Thursday, April 20, 2017

The last molecular evolution exam: Question #6

How can alleles be fixed in a population by positive natural selection (i.e. adaptation) if the environment remains constant for thousands of years?

Question #1, Question #2, Question #3, Question #4, Question #5, Question #6


10 comments :

  1. Beneficial mutations can often be contingent on other mutations. Therefore, adaptations can continue to improve over time with the addition of new beneficial mutations that interact synergistically, even in environments that don't change over that time period.

    ReplyDelete
    Replies
    1. You would get a good grade for such an answer as long as you mention that fixation of the "other' mutations can take a very long time, especially in a large population.

      Delete
  2. If we learned anything by the Lenski experiment it's that there doesn't seem to be any upper limit to adaptation (at least, not any limit within *realistic* timeframes).

    The overall adaptive effect (it's quantitative effect on fitness) of an arbitrary beneficial mutation might diminish, on average, over thousands of generations, but nevertheless it seems to be the case that among novel mutations there are still adaptive ones happening, even tens of thousands of generations down the line.

    As Eric brings up, epistatic effects is one aspect of the explanation for this phenomenon. Since the total combinatorial space for mutations scales with genome size, and already for a genome of a hundred thousand basepairs (which to my knowledge is smaller than the smallest known free-living prokaryote), the total number of sequences of that length is about 4^100000 = 10^60205 (ten uplifted to the sixty-thousandth power). An incomprehensibly vast number. Such a number of combinations could not have been exhaustively searched by even a planet-sized bacterial colony in the known age of the universe. So in effect, there will *always* be new mutational combinations to try out.

    ReplyDelete
    Replies
    1. We read several of the Lenski papers. Some of the important lessons are contingency, epistasis, and potentiating mutations. Sometimes selection for particular alleles depends on previous neutral (or deleterious) mutations that have been fixed by random genetic drift. That can be slow and haphazard, therefore it's unlikely that any population has reached the top of an adaptive peak where further adaptation is impossible.

      This concept, by the way, is one that IDiots have failed to grasp even though it has been explained to them many times over the past 25 years. It's the key to understanding why Behe is wrong in both his books and why Meyer and Jonathan King are wasting their time criticizing their straw man version of evolution.

      Delete
  3. Not clear if what's being asked is whether existing variations can become fixed by selection in the absence of environmental change, or whether selection, full stop, goes on in the absence of environmental change. Seems as if the answer to the second would be very simple - mutations and different combinations (particularly very rare combinations, as in the Lenski experiment) proceed whether the environment changes or not. The first is a little more difficult (to my layperson's mind) and interesting. Does "environment" in your question include, for example, sexual selection, and if not, is there any evidence "styles" change over time in sexual preference, even in the absence of changes in the rest of the environment?

    I also wonder how often we really see an environment that can be said to be unchanging from an evolutionary perspective. Disease, parasites, predator-prey relationships, food supply - for how long can such things stay stable enough not to affect fixation?

    ReplyDelete
    Replies
    1. Students would lose marks for discussing whether the environment could really be constant. They were warned that the basis of think questions and thought experiments is accepting the premise and assumptions.

      Delete
    2. Assuming that the genetic context counts as part of the environment, and assuming that it's impossible for at least neutral evolution to stop, it's also impossible for the environment to remain completely unchanging.

      Delete
    3. I guess that (the genetic context counting as part of the environment) is an assumption that Larry's students weren't to make in this thought experiment, perhaps to highlight the absence of "classic" adaptation opportunities to a changing external environment.

      Delete
  4. An additional possibility is provided by deleterious alleles fixed by drift. The back mutation, and any other that responds to the same selective pressure, can fix by positive selection.

    ReplyDelete
    Replies
    1. That was definitely one of the answers I was looking for. The main point of the question related to our discussion of Lenski's long term evolution experiment and our discussion of whether evolution (or natural selection) has stopped in humans. We also addressed the widespread misconception that evolution is intimately linked to changes in the environment.

      Delete