Nature Often Defies Sharp Categories

In the natural world, sharp division into categories reflects how our minds work rather than what is actually there.

One of the most basic divisions in visual perception is that between figure and background. For a predator, this involves picking out a prey animal that may be selected for camouflage coloring.

Categorical Thinking

Gestalt psychologists were the first to articulate the view that our visual perceptual system automatically divides visible scenes into figures that are interesting and catch our attention against a backdrop that is duller, and often flat, or repetitive.

Once we have determined what is interesting in the scene, we may categorize it based on generalizations about similar items viewed in the past. It might be a person, a fish, a house, a tree, a mountain, or a machine.

All complex animals seem capable of recognizing such categories. For example, pigeons can be trained to peck a key when viewing pictures that contain some item like a fish or a bicycle. Of course, machine learning visual recognition programs can not only detect people in pictures but can also identify them fairly reliably.

While such programs are error-prone, they have also been used to spot wanted criminals on busy city streets in London and other cities that use these Big-Brother strategies.

Categories help us to organize the natural world, but these categories are imprecise. One of the more remarkable cases of category fuzziness involves the division of animals into species, as initiated by Carl Linnaeus who used the genus and species designations that are central to biology.

The Fuzziness of Species

While everyone uses species categories, it has never been possible to define them satisfactorily.

Perhaps the most startling failing of the species concept is that animals ignore it. Mating across species boundaries is surprisingly common.¹ Even more remarkable, some of these matings produce viable offspring such as the mule given birth to by a female horse impregnated by a male donkey. Of course, mules are sterile, so inability to reproduce is merely delayed by a generation.

Closely related species may routinely interbreed so that there is no clear dividing line. This is true of seagulls that interbreed in a ring surrounding the Arctic but end up as distinct species, namely the large black-backed gull and the smaller herring gull.

The species concept uses literally dozens of different criteria of similarity, such as descent from similar ancestors, similarity in appearance, adaptation to a similar niche, breeding isolation, genetic similarity, and so on.

Nature is oblivious to our species categories and there is also a surprising amount of fuzziness in biological gender.

The Fuzziness of Gender

Male mammals are distinguished by carrying a Y-chromosome whereas females only have X-chromosomes. Since the Y-chromosome is what differentiates biological sex, it is the sex-determining chromosome. Among birds, it is the females who carry the sex-determining chromosome. In reptiles, gender is decided instead by the temperature of the place where the eggs are laid.

Some fish have the strange capacity to change gender as adults. Individuals that are socially dominant in a group of females, transition to being the breeding male.

Even mammals pull remarkable gender-bending stunts. Hyena females look very like males, complete with a pseudopenis that is used in social display. A bear that was initially misidentified as male surprised observers by giving birth to a cub despite the anatomical implausibility of this feat.¹

Of course, ambiguous gender is a feature of human development also for a variety of causes, including prenatal exposure of mothers to unusually high levels of sex hormones, and androgen insensitivity where genetic males closely resemble females. With our penchant for neat categories, we may want gender to be binary, but it is also fuzzy.

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The number of people who experience gender dysphoria and seek medical interventions to alter their gender appearance is increasing. This phenomenon may have many different explanations. One is that it happens at a time of greater tolerance for gender nonconformity and acceptance of gender fluidity.

There is also an explanation in terms of environmental influences, specifically the presence of hormone mimics in common industrial chemicals, such as the phthalates used in food packaging. When mice are exposed to these substances early in development, biological differences between males and females are diminished.²

The Restroom Controversy

In some ways, the controversy over who uses which bathroom is intensely silly because no one has demonstrated any harm arising from transgender people using any particular restroom. The real issue is that some people like to draw sharp gender distinctions, and ambiguity makes them uncomfortable.³ They may think in clear categories. Nature does not.