1a) From time to time, new species evolve from pre-existing species.
This is true and important.
1b) From time to time, old species go extinct.
This is true and important.
2) Many of the observed facts about biology can be understood in
terms of evolution and not otherwise.
This is true and important.
Remarks:
Items (1a) and (1b) are, in this context, the definition of evolution.
Larry W. mentioned the DNA evidence that has become available in recent years.
He aptly described it as "overwhelming". In a later note I will suggest a way
of quantifying just _how_ overwhelming it is.
In addition to the DNA evidence, there is macroscopic evidence. This has been
available for a long time. Many years ago I read an interesting and well-written
book about the origin of species. I think it was called _The Origin of Species_.
Another recommended book is _The Beak of the Finch: A Story of Evolution in Our Time_
by Jonathan Weiner.
In addition to all the observed evidence, we have a _comprehensive logical framework_
for understanding evolution. (I call it a comprehensive logical framework rather
than a "theory", because the latter term is ambiguous and often abused by persons
of bad faith.) This framework allows us to make nontrivial predictions, such as
the prediction that new species will evolve more rapidly in small isolated niches
(such as small islands) than they will in larger niches.
As another example, logic plus the notion of within-species inherited change leads
us to expect speciation. Hugh H. gave one version of this argument. A different
version -- which I find concise, elegant, and powerful -- concerns a _ring of races_: http://www.google.com/search?q=ring-of-races
Given a ring of races A, B, C, ... Z it is quite possible that the end-members
A and Z are so dissimilar that they are unable to interbreed. If they are separated
from the other members of the ring they will instantly become distinct species.
That is a classic argument, and is important conceptually and pedagogically,
because students often subconsciously (and less often consciously) worry that if
offspring are always of the same species as their parents, new species cannot
arise. That is a seductive fallacy, but nevertheless a fallacy. This gives you
an opportunity to remind them of the proverb: _proximity is not transitive_.
The neighbor of my neighbor is not necessarily my neighbor.
In particular, the ring-of-races argument tells you what you expect to _see_ on
the day that speciation occurs. Non-experts expect to see the birth of some new
creature. In contrast, it is vastly more logical to say that speciation occurs
on the day that some intermediate form dies out. (This throws a stark light on
the relationship between ideas (1a) and (1b) above. More specifically: evolution
involves both change and selection.)
Finally, one or two more ideas:
3) All extant species have a _single_ common ancestor.
This appears to be true; the evidence is overwhelming. As a taste of the evidence,
consider tube worms. They are ecologically about as weird as anything you can
imagine; indeed they are more alien than most science-fiction aliens. Yet if
you look closely at their biochemistry, you find they use remarkably conventional
means despite unconventional ends. Also, they fit easily and naturally into the
phylogenetic tree: http://sn2000.taxonomy.nl/Main/Classification/15764.htm
There is also the planet-wide left-handedness of molecules (including sugars and
nucleic acids) that didn't have to be that way. Is that a coincidence? I don't
think so!
Item (3) is fascinating, but IMHO less important than the basic ideas of evolution.
In particular, if somebody showed up tomorrow with evidence of multiple independent
lineages, it would not call into question ideas (1a) and (1b).
4) Life arose on earth only once. (?)
This is IMHO utterly unimportant. It is unprovable. It might some day be disproved,
perhaps as a corollary of disproving idea (3). I consider it perfectly plausible
that life arose more than once, but the Others died out long ago. I don't know, and
I don't much care.
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