Miscellany: defining 'species' is difficult (General)

by David Turell @, Saturday, September 04, 2021, 18:54 (345 days ago) @ David Turell

A review essay:


"An individual which belongs to a given species will always belong to that species, come what may. And assigning an individual to a species is for the most part pretty straightforward. But finding something which defines a species, as the number of protons defines an element, is impossible.

"The practical identification process depends on a sample, known as a type specimen, which is typically held in a specialised collection and described in a published paper. That specimen defines the species. Subsequent discoveries either belong to the same species as the type, or they don’t. Various international bureaucracies keep track of this process, providing ever evolving lists of the constituents of the living world. At the moment, about 1.2m species have been given recognised scientific names by this process. Most are multicellular eukaryotes (animals, plants, algae and fungi).


"Darwinism explains a huge amount about life, and about species. But it still does not say where, on the journey from twig-tip to trunk, the juncture is which marks a species-defining common ancestor. One answer was provided in 1942 by a German-American zoologist called Ernst Mayr. He suggested that a species is a group of individuals which can interbreed to produce fertile offspring only among themselves. In other words, they are part of an exclusive gene pool. If they try to breed outside that group, their offspring, if any, will be either sterile or unviable. A new species arises when that inter-fertile group is divided in such a way that some can no longer breed with each other.


"Mayr’s idea provides a useful way of thinking about how far back you need to go to find the common ancestor of a species. As long as all of a creature’s descendants can interbreed, you can go back further. Once its descendants form two groups which cannot, you have gone too far. But in practical terms, there are problems.


"There is also a problem of differential attention. Animals in which people are particularly interested tend to become ever more finely split up. Once-unitary species are divided either into multiple species or subspecies, a way for taxonomists to hedge their bets when they are not sure how different two populations are.


"There was a time when textbooks said, on the basis of fossils, that Homo heidelbergensis evolved from Homo erectus, and that Homo neanderthalensis and, later, Homo sapiens both evolved from Homo heidelbergensis, with the former going extinct. But more fossil finds and the advent of genome sequencing have muddied that all up considerably. It is now clear that modern humans and Neanderthals could and did interbreed with each other. They also both interbred with another group, Homo denisova. Hardly any fossils of these Denisovans have been found, but Europeans and Asians carry their genes. And the genomes of some Africans suggest that their ancestors crossbred with another population at least as distinct as the Denisovans, but which is not seen in the fossil record at all.

"Species, then, may look like natural categories. But their definition depends as much on what you are interested in saying about them as on the biology behind the answer. The results may be coherent biological entities. But you certainly should not bet on it."

Comment: the answer has to lie in sequential genetic studies, which are currently clarifying human evolution as noted in the article.

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