What is a species?

When I was writing about chickadees last week, I kept thinking to myself: “What really is a chickadee?” Yes, I know what they look and sound like, but do our diminutive black-capped chickadees really have the same “black-capped chickadee-ness” as those slightly larger 19th century chickadees? And if our Vermont chickadees continue to shrink, at what point do they reach that critical morphological/genetic tipping point where we no longer classify them as Poecile atricapillus and instead mark them as some novel new species, perhaps P. minimus or P. anthropocenii?

Orange hawkweed flowers in full bloom

Mayr’s species concept

In high school, I was taught Ernst Mayr’s definition of a species: “groups of interbreeding natural populations that are reproductively isolated from other such groups” (this is from Mayr’s dense, but fascinating 1942 book, Systematics and the Origin of Species). Implicit here is that breeding produces fertile offspring. Some species can and do interbreed, but as with mules, the cross between horses and donkeys, their offspring are typically sterile, so we treat horses (Equus caballus) and donkeys (E. africanus) as distinct species.

Under scrutiny, Mayr’s definition starts to break down rather quickly. Because of climate change, the overlapping portion in the Venn diagram between polar and grizzly bears’ ranges has expanded. And in that overlap these historically isolated species (populations?) interbreed, and fertile pizzlie (or grolar) hybrids are an increasingly common occurrence (link). Bacteria aren’t even capable of reproducing sexually, so would every individual bacterium be its own species? Plants can reproduce asexually (like when a diseased beech tree sends up a sucker from its roots), but other plants reproduce entirely via apomixis (production of a seed without fertilization), as is the case with some hawkweeds (source). While asexual reproduction isn’t all that common in vertebrates, it still occurs (in animals, asexual reproduction is called parthenogenesis). The entire population of desert grassland whiptail lizards (a hybrid of little striped and canyon spotted whiptails) is female and infertile. But these “infertile” females are still fully capable of producing offspring without fertilization.

Because of the inadequacy of Mayr’s definition, there are over two dozen other definitions that have been put forth by taxonomists trying to pin down what exactly a species is (link). But despite taxonomists’ best efforts, there is no one unifying concept that applies to all organisms. This is known to biologists as the species problem, and we see this pattern emerge whenever we try to generalize across all organisms (even defining what an organism is can be tricky: source). The challenge becomes even more difficult when we think of an interbreeding population persisting through time, as with our shrinkadees. At what point does a historic population become a distinct enough point in the past to draw that rigid species line?

Anthropocene species

In delineating geologic epochs (we’re in the Holocene, having left the Pleistocene some 11,700 years ago at the end of the last Ice Age), geologists look for two main criteria: (1) there is a long-lasting and significant change to the Earth and (2) that this change is evident in rocks, sediment, and/or ancient ice (source). There’s ongoing debate right now about whether we’ve entered the Anthropocene, and if so when did the epoch begin. You can find nitrogen from fertilizers, ash deposits from fossil fuels, and nuclear plutonium in sediments around the world dating back to around 1950. It seems quite clear that there’s a distinct signature of human activity that is detectable in the geologic record (and would be to an observer millennia from now). 

Might we think more like geologists in drawing distinctions between historic and modern populations so that we think about these as separate species? Back in 1972, Stephen Jay Gould and Niles Eldredge published Punctuated equilibria (source), their counter to Darwin’s idea that species evolved gradually through time. Instead, they proposed that species are largely stagnant through time until some major catastrophic event exerts significant evolutionary pressure on a species. In response, that species either goes extinct or adapts (often radiating into multiple species). 

The same forces at play marking the edge of the Anthropocene act as the major catastrophic event shaping the modern evolution of many organisms. Will biologists factor in the fourth dimension to the species concept, using the fingerprint of human induced adaptation in the genetics of species to re-classify modern organisms? I’m thinking here of the many punctuation marks wrought on species over the past few centuries: artificial selection of blight resistance in American chestnuts (source), the variation in house finch song from urbanization (source), color change in the pepper moth from industrial pollution (source), mortality rates from acid rain in spruce trees (source), isolation of mosquito populations in London’s underground tunnels (source), genetic modification of wild populations of wolves into domesticated dogs, the potential de-extinction of species like the woolly mammoth and passenger pigeons (source), and so on.

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