Re: On the Origin of a Species

John Harshman wrote:

dkomo wrote:


John Harshman wrote:



dkomo wrote:




explainer wrote:





Again, if this is a dumb query, I will delete it fast or you can ignore
it. As I read the posts for compliance with the original intent of TO
I think this one is closer than many.

I have been trying to understand, in non-scientist language if that's
possible, how a new species comes into being. Everything I read that I
understand says, "nobody knows and there aren't any good guesses".
What I read that seems to hold the ore I seek is beyond me.

Must I get an advanced degree in biology to fathom what may be going on
in evolution?

Is it as simple as A(m) X A(f) => B(m or f) and another A(m) X A(f) =>
B(m or f) with the new B(m) X B(f) => B(m or f) with enough B(?) coming
into being to start a new species. This is the simple (minded?)
thinking of many, but I think it is much more complicated, that a
population of A, probably a small one, interbreeds for many generations
causing mutations, many of which do not survive, but eventuallysome of
the mutants breed moving the DNA another notch away from the A species
with the eventual outcome of a new species, B, coming into being.



Here's speciation made very simple. A small group of animals migrates
to a different location, and they no longer interbreed with the main
population (they become reproductively isolated).


All fine except that the group doesn't have to be small and it doesn't
have to migrate. It could migrate. Or it could just stay in place while
a barrier of some kind arises in the middle of the population.



Yes, I was just trying to provide a straightforward example. I'd guess
that migration of small populations is the most common way that
speciation occurs. A barrier arising in the middle of the population is
probably relatively rare.


I would guess the opposite. What data exist on this support me, but I'm
not sure the assumptions required are valid.


Surprises me because migration is so universal. Even plants "migrate"
-- their genes can be spread by pollinators, by animals eating fruit, by
the wind, etc.


While we're splitting hairs, it should be mentioned that there is no
necessity for *both* populations to genetically change, only that one
does so.


True. Of course both populations will change. The question is whether
any of that change is relevant for the origin of reproductive isolation.
If isolation results from drift, we expect equal change in each
population.

Untrue. If the populations are of different sizes, the rates of drift
will be different. Even if they are the same size, the allele frequency
changes will still be different because drift is a random phenomenon.

If from selection, then if one population remains in the
original habitat and the other does not, we expect more change in the
second population.


An example here is again the migration of a small population
to a location that is ecologically distinct from the original location.
Because the population is small, the founder effect will be strong, as
will be genetic drift.


The latter is not true. The rate of neutral evolution doesn't depend on
population size.

Huh? The smaller the population the greater is the sampling error on
the gene pool from reproduction. How can this not affect neutral
evolution, which is strictly caused by statistical effects?

Nearly neutral evolution does, though, and there is
some evidence that it may be important.


Selection and genetic drift are entirely separate processes. So if
there is some selection in nearly neutral evolution, the selection
shouldn't be dependent on population size. The population genetics
equation I've seen that governs selection does not include population
size as a variable, only initial allele freqencies and fitnesses of
those alleles.


--dkomo@xxxxxxxx


Too, because the migrating group can exploit new
ecological niches, it is liable to evolve rapidily. What is likely to
happen, then, is that the small group can change rapidly in a relatively
few generations while the main population, if it is large and its
enviroment stays stable, changes hardly at all. We still get speciation.


True. Whether this is a frequent event is the question.


.

Relevant Pages

  • Re: On the Origin of a Species
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