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Thread: Species and Fertility

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    Post Species and Fertility

    Replacement theorists do not seem to have recognized the degree to which the structure of their argument hinges on the interfertility issue. Rapid (single-digit numbers of generations) population replacement would require either that the replacing group be uniformly homicidal, i.e., actually killing native hominids, or possess a degree of competitive advantage which is, ecologically speaking, difficult to imagine in a situation in which both groups are hunter-gatherers. The third option, that the replacing groups possessed a slight competitive advantage which won out over time, is only viable if the two groups are non-interfertile. I will return in a moment to the question of what kind of isolating mechanisms may be relevant.

    It has been shown that even a slight competitive advantage can result in the replacement of one population by another (Zubrow 1989). Indeed, taken in broad form this is the principle justifying much of adaptationist evolutionary theory. But this kind of competitive replacement takes tens (or hundreds) of generations. Zubrow (1989) concludes that a two percent advantage in terms of mortality rate would result in extinction after approximately 30 generations (1000 years). But, in order to compete, even indirectly (e.g., for scarce resources) two populations must coexist geographically. If genetic exchange can begin between the populations, the competitive advantage of one genetic group is soon removed. Zubrow assumes no migration, and thus no gene flow, between the groups even though he discusses them as subspecies.

    What kind of reproductive isolating mechanisms might be relevant? The first is simple biological infertility. Either no offspring could be produced or offspring were sterile. Another is hybrid maladaptation, i.e., offspring live and are fertile, but are less successful than either parent population. A third is behavioral isolating mechanisms, i.e., members of the other group are simply not attractive. The last is a cultural mechanism of isolation.

    Beyond this admittedly vague impression, a survey of the evidence concerning large free-ranging mammals suggests that simple biological infertility rarely develops between sibling species of these kinds of animals. Lions and tigers are fertile, as are zebras and horses, and coyotes and wolves.

    To look at one example, coyotes and wolves interbreed and produce fertile offspring where their ranges meet. However, the two species are adapted to very different environments; wolves to woodlands and coyotes to scrubland. Wolves are highly social while coyotes are, for the most part, solitary. Also, wolves are, on the average, twice the body weight of coyotes. The two species diverged evolutionarily more than 5 million years ago (Hall and Sharp 1985).

    Despite these differences (social behavior, morphology, body size, ecological adaptation, and time since evolutionary divergence) wolves and coyotes have not become biologically infertile. Hominids can certainly be credited with greater flexibility in sexual behavior than the canids, and thus deemed unlikely to have developed rigid behavioral barriers to mating. And if it is granted that large free-ranging mammals are unlikely to develop zygotic mechanisms of infertility, then arguing that a classic speciation event produced modern humans is a risky proposition.

    Any hybridization between replacing and replaced populations would introduce new mtDNA lineages. It would not matter how 'swamped' the gene populations being replaced were. Even if archaic morphological characteristics (governed by nuclear DNA) were rapidly selected out due to hybrid maladaptation, survival for two generations would be sufficient to introduce foreign mtDNA lineages into the replacing population. Such a state of affairs has been studied in two related mouse species in Denmark (Ferris et al. 1983; Wolpoff 1992:32) where a standing hybridization zone is quite narrow (as measured by nuclear DNA) but mtDNA variants of each species are widespread within the range of the other. Indeed, it is just this disjunction between nuclear and mitochondrial DNA which recommended its use for such studies.

    A contrary example using the Africanization of Brazilian honeybees has been offered by Cann (1992). Though hybridization between the invading African species and the native species occurs, non-African mtDNA lineages are rapidly becoming extinct. However, the breeding structure of bees relative to that of mammals suggests that they may be a poor example for comparison to late Pleistocene hominids, a point that even Cann admits (1992:69). With a single queen parenting all member of a given hive, and queen migration driving the Africanization process, the extinction of mtDNA, i.e., maternal, lineages is hardly surprising.


