Now, let's return to the quail thrushes...
If we treat all of these six forms as phylogenetic species and then just simply ask the question, how are they interrelated, we come up with this hypothesis. We find out, for instance, that marginatum, the species out in western Australia, is related to castaneothorax, which is in eastern Australia. So, two forms that are found in the western desert region and the eastern desert region are each other's closest relative. If we look at alisteri right along the southwest coast desert area and cinnamomeum, which is in the eastern desert area, we see that they are each other's closest relatives. In other words, these are both allopatric populations; they're not geographically close to one another (a stiuation that would be called parapatric)- they are separated or allopatric. Finally, castanotum, which is widespread across southern and western Australia is related to puntatum which is distributed in eastern Australia.
The color brackets on the slide identify species that hybridize where they come into contact. So, marginatum and castanotum hybridize in their area of overlap. Under the biological species concept, because of occasional interbreeding, scientists often put these two forms in the same biological species but they're not each other's closest relative. The same thing for castaneothorax and cinnamomeum which also hybridize occasionally along their zone of contact. They also are not each other's closest relative. They're not the same species the same biological species even though they produce hybrids.
If you recognize these forms as one or more biological species, you would not be able to understand how this situation has come about, how this group of species has evolved. So this is one example of how the two species concepts lead you to ask different kinds of questions about the history of a group and they lead you to understand hybridization events differently. There are many such examples like this within other organisms that have been discussed in the literature.
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