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EVOLUTION AND S P E C I AT I O N
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The term evolution describes a change in genotypic frequencies, which usually results in a population of individuals better adapted to the environment than their ancestors were. Speciation comes in two different forms. (1) It may be the evolution of a population over time until the current population cannot be classi ed as
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Charles Darwin (1809 1882). Darwin was an English naturalist who rst established the theory of organic evolution by natural selection. (Painting by George Richmond, 1840.
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Tamarin: Principles of Genetics, Seventh Edition
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IV. Quantitative and Evolutionary Genetics
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21. Evolution and Speciation
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Evolution and Speciation
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BOX 21.1
rom time to time, attacks on neo-Darwinism are mounted, usually by persons who either view evolutionary theory as antireligious or who misunderstand Darwin s theory. One attack, entitled Darwin s Mistake, by Tom Bethell, was published in Harper s magazine in 1976. Bethell began by pointing out that Darwinian theory is a tautology rather than a predictive theory. (The term tautology means a statement that is true by definition.) That is, evolution is the survival of the ttest. But who are the ttest Obviously, the individuals who survive. Thus, without an independent criterion for tness other than survival, we are left with the statement that evolution is the survival of the survivors. This, indeed, is a tautology. But it is possible to assign independent criteria for fitness. Darwin wrote extensively about arti cial selection in pigeons, in which the breeders choice was the criterion for tness. (Many novel breeds of pigeon have been created this way.)
Ethics and Genetics
Attacks on Darwinism
Plant and animal breeders have practiced arti cial selection extensively. Here, survival is not the criterion for tness; productivity is. It is more dif cult to establish a priori independent criteria of fitness in nature. Often, uncontrolled or unseen vagaries have major impacts on the course of events. Surely the temperature became colder before the mammoths became woolly. Is it then reasonable to predict that elephants would get woolly if the climate became colder in Africa today The answer is no, for several reasons. First, the elephants might adapt to colder weather in any of a large number of different ways they could get fatter, they could migrate, and so on. To some extent,
adaptation depends not only on the changing environment, but also on the reserve variation within the gene pool of the species. Second, the elephants could become extinct; they might not be able to adapt at all. And third, if the climatic changes were not severe, the elephants might not change at all. Predicting the exact course of evolution is nearly impossible.To provide independent criteria for tness in nature is, therefore, very dif cult. Some modern evolutionary biologists, although not doubting neoDarwinism, do worry to some extent about the dif culties in testing modern evolutionary theory. However, lower-level experiments are done to test various aspects of evolution in speci c systems. For example, in 1993, B. Grant and P. Grant hypothesized that changes in bill size would occur in the nch Geospiza fortis (see g. 21.6) because of changing food size due to changing weather on the Gal pagos Islands. Their proposal seems to be correct. In addition, the support for Darwinism (the fossil
belonging to the same species as the original population. This process is known as anagenesis, or phyletic evolution (an is Latin for without, genesis is Latin for birth or creation). (2) Speciation may also be the divergence of a population into two distinct forms (species) that exist simultaneously. This branching process is known as cladogenesis (clado is Greek for branch; g. 21.2). What do we mean by the term species Before Darwin s time, typological thinking prevailed, and a species was de ned as a group of organisms that were morphologically similar. All variants were considered imperfections of the model or type. One of Darwin s greatest contributions to modern biological theory was to treat variation as a normal phenomenon in a group of organisms. The modern biological species concept groups together as members of the same species organisms that can potentially interbreed. A species, therefore, is a group of organisms that can mate among themselves to produce fertile offspring.
Figure 21.2 Forms of speciation. In anagenesis (a), a species changes over time until it is so different from its progenitor that it is classi ed as a new species. In cladogenesis (b), speciation takes place as a branching process wherein one species becomes two or more.
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