Evolution
From NECSIWiki
The basic concept of evolution is a process of population change over generations through differential reproduction and survival. The required conditions for ongoing change over many generations are that reproduction includes inheritance of traits with variation, and survival depends on comparison to the current population, i.e. is competitive. Traditionally evolution is considered part of biology, however, it more generally applies to many other, if not all, complex systems. Still, biology continues to provide us with the most extensive information about evolution. In placing evolution in the realm of complex systems, it is well to define it in a general way so that not only organic (biological) evolution is covered, but also cosmic evolution and cultural evolution. I (S. Salthe) have advanced the definition: 'the irreversible accumulation of historical information', with a synonym 'individuation'-- in contrast to development, defined as 'predictable (or constitutive) directional change. These definitions work for any subject matter whatever.
Among the key topics in evolution that have become of central interest in the field of complex systems because they do not lend themselves to traditional approaches are:
Additional topics for description / discussion:
- artificial selection
- spontaneous generation
- historical conflict with fundamental religion
- Neo-Darwinism and the gene-cenered view
- group selection
- fitness
- quasi-species
- diversity
- computer based approaches: genetic algorithms, evolutionary programming
- evolutionary channel capacity
- relation to development
Author: S.Salthe COMPLEXITY IN THE EVOLUTION OF DEVELOPMENT: Selective limits on early embryonic traits imposed by genetic drift and reproductive value. (S.N. Salthe, March, 2006)
Consider quantitative traits of organisms from the point of view of their getting ‘improved’ by natural selection. So the elimination of the totally unfit will not be under consideration here. The individuals of concern are those who just don’t leave as many offspring as the best type, but who are well and fitting enough to reproduce.
Thinking about embryos, we can see that they pass through quite a few different stages, each with its own, quite different, “way of life”. Now think about the evolution of development. It appears that there are embryonic adaptations, but even more important is the question of to what degree development can be “scanned” by natural selection (as suggested by, e.g., Leo Buss in his book ‘The Evolution of Individuality’).
Consider GENETIC DRIFT. In the example of embryos, drift would be a consequence, not of small population size, but of prolonged delay between selective actions and reproduction. That is, the viability component of fitness is quite peculiar in embryos. As an example, consider just three stages of development of a frog: blastula, tailbud and larva. Selection distinguishing among blastulae will presumably be based upon some of their characteristics, and will sort them accordingly. Those that pass on into being tailbuds will now be subjected by a whole other group of selection pressures, while the pressures that had impinged upon them as blastulae have effectively become subliminal. The individuals will again be sorted -- this time on criteria related to being tailbuds -- completely irrespective of the criteria that affected them as blastulae. Moving on to the larval stage, we have the same phenomenon, this time with selection acting irrespective of criteria for BOTH blastulae and tailbuds. Traits reflecting selection by the earlier selective pressures must as a consequence drift at random all over the place, with those of the earliest stages being, as it were, DEselected to a greater extent than later stages.
Begin by considering fertilized eggs. Looking at some quantitative trait (here it could be a molecular concentration, some relation between landmarks, or granule size differences), we find that it is initially broadly distributed in the population of ova -- that is, there is lots of variability in the unselected offspring, and most possible measurements are well represented except at the very extremes.
Now we get selection in Stage 1 (say, among blastulae), shaping us a nice unimodal normal curve around a selected mean for one of the above traits.
Now we move on to selection in Stage 2 (gastrula), on different traits in what is effectively already a different kind of organism. As a result, our selected curve from Stage 1, measuring a trait no longer under selection, changes -- say it gets platykurtic around the original mean. This is the effect of a kind of genetic drift.
Now we move on to selection in Stage 3 (Neurula), on yet more different traits. As a result our curve selected in Stage 1 has changed again -- this time, say, it got skewed to the left, even changing the mean somewhat.
Then we move on to selection in Stage 4 (tailbud embryo), on yet more different traits again. As a result our curve selected in Stage 1 gets modified again -- this time suppose the mode gets hollowed out, producing a bimodal curve. And so on, through subsequent stages up to the stage where the organism is definitive for its kind -- here, a frog. Note that, as development continues the cohort size decreases, gradually bringing in more traditional genetic drift effects (of small population size) on the original trait found in Stage 1.
