Well, the OP is more about how evolution works, not whether it is real or not. Protein homology is good evidence for evolution, but it doesn’t, AFAIK, explain what causes speciation.
jahrding: You can see me blathering on at length about species concepts, subspecies concepts, and that old GD bugaboo, race, towards the end of this thread:
http://boards.straightdope.com/sdmb/showthread.php?threadid=64664
Re: point #1 - In brief, the Biological Species Concept (i.e. freely interbreeding populations ) has quite a few practical difficulties. Actually they all do, but the BSC seems to have more than most. The Evolutionary Species Concept is more to my preference, i.e. species as distinct allopatric lineages. But there’s others. And they don’t always agree( which is the point, of course ). Homo sapiens are a rigorously defind species no matter which concept you use. But other critters…
Take the Bullsnake/Gopher Snake/Pine Snake for example - the Pituophis melanoleucus “complex”. It’s long been recognized by systematists as a single, highly variable species ( like us 
), subject to considerable clinal variation in multiple directions across the length and breadth of North America. But around 1990, some workers pointed out that in fact there was a small gap in the Great Plains that in effect partitions the range into two enormous blocks - one comprising a complex of Western subspecies, one a complex of Eastern subspecies. Under the ESC, they declared they should be considered separate species - sure they can and will interbreed freely if given the chance - but the point is they aren’t. There is minimal, if any, gene flow between these two huge blocks. So they are, in effect, on separate ‘evolutionary trajectories’ and hence should be considered different species. But, and here’s the kicker, if those populations were to come back into contact with each other, there is little doubt they would then be considered a single species again, because they will interbreed freely and as gene flow between the two populations recommenced, they’d be back on a single evolutionary trajectory.
So what do we have? One species? Or two? Actually some recent molecular evidence ( which I accept ) indicates that we may have three . But molecular evidence is all arbitrary to some extent and varies from group to group. The only ( presumed ) constant is internal consistency.
Re: Point #2 - A smartassed question on my part about a highly unusual situation . But it points out the difficulty in applying different species concepts. In unisexual Cnemidophorus the BSC doesn’t work period, for obvious reasons. Can’t apply a interbreeding test to parthenogenetic clones 
. The ESC may work. But from a practical standpoint it’s a nightmare. A given unisexual “species” is generally not descended from a singly hybridization event, but rather multiple ones. As clones the descendants of each event are on their own little separate evolutionary trajectory. Yet they’re morphologicaly, ecologicaly, even karyologicaly identical to the other members of the “species”, that are descended from separate events. The individuals from these different lineages can be distinguished with a full lab. Otherwise, nuh-uh. So for purely practical reasons they are defined morphologicaly ( i.e. the MSC ). This is one of those cases where numerical phenetics, which is generally rather ahistorical, becomes necessary.
Re: Point #3 - Here’s the main problem I have. Yes, you can have fairly distinct species boundaries ( i.e. us ). But we’re taking a snapshot in time. And many boundaries are indeed very fuzzy. We unfuzz them when we define a species for our own hiearchical needs. Now for practical reasons, I think it is necessary that we do so. Cartesian reductionism does have it’s place 
. But we should be aware that at least some of our rigidly defined groups may not really adequately model the true situation in the natural world.
So to say as a blanket proposition that species are “discrete, natural units”, is, I think, overreaching. Maybe we could offer up the following highly equivocal compromise:
Species are discrete, natural units - Sometimes.
- Tamerlane
Yep, I agree with difficulties with the BSC. And thanks for the link.
Interesting example. I guess I’d say as long as they can interbreed then no problem–still one species in the BSC book. However, I’d be hesitant to even recognize a nub (rather than a full branch) on the evolutionary bush because I’m not sure there is any evidence that geographic separation would ultimately lead to two (or more) clearly divergent species. BTW I’m not married to the BSC so I don’t mean to come off as its champion. And after reading a bit more on your ESC, I like what’s in this too.
Would this make me a racist if were to attempt some taxonomy of these critters?
::cough, cough::
::spitting out water::
Whew!! Thanks for the life preserver! 
The snapshot analogy works well for me. But my sense is that those ‘fuzzy boundaries’ don’t make up too much of the mass of the bush. IOW at most points in time in which we take our snapshot, there aren’t a lot of “Is it an ‘A’ or is it a ‘B’?” At any given time, most species are clearly identifiable as discrete entities and those cases where the jury is still out number in the very small minority.
BTW I hate Cartesian dualism/reductionism but bow to its power and utility in a small number of instances and like you said, this is one of them where it does seem useful. Sob–I’m such a slave to the enlightenment.
