Home: Scrying: For the love of species III: Natural Selection

For the love of species III: Natural Selection

February 20, 2006

Last week, I offered a hesitant definition for “species”. (I say “hesitant” because I’m going to tear it apart and rewrite it in the coming months, as directed by my biology professor.) Part of my assignment in defining speciation was to describe how natural selection applied to my definition, and discuss some of the involved aspects. I’ll focus on some more detailed aspects later on, including my favorite, the fossil record, but for now, here is a brief summary:

Natural selection applied to my definition of speciesFour species of juncos winter in Colorado, according to a 1969 Denver Post special by Bernard Martin. Since Colorado provides a diverse array of habitats, it is likely that natural selection has led to speciation in Juncos here. (From my late Grandpa Torkelson's scrapbook)










Natural selection relies on adaptation to a changing environment in order for a species to survive. If a species can’t survive, it gets selected. The adaptation to survival must be recorded somewhere in the genetic code, so the adaptation can be effective in future generations. This is an essential point: if the change did not occur as a result of a point mutation in the DNA, a mistake in the code which will be passed on to the offspring, the change is meaningless in the face of evolution. So, if a bird eats some seed that makes him fly higher, his offspring will not fly higher as well. If, on the other hand, a point mutation (or a series of mutations) causes a change in the occipital receptors in the bird’s eye, allowing him to spot such the special seeds more readily, then his offspring may also carry the trait. Soon, you may have a population of high-flying birds. If they are “distinct enough” to isolate themselves from another group of birds, lacking the seed-spotting variation (perhaps because they can’t fly high enough,) they can become a distinct species.

According to this definition of species, evolution can occur at any rate, depending on the isolation mechanism. This fits the punctuated equilibrium theory, as proposed by Stephen Jay Gould. This idea suggests that evolution can be caused by sudden environmental changes, as in mass extinctions, as well as happening gradually among dividing groups. Some factors, such as gene flow between groups undergoing speciation, can take many generations to take any effect. Evidence for evolution at both paces, whether abrupt or gradual, has been found. Any definition of species should allow for such varying rates. Since I don’t clarify “eventually” at all, this should fit—isolation could happen today by a tsunami or next century by gradual changes in styles and preferences.

Most people argue that environmental pressures play a key role in speciation. If the environment changes, those groups of organisms that are ready to fill new niches will do so, and if changes are distinct enough, they will be considered a new species. Of course, a new species can put pressure on the environment. An ecosystem depends on an elaborate symphony of organisms, and subtle changes can have great effects. Look what happened when good ol’ Homo sapiens came along….