Question on Speciation

[Soapy Sam]

Current thought is that the two surviving chimp species separated from each other about 2 million years ago- though that's based on genetic clocks rather than fossils. The common chimp line and the human separated about 6 million years ago.

This is roughly a quarter of a million generations ago. Plenty time for gradual drift to accumulate the .5-2% DNA difference.

On this question of ratchets and "hopeful monsters"- It seems to me that while a sudden, major mutation in a germ line is most likely to result in the non viability of any generated offspring- ie they die before birth, or before sexual maturity, or are themselves infertile- there is another possibility. If there is a chemical or structural reason why a particular chromosome might split (for example) into two- and if that tendency results in a small percentage of individuals in a population carrying the mutation at any time, then
1. Those individuals may be unable to breed with "normals" of their own or previous generations, while appearing in every way
normal themselves.
but
2. They may be able to breed with other individuals carrying the same mutation.

If this happens even once in a population, the mutation may become sexually inherited in several members of the next generation. We now have two breeding groups in the same population, physically together, but genetically isolated.
In small, nomadic browsing populations, this could rapidly lead to the development of two separate species.

 

[neutrino_cannon]

A good example of the essential arbitrarity with which the term "species" is applied to animals is the genus canis. To the best of my knowlege, there are no members of the genus which can't interbreed to form fertile young, so you could in theory mix a greyhound and a dingo if you were so inclined. According to some, certain dogs will even sucessfully interbreed with foxes (which belong to several genera).

In my bit of the states (and many others) the ubiquity of coyotes, wide open spaces and unleashed dogs often results in a beast called a "coydog", which can be almost any mixture of it's parentss traits. I personally suspect that the furtive beasts I see darting through the fields aren't necessarily "pure" coyote by any means.

 

[RussDill]

Thanks guys, this is making sense to me.

So with the ratchet step thing ... I understood the difference between us and chimps (preventing interbreeding) to be down to having a different number of chromosomes. Is that incorrect? Are they just indicators of genetically which one is which or would you need a matching set of pairs to reproduce?

That would probably be a big enough difference, but I think usually, its other possibilities. There is a species of bird that lives about the lattitude of canada, alaska, etc. If you start in alaska, you can find a slightly different breed of the bird in canada, and these two breeds can produce offspring. Same with canada to a breed in greenland, and you can go around and around the longitudes this way, util you get to serbia. There is a breed in serbia that cannot produce offspring with the breed in alaska. There isn't one difference that makes them unable to breed, but a series of changes.

 

[Soapy Sam]

Dogs are an interesting example because for thousands of generations, their mates have often been selected for them by another species entirely. (Us). In that case, we would expect some strange results. And we've got them.

 

[SGT]

An example of a case where speciation was observed in the laboratory:

From www.talkorigins.org

Speciation in a Lab Rat Worm, Nereis acuminata In 1964 five or six individuals of the polychaete worm,
Nereis acuminata, were collected in Long Beach Harbor, California. These were allowed to grow into a population
of thousands of individuals. Four pairs from this population were transferred to the Woods Hole Oceanographic
Institute. For over 20 years these worms were used as test organisms in environmental toxicology. From 1986 to
1991 the Long Beach area was searched for populations of the worm. Two populations, P1 and P2, were found.
Weinberg, et al. (1992) performed tests on these two populations and the Woods Hole population (WH) for both
postmating and premating isolation. To test for postmating isolation, they looked at whether broods from crosses
were successfully reared. The results below give the percentage of successful rearings for each group of crosses.
WH X WH - 75%
P1 X P1 - 95%
P2 X P2 - 80%
P1 X P2 - 77%
WH X P1 - 0%
WH X P2 - 0%
They also found statistically significant premating isolation between the WH population and the field populations.
http://www.talkorigins.org/faqs/faq-speciation.html (11 of 16) [31/8/1999 1:45:18 PM]
Finally, the Woods Hole population showed slightly different karyotypes from the field populations.

 

[Paul C. Anagnostopoulos]

Soapy mentioned drift. Even if two environments were identical, random drift could ultimately cause speciation. For example, one population could undergo a harmless chromosome split that increased their chromosome count by one, but was otherwise neutral or beneficial. Except . . . it might prevent the two populations from mating.

~~ Paul

 

[CBL4]

I believe that it would be possible to have speciation without mutation. For example, an isolated population could lose certain genes through genetic drift resulting in animals that become too small (or too large or too orange) to mate with the standard population. In other words, if all the genes that favor large animals are removed from a population, a pygmy species could result. It would no longer be able to interbreed even though no mutation has occurred.

CBL

 

[RussDill]

I believe that it would be possible to have speciation without mutation. For example, an isolated population could lose certain genes through genetic drift resulting in animals that become too small (or too large or too orange) to mate with the standard population. In other words, if all the genes that favor large animals are removed from a population, a pygmy species could result. It would no longer be able to interbreed even though no mutation has occurred.

CBL

I think this may have occured at the high school I attended.

 

[Soapy Sam]

CBL4- Speciation certainly can occur without major mutation. Just slight variation in appearance can separate breeding groups. The only requirements for evolution are a replicator and heritable variation. Check out the herring gull / lesser black backed gull ring species as a good example.

http://www.don-lindsay-archive.org/creation/ring_species.html

Edited because I cutted & pasteded part way and it no longer made sense. (Assuming it does now.)

 

[epepke]

I couldn't quite see how the species variation and ability to interbreed were necessarily related?

In populations with sexual reproduction, species is more-or-less defined as the ability to interbreed.

Some people define it as "can and do interbreed," because there are some groups that could interbreed but just don't.

With asexual reproduction, the notion of species is a lot harder to pin down.