Phylogeny-07-web - Phylogeny 1 Speciation(continued...

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Phylogeny Speciation (continued) Definition of Species: Last lecture I defined the Biological Species Complex (BSC) and introduced the concept of Reproductive isolation. There are two broad categories of reproductive isolation: prezygotic barriers and postzygotic barriers. Prezygotic barriers prevent fertilization from taking place. Postzygotic barriers reduce either the viability or the fertility of hybrid offspring. Prezygotic barriers include temporal, behavioral, mechanical, gametic isolation, as well as isolation by habitat. Postzygotic isolation includes hybrid inviability and hybrid sterility (see Table 25.1) Two other definitions of species include Morphospecies and the Phylogenetic Species Concept. Morphospecies is a very simple idea; organisms that look different enough represent different species. The thought is that populations of organisms that look very different probably do not experience gene flow, probably because they are reproductively isolated. Although a simple idea, it is very helpful in some cases. For example, paleontologists do not have the opportunity to observe the mating behavior of their organism of interest. They need to look at the fossils and then determine, solely on appearance, whether two different fossils represent two different species. Another place where this concept is helpful is in plants. Plant populations often look different and in nature do not reproduce. In fact, though, many different plant “species” can be made to reproduce in the greenhouse, even though it appears they do not reproduce in nature. They are thus not strictly reproductively isolated. We are going to ignore the Phylogenetic Species Concept. The process of speciation: Sometimes biologists argue about what is and isn’t a distinct species. However, it is clear that there are millions of different species on earth. According to the theory of evolution, the species arose by the process of speciation.
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