    Assessment

    All these criticisms could plausibly be swept aside if there were no other possibilities for explaining the genetic homogeneity of modern populations. However, Weiss and Maruyama's (1976) model demonstrated that sufficient intra*species gene flow could produce patterns of genetic distance identical to those produced by a putative replacement. And Avise (1984) has shown that stochastic mtDNA lineage loss could explain modern homogeneity without recourse to speciation or bottlenecks.

    Given the assessment above of the genetics of speciation in large free-ranging mammals, the evidence of genetic homogeneity in modern humans serves better as an argument that gene flow rates were simply as high or higher than proposed in the models than as an argument that modern humans were the product of a speciation event.

    The unfortunate situation for molecular geneticists in their search for reliable avenues of inference is that nDNA (nuclear) is subject to selection, drift, and recombination; and mtDNA studies, which sought to avoid the problem of recombination (Cann et al. 1987:31), suffer from the problem of stochastic lineage extinction precisely because they do not recombine. Beyond that, as far as absolute divergence dates, both methods suffer from extreme theoretical uncertainties whenever gene flow is greater than zero (Weiss and Maruyama 1976).

    I find it particularly interesting that some of the most damning criticism of the molecular evidence as relevant to a recent human replacement event has come from within the mathematical biology/molecular genetics region of the theoretical spectrum. Maruyama, Weiss, Templeton, and Cann are all well-trained mathematical/population biologists.


    Fossil Evidence for Regional and Temporal Continuity

    Although it might not seem that it should, fossil evidence can play two roles in the discussion of the various theories in this dispute. One of those roles is when the evidence is examined in negative terms, i.e., as critical of a given theory, and the other is when it is examined in positive terms, i.e., its consistency with and support of a given theory. At this point in the discussion, I will discuss the evidence only in terms of how it functions to cast doubt on replacement theory, since I believe it does so unambiguously. One should avoid implying, however, that it then necessarily supports any other particular theory, since I believe it does this far less unambiguously.

    Fossil evidence has always been claimed as its strongest suit by the opponents of a replacement theory. Indeed, as noted above, there were replacement theorists prior to the investigation of genetic evidence but they were solidly in the minority. Moreover, due to the drastic nature of the replacement theory, any evidence of continuity between archaic and anatomically modern humans in any region outside of Africa would lead to a fairly conclusive refutation of the replacement theory. Further, as described above, any admixture between putative replacing populations and native regional populations would invalidate much of the genetic evidence purported to support a replacement event. Clear evidence of admixture (in the form of continuity) would logically shift the evolutionary (and taxonomic) pattern from one of interactions between species to interactions within a species.


    Two primary issues arise in the examination of regional fossil evidence:

    The first is one of distinguishing between grade vs. clade characteristics.

    The second is the related problem of elaborating a valid morphological and metric definition of Homo sapiens. Both issues arise over the desire to establish unambiguous criteria by which decisions can be made regarding whether there is disjuncture between individual fossils or groups of fossils. Note that the obvious implication of all the detailed analysis going on here is that, on its face, there is no obvious, unambiguous evidence of disjuncture in recent human paleontology. Geographic and temporal morphological overlap is such that more detailed analysis is necessary to the argument.


    The issue of grade vs. clade characteristics is one which Wolpoff has reiterated in his rebuttals to critiques, but is equally important to replacement theorists. Replacement theory suggests that early moderns outside of Africa should, in some ways, resemble early modern Africans more than regional archaics. However, if these characteristics are simply those which define modern humans in general, i.e., grade characteristics, then how can the regional archaics be expected to possess them? Related to this is the issue of generally possessed primitive characteristics. The demonstration by a phyletic theorist that a given characteristic was possessed by both the archaics and moderns in a particular region means nothing if that characteristic is not derived, i.e., replacing African moderns would be expected to possess the feature as well.

