Re: [asa] Speciation

> 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"
To unsubscribe, send a message to majordomo@calvin.edu with
"unsubscribe asa" (no quotes) as the body of the message.