Re: Drosera evolution II

Jan Schlauer (
Tue, 20 Jun 1995 15:12:41 +0100


First, a hearty welcome to the list. The following is just to find a common
terminology. I do not believe we are really far apart from each other,

>how were climates so different in the past in Australia and South America?

You have obviously followed Fernando's and my previous postings, so how can
you ask such questions now?

> But climate differences aside, how could climate
>influence rates of mutation within a Drosera (or any organism)? If I
>follow your argument right you say that Drosera are evolving at a faster
>rate on Australia than in the rest of the world. How?

Please do not forget that evolution is marked by essentially two phenomena:
variation (as a consequence of mutations) and *selection*. In a changed
environment (e.g. by climatic change like in AU:
temperate>mediterranean>desert>humid tropical), there was a *selective
pressure* on the organisms, and those taxa survived (or reproduced better)
which had some (adaptive) evolutionary advantages (= phenotype, acquired by
*previous* random mutations of the genotype, and NOT in wise expectation of
environmental change - to prevent another unnecessary reply), which enabled
them to live (better than other taxa) under the new (drier) conditions.
These taxa or "favoured races" (sensu DARWIN) were now able to conquer new
biotopes (in the case of _Drosera_ etc. especially in the mediterranean
zones) which were "empty" (or did not exist) before, which was a
prerequisite for adaptive radiation (for the definition of this term cf.
any textbook on evolution), which had a positive influence on evolutionary
rate. Subsequently (or rather during the whole process of climatic
changes), the taxa were redistributed and (at least partially) concentrated
(in SW AU), which opened new possibilities for hybridization/introgression.

> I would think that
>the rate of mutation would be constant at the genus level.

It is not.

>And if they aren't - why not?

There are several reasons. The most simplistic explanation is that a given
mutation in a given position in a given gene may be lethal in one
biological environment (cell, organ, individual, biotope, ecosystem) and
non-lethal in another. Lethal mutations are eliminated (this is what
"lethal" means) in a population (of cells, organs...), so the respective
positions are more strictly conserved than others. So even if the
probability of exchange of bases in a DNA was fairly homogeneous through
the whole genome, the probability of establishment/reproduction of specific
mutations in a given biological environment would be widely inhomogeneous.

>So if random mutations within a genus is normalized

....which is not the case (v.s.)...

>that leaves only mutations generated by external forces (i.e.. radiation).

Again, you forget selection.

>If a mutation happens that allows a single
>plant to reproduce better (evolve), and for it offspring to reproduce
>better (evolve) then it will dominate an environments in a relatively short
>period of time (in a matter of years or decades).

Not really. Non-lethal mutations are inherited from generations to
generations without any noticeable effect on reproductivity (which is even
facilitated in diploid or polyploid organisms which need only one "correct"
copy of a given gene although they have at least 2 copies of it). Many of
these "variations" are amassed through time. They become decisive only in
the comparatively few cases (e.g. climatic change!) where they convey some
(adaptive) evolutionary advantage. So adaptation does *not* mean the
necessary genetical changes to occur in response to selective pressure
(e.g. mutations as a consequence of climate change - LAMARCK), but it means
some feature which did exist already in the organism to become an
evolutionary advantage (- DARWIN).

>You would not expect
>mutations to occur in any order between any two plants so you shouldn't
>expect to see the same adaptations in multiple areas.

I do not need to cite examples of convergent evolution, do I? Here we are
not talking about mutations but adaptations (or evolution as a whole).
These are *NOT* the same thing. A given adaptation (phenotype) can even be
caused by completely different mutations (genotypes), and it is only (!)
the phenotype which is subject to selection.

>There is no reason
>why the same mutations that produced tuberous Drosera in Australia couldn't
>produce tuberous Drosera in South America.

On the contrary: there is ample such reason because there was never a
selective pressure which favoured _Drosera_ with bulbs/corms (tubers only
in S AF!) in S AM, so the (hypothetical) individuals carrying the same
mutations (the ancestors of bulbose _Drosera_ in AU) were not "favoured
races" here because habitats with sufficient humidity were always within
reach (cf. 1. the relative shapes and movements of AU and S AM which
are/were *NOT AT ALL THE SAME*, and 2. all the stuff I have written on that
subject already).

>But all the southern continents were connected when Drosera
>evolved and I don't think you'll be able to tell exactly where it came

Noone (perhaps sauf DEGREEF) has ever written anything else.
So what "bit more" do you have to say? Any new (& serious) idea?

Kind regards