why are there so many different kinds of animals?
All told, there are somewhere between two million and thirty million species of animals and plants alive on the planet today, and something like a thousand times that many species have evolved, struggled, flourished, and gone extinct since the Cambrian explosion. (The Beak of the Finch) And the great question of evolution is, Why? How do the lesser differences between varieties grow into the greater difference between species?
For Darwin, natural selection supplies the principle of divergence and tends to make nature "more & more diversified." Darwin realized that two varieties living side-by-side are thrown into competition, and unless one variety dominates and drives the other to extinction, the selective pressures will eventually move the varieties apart . Natural selection will lead to adaptive radiation -- the mutual adaptation of two neighbors to the pressures of each others' existemce. The occasional individuals with some character that sets them slightly apart -- that find a different seed to eat, a different nook or niche, will tend to flourish and so will their descendents. Over generations the varieties will move so far apart that competition will slack off.
How are species defined? Like other taxa, they have traditionally been treated as classes (universals), with defining properties that provide the essential, typological, or morphological bases for determining whether an organism is an instance of a species.
For Louis Agassiz, species were thoughts of the Creator, which are real. But evolutionary theory, as Darwin realized, called into question any stable definition of a species, since they were subject to constant change. For neo-Darwinists they came to be defined simply as "groups of interbreeding natural populations that are reproductively isolated from other such groups." (Mayr) In Michael Ghiselin's terms, these "composite wholes -- not to be confulsed with classes of organisms sharing certain intrinsic genetical qualities." would be "the most extensive units in the natural economy such that reproductive competition occurs among their parts," (i.e. subjects of sexual selection). It would be not only difficult, but logically impossible to list the attributes necessary and sufficient to define their names. For Ghiselin, "None such exist, and the only way to define these names is by an ostensive definition. That is to say, by "pointing" to the entity which bears the name.
speaking of pointing, the "phallus impudicus" mushroom is irreverently defined by its name and serves to remind us of the subjectivity of naming...It is also known as the common Stinkhorn. Gwen Raverat, a Victorian granddaughter of Charles Darwin's describes burning the day's collection of the mushroom "because of the morals of the maids." A discussion of that part of female anatomy known as the “pudendum” would take us too far afield, and into highly contested territory.
