philosophy

Recent models of niche construction contain channels of ecological inheritance that covary with channels of genetic inheritance. Some critics contend that the ecological inheritance channel lacks the necessary requirements for an evolutionary system and ought not be represented as an inheritance channel in niche construction models. Specifically they point out that ecological traits are often vertically diffuse. Such traits do not then vary consistently between genetic lineages over evolutionary time. Thus natural selection cannot act on the ecological/genetic combination. I provide three responses to the ecological inheritance skeptic. In the last and most important of these responses I argue that ecological inheritance is most consistent at the level of the population and has an important role in maintaining and explaining differences between groups. This level of analysis is of particular relevance to anthropologists given the population focus of much anthropological research.

Niche construction is a theoretical descendent of Lewontin’s (1985) earlier criticism of the one-sided causal assumption made by biologists in many of their models. Biologists had often worked as if the organism was passively molded by natural selection to its environment. They ignored the possibility that the organism could significantly alter their own environment and that these alterations could feedback to affect the evolutionary trajectory of the organism. Lewontin proposed that we describe the evolution of the organism (O) and environment (E) with a pair of linked differential equations:

(Levins and Lewontin 1985:105) where change in the environment over time is a function of the past environment and the previous organisms. Niche construction then expands upon this insight by providing an explicit model of feedback between inheritance channels in order to identify the effects of organism-constructed environments. (Laland et al. 2001:117)

The addition of an ecological inheritance channel is one of the novel contributions of niche construction theory. It is part of a recent trend to incorporate the dynamic aspects of the traditional organism/environment dichotomy into evolutionary theory (Dawkins 1982, Griffiths and Gray 1994). Unlike the previous authors, Laland et al. (1999, 2001) allow the environment to occupy a separate theoretical channel in models; this is in contrast to extending the domain of genetic effects as in The Extended Phenotype (Dawkins 1982) or clumping the environment and the organism into a single package called the “life cycle” in developmental systems theory, DST (Griffiths and Gray 1994, 2001:196). Niche construction then positions itself as an intermediate between holist theories and naïve environment/organism dichotomies; this position allows us to more feasibly model the interaction between the two (escaping a criticism often directed at DST, which includes so many causal interactions that the model becomes unwieldy (Godfrey-Smith 2001:283)).

Ecological inheritance can be thought of as the inheritance of environmental features (e.g. birds’ nests) from a niche constructing progenitor to its descendants. However, not just any feature of the environment can be inherited; the models of Laland et al. (2001:119) restrict ecological inheritance to those heritable traits which are the result of niche construction activities (e.g. ecological engineering of burrows) from either a genetic or ecological ancestor. An ecological ancestor is one who modified the past environment and such modifications exist in the present environment of the descendant (who need not be genetically related.) For example, the past actions of earthworm soil processing can effect the current environment of azaleas. Such niche constructing activities gain particular importance when they change selection pressures between generations. In fact the ecological inheritance channel is regarded as a channel of inherited selection pressures. (Laland et al. 2000:133, 2001:121) Laland et al. (2000:136-137) also distinguish between ecological evolution and cultural evolution (often regarded as a special subset of the environment, see Durham 1991:160 for discussion.) Cultural evolution then is given its own inheritance channel as an acknowledgement of the significantly different dynamics which categorize cultural inheritance.

In this paper we will be concerned with the question: does ecology constitute a legitimate and effective inheritance channel? Before addressing issues of effectiveness, we must first show that ecology can in fact qualify as an inheritance channel. Unlike the genetic or cultural channel, the ecological channel does not consist of (somewhat) discrete, replicating units like genes or memes. That is, the ecological channel is transformational rather than variational. The ecology is conceptualized as a pervasive entity experiencing ontogenic changes rather than as an ensemble of disappearing and reappearing units. Without the replication of modules, the ecological channel lacks generations and without generations there obviously cannot be inheritance from one F₀ to the next F₁. Thus there is an important asymmetry then between the ecological channel and the genetic or cultural channels. But once the ecological channel is coupled with the genetic channel (which contains replicating entities) the ecological traits can then be inherited between entities (genes or organisms). So the ecological channel may be an inheritance system but it is not by itself a replication system. Sterelny (2001:339) equates an inheritance system and a replication system, which is one reason ecological engineering does not qualify as inheritance on his view.

