Re: [asa] Speciation

I'd like to ask two questions, just for clarity.

1) Is it true that an allele must be expressed (ie, produce a feature) for
natural selection to operate on the organisms because they contain that
gene?

2) Must NS (in general) be operative (in general) for speciation to take
place? (except for the instantaneous case you mentioned).

Thanks,
Dave C

PS, I am deliberately ignoring the case where an unexpressed gene may be
somehow associated with an expressed gene that is undergoing NS. Simple
cases first please?

PSPS Gee, rats, I guess I really am going to have to take a class on
genetics. I'd rather study engineering. The ARM/Cortex-A8 cpu core in your
phone is way more interesting to me, but this genetics stuff keeps coming
back again.

On Mon, Oct 5, 2009 at 4:36 PM, David Campbell <pleuronaia@gmail.com> wrote:

> > Single generation speciation can and does occur. It is especially common
> in
> > plants in which chromosomal duplications can occur.
>
> This happens easily when a hybrid is produced that can reproduce
> itself but that can't breed with either parent. As Keith noted, this
> is extremely common in plants, but also is known in a wide assortment
> of animals. The reproduction may be sexual, if the hybrid is able to
> sort the entire set of chromosomes from each parent separately (e.g.,
> wheat), or asexual. This is probably the clearest example of
> instantaneous speciation; claims that new species do not form without
> miraculous help are simply wrong.
>
> > However, probably one of the most common modes of speciation is through
> > reproductive isolation from a parent population. This process results in
> > the splitting of lineages. This can occur in a variety of ways:
> isolation
> > as peripheral isolates; colonization of "islands;" or isolation by
> spatial
> > separation within the same environment, food choice, etc. In these
> cases,
> > the isolated population diverges such that development of other
> reproductive
> > isolating mechanisms prohibit interbreeding when the isolate makes
> contact
> > with its parent population. Although the new species evolved gradually
> (at
> > least in ecological time) it is an objectively distinct species.
>
> A recent study on some Drosophila flies found that several different
> genetic markers were involved in producing genetic incompatibility
> between two species.
>
> This process varies a good deal in detail. Suppose the two
> populations have specialized in ways that make hybrids do worse than
> either parent form. There will then be strong selective pressure to
> try to prevent wasting reproductive effort on such crosses. Barriers
> to reproduction could be behavioral, physical, genetic, etc.
>
> If the species do not generally come into contact, then there is less
> pressure for developing other ways to prevent breeding. E.g.,
> Baltimore and Bullock's orioles sometimes hybridize, but the hybrids
> seem not to do as well. They turn out not to be each other's closest
> relatives; presumably they are only recently coming into contact with
> the glacial and human-caused changes to the habitats of central North
> America over the past 10,000 years.
>
> This does involve new information-a novel total genome, whether it
> involves a new mix of existing genes, a novel lack of certain genes,
> or new alleles or new genes.
>
> > In the case of phyletic evolution, a whole population evolves as it
> tracks
> > environmental change. In this case there is no splitting of lineages.
> > After some period of time, the population may have changed sufficiently
> > anatomically to be classified as a new species. However, the point of
> > distinction between the species is arbitrary -- there was no point at
> which
> > the derived species was reproductively isolated from its parent. There
> is
> > considerable debate about how often this actually occurs. Some think
> that
> > this is rare.
>
> In this case, the division is very arbitrary, so the perceived rarity
> of frequency will depend a lot on whether the group was classified by
> a lumper, a splitter, or someone in between. The other difficulty is
> that this will happen over a considerable amount of time, so in
> practice it is impossible to determine whether there is reproductive
> isolation. The fact that Mulinia congesta and Mulinia lateralis
> (dwarf surf clam) consistently look different suggests a genetic
> difference of some significance, occurring during a mid-Pliocene
> sealevel lowstand and cooling event, but we would need a time machine
> to get individuals from before and after to compare against any
> species concept involving reproductive isolation.
>
> --
> Dr. David Campbell
> 425 Scientific Collections
> University of Alabama
> "I think of my happy condition, surrounded by acres of clams"
>
>
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Received on Tue Oct 6 20:52:53 2009