If there is pleiotropy connecting the original trait to some of the other stages' traits, the curve selected in Stage 1 could be messed up even more by subsequent selective episodes.
A key point here is that the organisms in the different developmental stages are very different kinds of beings -- say, larvae versus gastrulae. This is very different from the case comparing juveniles with adults, as in the usual studies of natural selection.
Punch line -- The results of selection in Stage 1, functional at the time, have been effectively nullified by subsequent other selection pressures. That is, the results of selection in Stage 1 could not be inherited, as such, by embryos in subsequent generations, because they can’t be preserved in the cohort's population during development.
So this would be a type of "developmental constraint", the constraint here being a limitation on the degree to which selection can hone embryonic traits, especially in the earliest stages of development, where the effects of distortion via drift would be more pronounced. Perhaps development can continue can operate in subsequent generations because those early stages don’t need to be other than relatively simple in form ('vaguely embodied' might be a better description).
One possible mathematical objection to the idea is that, if the populations here are very large (as in the eggs of fishes or frogs), since we are looking at random events from one stage to another, then because of the randomness itself, no substantial change in distribution of the early selected trait should accumulate -- just back and forth fluctuations over time. My counter to this is (1) this nullifying effect would be more powerful the more different stages were traversed cumulatively, but there just aren't THAT many different 'stages' during embryonic development. Also (2) as development continues, cohort size diminishes rapidly in many species, such as fishes and frogs.
As well, (3) I would bring in my original insight here -- FISHER’S REPRODUCTIVE VALUE (RV). This is used by neoDarwinians to explain organismic senescence, because the RV of an individual (its value to the reproduction of the population) decreases as it ages (it contributes less and less to the selective responses of the population as it ages). On this argument, properties of individuals at the age of first reproduction are the most intensively scanned by selection because many individuals do not survive to later breeding seasons, and so the population size of those reproducing for the first time are the largest of all, making up the greater proportion of the effective population size. My point is that the effect of distance from the reproductively most important stages in early maturity should work going forward into embryonic stages as well. The RV of a blastula is miniscule, and improves as development proceeds as an individual’s likelihood of breeding improves, but it is low during all embryonic stages, so that any selection they have been subjected to would have little effect on the next generation compared to the selective pressures bearing upon reproductive aged individuals. Only in really huge populations could there be enough individuals to be sacrificed for improvement of earlier life history stages in addition to the improvement of the definitive, reproducing stage -- and living systems with huge populations (say, bacteria) do not have life history stages. (Once again, it is necessary to to note that we are not considering the selective elimination of the unfit, which is just as potent anywhere in the life trajectory, but on selection as a force in Darwin's notion of the "improvement" of traits.)
SO, IT SEEMS THAT THE EVOLUTION OF DEVELOPMENT MIGHT NEED SOMETHING MORE THAN DARWINIAN NATURAL SELECTION. Then, how is development kept orderly? Here we must note that, since development is epigenetic, with one trait being built upon another materially, the earlier forms are used as templates or palimpsests upon which to build later ones and so would leave a footprint in the future. Again, this could obviously be important for properties of definitive stages (juveniles) the closer they are to the age of reproduction, but the effects of selection in the earlier, embryonic, stages must still be defaced to some extent. Selection in early stages could be ‘ball park’ only, with no refinements being possible. And, of course, larvae (in “complex life histories”) come in to this story as well. For example, how could selection scan the properties of frog tadpole lips? Nothing in the adult (as far as we know) is templated upon these, and yet these complicated species-specific characteristics continue to be preserved in the populations of each species.
Finally, a thought worth considering here is that the absence of a possibility for selective refinement of traits in early embryos may be one factor in these stages retaining the generativity (flexibility) necessary for generating evolutionary change.
References:
Buss, L.W. 1987. The Evolution of Individuality. Princeton U. P. Fisher, R.A., 1958. The Genetical Theory of Natural Selection. Dover Publ. Hamilton, W.D., 1966. The moulding of senescence by natural selection. J. Theoret. Biol. 12: 12-45. Richardson, M.K. et al, 1998. Haeckel, embryos, and evolution. Science 180: 983-985. Stearns, S. C. 1992. The Evolution of Life Histories. Oxford U.P.
