Some aspects of speciation and extinction

[STYRBJORN]

I find that most people who accept the fact of evolution assume that it occurs over extended periods of time. (Bible Bashers are invited to leave now.) While this is generally the case, there are instances wherein we can see evolution in observable time, extended spatially. The paradigmatic case occurs in Britain, so makes an appropriate starting point.
I quote from Mark Ridley*
"As we look at the herring gull, moving westwards from Great Britain to North America, we see gulls that are recognisably herring gulls, although they are a little different from the British form. We can follow them, as their appearence changes, as far as Siberia. At about this point . . . the gull looks more like the . . .lesser black-backed gull. From Siberia, across Russia, the gull . . . looks more like the British lesser black-backed gull. Finally, in Europe, rhe ring is complete: the two . . .forms meet, as two perfectly good species . . .which do not interbreed."
My point here is that there is continuous interbreeding along the chain. If you identified separate populations as "A", "B", "C", . . . "Z" you would find that "A "breeds with "B". "B" breeds with "C". Maybe "A" breeds with "C". "Y" breeds with "Z". "A" does not breed with "Z" because they are different species. At what point does "speciation" occur? Where on that 10,000 mile ring do herring gulls become lesser black-backed gulls?
Now shift your perspective. Imagine the process happening in time rather than space. When do herring gulls become extinct?
This post is already over long, so I will defer looking at open line speciation.

Ric
*Ridley,Mark. The Problems of Evolution OUP 1993

[Dorts]

Try looking at Betula utilis from western europe to eastern asia if you want to see speciation in action.
You can either make the case that they are one large complex under the general specific of 'utilis', or you can readily split them into individual species as you move across the continents. Though exactly where you place the dividing lines presents a problem.

Every 1,000 miles travelled, from any point, will give you a plant quite distinct from the one at the first point. The smaller the distance travelled, the smaller the difference between plants, until in just a hundred miles or so, no difference can be detected.

Any natural or un-natural isolation of plants along this line, so as to no longer allow cross-pollination between the variiations could result in a new species being formed.

Orchids are, in an evolutionary sense, are the most active plants we have. There are twice as many orchid species as there are birds, around 25.000 species recorded.
Orchids readily cross-breed within and to a lesser extent without their genera. Plant breeders have jumped at this over the years and around 100.000 new orchid cultivars and hybrids have been produced.

In the wild, even here in Britain, we can see orchids doing there stuff.
In my 50 years of studying our native orchids I have seen, particularly among the dactyl-orchids, new 'types' being formed that I believe one-day will almost certainly become sub-species and maybe in some cases go on to be classified as distinct species.

Evolution is a continuous process, sometimes moving slowly, sometimes almost at the blink of an eye.
Dorts.

[Adam Cheeseman]

Originally Posted by STYRBJORN

"As we look at the herring gull, moving westwards from Great Britain to North America, we see gulls that are recognisably herring gulls, although they are a little different from the British form. We can follow them, as their appearence changes, as far as Siberia. At about this point . . . the gull looks more like the . . .lesser black-backed gull. From Siberia, across Russia, the gull . . . looks more like the British lesser black-backed gull. Finally, in Europe, rhe ring is complete: the two . . .forms meet, as two perfectly good species . . .which do not interbreed."

My point here is that there is continuous interbreeding along the chain. If you identified separate populations as "A", "B", "C", . . . "Z" you would find that "A "breeds with "B". "B" breeds with "C". Maybe "A" breeds with "C". "Y" breeds with "Z". "A" does not breed with "Z" because they are different species. At what point does "speciation" occur? Where on that 10,000 mile ring do herring gulls become lesser black-backed gulls?

Ric

Herring Gulls wouldn't become Lesser Black-backed Gulls because they are different species with their own genetic fingerprints. You have different races of Herring Gulls and of Lesser Black-backs, which interbreed between races of their own species, and occasionally with other species. Looking at the Herring Gull, you only have to look at the recent splits of Herring Gull races into definitive species of Yellow-legged, American Herring and Caspian on subtle structural and distribution differences. Not an expert on evolution but surely it would take repeated interbreeding between two species to create a new species whilst diminishing the populations parent species.

Cheers,

Adam

[amycaramel]

Originally Posted by Adam Cheeseman

Not an expert on evolution but surely it would take repeated interbreeding between two species to create a new species whilst diminishing the populations parent species.

