Evolution in the absence of selective pressures

I would like to hear people thoughts as to what sort of evolution might be seen in a population in an ideal environment which is totally permissive and has no selective pressures. There is infinite room, all waste is magically removed on the backs of pink unicorns (invisible ones naturally), little pixies come and feed you nourishing food etc.. some of you may have come across similar thought experiments before or possibly actual experiments hoping to simulate such an environment. What sort of evolution would we see.I should give the source of this idea as well, for a bit of context. It sprang up from my ongoing debate with syamsu, another poster on this forum with controversial theories on how evolution should be studied, about Natural selection. Syamsu suggested that the Malthusian assumptions of Darwin's theory were problematic. I certainly don't want to recapitulate the entire argument here but this got me to thinking about what sort of evolution we might see in a non-malthusian environment.My first thoughts were that there wouldn't really be any evolution really, just what you might expect from random drift and that would be pretty inconsequential due to the unrestricted size of even the most feebly reproducing populations. But then I got to thinking about it a bit more, specifically in the context of the Li growth formula.This formula came from a paper which Syamsu used to start the discussion. In that paper it was used as the basis for an approach to modelling natural selection which would allow, but not require, the specification of a number of specific factors affecting selection.The formula supposes that the rate of change in overall growth of a population is proportional to the variance in growth rates of the sub-populations within that population. This means that over time the overall growth rate of the population will tend towards that of the sub-population with the highest growth rate as that sub-population comes to predominate.This made me think that while you would be unlikely to see any dramatic changes in gene frequency, and certainly no extinctions, you might still be able to notice clear trends over a very large number of generations. My main thought was that the only usefull mutations would be ones which directly acted to maximise fecundity. The population would become super R selected, although not for the usual reasons, and just pump out more and more organisms.What do you think would happen in such a non-selective environment? Would there be anything we could recognise as evolution and what shape do you think it might take?TTFN,WK

I'm a fruitcake, so don't listen. But isn't this a bit like normalized selection? Okay, is the earth an ideal environment? Maybe not because of the odd lion or two. If you're a dear or whatever. But we do see species that don't change much over time, yet that is because they are at the "height" of their evolution I suppose.But also, I am wondering, would this perfect environment mean that, okay...let's say abiogenesis happened, what then would happen in this perfect environment?

In the absence of any other selective pressures there will be a pressure to reproduce faster. Unless the population growth rate is fixed the varieties which reproduce fastest will tend to dominate in numbers. Organisms that use the strategy of producing large numbers of young but investing little resource in each one will tend to dominate unless we randomly kill most of those young in a non-selective way.As well as the obvious factors sexual selection might still be possible, provided sexually reproducing organisms already existed. The "recognition" element of not wasting reproductive effort with incompatible partners would likely be beneficial in many species (unless we are articicially restricting reproduction). And from that it is likely that some potential partners will be more sexually attractive than others (Consider the case of individuals that are "borderline" by recognition criteria - obviously they are a greater "risk" then individuals more clearly recognisable as the same species but on the other hand they also represent a possible opportunity for reproduction. The best option then is to not rule out such individuals as potential partners but to prefer those which are more clearly identifiable as viable reproductive partners).Extinction becomes much less likely, but not impossible unless there is no death at all (random death is entirely acceptable and old age could be permissable provided no major variations are disallowed or allowed only to compensate for other factors that would produce a selection pressure). In the presence of death a failure to reproduce fast enough to replace population will always end in extinction. Removing death as an element of selection does not change that, although it does tend to reduce the possibility.The other thing we would lose is stabilising selection. Since all mutations are neutral all mutations are equally likely to spread through drift which means that mutations we would consider detrimental would become more common. Of course they would not be detrimental in evolutionary terms given an environment which compensates for any loss of capability - which is why they spread.It is worth noting that AVIDA runs still produce evolution when the only selective element is the "reward" of "advantageous" behavior by granting the digital organisms a greater share of the "run-time" (and hence permitting faster reproduction). By the same token unless an option to provide compensating "rewards" is used AVIDA tends to select for shorter programs (which require less time to reproduce). AVIDA does have a maximum population but even if this is enforced by random death (which therefore has no selective effect in itself) evolution is still observed.

