A couple of days ago, I got a rather nice email from a creationist. Not, I got the feeling, a young-earth creationist, but someone who believes that a deity directed the creation of the Earth (whenever that happened), and that species can't change because they're the work of divine hands.
"I just can't believe in evolution," the writer said. "It's impossible that species can change. They go extinct sometimes, like the dinosaurs, but how could one thing change into another, like apes into humans? It makes no sense."
Again, this was light years from the snide, spittle-flecked screeds that I've sometimes received regarding this subject; I very much had the impression that the writer was simply curious as to why I find the idea of evolution persuasive. And as such, it deserves an answer.
I'm going to approach the idea of supporting evolution a little differently than most folks do. It seems like the majority of evolutionary biologists, when confronted with questions about the plausibility of the evolutionary model, usually discuss the tried-and-true body of evidence (genetic homology between related species, homologous structures, vestigial organs, the fossil record, and so on). These are well known, and in my opinion either you buy them or you don't. Those folks who don't also usually fall back on a few tried-and-true arguments against them (vestigial organs actually have a use which we just don't happen to know, the fossil record lacks transitional forms, radiometric dating is inaccurate, and so on).
More to the point, one of the usual anti-evolutionist arguments often centers around the question, "if evolution happens, why don't we see new species?" and the ordinary answer is, "because evolution occurs so slowly."
Well, sometimes. Maybe usually. But my contention is that rapid, observable evolution has happened many times, and if you don't buy the evolutionary model, there are a few real-world situations that really allow no other explanation.
So, to quote my dad, let's run them up the flagpole and see who salutes.
First, though, a definition. My understanding of creationism is that, at its basis, it states that new species cannot form. Species can become extinct, but god created the species that are here, and that's what we're stuck with. (If this statement is erroneous, I'd appreciate a correction.) So as an evolutionist, I have a twofold job; to define species (so that we all know what we're talking about), and to show that there have, in fact, been new species evolved on the Earth. If I can accomplish those two things, then I think I'll have made a pretty potent case that evolution happens.
The first task is relatively easy. While there is an increasing push to define the term "species" genetically, at present most of us (evolutionist and creationist alike) define "species" as meaning "a population of morphologically distinct individuals, all of whom are potentially capable of interbreeding and producing fertile offspring." By this definition, horses and donkeys aren't the same species because although they can mate and produce offspring, the offspring (a mule) is not usually fertile. All breeds of dog are theoretically a single species, because although there are morphologically distinct sub-populations (breeds), they are all more or less mutually interfertile, even though a mating between a male chihuahua and a female St. Bernard raises a mental image which is simultaneously a little disturbing and strangely hilarious.
Okay, now for the next part. Have there been any new species that have formed recently? If you buy the definition of species from the last paragraph, the answer is undeniably "yes." I know of three off the top of my head, which I'll describe below. The first two are simple, the third more complicated (but well worth the effort to try to understand, because it's way cool). And last, I'll describe a population phenomenon that I don't think is explainable unless you do accept evolution, although it's hard to classify exactly where it falls apropos of the definition of "species."
Number 1: The Faeroe Island House Mouse
About 250 years ago, mice were accidentally introduced onto the Faeroe Islands, an isolated island chain (way) north of Scotland. In the intervening years, the mice were isolated from their mainland kin, and the harsh climate was a powerful selection mechanism. Recent studies have shown that the Faeroe Island House Mouse is now no longer even potentially interfertile with mainland House Mice -- matings in the lab have resulted in no offspring or sterile ones, and the Faeroe Island mice are discernibly smaller and lighter in color than the mainland species. If you accept the definition of "species," the Faeroe Island House Mouse is a new species -- morphologically distinct, and unable to interbreed with other populations -- and it's arisen in only 250 years.
Number 2: The London Underground Mosquito
When the London Underground (subway) was built about a hundred years ago, a population of mosquitoes of the species Culex pipiens was trapped in the tunnels. Being that subways are warm and moist, the mosquitoes flourished. Culex pipiens, which mostly preys on birds, is reluctant to bite humans and will only do so if there is no other food available; in the 100 years since the isolating event took place, natural selection has favored the individuals underground who are more attracted to mammals (mostly rats and humans), and the result has been a rapid speciation event producing the aptly-named Culex molestus. C. pipiens and C. molestus will not interbreed -- in fact, in the lab they won't even mate. Genetic studies have shown that their genetic makeup has diverged rapidly (due to the heavy selection underground and the fact that mosquitoes breed quickly) -- so by any conventional definition of the word "species," they are different species.
Number 3: Cordgrass
This one is fascinating. Cordgrass (Spartina) is a genus of marine grass, with a number of morphologically distinct species. In England, Spartina maritima was the most common species, until the 19th century, when the American species S. alterniflora was accidentally introduced. The two occasionally hybridize, producing an infertile (although vigorous) hybrid, S. x townsendii.
Okay, so far, nothing amazing; it's just the botanical version of the horse and the mule. Normally these interspecies hybrids are infertile because they lack paired chromosomes, and during meiosis (sex cell formation) the process goes awry because it is impossible to evenly divide the genetic material without this pairing. But apparently at least once (possibly more), an individual of S. x townsendii underwent an odd transformation; in one of its flowers, the chromosomes spontaneously doubled. This phenomenon, called allopolyploidy, is rare in the wild but rather easy to induce in the lab (it's how the huge tetraploid and triploid daylilies you often see in gardens are created, for example). What this did was instantaneously produce an offspring with paired chromosomes, and a different number of chromosomes from either parent. It is completely fertile with others like it; is not back-fertile with either parent species; and is morphologically distinct. It's been accorded species status (as S. anglica), and for good reason, because if this is not a new species, I don't know what is. Furthermore, it's an amazing competitor, and is in many locations outcompeting both its exotic and its native parent.
And one more, just for lagniappe, as my mom used to say (lagniappe is Cajun French for "a little something extra"). If none of these convince you, then look into the concept of a ring species. A ring species is a set of morphologically distinct populations, which encircle a geographical barrier of some kind. Each sub-population can interbreed with the ones adjacent to it, except at one point in the ring. This has been observed at least three, possibly more times -- in Himalayan Greenish Warblers, in a group of salamanders (genus Ensatina) in California, and in a group of gull species (in this one, the ring goes all the way around the world!).
Let's just make it clear how weird this is; picture a group of populations (call them A through G) which go around some sort of geographical barrier (the Himalayas, the Sierra Nevadas, and the Arctic Ocean, respectively). A can breed with B, B with C, C with D, and so on. And you ring your way around the barrier, and find that A and G are right next to each other -- overlap, even -- but A and G can't interbreed!
So which are they -- one species, or many? If you say "one," then why can't A and G interbreed? Breaks the definition. If you say "several," where do you draw the line(s)? No matter where you draw the line(s), you will separate populations that can interbreed, and produce fertile offspring (and therefore should be part of the same species). So, once again: what is this? And if "species" are all divinely created, immutable little populations which don't change, how on earth did this come about?
Myself, I find it impossible to explain any of these without recourse to the evolutionary model. If anyone has a plausible alternative explanation, I'd love to hear it. Encouragement of all viewpoints, as always, is the watchword hereabouts.