On Mon, Nov 1, 2010 at 2:55 PM, Ralph Reid <> wrote:
> 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?
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.
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