In the Light of Evolution Volume IV: The Human Condition (2010) / Chapter Skim
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14 A Role for Relaxed Selection in the Evolution of the Language Capacity--Terrence W. Deacon
14 A Role for Relaxed Selection in the Evolution of the Language Capacity--Terrence W. Deacon
Pages 275-292
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From page 275... ...
These may play important roles in the many levels of evolutionary pro cess contributing to language. surprisingly, the relaxation of selection at the organism level may have been a source of many complex synergistic features of the human language capacity, and may help explain why so much language information is "inherited" socially.
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From page 276... ...
short of appealing to divine intervention or miraculous accident, we must look to some variant of natural selection to explain it. By paying attention to the way Darwin's concept of natural selection can be generalized to other systems, and how variants on this process operate at different interdependent levels of organism function, explaining the complexity of language and the language adaptation can be made more tractable.
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From page 277... ...
Thus, such highly complex functional capacities as human cognition and language would intuitively seem to be the least evolvable of life's products. indeed, so-called "intelligent design" critiques of Darwinism have focused on far simpler molecular and cellular mechanisms to make their argu ment that the complexity of organism design is not evolvable, indirectly implying that our vastly more complex cognitive abilities are all the more beyond the explanatory power of natural selection theory.
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From page 278... ...
of course, nonadaptive traits, functional compromises, and inefficiency are also common to other biological adaptations. so this does not in itself disqualify the human language faculty as a biological adapta tion honed by natural selection.
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From page 279... ...
. Today, sexual selection theory is again considered an important adjunct to the theory of natural selection; however, its reinstatement has not resus citated the power of Darwin's account of language origins.1 even though Wallace's critique of sexual selection has been answered and its power to explain the evolution of certain exaggerated traits is now recognized, there 1 There are, nevertheless, contemporary theorists who have offered variants on Darwin's proposed sexual selection account of language origins [see, e.g., G
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From page 280... ...
This is because sexual selection inevitably produces complementary divergence of male and female traits, as is exemplified by peacock tails and moose antlers, which are exhibited only by males. Though there are indeed a few highly divergent traits distinguishing women from men (e.g., patterns of fat deposition in breasts and hips, etc.)
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From page 281... ...
. 2 This was a variant of natural selection theory that theoretically might lead to pseudo-lamarckian effects, such that the functional utility of a specific acquired habit of behavior (e.g., a language behavior)
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From page 282... ...
Therefore, although it seems beyond doubt that the human language capacity must have evolved due to extensive selection affecting multiple levels of adaptive mechanisms, both the form of the variant of natu ral selection that was involved and the nature of the cognitive capacity that it produced remain topics of intense debate in evolutionary biology. Whatever account is given, however, it must explain the evolution of the complex interdependence of the neurological, behavioral, and social transmission features of language.
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From page 283... ...
This process appears to be fairly species-general, with many mechanisms shared by a wide range of vertebrates. Although slight tweaks of this species-general brain architecture likely play important roles in producing the structural and functional differences of different species' brains, a significant contribution also comes from selection-like processes that incorporate both intra- and extraorganismic information into the fine-tuning of neural circuitry.
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From page 284... ...
These mechanisms are almost certainly relevant to human brain evolution for language, especially considering that language is such a significant contributor to early experience. This neuroepigenetic variant of selection logic is only one among many processes that might more generally be described as intraevolutionary mechanisms -- that is, intraorganismic morphogenetic processes that parallel attributes characteristic of phylogenetic evolution.
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From page 285... ...
in this process too, duplication allows variants to evolve, but largely because the presence of a redundant copy can relax selection that otherwise would tend to eliminate variant forms with mutations that alter critical functions. Where a redundant copy is not itself a source of maladaptation, single nucleotide substitutions and other noncatastrophic modifications to its sequence tend to progressively and incrementally degrade the functions of its protein product over evolutionary time.
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From page 286... ...
But because a random walk produces incremental deviation, there is a significant non zero probability that one or more of the increasingly variant forms within a population of organisms will "wander" into a related interaction rela tionship with some duplicate counterpart, and again become subject to selection for any interactive deleterious or synergistic effects. it is no surprise, then, that gene families descended from a common ancestral gene often form synergistic functional complexes.
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From page 287... ...
regular dietary substitution of ascorbic acid from fruit relaxed selection that would otherwise have regularly eliminated mutational variants with reduced ascorbic acid synthesis. relaxation of this stabilizing selection allowed functional degra
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From page 288... ...
What was once selection focused on a single gene locus became fractionally distributed across a great many loci instead. one striking and plausible correlate is the evolution of three-pigment color vision in anthropoid primates, which coincidentally also involves gene duplication effects, the first of which appears to have occurred just before the divergence of old and new World primates (shyue et al., 1995; nei et al., 1997)
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From page 289... ...
Because sensory and motor biases can be significantly affected by experience, song structure could also become increasingly subject to auditory experience and the influence of social stimuli. in this way, additional neural circuit involvement and the increased importance of social transmission in the determination of song structure can be reflections of functional dedifferentiation, and yet can also be sources of serendipitous synergistic effects as well.
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From page 290... ...
They are almost certainly crucial to the evolvability of highly complex synergistic adaptations, such as human language. recognition of the potential contributions of each of these processes to evolvability should warn against monolithic natural selection accounts of language evolution that ignore the contribu tions of these interlinked levels of selection and drift processes.
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From page 291... ...
not only will this process be subject to selection with respect to semiotic and pragmatic demands of symbolic communication, it will also favor structures that are more easily acquired by immature brains undergoing activity-dependent intraselection of neural circuitry. indeed, just as evolvability is aided by evolution-like processes involved in ontogenesis, we should expect that the social evolution of language should itself exhibit analogous processes due to redundancy, degeneracy, and functional interdependency.
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From page 292... ...
/ Terrence W Deacon acquisition process, the deep integration of language and human cogni tion, and the involvement and synergistic interaction of widespread and diverse brain systems in language processes together imply that there has been long-term adaptation involving a very broad suite of genetic loci and the involvement of many levels of intraevolutionary mechanisms.
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