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Evolutionary Genetics - Evolutionary Genetics Introduction...

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1 Evolutionary Genetics Introduction Biological evolution is a heritable change in one or more characteristics of a population from one generation to the next. Microevolution examines the changes in the gene pool over a measurable period of time. Macroevolution refers to the changes that produce new species and higher taxa. Studies of molecular evolution examine changes in the genetic material that are responsible for the phenotypic changes associated with evolution. Species Concepts The morphological species concept Careful analysis of phenotypic differences are the basis of identifying morphological species. This concept can be used to define species that are extinct or living, or species that reproduce sexually or asexually. Unfortunately, it is based mainly on the judgment of the investigator (there’s an old saying that a species is whatever a competent taxonomist says it is). Therefore, different investigators often come up with species designations that are not comparable. Also, it is difficult to apply to organisms that are small and have few measurable morphological characters to assess. Other difficulties include dealing with fossil species that differed in characters that are not preserved (e.g., color and/or anatomy of soft tissues), populations that are similar in morphology but differ in other characteristics such as song, temperature or drought tolerance, habitat use, or courtship display. Such populations are referred to as cryptic species. The biological species concept Under the biological species concept, the criterion for identifying evolutionary independence is reproductive isolation. If populations of organisms do not hybridize regularly in nature, or if they fail to produce fertile offspring when they do, then they are reproductively isolated and considered good species.
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2 It is used by many zoologists, and is the legal definition employed in the Endangered Species Act. A great strength of this concept is that reproductive isolation is a meaningful criterion for identifying species because it confirms a lack of gene flow. However, if populations do not overlap, there is no way to know whether they are reproductively isolated. Of course, it might be possible (though often impractical) to bring individuals from non-overlapping populations together and experimentally see if they can effectively interbreed. One often hears the biological species referred to as a population of “actively or potentially interbreeding” individuals. Other shortcomings include the inability to test the concept in fossil forms, it has no meaning at all with regard to asexual populations, and it is difficult to apply in many plant groups where hybridization between strongly divergent populations is routine.
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