Saturday, April 08, 2006
Genetic Assimilation and Directional Change
Basically, the idea is that organisms have response strategies for stress, and some of these are or can be heritable, especially after multiple generations experiencing the stress. The paper linked above concentrated on responses to predators, while this one concentrates on other types of stresses during development.
This sounds (and is) incredibly Lamarckian. Obviously organisms can respond to stress in their lifetime, but how is that passed on to future generations? The paper suggests several mechanisms:
- transferring physical substances (this was not expanded on)
- hormonal effects that affect the genetic expression of offspring
- heritability of epigenetic structures
- development incorporation of a stressor
- canalization of mutations in the offspring
While the other ones are interesting, the last one is the one I am most interested in. It is interesting to think about a change in hormones in one generation may change the way the genes of children are expressed. However, for me, looking at changes in the actual coding of the genome is what interests me the most. Canalization refers to the focusing of mutations on a specific region. The paper suggested that the developmental systems are organized to channel stress-induced variation into useful areas without damaging the primary function of the organism. It had a table listing several types of patterns of stress-induced changes. Here are the ones listed under stress-induced genetic variations:
- directional and locally adaptive mutations
- increase in the evolutionary rate of a gene
- increase in the frequency of sexual recombination
- increase in mutation/recombination rates
- appearance of primitive, ancestor-like forms
While we have discussed the mechanisms of directional mutations in single-celled organisms or somatic tissue, the question remains (RESEARCH SUGGESTION ALERT FOR YECs) how do directional germ-line changes occur in multicellular organisms? Or even an increase in the evolutionary rate of a single gene?
Chris Ashcraft has previously pointed out the ability of populations to increase their own diversity. It seems that stress likely causes such diversity-generating mechanisms to increase.
Some other interesting tidbits from the paper that I found interesting:
- Stress reveals "hidden" variability that already exists within a population.
- The lack of phenotypic plasticity will result in organismal extinction in stress situations
- A lot of these things are aided by developmental complexity in ways that the paper did not make entirely clear
- Stresses occurring during ontogeny are often accomodated by organisms without a reduction in functionality
- Organisms continually exposed to unfamiliar stressors wound up in "stressful helplessness" where they couldn't cope with any stresses at all, indicating that organisms need to be able to have time to develop stress-avoidance strategies to specific stressors
Anyway, there is a lot of interesting things to think about in the paper, although unfortunately it did not go into detail on a lot of the mechanisms (but it did have an extensive, and very interesting, reference list). It also asked several questions which are easily answered by removing ateleological assumptions. The primary one being that in order to respond to stress, an organism has to be able to detect it and organize an appropriate response. So then how does an organism develop detection and response mechanisms? It wouldn't be directionalized without the stress, but the stress itself would in many cases kill the organism without the response already being in place. As Creationists, our questions become (a) what are the mechanisms of stress response, (b) how general are the strategies, and (c) is any vitalism involved in such a response?
One paper that this one refers to is Chance Favors the Prepared Genome which I plan on requesting from inter-library loan soon.