than those with wider bills. At end of drought, however, selection for deeper bills and bigger body size resulted in wider beaks even through deeper narrower beaks would have been a better solution. Presumably same genes control all three traits. And solution is not perfect. Natural selection does not produce ‘perfect’ solutions Similarly, many characteristics of organisms are the result of compromises between different selection pressures. The wings of various species of auk (seabirds that dive and swim) are a compromise between the need to “fly” (i.e. swim) underwater and in the air, two very different media. Little Auk Razorbill polar.alaskapacific.edu/aharding/images/Littl... http://media-2.web.britannica.com/eb-media/16/26016-004-13D8FA4C.jpg Natural selection is nonrandom, but not progressive There is no “goal” of natural selection. Evolution makes organisms better adapted to their environments, but there is no trend towards being more advanced. E.g. Tapeworms have no digestive tract. They are simpler than their ancestors. Selection does not act for the “good of the species” Apparently altruistic acts (e.g. giving an alarm call) are favored because they enhance relatives’ survival. Infanticide in lions benefits individual male lions not the species as a whole.