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Douglas Davidson

 Species are named on the basis of statistically significant differences in the traits used to estimate the phylogeny  Recent analyses have found that the phylogenetic species concept often distinguishes a series of cryptic species in populations that were formerly considered a single species. Applying Species Concepts: Two Case Histories  Species can be identified by distinctive morphological traits, reproductive isolation, and/or phylogenetic independence. Each species concept has advantages and disadvantages.  Employing more than one species concept can help biologists recognize diversity and organize research on its consequences  Read example on Marine Copepods and Elephants on Pages 609-610 16.2- Mechanisms of Genetic Isolation  Speciation has been hypothesized to be a three-stage process: o Initial step that isolates populations o Second step that results in divergence in traits such as mating system or habitat use o Final step that produces reproductive isolation  Isolation and divergence steps were thought to take place over time and to occur while populations were located in different geographic areas  Final phase was hypothesized to occur when these diverged populations came back into physical contact- an event known as secondary contact o Secondary contact- when two populations that have diverged in isolation from a common ancestor are reunited geographically  It is now clear that, isolation and divergence steps that initiate speciation frequently take place at the same time and in the same place o Also it appears likely that in a significant number of speciation events or even a majority, the third phase never occurs  Isolation/divergence/secondary contact hypothesis provides a useful framework for analyzing how speciation takes place  First step in speciation is genetic isolation  Physical separation or changes in chromosome complements can reduce gene flow between populations  Once gene flow is dramatically reduced or ceases, evolutionary independence begins and speciation is underway  Therefore, speciation process begins when gene flow is disrupted and populations become genetically isolated Physical Isolation as a Barrier to Gene Flow  Geographic isolation produces reproductive isolation, and thus genetic isolation  Allopatric Model- the hypothesis that speciation occurs when populations become geographically isolated and diverge because selection and drift act on them independently  Essence of allopatric speciation is that physical isolation creates an effective barrier to gene flow  Geographic isolation has been an important trigger for the second stage in the speciation process: genetic & ecological divergence  Geographic isolation can come about through dispersal and colonization of new habitats or through vicariance events, where an existing range is split by a new physical barrier  Dispersal is when a population in one area splits into two and one moves to another isolated area like from mainland to an island  Vicariance is when a large area is populated and some natural cause occurs and divides the area into two separate lands, dividing one from the other by nature  Look at page 612 for better understanding.  The dispersal-and-colonization hypothesis makes two predictions based on these facts: o Closely related species should almost always be found on adjacent islands o At least some sequences of branching events should correspond to the
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