Answering Wallace's challenge: Relaxed Selection and Language Evolution

ResearchBlogging.orgHow does natural selection account for language? Darwin wrestled with it, Chomsky sidestepped it, and Pinker claimed to solve it. Discerning the evolution of language is therefore a much sought endeavour, with a vast number of explanations emerging that offer a plethora of choice, but little in the way of consensus. This is hardly new, and at times has seemed completely frivolous and trivial. So much so that in the 19th Century, the Royal Linguistic Society in London actually went as far as to ban any discussion and debate on the origins of language. Put simply: we don’t really know that much. Often quoted in these debates is Alfred Russell Wallace, who, in a letter to Darwin, argued that: “natural selection could only have endowed the savage with a brain a little superior to that of an ape whereas he possesses one very little inferior to that of an average member of our learned society”.

This is obviously relevant for those of us studying language evolution. If, as Wallace challenged, natural selection (and more broadly, evolution) is unable to account for our mental capacities and behavioural capabilities, then what is the source behind our propensity for language? Well, I think we’ve come far enough to rule out the spiritual explanations of Wallace (although it still persists on some corners of the web), and whilst I agree that biological natural selection alone is not sufficient to explain language, we can certainly place it in an evolutionary framework.

Such is the position of Prof Terrence Deacon, who, in his current paper for PNAS, eloquently argues for a role for relaxed selection in the evolution of the language capacity. He’s been making these noises for a while now, as I previously mentioned here, with him also recognising evolutionary-similar processes in development. However, with the publication of this paper I think it’s about time I disseminated his current ideas in more detail, which, in my humble opinion, offers a more nuanced position than the strict modular adaptationism previously championed by Pinker et al (I say previously, because Pinker also has a paper in this issue, and I’m going to read it before making any claims about his current position on the matter).

At its core, Deacon’s proposal is that the relaxation of selection pressures allows genetic control to be offloaded onto epigenetic processes. This in turn allows for a greater influence of social transmission due to development being open to experiential modification. Our capacity for language, then, is a story of how developmental and evolutionary dynamics interact. As a recent post over at Babel’s Dawn notes, this is basically a three-phase scenario:

  1. Standard primate brain in which midbrain areas (older parts of the brain) control vocal emotional communications.
  2. A duplication of a section of the genome leads to “relaxed selection” and extensive cross talk between many cerebral cortical systems (newer parts of the brain).
  3. “Unmasked selection” fixes new functional coordination and drives the brain’s anatomical reorganization.

The first aspect we need to appreciate is how Darwinian-like processes operate at the developmental-level. Deacon cites many instances, such as the fine-tuning of axonal connection patterns in the developing nervous system, where developmental processes are achieved through selection-like operations. Importantly, though, the logic differs from natural selection in one respect: “selection of this sort is confined to differential preservation only, not differential reproduction. In this respect, it is like one generation of the operation of natural selection”.The point he’s trying to get across is that these intraselection processes are taking place right across nature. Take, for instance, the genus Spalax (the blind mole rat): during development its thalamic visual nucleus is dominated by brainstem auditory and somatic projections. This is because the blind mole rat has vestigial eyes (hence the name), with projections from their small retinas being out-competed in favour of somatic and auditory functions. As Deacon notes:

Experimental manipulations in other species, in which projections from one sensory modality are reduced in early development, likewise exhibit analogous takeover effects, and manipulations of the sensory periphery likewise demonstrate that intraselection adapts neural functional topography with respect to functional experience.

Such developmental flexibility is crucial in that it provides a general mechanism for natural selection to recruit in brain evolution. And as such, it is almost certainly relevant to the evolution of the human brain in relation to language. From this evodevo perspective, Deacon goes onto highlight how Darwinian processes characterising natural selection (replication, variation, and differential fitness) have analogous counterparts in intraorganismic processes (redundancy, degeneracy, and functional interdependencies):

First, they involve processes that produce functional integration and/or adaptation even though they are generated by mechanisms that are dissociated from this consequence. Second, they all involve the generation of redundant variant replicas of some prior form (gene, cell, connection, antibody, etc.) brought into interaction with each other and with an external context in a way that allows these differences to affect their subsequent distribution. And third, their preservation and expression are dependent on correlation with context.

According to Deacon, these parallels with evolutionary processes are generally distinguished through the level at which selection operates — and how these interactions generate functional redundancy. Specifically, he looks at three types of redundancy: (i) internal redundancy; (ii) external redundancy; and, (iii) global external redundancy.

Types of Redundancy

In internal redundancy, duplicated genes enable relaxed selection. It’s an odd inversion natural selection, where instead of the competitive elimination of gene variants, evolution favours preservation. Under this scenario, the original gene will continue its functional role, whilst the duplicate gene is allowed to accrue mutations, subsequently increasing the possibility of deleterious interactions and exposing synergistic possibilities. Selection then either removes these deleterious interactions or takes advantage of the these new synergistic interactions. Furthermore, this is happening both within and between organisms, as Deacon explains:

For example, the duplication and differentiation of regulatory genes, such as the well-studied homeobox containing genes that control segmental organization in insects and vertebrates via their regulation of the expression of a diverse range of other genes, enables duplication-degradation-complementation at the phenotypic level. The generation of structural redundancy of body parts (e.g., limbs) via segmental duplication similarly relaxes selection on some with respect to others. Again, this increases the probability that random walk degradation will expose synergistic possibilities (e.g., locomotor function) that will become subject to selective stabilization in their own right.

