Wright’s Shifting Balance Process: Phase 3 part 2

Last week I tried to establish that group selection by differential migration can work.   On both experimental and theoretical grounds we find it does work, and in fact will frequently be stronger than individual selection. The question comes where does it fit into Wright’s shifting balance process.

The first problem we need to confront is that Wright apparently thought that the group level trait would be concordant with the individual level trait. That is, he thought that with greater absolute fitness would come the production of more individuals, and with that a greater emigration rate. There really is no reason for this to be true. For one thing, the important factor is relative fitness, not absolute fitness. The problem is that as overall fitness increases in population so does the resulting competition. Thus, a population may experience a steady increase in some measure of absolute fitness, but no overall change in population size. This is actually the basis for the Alice in Wonderland (AKA the red queen) hypothesis. That is, Fisher’s idea that the environment is always deteriorating is largely due to the fact that other individuals are always evolving. Thus, the improvement in absolute fitness (survivorship, number of offspring produced) is completely offset by the similar improvement in other individuals with no resulting increase in apparent fitness or population size.

cheshire-cat

who , Me?

So, how to resolve this? I think the answer may lie in the Wright’s words. (I am away from my books at the moment, so this is a paraphrase). When Wright was investigating the effects of migration on population differentiation he stated something to the effect that it would appear that one migrant every other generation would be sufficient to destroy population differentiation. After uttering this now famous platitude he went on to say: However, given that immigrants will have a much lower survival and mating success than residents there could be many thousands of migrants and yet there will be population differentiation. OK, I have no idea what he really said, but this is how I remember it.

What this has to do with phase three is that even if migrants are moving randomly among subpopulations, they have to survive and reproduce once they get there.   Thus, if one group has a lower absolute fitness than another group migrants from the low fitness group will have a low relative fitness in the group they move into, and as a result may not survive and reproduce. On the other hand, migrants from the high absolute fitness group will have a high relative fitness in the low fitness group, and they will have a better than average chance of surviving and reproducing. Thus, even random migration can potentially lead to differential migration once within group individual selection is added in.

The second issue is rather interesting. When talking about phase three it is convenient to say something to the effect that migrants from high fitness populations send out migrants that lead or convert the low fitness population over to the new higher peak. This sounds something like the five rusty rats leading the founding of the village of cream puffs.

rusty rats

“And so, while the wind and the snow blew and the blizzard beat its icicles in their faces, they held on to the long curved tails of the rusty rats till they came to the place where the Village of Cream Puffs now stands.” If you don’t know the rutabaga stories by Carl Sandberg you should. (http://www.josephperry.net/rootabaga/01-03rustyrats.html)

The reality is that the offspring of these migrant individuals will be hybrids between the two peaks, which ought to put them smack dab in the middle of an adaptive valley. In other words, these migrants are more like genetic terrorists than saviors. What they do is trash the adaptive gene complex by adding genes that are generally bad for the current population.

chu red

I was looking for a non-controversial revolutionary. I came up with Pika Ché (http://www.redbubble.com/people/meganegi/works/8351063-chu?p=t-shirt)

So, my thought on phase three is that at the “end” of phase 2 you have a set of populations distributed about two or more peaks (I put end in quotes, because the phases all go on simultaneously). At this point they are all sending out and receiving migrants, but the ones on the higher peaks are net senders, and immigrants tend not to survive and reproduce. Those on lower peaks are net recipients of migrants since immigrants tend to have higher fitness than the locals. That said, the offspring of these migrants is low, and has the effect of dragging the low fitness population off of its local peak, and basically allowing drift to have its effect. However, in a sense it might be called directed drift, since there will be a continued input of new migrants from the higher peaks. These migrants will “encourage” drift in the direction of the new peak, but by no means guarantee it. In other words rather than the lower peak being led to the new higher peak, I see it getting dragged kicking and screaming through the adaptive valley.

jones dragged

Indiana Jones being dragged kicking and screaming to the lost ark (http://www.propstore.com/cms/the-prop-store-collection/indiana-jones-and-the-raiders-of-the-lost-ark/harrison-fords-whip/)

So, then we can start to see what phase three is really doing. When a metapopulation is spread across multiple adaptive peaks migration will have the effect of moving populations off of those peaks. Because the fitness of the incoming migrants will reflect their population of origin, by in large low fitness populations will have more effective migrants (migrants that enter and survive and reproduce) than high fitness populations. Thus, the low fitness populations will be more likely to be driven away from their peak, and on average, they will tend to be dragged towards the new higher peak. Will they make it to the new peak? That is hard to say in a complex stochastic world. Some will at least temporarily climb back to their old peak, others may climb the new peak, and still others, having been dragged off of their local peak may drift around and discover yet a new and even higher peak.

6 Responses to “Wright’s Shifting Balance Process: Phase 3 part 2”

  1. “However, a lot of fitness increases resulting from individual selection are associated with increased competitive ability, which frequently will not be associated with increased population size.”

    That just sounds like relative fitness, no?

  2. Bjørn;

    I think you are correct that any increase in absolute fitness that is associated with increased efficiency of using resources should be associated with an increase in overall population size. As such it fits in well with Wright’s original model, and there is no problem. However, a lot of fitness increases resulting from individual selection are associated with increased competitive ability, which frequently will not be associated with increased population size. The “mean plants” example is one such situation, but I am sure I could come up others. So then the question becomes can phase 3 work even without the correlation between increased fitness and population size? I was speculating that it could.

    Mean plants note: Note that if a plant that evolved to compete in a high competition situation migrated to an area where the plants were not mean it would exceedingly well since this would essentially be a “selfish” individual moving into a comparatively “altruistic” population.

  3. In the example with mean plants as I understand it the absolute fitness of individual plants is depressed by the fact that space is limited. This effectively means that beyond some point the absolute fitness doesn’t increase even when individual yield does.

    So, I guess I still have trouble understanding how an increase in absolute fitness should not lead to an increase in population size. Maybe we are not in agreement about what “absolute fitness” means?

  4. Bjorn;

    Obviously the SBP does not require sexual species, but from all his examples it is pretty clear that Wright was thinking about diploid sexual species.

    There are plenty of examples where increasing absolute fitness does not affect or even lowers population size. The best examples are increasing competitive ability. Often out-competing your neighbor demands putting more energy into offense and defense, and less into growth and reproduction. This has been experimentally seen with plants and individual selection for increased productivity.

    http://blog.uvm.edu/cgoodnig/2013/10/23/indirect-effects-bruce-griffing-and-mean-plants/

  5. “The reality is that the offspring of these migrant individuals will be hybrids between the two peaks, which ought to put them smack dab in the middle of an adaptive valley.”

    Was Wright exclusively thinking about sexual species?

  6. “The problem is that as overall fitness increases in population so does the resulting competition. Thus, a population may experience a steady increase in some measure of absolute fitness, but no overall change in population size.”

    Hmm. As I understand absolute fitness, increasing it should lead to a larger population. What is “some measure” of it that doesn’t?

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