To speak of creation categories in terms of overall form rather than just say due to small characteristics? What I'm getting at is: Is it reasonable to call organisms that are said to arise via parallel evolution to appear the same different just because they are said to have different lineages? Sometimes I wonder why we can't just call euphorbias and cacti different forms of the same thing rather than totally different plant families.
At least an evolutionary classification could be said to be objective. If we take that away, who's to say which classification is right? The one that groups all cacti together based on their flower structure or the one that groups
some cacti with
some euphorbias and
other cacti and
other euphorbias with other plants based on their shape? Or something completely different?
why are there so few transitions between species if there is no status quo.
IIRC Darwin reasoned this out quite neatly in
Origin. Anyway, a few things to consider:
(1) Intermediates would not be between two
extant species but between either of them and their last common ancestor. (Unless one extant species is the ancestor of another)
(2) If a species splits in two to occupy two different niches, the intermediates will be less fit in either niche than the extremes and therefore would not prevail. (
Disruptive selection)
There are two exceptions to (2) I can think of: first, intermediates in traits that do not influence fitness, or intermediates in
intermediate habitats, could be present in the long term. I don't know how often each occurs in the real world, though.
(3) If a species evolves along with a single, changing environment (
directional selection), then better adapted forms of it will outcompete previous versions. At any point in time the dominant form will be the most recent one - intermediates will only exist in significant numbers in the past (of course, the most recent form can easily be transitional towards something that comes after it, but that's a bit hard to tell without functioning crystal balls
).
Gradual evolution
can be observed in the wild if you have a few decades to watch and measure finches (like
the Grants did) or something*, but larger differences, those that are traditionally used to distinguish species**, normally accumulate slowly (on human time scales, that is). What you see when you look at living organisms is just a few frames in a very long movie. Many of them could very well be going somewhere, but you'll never realise that because humans don't live long enough to see where (or where from).
And punctuated equilibrium...
Punctuated equilibrium? Well... There is no evidence at all for that.
... is why the fossil record can also easily miss species-level transitions. New species may arise slowly by human measures, but they can arise very quickly on a geological time scale. If most new species arise from small populations split off from the edge of a larger one (
peripatric speciation), there is also a problem of numbers: fossilisation is a rare event, after all.
Islands are wonderful natural experiments on PE: small founder populations in a new habitat, rapid evolution, lots of new species in a "short" time. (In this sense, lakes can be islands too; IIRC the cichlids of Lake Victoria boast the fastest speciation rate of all organisms examined.) I don't know how much evidence exists for the
equilibrium part of PE, though.
take a look at my avatar. It changes from day to day even within the genus centropyge. Is there an intermediate angelfish between all these? Why do they all appear so different. Sorry to be so simpleminded.
Not simple-minded at all. In fact, refreshingly intelligent
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NOTE: this all makes sense to me but most of it comes off the top of my head, so if I said anything stupid or untrue, let me know.
*Note that in this example, finch beaks don't change steadily in any particular direction but rather fluctuate back and forth as the rains dictate. But it does make the point that evolution is an observable phenomenon even in the wild.
**Looks are a lousy way of defining species, but I digress...
