Wednesday, January 14, 2009

Climbing Pit Improbable

In the ongoing Adaptationist v. Pluralist debate, both sides agree on a surprising amount. Both sides agree that there is more to evolution than adaptation by natural selection. However, Adaptationist would argue that adaptation by natural selection is the most important, or even the overwhelming, evolutionary process, and that evolution can be described as climbing Mt. Improbable – with adaptation to environment similar to climbing a fitness landscape peak towards optimal fitness (but please note, never, ever, reaching the top!)

I disagree. I think genetic drift accounts for most of the evolution that occurs, and natural selection, while very important – especially in creating diversity – accounts for a smaller percentage. However, both have worked together to produce the diversity of life on Earth.

I also have a problem with the Mt Improbable analogy . . . well, actually I have two problems.

1) It perpetuates the idea that evolution is an upward striving process, and that derived or adapted groups are higher, than the less derived or less adapted and, as a consequence, fitter, advanced . . .better. (the old Tree of Life analogy problem.)

OK, maybe it is applicable to a fitness landscape, but there is no reason that the landscape has to have the peaks pointing upwards, . . . is there? Surely it's the distance between where you are on the peak and the schmucks on the fitness plane that is important, not the direction of that distance?

Plus, fitness landscapes, are not permanent, or even solid. They change with the environment. A population/species, or whatever, may be quite "high" (see how hard it is to use neutral language)on a fitness peak one minute, and find itself down on the plane, or even in a fitness trough, with hardly any change in allele frequency, but a significant change in environment. In other words the fitness landscape moved underneath it.

2) The real problem with the Mt Improbable analogy though, is that it gives the impression that as hard as it is to ‘climb up’ (and it is), the analogy suggests that it is relatively easier to ‘climb down’ - and it isn’t because its actually harder. OK that might be pushing the analogy a bit far – but that’s the point, it doesn’t hold up to detailed scrutiny.

The real problem is that adaptation, in the broader picture, is an evolutionary cul-de-sac.

Adaptation means that certain alleles are being selected for because they confer an advantage in a particular environment. If the environment changes, then the alleles that conferred an advantage may no longer do so. Worse, the very process of selecting for certain alleles may well have stopped other alleles getting fixed through drift – alleles which might be beneficial in the new environment. Even worse, the alleles that were originally selected for may be costly to produce and maintain where they confer no advantage, and thus be deleterious.

But the really bad thing is that, as hard as it is to gain the alleles that provided an advantage, it is even harder to loose them, as this would require specific mutations to affect those particular alleles (rather than the random process that produced them). You could reduce them to a vestigial level, provided you survived long enough to do so. Difficult though, if you are struggling to survive in a new environment where the competition does not have the adaptive dead weight (unless you have some other advantage.)

The more adapted a group becomes, the more imbedded it is in a particular environment, and the more sensitive it is to changes to that environment.

Eventually all strategies lead to extinction, but during environmental change, it’s the generalists that survive, not the specialists. Adaptation generally leads to extinction. Highly adapted groups/species and ecosystems delicately balanced on a web of interconnected adaptations, will crash once environments change.

Adaptation is not climbing up Mt Improbable, it’s climbing down Pit Improbable! The pits are hard to find, but once in, it’s easier to go down than it is to back out, and if you adapt too far, you are trapped in a cul-de-sac with no way out when the environment changes. The generalists that flirt with the rim of the pit, or on the fitness plane have a better chance of surviving to become the stem stock for new adaptations.

It may well be that some species or groups of species in a pit break through to new fitness landscapes and produce new groups (e.g. birds and mammals from reptiles) because fitness landscapes are not flat, but curved.

But for most populations/species, adaptation is a pit of no return.

Photo credits
Mountain image from copyright-free-photos.org.uk
Pit image from larrydsmith.com


UPDATE
There are two discussions here and here, and a new blog post here.

19 comments:

  1. The notion of an adaptive landscape is itself already an abstraction, which was held constant as a temporary simplifying assumption in the early mathematics of Fisher and Wright. In fact there is no adaptive landscape, just the interactions of all the populations of organisms of different taxa in an area under certain abiotic constraints (space, heat, light, materielle).

    The early synthesisers treated the landscape as a useful tool to visualise all these interactions from the perspective of a single population under investigation. But the landscape as such does not exist, and as you note, if we were to model it accurately it would itself be changing both independently of the population we are studying, and as the result of what that population does to its environment, living and abiotic. The "landscape" is a bit like the Einsteinian spacetime rubber sheet in that respect.

    ReplyDelete
  2. Interesting; perhaps you have enough material to challenge Dawkins' book?

    ReplyDelete
  3. Awesome, I've been skeptical of Dennett's and Dawkins' strict adaptationalism for a while now, and I finally have a clear reason why. I'm an evo bio major so I can't wait to learn more!

