Postzygotic or postmating isolation

Postzygotic or postmating isolation - higher values than an...

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Postzygotic or postmating isolation can be measured as in the following example. Consider two species, A and B. These can be crossed two ways (reciprocally) to produce hybrid offspring . We can also examine the viability or fertility of the two sexes of these hybrid offspring, hence four contexts are examined to score postzygotic isolation: Case Female parent Male parent Offspring Inviable or sterile? 1 Species A Species B Male No = 0 Yes = 1 2 Species A Species B Male No = 0 No = 0 3 Species B Species A Female No = 0 No = 0 4 Species B Species A Female No = 0 No = 0 I = 0 I = .25 Note: Isolation index is the average score for the four cases. In any particular case one could choose to score isolation in terms of the presence or absence of either isolation or sterility. Normally hybrid sterility evolves before hybrid inviability (mules are sterile but viable). Hence an index based on sterility would have
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Unformatted text preview: higher values than an index based only on evidence for inviable hybrid offspring. Coyne and Orr extracted these two types of data from the literature and tested some important ideas about the genetics of speciation. The general idea is that genetic distance (D) is positively related to time (the molecular clock hypothesis) and thus species pairs showing different degrees of genetic distance should be at different degrees of completion of the speciation process (be aware that many organisms are in the process of speciating as you read these notes). Coyne and Orr show that there is a significant relationship between genetic distance and both premating and postmating isolation...
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