Question on Speciation (help)

[redfoxindigo]

Hi all,

I'm having some trouble with this one question on speciation.

2) Using the full name, place the following events in the proper order that will cause and then ensure speciation.
a) Geographic isolation occurs
b) Reproductive isolation occurs
c) Speciation occurs
d) Distruptive isolation occurs

My immediate thought was a)d)b)c) .... but now i'm not so sure.

Any help you can give on this question would be most appretiated.

Sincerely,
Redfox

[AstusAleator]

d,b,c,a

insert *extra space*

[Locus]

I think, that most significant think within speciation is disraptive selection caused by differences at the ecologycal conditions within the ecologycal space were initial spechies exest.

[Darby]

"Disruptive isolation" is not a common term in evolution - what's it supposed to mean here?

You may have your order right, depending upon the definition of that term.

[AstusAleator]

I think Locus just defined it...

[Darby]

No, that context doesn't fit this list at all - it's dealing with adaptive radiation, while the question obviously is about geographic isolation.

Really, the original question itself doesn't make complete sense - if you've got geographic isolation, you've got reproductive isolation, and I'd lean toward whatever's going on also being disruptive isolation (which is itself a bit of an oxymoron). They happen simultaneously.

[AstusAleator]

Hm, I guess you're right. Disruptive isolation is a tricky thing to define. I guess that it might mean ecological disruption (like a fire? or a new population of predators?) that causes a barrier to gene flow?
I think that maybe there was a mistake in the wording of the question? Are they talking about disruptive selection, ie selecting for extreme phenotypes?

anyway

The question is not specifically about geographic isolation/speciation. It is simply about speciation, which can occur through geographic or reproductive isolation. They may seem to overlap, but any of them can occur independently of the others.
It's been a year or so since I had to define these, but I'll give it my best shot.

Geographic isolation occurs when a change in the geography of a region splits an original population apart, and causes a barrier to gene flow.

Reproductive isolation occurs when behavioral patterns, chromosomal issues, or body types don't allow successful reproduction between two or more segments of an original population.

So, if you have geographic isolation, you don't necessarily have reproductive isolation because if put back together, individuals may still successfully interbrede.
And, if you have reproductive isolation, you obviously don't necessarily have geographic isolation.

[Darby]

The problem here is that you're not applying your definitions to the original question: steps leading toward speciation. For that, the geographic isolation becomes the source of reproductive isolation, which sets up speciation.

If "disruptive isolation," using your definition, means that once the geographic isolation is in place, then changes occur in ecology, driving adaptive changes leading to speciation, then that's where it would fit in the sequence. But genetic drift can produce speciation without changes in ecosystems...wow, it is easy to wander off the question!

[AstusAleator]

The problem here is that you're not applying your definitions to the original question: steps leading toward speciation. For that, the geographic isolation becomes the source of reproductive isolation, which sets up speciation.

I think the question is purposely open-ended, in that the sequence of events does not necessarily have to be in one order. I think the key part is that it must first cause speciation and then ensure speciation, and that any argument given within those parameters is acceptable.
So you see? it isn't "steps leading toward speciation" as much as steps involved in speciation.

The inability of two populations to reproduce after geographic isolation is not reproductive isolation. Reproductive isolation occurs in individuals that can encounter each other within their natural range. It is the result of pre-zygotic (behavioral patterns, body size, life-history traits, etc) or post-zygotic (incompatible chromosomes, deleterious mutations, offspring sterility, etc) traits.

So, the two can be mutually exclusive (although I'm sure it's possible for them to be linked as well).

I think that the question is somewhat faulty, in that both reproductive and geographic isolation can cause and ensure speciation, and arguments could be made that either is more effective than the other at doing so.
Also, like we've already mentioned, disruptive isolation is something that I can't readily define. Like I mentioned before, maybe they meant disruptive selection...