Next is external redundancy which is the product of functional duplication, rather than gene duplication. Here, an extrinsic source provides the organism with a function previously supplied by a particular gene. Although these forms of duplication are analogous to one another in influence, they can produce significantly different consequences. Take, for instance, the loss of endogenous vitamin C synthesis in some lineages. Instead of internally synthesising vitamin C, humans, and some other species, must frequently acquire vitamin C from external sources. But as Deacon notes, “the human genome includes a pseudogene for the final enzyme in the ascorbic acid synthesis pathway: l-gulono-gamma lactone oxidase (GULO)”. Thanks to external sources of vitamin C, the human GULO gene underwent functional degradation and, if you think about it, shifted selection pressures onto a form of dietary addiction:

Because this essential nutrient was only available extrinsically, selection to maintain its antioxidant function shifted to any sensory biases, behavioral tendencies, and digestive-metabolic mechanisms that increased the probability of obtaining it. What was once selection focused on a single gene locus became fractionally distributed across a great many loci instead.

His last type, global external redundancy, is basically when the relaxation of selection produces global dedifferentiation effects. An example of this is when a species is not under strict reproductive and survival limitations, as seen in domestication. One example Deacon focuses on is the White-Backed Munia: over the course of approximately 250-years, Japanese breeders bred the Munia for colouration, and eventually came up with Bengalese Finch. What’s relevant about these domesticated hybrids is in their ability to acquire songs via social learning. Conversely, the White-Backed Munia does not acquire its song via social learning; rather, it’s genetically inherited. This leads to differences in the rigidity of the songs — the Bengalese Finch having far more variability within and between individuals than their wild cousins.

Oddly enough, it appears that by inhibiting the stabilising effects of natural and sexual selection for birdsong, the Bengalese Finch actually underwent behavioural complexity. This is clear in how the socially-acquired songs require far more forebrain nuclei and their interconnections, than the innately pre-specified song of the White-Backed Munia. As Deacon explains:

As constraints on song generation degraded with prolonged domestication, other neural systems that previously were too weak to have an influence on song structure could now have an effect. These include systems involved in motor learning, conditionally modifiable behaviors, and auditory learning. 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.

By appealing to the dedifferentiation and redistribution effects of relaxed selection, Deacon argues for a tendency to shift from an innate, localised function onto a more distributed array of systems. Of course, Birdsong is far simpler than human language, and we’d be careful to avoid drawing too many conclusions. But there are, however, some interesting parallels between the differentiating aspects of human language and primate vocal communication and the Finch/Munia example:

These include (i) a significant decrease in the specific arousal-coupling of vocal behaviors, (ii) minimization of constraint on the ordering and combination of vocal sounds, (iii) reduction, simplification of the innate call repertoire, (iv) subordination of innate call features to a secondary role in emotional tone expression via speech prosody, (v) a significantly increased role of auditory learning via social transmission, (vi) widely distributed synergistic forebrain control of language compared with highly localized subcortical control of innate vocalizations, and, of course, (vii) an increased social-cognitive regulation of the function of vocal communication.

All this basically leads to the following question: Are humans a self-domesticated species? If so, genetic dedifferentiation of the nervous system may not only have led to the functional complexity in human language, but also contributed to more widespread degeneration; influencing our suite of seemingly unique cognitive, social and emotional abilities. Deacon’s central point is that these processes are not exclusively the products of natural and sexual selection. Instead, we must also appreciate the many levels of inter-linked dynamics at play, including phylogeny and ontogeny. It is also the case that language itself exhibits an evolutionary dynamic. This additional twist allows language to evolve and adapt irrespective of human biological evolution, which, as I mentioned here, can account for the arbitrary features of language, such as X-bar theory and case marking, without appealing to a domain-specific language module.

This is somewhat of a return to Deacon’s older argument: a coevolutionary scenario between language evolution and biological evolution. So that whilst our brains have undergone adaptation to the special demands of language processing, languages themselves are also utilising similar evolutionary mechanisms to favour advantageous variants. We should therefore see language adapting to constraints like learnability: where languages become increasingly learnable for their speaker-hearer community. Importantly, language is generally evolving faster than biology:

This means that brain functions selected for the special cognitive, perception, and production demands of language will reflect only the most persistent and invariant demands of this highly variable linguistic niche. This is another reason to expect that the synergistic constellation of human brain adaptations to language will not include specific grammatical content, and to suspect that much of the rich functional organization of any language is subject to influences on this extragenomic form of evolution. In other words, the differential reproduction of language structures through history will be dependent on the fidelity and fecundity of their transmission.

We’re only dealing with the basic framework here, and not any systematic treatment for how language evolved. What’s now needed is to incorporate findings, and test specific hypotheses, relating to these three basic evolutionary systems: phylogeny, ontogeny and glossogeny (picture taken from Kirby & Hurford, 2001).

Citation: Deacon, T. (2010). Colloquium Paper: A role for relaxed selection in the evolution of the language capacity Proceedings of the National Academy of Sciences, 107 (Supplement_2), 9000-9006 DOI: 10.1073/pnas.0914624107

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