Species essentialism

Species essentialism can be defined as the view that each life form that exists or ever existed can be unambiguously and non-arbitrarily classified into exactly one species, its "essential" type. (And implicitly that each species has multiple members, rather than being trivially defined as consisting of a single organism.) This can also be called biological essentialism, but that can be confused with unrelated notions that "biology is destiny" (especially in contexts like evolutionary psychology).

Species essentialism may seem like common sense, and for practical purposes it is how biology operates, but due to the reality of evolutionary gradualism and the broader tree of life, it is actually a fallacy. Even without considering fossils or DNA, counter-examples are seen today that demonstrate the fallaciousness of taking this idea literally. As with much of the universe, the biosphere doesn't try to conform to our neat little categories.

Ring species
Biologists frequently define species (if not asexual) in terms of interfertility; two specimens are part of the same species if they are closely related (like parents and children) or if they can in principle interbreed to produce fertile offspring. (Horses and donkeys come close, but their offspring, mules, are almost always infertile.) This definition works reasonably well, but it hits many a wall. Suppose two similar-but-distinct groups A and B are interfertile (implying they are subspecies of the same species), and that B and C are interfertile, likewise C and D -- but D and A are not, D's family line having changed genetically and physically so much that it cannot reasonably be considered part of the same species as A.

This phenomenon is called a ring species. The most famous example is with Larus, a group of seagulls whose distribution forms a large physical "ring" on the planet. (The actual details of their family tree are even more complicated than is usually given in textbook discussions.) Perhaps a more familiar instance of dramatic difference within a single species is the domestic dog; chihuahas and great danes (respectively the smallest and largest dogs) are interfertile only in theory (genetics are less of a problem than the hypothetical offspring's development and health), but they can be linked by a series of interfertile dogs.

Creationism and confusion
Creationist "baraminologists" like to define their baramins by interfertility, similarly to how biologists define species. But unlike biologists, they retain an illusion that their definitions point to genuine lines in the sand of nature.

Folk taxonomy and baraminology
When it comes to broader groups, especially the popularly-recognized animal classes like "mammal" and "bird", creationists demonstrate the same essentialist mentality as with baramins, even though they don't believe that classes point to any deeper biological truths (eg, relatedness) other than the whims of the Creator to make "common designs". They insist that transitional fossils represent organisms that are "fully" part of one such group or another (for example, that Tiktaalik is "fully fish"… not just "fully Tiktaalik").

The microevolution-vs-macroevolution argument relies on species essentialism, especially with the phrase "it's still an X". A kind of folk taxonomy, whereby categories become broader the more distant from humans, becomes relevant in determining what sort of X (an essentialist category) is allowable by creationists:
 * "It's still a dog" (despite previously mentioned drastic differences within the species)
 * "It's still a fruit fly" ( verbatim from David Klinghoffer)
 * "It's still a bat" (an entire order of mammals — for comparison, the order we belong to is primates)
 * "It's still a lizard" (an enormously diverse group of reptiles, non-monophyletic only because it excludes another folk category, snakes)
 * "It's still a bird" (well, maybe flightless birds are special, but all those little tweety ones are essentially the same, right?)
 * "It's still a horse" (some creationists have started to change their tune on the famous horse series, labeling it a single baramin)
 * But never: "It's still an Australopithecene ape with a few modifications."

All of these examples are meaningless in the absence of an ironclad definition of each of these categories, and thanks to the reality of gradations rather than essentialism, one is hard-pressed to identify such distinguishing characteristics (one of many headaches for taxonomists, of course). How does Klinghoffer know "it's still a fruit fly"? Because it looks like one to him?

Transitionals as counterexamples
Species essentialism can be difficult to unlearn, partly because of its refusal to even grant the logical or coherent possibility of a counterexample. (Suppose it were intuitively obvious, or ideologically compelling, to think that all odd numbers are prime; one could become totally convinced that either 9 isn't odd, or it is prime, "by definition", or at least that any odd composite number would have to be much more unusual than 9.) Transitionals can be dismissed a priori on the grounds that any "in-betweenish" creature would be too freakish to survive long enough to reproduce. The essential categories are seen as places of comfort and normalcy (which is why egg-laying mammals can seem "weird" from a superficial perspective).

Furthermore, creationists like to list numerous X-suggestive features of an organism as proof that it is "fully X", as if enough such traits tip the scale entirely into one of the two distinct categories. This line of argument requires species essentialism (or at least a kind of "rounding-off essentialism") to make sense, and is therefore circular or at least redundant (since species essentialism and transitional forms are incompatible ideas in the first place). Outside of the essentialist mindset, one can see how the same creature may demonstrate aspects of two "groups" at once, since the groups aren't real (except as broad ranges) to begin with.

Often, biologists disagree over how to classify something, or the consensus on a classification changes. Sometimes creationists treat this as a point against evolution, implying either that species essentialism is a part of evolution (which is the opposite of correct) or just that it must be true by definition, and any discipline that can't even distinguish between something as basic as ungulates-vs-whales or apes-vs-humans can't be worth much for biology. Never mind, of course, that creationists themselves are both very divided on the precise classification of hominid fossils and very united in the principle that all such fossils ''must' be "fully human" or "fully ape". So much for species essentialism.

Philosophical background
Essentialism is a broader category of philosophical ideas, of which the first major variant was Platonic ideals, the notion that everything we perceive is a crude representation of an abstract "form" corresponding to its basic type of thing: Platonic rocks, Platonic lions, and Platonic triangles all exist in some rarified realm beyond our perception. (A bit like how classes and instantiation work in object-oriented programming.) Close scrutiny of this idea raises the (rhetorical) question of whether there is an ideal form of, say, animal droppings.

A closely related issue is the continuum fallacy, first posed in ancient Greece as "the paradox of the heap": Exactly how many grains must be removed from a heap of sand before it is no longer a "heap"? Equivalently: How many hairs must a bearded man lose to be clean-shaven? How many minutes does it take for a baby to grow old? How many pennies does it take for a poor person to be rich? And over exactly how many generations did the first mammals evolve from reptiles?

The answer, my friend, is blowing in the wind; the answer is blowing in the wind. More specifically, the answer is that "heap", "old", "rich", and "mammal" are all mere human labels for roughly-defined ranges in continuous phenomena. In the case of evolution, extinctions tend to ensure the absence of intermediates, leaving a false impression of discontinuity. (Fashion and economics sometimes do the same thing with beard lengths and social class.)

The establishment of arbitrary "bright lines" on fuzzy phenomena can also be of use in areas outside of biology. For example, a body of law usually has to determine whether someone is an "adult" and whether their income is "high". These definitions are established by midnight-of-birthday and tax-bracket cutoffs, both of which can technically be measured to the nearest minute or dollar, even though nothing magically significant happens to someone's maturity or to their net worth at either of these arbitrary dividing lines.