    ReplyDelete
  4. I think there is a landscape but it's dynamic, more of a bouncy castle floor than a landscape.

    ReplyDelete
  5. Mount Improbable does have sheer cliffs, whose unclimbability is the main mental obstacle for creationist-by-default people. Dawkins' emphasis was to demonstrate the possibility of the long way round, even though his subsequent description of "how" was a bit less obvious. The Pit of Adaptation is much better at the how, but I'll stick with the cliffs and mountains version if doing Evolution 101 For A Doubtful Audience.

    ReplyDelete
  6. Being more of an adaptationist than perhaps I should be, a couple of thoughts come to mind…

    I’ve cut, copied and posted this article on my blogsite with added commentary (for the purpose of discussion, no bad intent).

    I would have felt guilty inserting a full page here in the comment box – though I’m happy to due so if you’d prefer.

    http://ecographica.blogspot.com/2009/01/improbable-mountains-in-landscape.html

    ReplyDelete
  7. Bayesian Bouffant, FCDJanuary 15, 2009 at 6:28 AM

    because fitness landscapes are not flat, but curved.

    Maybe fitness landscapes are more like an 83-dimensional manifold.

    ReplyDelete
  8. The long term advantages of being generalist are countered by the short-term disadvantages when competing with species that are better adapted to specific environments.

    So being a generalist might or might not get you further down the road.

    ReplyDelete
  9. I wish you had spent more time on the differences adaptionist versus pluralist. The objections with the idea of a mt. vs. a well are non-issues to me. Both are analogies, visualizations that are simplifications and not rigorous at all. Perhaps a well is better than a mountain, but neither changes what actually *is*.

    I think you are right about generalist vs. specialist. I have been thinking lately that what we think of as more "highly" evolved are actually organisms taking advantage of periods of time that are by chance more stable, possibly ending at any moment.

    ReplyDelete
  10. gawp,

    No, I'm with John on this. The 'landscape' is a function of the populations of taxa. It occurs where the populations are, and expands and contracts with those population. Where there are no populations there is no 'landscape'

    ReplyDelete
  11. Lurker #753,

    Regarding the cliff analogy, the corresponding analogy would be that the pit is formed gradually by the population. The concentration of the new alleles causes the 'landscape' to sag and forms the pit at that point (similar to the Einsteinian spacetime rubber sheet that john mentioned). Subsequent generations create an even deeper pit as separation from the rest of the population causes a concentration of separate alleles.

    So the pit is formed gradually over generations. Forming the pit in one jump would be like jumping into a sand box and trying to produce a Grand Canyon.

    ReplyDelete
  12. Don,

    You are correct that adaptation is a short term strategy, and a very good one. Both strategies are valid in an evolutionary sense. The question is which will give you the better chance of surviving for a greater length of time?

    The answer is to be a generalist. Being less committed to a specific environment gives a better chance of surviving a change to that environment.

    ReplyDelete
  13. Brian,

    For more discussion on Adaptation v. Pluralist, check out Larry Moran's blog Sandwalk

    ReplyDelete
  14. John's relativity analogy is clever:

    The equivalent of 'matter tells Spacetime how to curve, and Spacetime tells matter how to move', would be something like:

    'organisms tell the fitness landscape how to curve and the fitness landscape tells organisms how to adapt'.

    In that phrasing the fitness pits work best.

    Now if only we knew the dimensions, let alone the geometry.

    ReplyDelete
  15. My own visual metaphor for it is bubbles under a sheet of ice. Functionally equivalent to yours of course, except with the added wrinkle that one bubble can split into two and explore two adjacent maxima - a rough approximation of speciation.

    ReplyDelete
  16. Alan Templeton proposed this idea in 1982. See "Perspectives on Evolution" ed. Roger Milkman chapter 2 "Adaptation and the integration of evolutionary forces. It is always worth repeating.

    ReplyDelete
  17. Chris,

    I enjoy your writing, and many thanks for the time spent clarifying your point at my blogsite. I sincerely appreciate your patience in talking me through the analogy and your perspective of it, but – I struggle still…

    Once again my thoughts are a bit bulky (pictures this time!) for posting here, so I've posted them at my blogsite.

    If your bored stop by, thanks

    http://ecographica.blogspot.com/

    ReplyDelete
  18. Regarding the cliff analogy, the corresponding analogy would be that the pit is formed gradually by the population. ....

    Well, a landscape peak too is formed gradually by the population. After all,there is no preexisting fitness peak that the population is aspiring to reach. Just as a pit, a peak too is formed gradually over generations as in your words " separation from the rest of the population causes a concentration of separate alleles."

    Since we are talking of an abstraction I dont see how a pit analogy is any more helpful over a peak analogy.

    ReplyDelete
  19. As a non-specialist, I think the pit analogy is better than the peak (bearing in mind that both are just that, analogies) because it is harder to climb out of a pit than walk down a slope.

    ReplyDelete