    Clade characteristics are those which are unrelated, per se, to the transition from archaic to modern Homo sapiens. They presumably should be evolutionarily neutral or plausibly maintained by regional sexual selection. A high frequency of an Inca bone in East Asia would fall into the first category. East Asian facial features would plausibly fall into the second category-- they are not necessarily ecologically adaptive, but divergence from the established norm would presumably effect the attractiveness (sexual and otherwise) of the individual.

    Complicating the issue is the fact that depending on the region in question, Australasia, East Asia, Europe, or Africa, the same characteristic may play different roles, i.e., it may be a viable clade characteristic in one case, and a viable variant grade characteristic in another.

    Wolpoff asserts that "none of the earlier Australasian, Chinese, or European 'modern Homo sapiens' specimens possess features which uniquely resemble archaic or modern Africans. Nor are the earliest 'modern Homo sapiens' samples in aggregate more like Africans than their successors" (Wolpoff 1992:53; emphasis added). This assessment however, relies on complex statistical tests and some subjective assessments of the character states of particular specimens. Such assessments (from both ends of the theoretical spectrum) are vulnerable to non-random selection of specimens to include in a sample.

    The issue of morphological and metric definitions thus revolves around developing objective standards by which character states can be determined in a specimen. Such standards, if effective, should produce results which accord with biological reality. Developing such a definition of anatomically modern Homo sapiens has proven frustrating. The problem is that definitions which divide the paleontological evidence with some efficacy often leave portions of the modern human range of morphological variation outside Homo sapiens! Definitions which solidly encompass the modern range of human variation become nearly useless with respect to the paleontological evidence.

    Using a multivariate analysis of European and Near East fossil evidence compared to a large baseline modern craniometric sample, James Kidder, Richard Jantz, and Fred Smith (1992) found that only post-26kya fossils unambiguously fall within the modern range of variation. Prior to that point, all fossils exhibit various kinds of characteristics outside the modern range. This, they argue, illustrates some genetic input from Neanderthal/archaic populations and mosaic evolution/spread of modern morphology.

    Regardless of the assessment of the significance of the fact, it thus nevertheless remains that there is no unambiguous metric/morphological definition of Homo sapiens which consistently divides the fossil evidence in a way consistent with complete replacement. It would seem from a general review that East Asia, particularly Northeastern Asia provides the best evidence of regional continuity. Hexian, Dali and Jinniushan, all pre- a.m. Homo sapiens, each possess characteristics which link them to modern Chinese populations and which separate them from archaic and modern African populations (see Young 1995 for a review).

    Wolpoff has issued a fairly strong challenge over what he sees as a fairly cavalier attitude toward published scholarship by workers directly familiar with the fossil material. "The data do exist, and it seems to me that the details of regional continuity that have been researched and published by workers familiar with the fossil specimens are sufficient to demand more than unsupported assertions for their valid dismissal. Refutation with new data is the acceptable procedure for validly dismissing observations and their implications."

    Other regions have evidence of regional continuity in clade characteristics, i.e., uniquely derived, but this evidence is less unambiguous than in East Asia. Wolpoff (1989,1992) and Brauer (1992) provide good reviews of that evidence and display an impressive first-hand familiarity with much of the fossil material. However, any degree of uniquely derived characteristics which survive across the archaic Homo sapiens - anatomically modern Homo sapiens divide invalidates the replacement assertion that speciation produced modern humans. Since the replacement theory posits an African origin for modern humans, evidence of continuity there does not conflict with that theory. However, the characteristics of African fossil humans can be used to assess the likelihood of direct relationship between African morphology and early modern human morphology outside of Africa. Unfortunately, the African human paleontological record reflects a great deal of variability, and few clear geographical or temporal trends can be identified. Few uniquely modern African characteristics can be definitively seen in the pre-Holocene African fossil record.


    Species Recognition in Human Paleontology

    The cruciality of the interfertility question almost seems to have been deliberately overlooked. Part of this is certainly due to the fact that taxonomic classifications have been made only with regard to ease of reference, resulting in a non*standard classification system. No other animal in history has had modifiers, i.e., 'archaic' and 'anatomically modern', added to its Linnean name in the way that Homo sapiens has. This would not be such a problem if it weren't for the fact that all the controversy is over the origin of modern humans, not the archaic model. Regional evolutionary continuity between Homo erectus and local archaic Homo sapiens is not disputed by any researcher-- a seldom-noted fact with profound implications.