Since jharding has given his response, I figure I should probably give mine
Here, let me first make a correction to your BSC definition (“i.e. freely interbreeding populations”):
When Ernst Mayr first developed the Biological Species Concept in 1942, he defined a biological species thusly:
This would, to my mind, put the snakes you used as examples in the same species (this fits the definiton of an allopatric species). The “potentially” is important in the definition - if it is accepted that yes, they might interbreed if brought together, then technically they are the same species, per the BSC.
As to the multiplicity, the BSC as defined by Mayr is perhaps the major one used today. While it may be difficult to apply, it is sound in theory. Even asexual groups, to which the ‘interbreeding’ criterion does not apply, are clearly reproductively isolated from other groups. One might even argue that each asexual organism constitutes its own ‘population’, and that each group of said organisms constitutes the aforementioned ‘array of populations.’
Despite the difficulty in applying the BSC, the ancestor-descendant relationship between organisms indicates that there must be some form or reproductive isolation between groups, and this isolation is one of the driving forces behind speciation.
Having said all that, my favorite species definition is “a species is a species if a competent specialist says it is.”
**
I’m afraid I don’t know enough about the organisms you’ve mentioned to give a qualified answer. Damn it Jim, I’m a paleontologist(-wannabee), not a Cnemidi-phoro-whatever-the-heck-it-isologist!
**
But, if they are not quite on the end of the continuum where they are separate species, isn’t it clear that they are not, you know, separate species?
Personally, I don’t have a problem with the “species-as-a-snapshot-in-time” concept, since we, as humans, are only capable of directly observing this geological snapshot in time, and we many other snapshots in tim in the form of the fossil record. But, because of the pacing of evolution (and, specifically, speciation), we are unlikely to actually witness many instances of speciation (which is, of course, not to say that we haven’t observed such instances, or cannot observe them, just that it’s likely to be comparatively rare).
If we were very long lived individuals (on the order of tens of thousands or hundreds of thousands of years, say), then your objection (stated in one of your responses to jharding) might hold. However, even then, I would argue that the point at which two populations (and there must exist such a point) become reproductively isolated, per the BSC, is the point at which you can say, “Ha! A new species has formed!” Up until that instant, you have but one species.
So, I guess my point here is that while the actual continuum between species A and species B may be fuzzy in reality, chances are we will only see species A and species B as separate entities, so I see no problem with declaring them as such.
In answering the OP, let me first draw a distinction:
“Prezygotic isolation” is a form of separation between species which takes place before conception: for example, the mechanical incompatibility of genitalia which you mentioned. In insects you can have species with different pheromones, different mating dances, etc., (not to mention a great deal of mechanical incompatibility of their “swiss army knife” insect genitals.)
“Postzygotic isolation” takes place after conception. Since you seem to be asking about genetic factors which prevent interbreeding between species, you’re interested in postzygotic isolation.
Now, in the case of trying to crossbreed a human and an octopus, the gross differences between the contents of their genetic material make a successful cross impossible: the resulting mix of human and octopus genes would simply be too jumbled for the individual parts to work together. The proteins which cause an octopus to grow eight arms would interfere with the proteins which cause a human to grow two arms and two legs, and the end result would simply be a nonviable mess.
On the other hand, the differences between the contents of the horse and donkey genomes are quite minimal. There’s no particular reason why you couldn’t have an animal in which half the genes come from horses and half come from donkeys (in fact, it’s called a mule.) Why, then, can horses and donkeys not produce fertile offspring? The reason lies in the arrangement of the contents of their genomes.
As you know, you have two copies of each chromosome: one from your mother, and one from your father. When you produce sperm or eggs in the process called meiosis, the chromosomes have to be sorted so that one chromosome from each pair ends up in a sperm or egg cell, so that you don’t have (for example) a sperm with no chromosome #3 and two copies of chromosome #12.
When this sorting process occurs, the maternal and paternal chromosomes line up, gene for gene, alongside each other and then are pulled apart and encapsulated in two different cells. Somewhere in the course of horse and donkey evolution, sections of their chromosomes got swapped around so that they can’t be lined up during germ cell production: one end of horse chromosome 3 might match the same end of donkey chromosome 3, but the other end of horse chromosome 3 matches the end of donkey chromosome 6. The end result is that a mule can’t produce viable germ cells, because it simply can’t sort its chromosomes properly. Bear in mind that there is some degree of tolerance for small chromosomal rearrangements. If a horse suddenly got the end of one chromosome rearranged into the middle (to draw an analogy, the sequence abcdef could be turned into abcfed,) the chromosomes would twist during meiosis to line up the rearranged regions, despite their slightly different positions.
