If Evolution is True, Why Are There Species?

A “species” is whatever humans say it is.

If evolution is true, why are there species? originally appeared on Quora, the place to gain and share knowledge, empowering people to learn from others and better understand the world. You can follow Quora on Twitter, Facebook, and Google Plus.

If evolution is true, why are there species? Because of evolution and the related nature of all life, species don’t really exist in nature, the term is entirely subjective.

In other words, a “species” is whatever humans say it is. Any animal or plant or microbe is an observable fact (as is evolution, by the way), but to claim certain of these are one species distinct from others is a hypothesis, by making a line. Sometimes the lines are obvious: humans are not the same species as chimpanzees. Other times they are not so clear. Scientists often disagree on whether certain groups are separate species or the same, and species revisions are very common, although obviously some species are pretty solidly accepted and have not been revised once since they were first described.

How is this possible? Because life is far stranger than non-biologists realize, and certainly more complicated than any creationist could imagine. To standardize things, biologists have species concepts that define where to draw the lines, but they don’t always work. For example, the “biological species concept” says a species can only form viable offspring with itself. This works for most animals, but not for plants or microbes, and not for ring species or species complexes: if A mates with B and B mates with C, but A cannot mate with C, are these three different species or one? What if they can mate and produce fertile offspring if you bring them together, but they naturally live in different continents and never meet? What if you cannot perform mating experiments? Do you look at how similarly they appear, or how similarly they behave? Do you focus only on reproductive traits? Do you look at geography? Genetics? The answer is that you look at everything you can, and try to decide which features matter most. Taxonomy, the science of defining and naming species, is very hard work!

Creationists like to say they believe in “microevolution” (evolving new traits) but not “macroevolution” (evolving into a new species). To a biologist, these terms have no meaning: where do you draw the line between the two? Once a population has acquired enough new traits, someone can decide it is a new species. Maybe it can no longer reproduce with the old population, filling the needs of the biological species concept. Maybe it looks sufficiently different, or is sufficiently genetically distinct. In all cases, the line is artificial: there are no “kinds” as in the Bible. Rather, all life is related, and humans make lines between them. Evolution is the process by which life diversified, and it is an observable fact, but each species/family/order/etc. is a human-defined name.

So, why have species at all? For the same reason we name anything: to tell things apart. Red and Yellow merge as Orange, and all colors are on a spectrum of light, but if I am driving a car I cannot treat a red light the same as a yellow one! You can think of colors as species, therefore, and the theory of evolution is like the wave theory of light. Both are scientific theories, which means collections of observed and experimentally proven facts that explain a natural phenomenon, not “guesses.” One explains the origins of the diversity of life, and the other explains the diversity of electromagnetic radiation. Humans artificially divide life into species and radiation into colors and other terms, and we can disagree on where to put the lines (what is the difference between “maroon taupe” and “mauve?”), but the existence of these living things and the explanations for their differences are both facts.

This question originally appeared on Quora. More questions on Quora:

* Evolution: Why didn't the big cats drive humans to extinction?

* Species: Are there any species where the male is nothing like the female?

* Evolutionary Biology: Could a large animal exist that has not yet been documented by humanity?

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