Not quite how it works. Although different species can, rarely, breed they can only produce infertile young. Speciation takes place when one species splits into 2 (or more) lines following evolutionary changes.

As for one type of gull evolving to look like another there are many cases of parallel evolution. 2 species on different continents with different ancestors finding the same solutions to the question of how to survive. They look and behave the same (or very similar) but are completely different species and as such cannot breed.

I wish I could think of an example ...sure other people can suggest some!

(I'll appologise now for the teacher in me coming out! :-) )

[amycaramel]

Originally Posted by amycaramel

I wish I could think of an example ...sure other people can suggest some!

Had a look. Many butterflies look the same, but evlved seperately. And Hedgehogs and echidnas. Different in many ways, echidna being egg laying for one thing, but look similar. ( awful example I know)

[Dorts]

Originally Posted by amycaramel

Had a look. Many butterflies look the same, but evlved seperately. And Hedgehogs and echidnas. Different in many ways, echidna being egg laying for one thing, but look similar. ( awful example I know)

What you discribe is Parallel Evolution. It occurs when organisms which are unrelated develop the same characteristics or similar adaptations due to their environment.

Regards to the fertility of hybrids; certainly in the plant world hybrids between different subspecies within a species, known as intra-specific hybrids are usually fertile.
Hybrids between different species within the same genus, generally known as interspecific hybrids are occasionally fertile.
Hybrids between different genera, known as intergeneric hybrids are rarely fertile.
Dorts.

[Bobbobthebob]

Originally Posted by amycaramel

Not quite how it works. Although different species can, rarely, breed they can only produce infertile young. Speciation takes place when one species splits into 2 (or more) lines following evolutionary changes.

Not entirely true. We try to categorise life itself into nice discrete packages when there are often many grey areas and quite a lot of viable hybridisation. This is the cause of lots of taxonomical arguments over splitting and lumping species.

The Herring/LBB gull thing is basically the suggestion that they're a ring species. The gist of it being that Western European Herring Gulls readily interbreed with their American counterparts who in turn interbreed with their nearest Siberian counterparts who in turn interbreed with other Siberian gull groups all the way back around to Western Europe with the Lesser Black-backed gulls we have here. Therefore gene flow takes place between the two species even though they rarely ever directly hybridise in Western Europe. As I understand it, this has turned out to not be totally true and that the situation is much more complex than that. The Herring/LBB gull complex is still used as a ring species example in quite a few textbooks.

Originally Posted by Dorts

What you discribe is Parallel Evolution. It occurs when organisms which are unrelated develop the same characteristics or similar adaptations due to their environment.

I think the term is actually convergent evolution? Parallel evolution is where two species from the same ancestry meet the same ecological niche while remain separate species. Convergent evolution being when unrelated species converge on a similar body plan.

[amycaramel]

I will happily admit to over simplification. The curse of teaching 11 year olds. One of many things that I love about nature is the constant ability to provide exceptions to every rule we try to use to make sense of it.

[Dorts]

Originally Posted by Bobbobthebob

I think the term is actually convergent evolution? Parallel evolution is where two species from the same ancestry meet the same ecological niche while remain separate species. Convergent evolution being when unrelated species converge on a similar body plan.

Parallel Evolution, the evolution of geographically separated groups in such a way that they show morphological resemblances.

Convergent Evolution, the process whereby organisms not closely related independently evolve similar traits.

Bob, you are right in that I didn't make myself clear about the same or different ancestry.

We also of course have Divergent Evolution where a quantity or accumulation of differences between groups can lead to the formation of new species. This is what I was trying to describe earlier regards to what I see happening in the Dactyl orchids in Britain.
Dorts.

[aeshna5]

Originally Posted by amycaramel

Not quite how it works. Although different species can, rarely, breed they can only produce infertile young. Speciation takes place when one species splits into 2 (or more) lines following evolutionary changes.

As for one type of gull evolving to look like another there are many cases of parallel evolution. 2 species on different continents with different ancestors finding the same solutions to the question of how to survive. They look and behave the same (or very similar) but are completely different species and as such cannot breed.

I wish I could think of an example ...sure other people can suggest some!

(I'll appologise now for the teacher in me coming out! :-) )

But different species interbreeding DOES sometimes result in fertile young. This is the reason for the recent cull of Ruddy Ducks in the UK as they were hybridising with Eurasian White-headed Ducks; the resultant young proving to be fertile.

Similar results have also happened with Mallard introduced to parts of the world where it wasn't native.