08_Speciation_and_Extinction (2) - BI SC 002 LECTURE...

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BI SC 002 LECTURE 8—SPECIATION AND EXTINCTION Draft: November 12, 2010 Up until this point, everything we have discussed involving evolution would be classified as microevolution . Microevolution is the evolution/patterns of change within a population or species. This is evolution on the small scale, over a short period of time. Today our lecture moves more into the idea of macroevolution . Macroevolution is the overall pattern of change over a much longer period of time. Macroevolution is the evolution that leads to the origin of new species and the elimination of outdated ones. Today we discuss speciation , the process by which new species arise. But, first we must define what a species is. Depending on which branch of science you are studying, you will get a different definition. I personally prefer the biological species definition. A biological species is a reproductively isolated community in which all individuals potentially or actually interbreed amongst themselves but are genetically isolated from other groups. Two key factors with that definition: 1) the individuals can POTENTIALLY interbreed. So, members who are displaced geographically are included in the same species. Mountain lions once range from Alaska to South America. The odds were against one from Alaska breeding with one in South America (because of distance) but, should they encounter each other, the potential is there for them to breed. 2) The individuals can interbreed—this eliminates any subjective judgment in defining what a species is—either two individuals can breed or they can’t. Dogs, no matter how varied the breed, can interbreed with each other. They are all the same species. However, if a horse and a donkey are bred, yes, a mule is produced, but that mule is sterile. We’ll see later on that because the offspring is unable to reproduce, this demarcates a horse and a mule as separate species. Scientists who are interested in body structures define a species as a morphospecies . A morphospecies, or morphological species, consists of individuals that look alike, that is, they have similar phenotypes. The classic delineation of species is done this way. It is easy to tell which organisms look alike and which do not. The only issue with this definition would probably be which characteristics to use as the classifying scheme. Scientists interested in fossils and paleontology would use the paleospecies definition as their definition of species. A paleospecies is a chronological series of similar forms. This is useful in determining what species extinct organisms belonged to. Scientists interested in genetics may use the agamospecies definition to define a species. Agamospecies are grouped together based on the percentage of genetic relatedness, that is, how much of the DNA between two organisms is identical. This definition is especially useful for asexually reproducing organisms. In asexual reproduction, the only chance for genetic modification is through mutation—there is no genetic recombination as is found in sexual
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08_Speciation_and_Extinction (2) - BI SC 002 LECTURE...

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