People like to think of species as linear, however when you look at subspecies and variations between populations, it is self evident (imho) that a bushier arrangement is appropriate, maybe even more of a weave when subspecies interbreed and produce hybrids.
I should have been more clear, but I was speaking of the literature and instruction. H. sapiens evolution was considered to be more 'linear' in the sense that it was thought that we evolved from one species, which evolved from a previous one, and so on. Few were advocating either hybridization or a multi-regional type gene flow. It has been a fairly recent development that hybridization and gene flow has been found to have occured during our evolution, and this recent paper helps 'bush' out our evolution from the stricter linear one.
Because it happened during the time H.sapiens was already established as a species. Don't know how it affected the H.neanders (anyone know?) ...
Again, I should have been clearer. I used to be of the impression that the relatively small amount of morphological variation in humans today, was due to the recent development bottleneck. The greater variation in the DNA of previous populations (before the bottleneck) would have been reflected in their greater morphological variation as compared to us today, and these populations should have been thought of as belonging in our species.
Be careful here. Multi-regional, IIRC, was originally about H.sapiens arising independently in several locations (an aspect I have always had trouble with -- such a scenario should result in different species or subspecies not the same one)
Which is NOT what I was taught in College, even in 1988. My understanding, and the textbook from that time that I still have, states that the Multiregional hypothesis posits that enough gene flow occured to allow the populations to evolve together, and NOT that they evolved independently.
I would expect that many hybrid offspring may have been infertile, thus reducing the impact of such individuals on the overall populations. After all some mules are fertile and can produce offspring with horses and donkeys.
As far as H. sapiens is concerned, certainly that is a possibility, but I should point out an example that fits a bit closer to home, so to speak. From a
2001 article by Clifford Jolly:
Another source of phylogenetic uncertainty is the possibility of gene-flow by occasional hybridization between hominins belonging to ecologically and adaptively distinct species or even genera. Although the evidence is unsatisfactorily sparse, it suggests that among catarrhines generally, regardless of major chromosomal rearrangements, intersterility is roughly proportional to time since cladogenetic separation. On a papionin analogy, especially the crossability of Papio hamadryas with Macaca mulatta and Theropithecus gelada, crossing between extant hominine genera is unlikely to produce viable and fertile offspring, but any hominine species whose ancestries diverged less than 4 ma previously may well have been able to produce hybrid offspring that could, by backcrossing, introduce alien genes with the potential of spreading if advantageous. Selection against maladaptive traits would maintain adaptive complexes against occasional genetic infiltration, and the latter does not justify reducing the hybridizing forms to a conspecific or congeneric rank. Whether reticulation could explain apparent parallels in hominin dentition and brain size is uncertain, pending genetic investigation of these apparently complex traits.
(my bold)
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