The Theory of Evolution
Evolution: organisms arise by descent and modification from previously existing organisms. Theories of Evolution: LaMarck: Due to use or disuse, structures become modified and these modifications are passed on to future generations. In general, LaMarck’s theory states that acquired characteristics can be passed on from parent to offspring. o Evidence for LaMarck’s Theory? The embryologist Charles Waddington treated fly larvae with heat shock. As a result of this treatment, some of the adult flies showed the abnormal condition “crossveinless” (some of the their wing veins were missing). After several generations of this treatment, he let a generation of flies develop without heat treatment and many of them were also crossveinless. Does this experiment provide convincing proof of LaMarck’s theory? Discuss. Darwin and Wallace: o Populations of organisms, if left uncontrolled, would overproduce and overpopulate. o Because of controlling factors, like predation, competition and other environmental controls; some organisms will have a greater likelihood of surviving to reproduce. o A heritable change occurs in a particular species in a population. This change makes it more reproductively successful. o These changes are passed on to the offspring. o These changes proliferate within the population. o When enough changes occur, a new species arises.
The stated theory above is based on the following hypotheses made by Darwin: o Earth is very old o All organisms are related by common descent o Diversity results from speciation o Evolution produces gradual changes in population o Evolution occurs by Natural Selection: differential reproductive advantage. Often, people describe Natural Selection as “Survival of the Fittest”. Describe the term “fitness” based on Darwin’s theory. What Evidence supports the hypotheses made by Darwin? o Artificial Selection: Differential reproductive success directed by humans and not the environment. o Example: Dogs, genetically identical to wolves but they had developmental mutations that were selected for by humans. Or, chickens were selected for white breast meat (which caused them to lose their ability to fly). o Fossil Record: o Dating Techniques: Half-lives of Radioisotopes are predictable times when the nuclei decays. By comparing the amount of radioisotopes in a specimen with a known age, you can measure the amount of decay of an unknown specimen and compare the relative ages through extrapolation. Common radioisotopes and their half lives are: 14 C = 5700 years, 40K = 1.3 Billion Years, and 238U = 10 Billion Years. o Biologists use the principal of relative position, that is, specimens found deeper in sediment are older than those found in more shallow sediments. o The fossil record shows that more primitive species are found deeper in sediment layers than more advanced organisms. For the most part, transitional species are found, but in some cases there seems to be larger evolutionary “leaps” seen. o Embryology: All vertebrate embryos look the same (all have the same features) supports the relative similarity found in related species.
o Comparative Anatomy o Homologous Structures: structures that are identical in two different species. o Analogous Structures: structures that do the same thing in two different species. The appearance of both homologous and analogous structures supports the idea that what is a reproductive advantage to one species is a reproductive advantage to most. These structures are often found in organisms that share similar environments. o Vestigial Structures: structures that exist in a species that have no apparent function.