What is the driving force of evolution??
What makes one species evolve into another??
Why does evolution occur??
Is it natural selection? or is it perhaps mutation?
The points below are what I understood after a bit of reading and thinking.
Natural Selection drives evolution. Since irrespective of the repertoire of different genes present in the gene pool, if one gene is not chosen above another gene (based on some criteria) then the relative frequencies of genes in gene pool will remain the same and consequently there wouldn't be any evolution.
So clearly some gene has to get selected over another, this selection criteria is called Natural Selection.
However for Natural Selection to work there must be sufficient diversity in the gene pool itself so that some or the other gene is good enough to pass the test proposed by selection. Thus rate of mutation also is a factor at driving evolution. Its observed that viruses mutate at a higher rate and (thus?) evolve at a higher rate too.
However, mutations can either make a gene better at passing the present selection test, or make it worse at it. If they make the gene worse then it will fail the test and disappear into oblivion. But generally mutations don't cause any significant change in the capability of a gene to pass a selection test. These 'neutral' mutations can accumulate to a large number over the span of time. Random drifts in such neutral mutations can result in some gene being replaced by another as result of random event. Might seem a little far fetched, but in the timescales that evolution works under its definitely plausible. And so random drifts in neutral mutations may drive evolution too.
Is there any bias for or against some particular mutation? Is it more likely for an 'A' to convert into 'G' or 'C' rather than a 'T'. Or for that matter any other combination of 'ATGC' in the above sentence. I don't know if such bias is proven, but this bias has been implicated as the reason for the observed GC content variations among closely related prokaryotic species.
Of course all these mechanisms are not mutually exclusive w.r.t gene or time. For different genes different factors would be responsible for evolution. But these forces probably are the reason why one species evolves into another with passage of time.
3 comments:
i think in the article you are missing one very imp point and that is the environment and without considering this the argument is incomplete. If the surroundings are static you dont even need to change so i guess no evolution! But that is not the case, this is exactly where selective pressure comes into picture, env is changing- the one's who have appropriate machinery to survive the change survive, others die or employ still different mechanism to tackle selective pressure and thus diverge.Hence without selective pressure there is no selection though i think there can be divergence(due to random mutations or changes in the genome but then you still require some kind of selective pressure there) Also, we need to consider processes like gene duplications, gene conversions, effect of mobile genetic elements and even parasites... cause its just not mutations i.e ATGC that bring abt changes in the genome. For that matter, its quite popularly known that parasites are the once which are major contributors in evolution of their hosts...quite interesting if you look at it!
P.S: did not get Systems biology in the labels...if u have intended to make some pt pertaining to it in the text...sorry i missed it ???
Actually in my article i have nowhere explicitly mentioned that selection pressure is because of environmental factors because Selection pressure can also be something other than environment for example a new virus, new competetive species, new predator or a new disease (which are not environment perse).
However I have throughout given due weight to selection.
And the point u make about random mutations accumulating diversity from which selection will choose is absolutely valid. But i was talking about another force which is 'evolution due to accumulation of neutral mutations and random drift' (without selection). Chk out http://en.wikipedia.org/wiki/Neutral_theory_of_molecular_evolution
As far as parasites are concerned:
Parasites are a major factor in evolution of a host BECAUSE they are constantly putting selection pressure on the host. And besides a parasite which has stayed on in the host for a really long time would itself evolve into a neutralist/mutualist/symbiont.
The systems biology tag is just for tp... it was there so i clicked it!!! :P
talking of neutral mutations...they obviously occur without selection...but the moment selection pressure occurs on a neutral mutation its no more a neutral mutation...what i mean is "most" of the mutations are in fact neutral or were previously (supposedly) called neutral until selection was introduced...
P.S: how m i supposed to track if someone replied after me... in cases like this one???
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