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Extrinsic inheritance

Back in August at the annual ESPP conference, Tom and Ben presented a paper arguing against the existence of non-genetic forms of evolution – environmental, cultural, mimetic – running alongside and independently of genetic evolution. I’m not sure I followed everything they had to say (there was a lot about information theory, whose relevance I missed). But one key element was that non-genetic inheritance, in so far it could be said to exist at all, is parasitic upon genetic inheritance – a consequence of sensitivities and dispositions that have been genetically coded for. Extrinsic inheritance (involving mechanisms external to the organism) depends on features intrinsic to the organism, and these latter are ultimately determined by genetic processes. So there is no separate source of design here, just some fancy consequences of intrinsic gene-driven design.

I’ve a question. Suppose extrinsic inheritance does depend on intrinsic features in this way. Why is this incompatible with it being a source of design itself? Why can’t we think in terms of levels of design? (After all, genetic processes themselves depend on chemical and ultimately physical ones.) Why can’t we accept that there are further levels of design that are ultimately the product of genetic processes but which are not salient at the genetic level? That is, why can’t we think in terms of Dennettian stances – a genetic design stance and (say) a higher-level mimetic design stance? Do the people Tom and Ben are attacking want to maintain something stronger than this? Or is this already too strong for Tom and Ben? I suspect the nub of their argument may lie elsewhere, with the considerations about information theory; but, if so, I need some help with that.

epigenetics

The difficulty with Lamarck is complex. He had two rules: 1. adaptive change is the result of use and disuse during an individual’s lifetime and 2. with the rather bizarre requirement that both parents show the same changes, these changes are inherited by the next generation.

Most evidence-based objections to Lamarck are focussed on the second rule. And Weissman’s classic experiment was to cut off mouse tails generation after generation. The tail docking never seemed to effect tail growth in subsequent generations as might be predicted from 2. But there is evidence for inheritance of acquired characters (I am trying to write a (categorising) review of this evidence at the mo) and this shouldn’t be too shocking. The first reason why is that there are plenty of taxa which don’t segregate their germline from their somatic cells (e.g. plants) and which therefore could spawn the next generation directly from cells that have undergone modifications in response to the environment. The second reason not to be surprised is that, as Tom suggests, natural selection might be expected to deliver this kind of flexibility. Why is that?

The real reason Lamarckism is wrong, I think, at least as a stand-alone explanation for adaptive evolutionary change, is that it requires some discrimination mechanism. In the Darwinian mode, natural selection does the discriminating between randomly occurring variants, but in the Lamarckian mode, what is it that allows the inheritance of the adaptive, but not the maladaptive or degenerative responses to the environment? (The latter category you might expect to dominate and indeed much of the evidence in human studies seems to be about disease.) The neo-Darwinian answer is that natural selection is the only discriminator. So there is no need to be nasty about Lamarckian mechanisms, and in some cases they may do a great deal of useful tinkering (which is all that can be said for selection too), but they are additionally constrained as described.

Epigenetics

Keith flagged up the issue of epigenetic inheritance and rightly pointed to the core issue of environmental triggers setting genetic expression and then this expression being heritable. Some have argued that this is a case of environmental inheritance, and therefore a second source of design running, at least, alongside genetic inheritance as understood in contemporary neo-Darwinian terms. [For a recent, and extensive treatment of this I recommend a look at Jablonka, E. and Lamb, M. (2005) Evolution in Four Dimensions. MIT Press.]

This is not the way to think about it. The genes that allow for this order of switching – such that different phenotypic outcomes are produced – are responding to key environmental stimuli, that they have been selected to respond to. This is to be seen as a conditional rule – like any other evolved system. Any system that can be so calibrated, and have its effects inherited across generations is clearly a system designed to respond to specific and medium- to long-term environmental contingencies. So, natural selection still is the primary source of design here, as ever.

More tellingly, those who argue that this presents a second source of design are pulling a form of rhetorical trick because really all they have to say is that there is a role for the environment – and everyone knows that genes need externally derived inputs. It is as if this kind of phenotype is being privileged over others in order to force a point about extrinsic inheritance.

In short, these inputs have no role without the genes. The genes are prepared to operate over these inputs in specific and finite ways. Therefore, the genes set the agenda and our best theory of this agenda setting is neo-Darwinism, with which all of this work is completely compatible. Extrinsic inheritance has nothing to do with evolution.

Epigenetics

Last night’s BBC2 Horizon programme (slightly less reliant than usual on flashing lights and portentous narrative) was on epigenetics – a subject of which I knew nothing. The message was startling: work by Marcus Pembrey, Professor of Clinical Genetics at the Institute of Child Health in London, and Swedish researcher Lars Olov Bygren suggests that environmental influences can switch genes on and off in an individual’s sperm or egg cells, possibly with profound effects on the health of their offspring. Moreover (and this is the really amazing bit) these effects will be inherited by their offspring. If a gene is switched off in one generation, it remains switched off in subsequent generations. In short, environmental influences have effects that are inherited. That sounds like Lamarkianism to me, and a serious challenge to neo-Darwinian orthodoxy. Is it?