In terms of evolution, then, the separation between species typically takes place in roughly four stages:
-
Geographic separation. A small population of geckoes is separated from the rest (let’s say they’re on gecko island.) This means that a) they can branch off into a new species, separate from the rest of the geckoes, and b) new mutations will be conserved through (to oversimplify a bit) inbreeding.
-
Chromosomal rearrangements. Rearrangements of the chromosomes slowly start to accumulate. At any particular step, the geckoes with new rearrangements can interbreed with the other geckoes on Gecko Island, since the changes at any step are small and can be accomodated by twisting the chromosomes during meiosis so that they can line up. Over time, however, the rearrangements accumulate until Island Geckoes can’t produce fertile offspring with mainland geckoes.
Bear in mind that at this point, the inability to interbreed has nothing whatsoever to do with selection pressure for beneficial traits. It just happens that in the wild, the chromosomal rearrangements accumulate at the same time that beneficial mutations do.
Now, suppose the island and mainland geckoes were reunited (for example by a climate change that exposed a land bridge.) At this point, it would be beneficial for the geckoes to be able to tell each other apart, so that they didn’t waste resources on producing infertile crossbreeds. That brings us to:
- Prezygotic isolation. The geckoes are now under selection pressure to develop incompatible genitalia, pheromones, or mating dances so that they don’t produce infertile offspring.
Now that the two populations of geckoes are no longer sharing genes, they can continue to drift further and further apart, until we reach:
- Gross differences in the contents of their genomes make it impossible to give birth to viable offspring, even sterile ones.
-Ben
That pretty much answered my question. I had surmised from the other posts, and from prior knowledge, that the eye color change wouldn’t lead to a new species in and of itself. It is just one of the many visible characteristics that gets carried along when a speciation “event” (in quotes because it’s not a 30 second shot) occurs. You explained in great detail what that “event” is, so you filled in the gap. Thanks.
Mike
Ben: Good post .
Darwin’s Finch: You too. But you’re wrong!!
Just kidding . It’s all about perspective and philosophy ultimately.
You see, given the snake example, I would disagree that they are the same species. Given the lack of gene flow between the two populations, and given the existence of characters ( however minor ) that could be used to separate them I agree with the argument that these populations, at least temporarily, are on separate evolutionary trajectories and therefore should be accorded separate status. As it happens, in this case there is molecular evidence ( circa 2000 ) to support the separation into Pituophis melanoleucus ( East ), Pituophis catenifer ( West ), and Pituophis ruthveni ( Florida - There appears to be a small spatial separation here, as well ). In otherwords I reject the BSC explanation and accept the ESC/PSC explanation.
A very brief example of this debate as applies to falcons can be found here:
http://www.falcons.co.uk/mefrg/Falco/13/Species.htm
A brief, but a little more thorough coverage of currently proposed species concepts can be found here ( you might find the Cohesion Species Concept interesting ) :
http://www.tulane.edu/~eeob/Courses/Heins/Evolution/lecture6.html
At the bottom of this page is a nice, concise chart that explores stengths and weaknesses of all of the above-mentioned concepts and then some:
http://www.ebi.calpoly.edu/BioSci/Courses/BIO/BIO414/MacroEvol/Mac03.html
I’ll agree that the BSC still remains dominant paradigm ( don’t you just hate that phrase? ). But I think it slowly continues to decline among professional systematists. But this…
…Makes perfect sense to me .
What!!! They’re not teaching Cnemidophorology as a lower-division requirement anymore? I tell ya, the decline in the state of higher education these days is just shocking!
Well, that’s the problem. I would submit, that:
a.) No, it’s not always clear . and
b.)It kinda screws up the “discrete”, in discrete, natural unit.
Now in the rest of your argument you start trying to get practical on me, completely screwing with my philosophical points - Damn you !
However I submit, that as a paleontologist, you ARE actually supposed to be searching for the fuzzy boundaries as a recoverer of evolutionary history. I fact, just to be a jerk, I hereby submit the following page which argues just this point of view ( while bolstering a point or two of yours ) for the field of Paleontology :
http://www-geology.ucdavis.edu:8000/~gel3/speciesconcept.html
- Tamerlane
You’re welcome! If you want a good evolutionary biology textbook, read Mark Ridley’s Evolution (be aware that there’s also a popular-level book of the same name by the same author.) It will answer plenty of questions about evolution that you wouldn’t even think to ask.
Tamerlane: thanks!
-Ben