Criticism and Responses

Sterelny (2002:344-345) has argued that the ecological inheritance channel proposed in niche construction models does not constitute an actual inheritance system. His strongest argument runs as thus: we need not bother representing ecology as an inheritance channel in evolutionary models because transmission can be both horizontal (between members of the same generation) and vertically diffuse. That is, the niche constructing activity of one organism (F₀) will affect its own progeny (F₁), but it will also affect the other organisms in the population. (ibid.) This argument can be translated into genetic language. Specific ecological trait variation is not strictly inherited; this reduces variation between genetic lineages with regard to their ecological traits. As a result, without consistent ecological variation, natural selection will operate on only on genes, not gene/ecology complexes.

One way to combat this criticism is to point out the natural selection describes population dynamics. “So because ecological inheritance has to do with bequeathing selection pressures [population level processes] to progeny, it is awkward to think of it as a relation between individual organisms and their parents.” (Weisberg unpublished:20) By this argument we simply avoid speaking of inheritance between particular individuals and instead describe ecological inheritance at the population level, which is perfectly admissible given that the niche construction models are population genetic models. (Laland et al. 2000, 2001)

A second reply to the critics might emphasize the legitimacy of the ecological inheritance channel by focusing on the cases of ecological inheritance which are specific to genetic lineages (not diffuse.) The critics acknowledge the validity of such cases but think they are infrequent. (Sterelny 2001:344) However particular aspects of the ecology may well be vertically inherited in many different types of populations and have a significant impact on fitness. For example, population wave of advance models place organisms in the geographic center or edge of an expansion. The location then determines (all other factors being equal) the relative fitness and geographic extent of the organism’s progeny over successive generations. The long-term effects of location only impact fitness if the location (e.g. population center or edge) is inherited by progeny for some period of time. Linking location to specific genetic lineages allows us to predict the differential success of particular genetic lineages (Lillie 2002). Ecological inheritance of location then becomes extremely useful for spatial autocorrelation models. (Wade 1996:382-383)

Despite the apparent usefulness of strict ecological inheritance in specific genetic lineages, I agree with Sterelny that diffuse inheritance may be the norm for many niche constructing characteristics; so a third response begins by biting the bullet. The effects of niche construction may often diffuse between vertical, genetic lineages thus homogenizing ecological variation within the population (e.g. earthworms mixing organic and inorganic soil and indiscriminately effecting each other’s lineages). However, homogenizing within-group variation can increase between-group variation.

This can be shown by describing an analogy with F_ST, a statistic used to summarize the genetic difference between subdivided portions of a population. More formally, it is a measure of the difference between the probability that two alleles from within a deme have a particular, identical genetic state compared to the probability that two alleles drawn at random from the metapopulation (which is composed of demes) have the identical state. (Gillespie 1998:98) In this case, F_ST will measure the ecological difference between two populations, substituting organisms for alleles and using ecological states rather than genetic states. Assume, for simplicity, we have a single ecological property with only two states (e.g. forest, savannah). By holding the total amount of ecological variance constant, and decreasing the variance within groups (i.e. homogenizing the populations), we increase the variance between groups:

(where T is total ecological variance, b is between group variance and w is within group variance.) Correspondingly, as s_b² increases, F_ST (the difference between groups) also increases. (Wade 1996:385)

Ecological inheritance then becomes a channel by which differences between groups can be established and heritably maintained. Natural selection then acts on the combination of genetic populations and their consistently inherited ecological traits. This allows us to make predictions about which groups will proliferate and in what circumstances. Such a response has great implications for the application of the niche construction model.