Dear Mike,Its nice of you to say I'm a dear but I suspect you meant if I was a deer.Its arguable how much we really see organisms which don't change over time. Certainly there are large periods of apparent morphological stasis but that doesn't remove the possibility of evolution occurring in a number of other areas not reflected in gross morphology. All that really suggests is that those organisms had adapted, morphologically at least, to an environment which remained relatively stable for a long time.As to the abiogenesis question, that is an interesting point, If you had an environment entirely amenable to the self-replication and stability of, say, an RNA sequence would there be any advantage in what in our world are more stable nucleotide forms such as DNA.It's a lot harder to hypothesise about that sort of scenario, we really don't know enough about the behaviour of these molecules. Trying to decide what a 'non-selective' environment for self-replicating RNA would be is a pretty thorny problem in and of itself.TTFN,WK

Dear PaulK,Your right of course, any sub-population not able to at least breed at replacement levels would become extinct.I'm not sure about the value of sexual selection. I tend to think that a broadcast strategy like wind dispersed pollen or the mating swarms of jellyfish would be favoured, a K-strategy as I suggested before. Without needing to worry about resource scarcity the survival of the young is much more likely leading to the capacity for a huge population expansion in only a few generations.Your example from AVIDA seems to be in line with my thoughts that in the absence of other selective factors fecundity itself would still be selected simply by the sort of process the Li growth formula modelled. Perhaps a situation analogous to your shortening of the progams would occur. Simplified organisms would be favoured which can reach reproductive maturity as quickly as possible.In some ways I suppose earth may show some of these affect. If we accept S.J. Goulds 'Bacterial Mode' argument that bacteria always have been and probably always will be the predominant form of life on Earth then we are really saying that all other forms are simply less fecund populations for whom there are enough resources available that their wasteful reproductive strategies can be indulge, which I guess ties in with Mike's suggestion that the world is a pretty ideal environment, although there are of course problems with this idea.TTFN,WK

Okay, fair enough. I just thought that we might atleast have an "experimental" example with normalized selection. It's not perfect but I thought it might reflect your scenario somewhat.As for speculation about abiogenesis, do you believe in abiogenesis then? Or is it not a matter of belief? Is the evidence for abiogenesis overwhelming? Does evolution confirm abiogenesis? Must we believe in A if we believe E?

The whole abiogenesis area is off topic in this thread.

It would be boring.

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Aslan is not a Tame Lion

Weellll.. there'd probably be a lot of sex.

Nice little bodyswerve into the old abiogenesis debate there MTW. Any thoughts yourself as to what might happen to an early self-replicator in the proposed environment? Allowing of course the premise that one could come into existence in the first place, by whatever process.TTFN,WK

No idea.Maybe if one came into existence, then anything's possible. Maybe any idea goes. I mean, if such a thing could happen and all. It's a bit of a hefty premise.

I should emphasise that my ideas about sexual selection assume that there are already sexually reproducing species. I don't think that selection relying solely on rate of reproduction is going to produce such species and would more likely work against it.Broadcast strategies have two problems - they are inefficient and they can still fail if there are no potential mates in range. While they are necessary for sessile species I am not convinced that it would be the best option for motile species. I would suggest that a strategy more like those of some fish where the female lays many eggs and then the male fertilises them would be more productive.

From what Wounded King describes, if we throw in random mating, you'd have a population in Hardy-Weinberg equilibrium. Gene frequencies would be stabilized, except for new alleles popping up occasionaly per the mutation rate. Even their frequencies would eventually stabilize as well.KC

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I would like to hear people thoughts as to what sort of evolution might be seen in a population in an ideal environment which is totally permissive and has no selective
pressures.

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Well, the evolution of obligate intracellular parasites" might give us a bit of a clue. These are bacteria (and a few eukaryotes) that live inside their host’s cells and have co-evolved with their host to the extent that they can’t live without themThe interesting point with regards to this topic is you basically have cellular life, living within a cell fill to the brim with all the things that are required for cellular life. All the parasite actually needs to do is copy itself, make its cell walls and steal the products of its host metabolism for the rest:What seems to happen by and large is a massive reduction in genome size, with many more deletions than insertions and the loss of a lot of genes that you’d thin where absolutely required for life. It’s hard to tell how much of the genome falling apart is just random with the lack of selectional pressure to weed out such mutants and how much is a selectional pressure to generate a more slim line genome.The Mycobacterium leprae (leprosy) genome provides a good example. It is massively reduced and size and overflowing with (actual) ‘junk’ DNA which is uncommon for a prokaryote.Oh yeah, first post, hi everyone.This message has been edited by biochem_geek, 06-24-2004 10:01 AM

and a very good first post it is! Thanks for it.