On 5/15/2016 2:37 PM, Bob Casanova wrote:
> On Sun, 15 May 2016 09:15:27 -0400, the following appeared
> in talk.origins, posted by Jonathan
> <
writeI...@gmail.com>:
>
>> On 5/15/2016 7:48 AM, Burkhard wrote:
>
>>> I remember I gave you last
>>> time a paragraph by paragraph account of Noor's actual paper, all his
>>> points were of the form: we understand a lot about speciation and have
>>> identified its basic mechanisms, here are the details that would be
>>> great to know more about.
>
>> Is this quote unclear to you?
>>
>>
>> "One of the greatest unsolved questions in biology
>> is how continuous processes of evolutionary change
>> produce the discontinuous groups known as species.
>
> It's clear to me. It's also clear, given the fact that you
> were provided with context by Burkhard and continue to
> ignore it, that you prefer quotemines to facts. I seem to
> recall the same sort of tendency on your part regarding
> complexity and how science uses it when relevant.
>
>> It says they don't know how speciation happens.
>
> Not in context, it doesn't.
>
>> So what is speciation's basic mechanism?
>
> It has at least two (selection and drift), both dealing with
> the change in allele frequencies in populations over
> multiple generations. Some of the details are still under
> investigation, but the basic mechanisms are known.
>
So all speciations involve drift /and/ selection?
Or can it be one or the other?
Can speciation occur without either?
Can there be other primary mechanisms?
What is the relationship between these two mechanisms?
When I read a paper on speciation, such as the
below, all I see is a bunch of anecdotes
without any generalized form that applies to
....all speciations.
Like with the theory of gravity, say, an underlying
theory that applies equally to all.
All this paper says is speciation can happen in
any number of ways and it depends depends depends
on all kinds of system specific details.
THAT IS NOT A GENERALIZED THEORY OF SPECIATION.
If you're going to approach the problem by
recounting each and every species one at a time
blow by blow, you'll never NEVER find a generalized
theory or 'see' the simplicity of life.
The correct answer will not be eleventy billion different
explanations one for each species, but one explanation for
THEM ALL.
I know about drift and selection, I'm talking
about a theory that never mentions any specific
example of speciation, but it's laws.
The paper by Korol et al. (1) addresses one of the most persistent
questions in evolutionary biology: How do new species arise? As with so
many apparently simple questions, there is no simple answer, only
complex answers to a number of interrelated questions. How do sexually
reproducing organisms become reproductively isolated? How do environment
and ecological interactions influence the formation of new species? Are
the processes of local adaptation and the evolution of reproductive
isolation the same (i.e., both resulting from the accumulation of small,
adaptive genetic changes) or are the genetic changes leading to
reproductive isolation fundamentally different (i.e., large and rapid
genetic changes such as chromosomal rearrangements, genetic revolutions,
transilience, or founder events)? Is the disruption of gene flow
necessary? What are the relative roles of chance events (e.g., genetic
drift) and selection in speciation?
The short answer to all of the above questions is that reproductive
divergence can evolve in a number of ways. Both drift and selection can
be important depending on the number, degree of interaction, and
magnitude of effect of genes involved in reproductive isolation; on the
relationship between genes controlling reproductive compatibility and
phenotypic characters that may be under ecological selection; and on the
historical effective population size of the diverging populations.
Reproductive isolation may evolve as a consequence of genetic drift in
populations of small effective size (see, e.g., refs. 3–5 for recent
treatments), or as a byproduct of adaptive divergence or genetic drift
in large populations in allopatry [the classic allopatric divergence
model of Mayr (6)]. Reproductive isolation may also evolve in sympatry
because of disruptive or divergent selection. For example, natural
selection on characters important in both ecological function and mate
recognition [such as bill size in Darwin's finches (7) or body size and
shape in sticklebacks (8–10)] can lead to premating reproductive
isolation without geographic isolation. Rapid chromosomal changes (as in
the origin of polyploid plants) can result in instantaneous reproductive
isolation, as can changes in genes of large effect that result in rapid
evolution of phenotypic characters important in reproduction (e.g., ref.