We would then expect selection pressures on niche constructing populations to be most effective in the context of isolated populations where the ecological inheritance channel is the most reliable. Or in other words, there must be low rates of migration between demes otherwise ecological traits will not be inherited vertically among genetic populations. (By most effective I mean, able to maintain population traits for some substantial length of time.) Explanations for differential reproduction of various groups will reference both the properties of organisms in the populations and the ecological contexts under which these population flourish.

I see this argument for the relevance of the ecological inheritance channel as particularly applicable to anthropology given the tendency for anthropological analysis to use the unit of groups.[1] Let us then move on to an anthropological example of ecological inheritance . While individuals on an island may not experience deep intra-population environmental variation which is maintained over several generations, comparing populations between islands can indeed maintain consistent environmental effects which vary between the localities over several generations (assuming low rates of migration). Take, for example, the Maori and Moriori of New Zealand. Both groups are descendants of Polynesian immigrants who boated to mainland New Zealand and the Chatham islands. This qualifies as “choice” niche construction. (Laland et al. 2000:132) The Maori of northern New Zealand reliably inherited warm, tropical ecology amenable to crop growing, dense populations etc. The Moriori, however, living on the Chatham Islands inherited cold, resource-limited ecology from population generation to population generation. These different island ecologies were instrumental in the varying population densities on different islands (see Diamond 1999:53-54 for other effects). Here the inheritance of particular selection pressures (or the relaxation of these pressures) via different island ecologies affected the genetic fitness of those inhabitants and perhaps the evolution of different cultural traits between the two populations.

Conclusion

Critics of the ecological inheritance channel have argued that natural selection could not act on ecological traits because they diffuse between non-related genetic lineages during inheritance. I suggested three responses to this criticism. The first is to point out that natural selection works on populations of individuals, so the inheritance of selection pressures is a population process and not one we would expect to be inherited from an individual to its offspring. Ecological inheritance should then best be thought of as a population level process. Second, I show that sometimes ecological inheritance is not diffuse, but is instead specific to genetic lineages and furthermore can affect genetic fitness (e.g. wave of advance). Third, I argued more strongly for classifying ecological inheritance as a population level process with significant consequences for increasing between-group differentiation. We then would expect selection pressures inherited from population to population. For example, ecological traits that have some reliably important effects on evolution might be: spatial location, environmental stability (Hewlett et al. 2002:315), and artifact construction such as dams, blockades, etc. The inheritance of the traits and their associated selection pressures affect the fitness of genetic populations. We should therefore continue to include and characterize ecology as an inheritance channel in niche construction models.

Gillespie J. 1998. Population Genetics, A Concise Guide. Baltimore: John Hopkins

Griffiths PE and Gray RD. 1994. Developmental systems and evolutionary explanation.

Griffiths PE and Gray RD. 2001. Darwinism and developmental systems. In Cycles of

Contingency, ed. S Oyama, PE Griffiths, RD Gray, pp.195-218. Cambridge MA: MIT Press

systems theory. In Cycles of Contingency, ed. S Oyama, PE Griffiths, RD Gray, pp.283-298. Cambridge MA: MIT Press

inheritance, and cycles of contingency in evolution. In Cycles of Contingency, ed.

replicator. In Cycles of Contingency, ed. S Oyama, PE Griffiths, RD Gray, pp. 333-350. Cambridge MA: MIT Press

Wade MJ. 1996. Adaptation in subdivided populations: kin selection and interdemic

selection. In Adaptation, ed. MR Rose, GV Lauder, pp. 381-406. London: Academic Press

[1] There may be some redundancy between the artifacts and products of ecological niche construction and cultural niche construction, but it is important to note that the cultural construction affects the ecological inheritance of selection pressures. (Laland et al. 2000:136)