Dickins, T.E. & Rahman, Q. (2012.) The Extended Evolutionary Synthesis and the role of soft inheritance in evolution. Proceedings of the Royal Society, B. doi: 10.1098/rspb.2012.0273
Here is the abstract:
In recent years, a number of researchers have advocated extending the modern synthesis in evolutionary biology. One of the core arguments made in favour of an extension comes from work on soft inheritance systems, including transgenerational epigenetic effects, cultural transmission and niche construction. In this study, we outline this claim and then take issue with it. We argue that the focus on soft inheritance has led to a conflation of proximate and ultimate causation, which has in turn obscured key questions about biological organization and calibration across the life span to maximize average lifetime inclusive fitness. We illustrate this by presenting hypotheses that we believe incorporate the core phenomena of soft inheritance and will aid in understanding them.
The paper is regarded by some in behavioural biology as controversial. Indeed, when I gave a version of it at the European Human Behaviour and Evolution Association in March this year the audience were sharply divided, and vocally so. The paper itself has caused a little chatter on various blogs, and a response to it has just been submitted by some of the characters we take issue with during the course of our argument.
So, what's all the fuss about?
The central theoretical structure of evolutionary biology is the modern synthesis which was a synthesis between Darwinism and Mendelism. It was not until the arrival of population genetics in the 1930s that the biological community realised that Mendelian models of inheritance and Darwinian models of evolutionary change through natural selection were both compatible and also different levels of explanation. Once this was apparent evolutionary biology influenced all of the subdisciplines of biology over a thirty or so year period, however, the fundamental statement of the synthesis was made in 1942 by Julian Huxley, as we explain in the paper.
In 1961 Ernst Mayr, one of the architects of the synthesis, further clarified thinking in evolutionary biology with his classic paper Cause and Effect in Biology. In this paper he drew the distinction between proximate accounts and ultimate accounts. The former are accounts of how a particular mechanism works, the latter of why it works in the way it does. His example was bird migration, where we can say that a bird migrates because of neurohormonal changes in its brain induced by temperature and daylight changes - which is proximate causation - and its brain is designed in this way, by natural selection, in order to enable the bird to track changing food resources and mating opportunities - which is ultimate causation.
Ultimate accounts most often invoke natural selection and the concept of inclusive fitness, but they can also be based on models of genetic drift, founder effects and so forth.
This distinction has been central to evolutionary biology, and behavioural biology ever since. However, it is extraordinarily easy to conflate these two kinds of causation.
In recent years much excitement has gathered around transgenerational epigenetic effects, and some have claimed that they allow for a Lamarckian model of evolutionary change. Larmarck is associated with the inheritance of acquired characteristics, where a variant of a trait is developmentally induced through some environmental input and this new variant is then passed on down the generations. The modern synthesis dismissed this idea, instead relying upon mutation (and drift etc.) to introduce new variants, where mutation was a random event, but once selection got hold of a mutant the outcome was anything but random. Given this, a group of scholars have been arguing for an overhaul, or extension of the modern synthesis.
Their argument is long, and to get it in detail our paper really should be at least skimmed, but the essence is this:
Developmentally induced transgenerational epigenetic traits have been reliably measured in mostly rodent models. Epigenetic effects rely upon the addition of methyl groups directly to portions of DNA, or to the histone proteins that DNA wraps itself around. As a consequence of this methylation or histone modification portions of DNA code are switched off. To be clear, DNA code is not altered, it is merely prevented from functioning. So, various environmental inducers can switch on or off DNA expression in developing foetuses, as well as perinatally, leading to different behavioural (and other) phenotypes in the next generation.
Those advocating an extension of the modern synthesis say that this looks like a non-genetic inheritance system that can caused changes across generations, and is therefore capable of generating an evolutionary dynamic independently of gene-based systems, which are the sole focus of the modern synthesis.
Very straightforwardly our argument is that epigenetic inheritance relies upon mechanisms that are most likely adaptations, to be seen as homologous to learning mechanisms, that are designed to calibrate organisms to changes in local ecologies. As such the accounts of their actions are entirely proximate, and have no bearing upon ultimate models - and we go to some lengths to spell this out in this paper, and I have elsewhere too.
The reason we are so exercised by all this is that without this distinction various hypotheses cannot be asked about the nature of developmental calibration, and various questions cannot be explored about evolutionary change. Worse still, no one from the extended synthesis camp has yet provided a novel hypothesis that we cannot generate.
Sociologically this is interesting as many in the extended camp are now claiming a Kuhnian revolution in evolutionary biology, based on their view, but as all philosophers of science know, such revolutions are noted long after the death of all the normal scientists who contributed to them.
Any how this will roll on. The latest move has been to conjure up reciprocal causation in response to the Mayrian dichotomy - I've just submitted a paper on the problems with this, with Robert Barton (Durham), to Biology & Philosophy which I hope they will take. If they do, I may mention it here...
Tom
http://dissentwithmodification.com/
No comments:
Post a Comment