    Thus, the field is faced with arguing over the origins of what is nominally a subspecies, typically using evolutionary theory ill*equipped to handle the subject. Indeed, it has been noted that "Our knowledge of the fossil history of non*human subspecies is almost negligible and Homo will probably furnish the first well-studied case of the evolution of several subspecies in geological time"(Van Valen 1966:382).

    While there is a tacit understanding that the genus-species portion of Linnean classifications do not necessarily adhere to a biological species definition, there has been little subsequent effort to clarify biological relationships. Thus, replacement theorists are arguing that anatomically modern Homo sapiens specimens belong to a new species, but they do not address questions that then arise over what are now called archaic Homo sapiens , but which would then presumably be called 'advanced Homo erectus' . Nor do they address all the issues that arise with 'Homo erectus neanderthalensis'. This dilemma has been noted by Trinkhaus (1989) and Brauer (1984:346-9). Stringer et al.(1979) have suggested dividing Homo sapiens specimens into three grades, with Grade 1 being very archaic specimens (Bodo, Broken Hill, and Petralona), Grade 2 being early Neanderthals and Grade 3 being late Neanderthals and a.m. Homo sapiens. This scheme clearly states that it is the result of the application of morphological criteria only and so carries no necessary biological or evolutionary implications. This being the case, it could be argued that its worth is limited.

    Some scholars have actually argued that more biological species have existed than have been recognized in the Middle to Late Pleistocene hominid fossil record. Tattersall (1986), noting that many true species differ only in soft parts which would be paleontologically invisible, concludes that hominid specific diversity may have been greater than current assessments allow. He also notes the taxonomic dishonesty inherent in having to make a distinction between "archaic" and "anatomically modern" types of (putatively) one species. However, recognizing the morphological distance between archaic and modern humans does not resolve the issue of the nature of the transition between them.

    The question of whether modern Homo sapiens was the product of a true, localized speciation event remains to be definitively resolved. Any fair-minded scholar on either side of the issue, even the partisans Stringer and Wolpoff, would admit that the evidence, both genetic and paleontological, is ambiguous to some degree. Thus, the issue cannot be finally and definitively decided based on the evidence currently available. However, the implications of a theory of complete replacement without admixture are so profound and disturbing that I believe prudence urges that until definitive evidence can be pointed to, a less extreme preliminary theory should prevail-- whether it emphasizes continuity or disjunction. Beyond that, a premature acceptance of the replacement theory would mistakenly turn all non-African fossil hominids prior to 40 kya into irrelevant curiosities. Again, scientific prudence would urge against premature exclusion of this fossil evidence as well as the archaeological evidence associated with it.


    Source:
    http://www.sinc.sunysb.edu/Stu/jhov...section1a-2.htm
    http://www.sinc.sunysb.edu/Stu/jhover/thesis/toc.htm
    http://www.sinc.sunysb.edu/Stu/jhover/index.html

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    Imo these questions can only be answered through DNA-analysis, not paleo-onthology or some deductions. I personally think that it would be highly unlikely that so closely related species as late hominids (like 1000,000 B.C. and forth) would be infertile, since even much more distant species actually are, like abovementioned wolves and coyotes, or, for example, big cats (tigers, lions, leopard all can procreate). I would go as far as saying that human and chimp would probably be fertile, if not for rare mutation that changed humans\' number of chromosomes, as compared to chimps\'.

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    Excellent thread! To add further, plants can procreate even with vastly different chromosomal count last I checked...

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    Great post. The really thoughtful prehistorians do try and consider all those things, but often get bogged down when explaining to a broader audience.

    Your post, thought, is scholarly and readable at the same time and explains so many of the variables so coherently.

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