The Way of Science

B. SPECIATION

We will consider now two major sources for an increase in diversity. The first, geographic isolation, has been shown to be very important for both plants and animals. The second, polyploidy, is very important for plant speciation, but insignificant for animals. (Who knows what new sources of speciation remain to be uncovered in the algae, the fungi, etc.: all those organisms that are neither plant nor animal.)

(1) Geographic isolation
Let us begin with a simple hypothetical example, and then look at what is perhaps the most famous example: Darwin's finches.

Suppose some hypothetical long and skinny island has two rather different environments, one at each end of the island. One environment is wetter, cooler and less windy than the other. Throughout the island, there lives a single species of snokkle (Snoccophilus hypotheticus). It should not surprise you that the snokkles at one end show a set of adaptations to the hot, dry and windy conditions. At the other end, there are plenty of snokkles, but they are different in many ways: perhaps they have pale, short hair (good at reflecting sunlight and as heat insulation), versus no hair and dark bare skin at the other end. There are other differences correlated with the geography: a tendency to nest high above the ground versus at ground level, food preferences, etc. Nonetheless, it is easy to see that we still have one species, because we can demonstrate that there is successful breeding going on between the two groups. We might demonstrate this breeding by high-tech means (DNA analysis), or by more traditional means (field work). Let us refer to these two sub-populations by their proper term: subspecies (sspp.). A ssp. is a geographically distinct subpopulation which shares many traits in common ("concordant variation"). Certainly, selection by different environments will differentiate these two groups, but as long as interbreeding persists, the two groups integrate at intermediate environments, and "wrong" traits will continue to appear in both areas, where they are less fit.

Suppose that some geologic event isolates the two ends of the island from each other. Perhaps there is a rise in sea level. Since snokkles cannot swim, and greatly fear getting wet, breeding between the two subspecies is stopped. What happens as time passes, and many new generations are produced at each end of the island? Surely, new mutations cannot be exchanged any longer. Selection can work more efficiently at each end, since the influx of "bad" genes is halted. The result? The two populations gradually diverge, genotypically and phenotypically. Perhaps 100 generations later, sea level subsides, and brave snokkles move into each other's area. Two different scenarios are possible at this point.

1. The two populations, though different, still find each other "breedable," and proceed to interbreed. Gradually, the differences decrease, and eventually we have two subspecies again, though they may be somewhat different from the ancestral two.
2. The two populations intermix, but find it impossible to exchange genes. Perhaps they have developed different mating calls or rituals, or they may have undergone chromosomal changes that make their hybrids sterile, etc. Now we have two unambiguous spp. of snokkle, neither one of which may be exactly like either original subspecies.

Between these two extremes are intermediate stages, partial sterility, for example. Humans like nice unambiguous populations on which to place formal names, species or subspecies names. There will always be cases where the pattern of Evolution is clear, but where we cannot get our naming system to reflect the intermediate stages. Your instructor may wish to present some examples of "gray" areas, such as the Asian and western plane trees (Platanus).

Now let us look at one very famous example of speciation via geographic isolation: Darwin's finches in the Galapagos Islands. A brief description with illustrations can be found on pp. 10-11 of Science and Creationism. This pattern of speciation is frequently called "adaptive radiation." Can you see why?

The Galapagos is a group of small islands several hundred miles from the Ecuadorian coast. When Darwin visited the islands, he found many very similar spp. of finches. These finches were clearly closely related, and differed primarily in beak size and proportion, and thus in feeding choices. After examining many individuals, and noting how they were distributed, he concluded that an original colonizer, or a very few, had been blown to the islands from Ecuador. This parental species was also noted by Darwin. With isolation came adaptation, and divergence from the mainland form. As the population on an island grew, some individuals migrated to adjacent islands, which are fairly close, where additional adaptation and divergence took place.

Since Darwin's visit, scientific work has continued on these birds. A very important recent work, done over the course of 20 years, has produced some fascinating results, including solid empirical evidence for rapid selection by the environment. This work was done by R. and P. Grant, and their results have been described for the non-specialist in a recent book (J. Weiner, 1994: The Beak of the Finch).

The Grants followed birds through periods of drought and wet, and found that beak length changed rapidly in succeeding generations in response to environmental changes. As moisture changed the sizes of seeds, their food, beak size changed rapidly as a consequence. Most important, the Grants demonstrated that changes of less than a millimeter affected survival rates (fitness). Again, detailed and careful field work produced solid empirical evidence in support of "Darwin's Dangerous Idea" (natural selection "guiding" random variation, and thus changing future generations).

It gets better, as a result of some very recent work by a biologist from the University of Arizona. He analyzed vocal-tract structure of these beak-shifting finches. As beak size changes, so do components of mating calls. Birds with larger and heavier beaks had a lower rate of repetition of calls, and fewer tones, than individuals with daintier beaks. In other words, here is solid evidence that small changes in beak size may contribute to speciation in these birds, by altering mating signals.

As you will see in the next section, the anti-Evolutionists constantly hammer the American public with the claim that there is no solid evidence for Evolution (either fact or theory). Their claim is based on enormous ignorance. The few specific examples given above are but a tiny fraction of the verification published in refereed journals. Unfortunately, these journals are difficult for the non-biologist to read, and the various news media are notoriously bad at reporting science. The majority of the public remains ignorant of the enormous mass of evidence for Evolution. All aspects of Evolution (fact and theory) are constantly re-examined in the literature, constituting a continuous series of tests (falsification) that have consistently supported descent with modification, common ancestry, and the power of natural selection.

There is one last point to be made before moving to the second means of speciation, polyploidy in plants. We have emphasized the role of subspecies in animal speciation, and now the term "race" needs examination. In the past, biologists used the terms "subspecies" and "race" as synonyms. Now, most biologists avoid the use of "race," because the public has picked up the term for human skin-color variants. Human "races" based primarily on skin color are not equivalent to biological races (subspecies). To establish solid biological races in humans is very difficult; some say it is impossible. In any case, the present four or five skin-color "races" bear little relationship to the biological reality. Why? Explain in class!