Greetings all,
A short poser.
Given that all existing species (particularly those of the animal kingdom) are descended from a somewhat similar ancestor species, but divergent enough to be considered a separate species; and given that the general definition of a species is that a species of animal does not/cannot interbreed with another separate species (although, admittedly, there are many exceptions – tigers and lions interbreed, horses and donkeys interbreed as just two examples, but with sterile offspring). Then, by this definition a species must generally have evolved/changed its characte ristics to the extent that members of this species now cannot inter-breed with members of the ancestor species.
Does this imply that the descendant species (generally being more fitted to exploit a particular environmental niche that the ancestor species) ultimately replace the ancestor species, driving that ancestor species into extinction?
Or do the two species continue to co-exist side-by-side, possibly each exploiting somewhat different environmental niches, due to sympatric speciation?
If one considers the genus homo, for example, why are all the previous variants of hominins not continuing to live today alongside homo sapiens?
So the question arises – are there any known, proven instances of an ancestor species continuing existing with a descendant species? Are there any known ornithological examples of this?
Regards
Ralph Reid
============================== To unsubscribe from this mailing list, send the message: unsubscribe (in the body of the message, with no Subject line) to: birding-aus-request@vicnet.net.au
http://birding-aus.org ==============================
It depends. A few points to consider. A good mechanism for speciation is the physical separation of a single species. There are a lot of ways for this to happen over geological time. For example, when more water is tied up in glaciers, sea levels rise and many archipelagos become a single land mass. As the globe warms and sea levels rise, the single land mass is inundated at low spots and a chain of islands re-emerges. Simultaneously, cold-loving species are now confined to isolated high-altitude refuges where previously they could roam over large areas. Imagine a population of, oh, squirrels separated on either side of a new ocean channel. They’re one species. Given enough time and chance, they’re likely to diverge to the point where they don’t have the capacity (or behavioral/habitat compatibilities) to interbreed. Now imagine that the islands are rejoined because the world is getting colder and these two populations can meet up again. Which one is the ancestral species? Neither? Both? It doesn’t really work that way – they’re each populations that have descended from another population – neither has primacy.
Living ‘ancestor species’ isn’t a meaningful concept a lot of the time. If you look at a population of animals 10,000 years ago that has never been split, chances are it’s quite different today. Why? Because of natural diversity in the population, random variation, and random selective pressures. The genome isn’t fixed even in a ‘stable’ population.
You can make a lot of predictions about diversity just by looking at a map. Anywhere that’s got islands (divided by water, temperature or other divisions) arising and subsisting regular over the eons is likely to have higher diversity. North American has something like eight nearly non-overlapping species of Chickadee, for example. Length of isolation can also make a big difference as then there’s less chance of members of the original population meeting up with their long-lost-fellow-descendants and interbreeding (gene flow), thus minimizing divergence. (The Mallard, as an example, is now swamping a lot of other Anas ducks with Mallard genes which is reducing the number of localized ducks around the world.) Hawai’i is an obvious example of radical divergence based on long isolation.
The thing about natural selection is that its fundamentally random, or so they say. That means that if you had a population of identical twins, split the pairs and created two isolated populations – they would have different fates. Let’s say you’re an early ancestor of the finches and you land on O’ahu. There aren’t any/many birds or mammals there. The number of niches you can eventually adapt to is huge. (Hawai’i and Madagascar are both commonly cited as good examples of adaptive radiation – the Galapagos finches, too.) Meanwhile, the original ancestor back on the mainland may have been in a more mature ecosystem with fewer opportunities to exploit and so it hasn’t changed as much. Likewise, absolutely random factors may make these populations differ by presenting different selective pressures. The soils in parts of Namibia are loaded with toxins that the local herbivores and their predators have built up an immunity to over a slow, selective process. What happens if you bring the same species in from populations on the eastern side of the continent at a similar latitude? They can drop dead.
Here in Australia, the birds are really distinctive – I guess the continent has been isolated for a long time. There just aren’t many birds here that are indistinguishable if you get a good look. That’s definitely not the case in North America or Europe (hard: raptors, new world warblers, old world warblers, sparrows, finches, gulls….) Then again, there’s nothing to say that there’s been any adaptive benefit to being readily distinguishable by humans, so that’s kind of an unreasonable bias on my part…
There are some “fossil” species that seem to have remained unchanged for unspeakable ages. It stands to reason that plasticity itself is a selectable quality – and that there are selective events that will favor either a population that’s very fixed in form (horseshoe crabs) or one that’s highly diverse (Hawaiian land snails.) Since there are different pressures out there in the mists of time, it’s easy to come up with examples of either outcome. ===============================
To unsubscribe from this mailing list, send the message: unsubscribe (in the body of the message, with no Subject line) href=”mailto:birding-aus-request@vicnet.net.au”>birding-aus-request@vicnet.net.au
http://birding-aus.org ===============================
The problem is we can’t usually tell the time-depth of species sufficiently accurately to determine whether one form or another is ancestral. (The case of humans and other homonoid species is unique because the fossil and sub-fossil evidence is subjected to a level of scrutiny that forms in other genera aren’t subjected to, plus we know human anatomy very well).
For example, take the Corellas, it’s a possible scenario to say that once an ancestral species ranged widely over Australia, but drought reduced it to three areas, a western refuge, a central refuge and a south-eastern refuge, from whence emerged the Western Corella, the Little Corella and the Long-billed Corella. With wetter climates the central population expanded and is now in contact again with the Western Corella, and Long-billed Corella.
Is the Little Corella the ancestral species? Or do the Long-billed or Westerns look more like the ancestor? I remember reading that a fossil of a Corella type parrot was found at Riversleigh from the Miocene that looked very like the modern Long-billed. So perhaps the story is that all Corellas were long-billed before the Pleistocene drying and we should call the Little the Short-billed Corella and the Western the Medium-billed Corella!
This is going to be a fascinating thread.
John Leonard
href=”mailto:birding-aus-request@vicnet.net.au”>birding-aus-request@vicnet.net.au