11). Host shifts in phytophagous insects are well-known to result in
essentially instantaneous reproductive isolation (e.g., refs. 12–14).
The challenge is to distinguish among alternative hypotheses for
diversification in natural populations and to determine the relative
roles of selection and drift in speciation.
Natural selection has always been considered a key component of adaptive
divergence and speciation (2, 15–17), but the importance of selection
has been eclipsed in recent decades by a strong focus on the geography
of speciation and on the purely genetic mechanisms by which reproductive
isolation evolves (see refs. 18–20 for reviews). Even though selection
was seen as critically important in sympatric speciation, sympatric
divergence was thought to be rare, and the role of ecology and the
environment in diversification received little emphasis. Currently,
there is resurgent interest in the role of ecology in speciation.
Several recent studies (9, 21–28) have emphasized the importance of
ecology and selection in speciation, regardless of the geographic
context in which populations diverge, and the term “ecological
speciation” has become an important part of the modern lexicon. In a
review of 40 years of laboratory experiments on speciation in
Drosophila, Rice and Hostert (29) concluded that founder events, drift,
and isolation played little role in speciation (but see ref. 20 for
critique). On the other hand, diversifying selection was found to
contribute substantially to the evolution of reproductive isolation,
even when populations were not isolated [see also Barton and
Charlesworth (30) who reached a similar conclusion]. This finding
bolstered earlier theoretical studies that showed how premating
reproductive isolation could evolve as a consequence of ecological
selection on characters involved directly in mate recognition or as a
consequence of pleiotropy or genetic hitchhiking of genes controlling
ecological and reproductive characters (31–33).
The paper by Korol et al. in this issue (1) is important in that it
demonstrates significant premating reproductive isolation among
populations of D. melanogaster experiencing divergent selection between
habitats, whereas there is no reproductive isolation among populations
occupying similar habitats. This result is consistent with the evolution
of reproductive divergence as a consequence of ecological selection
(ecological speciation). However, a key question remains unanswered,
namely, did reproductive isolation evolve in situ in response to
selection or is the presence of reproductive isolation among populations
a result of secondary contact between divergent populations whose
ecologies differ and whose ranges abut in Evolution Canyon? This
question is important because it bears on the mechanisms by which
reproductive isolation has evolved. If the populations are historically
divergent, then a degree of reproductive isolation may have evolved
because of drift or as a byproduct of adaptive divergence in allopatry
before their meeting in Evolution Canyon. If premating reproductive
isolation has evolved in situ as a result of local adaptation, then
traits under ecological selection must be either directly involved in
mate choice, or genetically correlated (via pleiotropy or linkage) with
phenotypic characters important in mate choice.
History, and the relative roles of drift and selection, are general
issues for empirical studies of speciation in natural populations. One
approach to examining the relative roles of drift and selection in
population divergence and speciation uses the correlation between
reproductive (or morphological) divergence and genetic divergence in
neutral molecular characters (Fig. 1). The null model is that
reproductive divergence evolves simply as a byproduct of the
accumulation of genetic differences among populations because of
mutation and drift. Under the null model, populations should show
similar levels of reproductive isolation for a given level of genetic
distance regardless of their ecological milieu. Several studies have
assessed the degree of reproductive and/or morphological divergence in
relation to genetic distance within and among populations occupying
different habitats and found support for ecological speciation (e.g.,
10, 19, 21, 22, 26, 28, 34). In addition, powerful evidence for the role
of natural selection in population divergence and speciation comes from
examples of “parallel speciation” (ref. 24; Fig. 1c) in which
reproductive and/or morphological divergence evolves repeatedly in
response to similar selective regimes in evolutionarily independent sets
of populations (9, 10, 19, 21, 28).
http://www.pnas.org/content/97/23/12398.full