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Can you define a ‘species’?

There seems to be no certain end to the debate on 'species', 'populations' and 'hybrids'

An illustration, titled ‘The Panther’, from the 12th century ‘Aberdeen Bestiary’. Photo: Aberdeen University Library/Wikimedia Commons
An illustration, titled ‘The Panther’, from the 12th century ‘Aberdeen Bestiary’. Photo: Aberdeen University Library/Wikimedia Commons

Once considered freaks of nature or biological oddballs by many naturalists and experts, natural hybrids are back in the conservation spotlight as key to evolution. In February, new research revealed that ancient elephant species (including Woolly and Columbian mammoths) interbred, and hybridization played a key role in their evolution. Hybridization is a biological process in which a new population is created from two genetically distinct source populations. In March, the hullabaloo surrounding the death of “Sudan", said to be the last captive northern white rhino (a subspecies of the white rhinoceros in Africa) in Kenya, intrigued me enough to revisit the age-old “what is a species" debate and the German zoologist Werner Kunz’s argument: “Do species exist?"

There are no easy answers. Recent genomic studies have shown hybridization shaped feline evolution—tigers, lions, leopards and jaguars have interbred since individual species diverged millions of years ago. Research has established that most species of bears have evolved in a similar manner—through inbreeding, hybridization, and isolated populations. There are now numerous examples of different groups—mammals, birds, fish, amphibians, and, lest we forget, man’s best friend, which did evolve from wolves.

Today, there are more than 20 “species" concepts in circulation amongst scientists. “The species as a taxonomic unit has been one of the most controversial topics in biology.... Modern biology has many triumphs to celebrate, but a generally applicable species definition is not among them," Kunz writes in his book Do Species Exist?

As an amateur birder, it’s a constant challenge to keep learning new names of familiar birds in repeated taxonomic overhaul. For example, what was called the Purple Swamphen—a plump blue-purple coloured bird with a bright red beak—for three decades now has a new name: Grey-headed swamphen. Similarly, in the past, common birds such as the White-breasted Kingfisher and Grey Partridge were renamed White-throated Kingfisher and Grey Francolin. The ornithological world is rife with “splits" and “lumps"—species getting separated or joined by taxonomy.

In India, there is no one agreed checklist for birds, although three Indian ornithologists, Praveen J., Rajah Jayapal and Aasheesh Pittie, have been hard at work for many years to bring everyone on the same page. They have tried combining four different sets of bird checklists that enjoy patronage around the scientific world.

Kunz argues that bird field guides (also true for animals and plants) misguide us into believing that individuals that differ clearly in traits must belong to different species. For example, if a goose has a uniformly pink-to-orange beak and pink feet, it must be a Greylag Goose (a common winter migrant in north India). However, if the bird has an orange beak with black margins together with orange feet, then it’s a Bean Goose. Nevertheless, each individual Greylag Goose or Bean Goose does not exhibit these differences because mutants occur. Hence, why are those mutants still members of the species? Why are they not different species?

“It’s time to shift focus from ‘species’ (as a concept) to ‘populations’ and have meaningful conversations around different groups of plants and animals so that we have the right conservation goals," says Anindya Sinha, primatologist at the National Institute of Advanced Studies.

Genomic research is breaking barriers, especially the conventional concept of “species" with a pure stock of genes being passed down through successive generations and hybrids as sterile misfits. Increasingly, new studies are revealing the importance of wild hybrids and how they play a critical role in evolutionary biology and species conservation.

Richard Prum, an evolutionary ornithologist at Yale University, US, says most of us have misunderstood Charles Darwin’s theory of evolution. Darwin’s second book, The Descent Of Man, And Selection In Relation To Sex, acknowledges that there are limits to the power of natural selection as an evolutionary force. And his theory of mate choice has been ignored or forgotten completely. It was Darwin who put forth the argument that biological species is a human construct, yet we still look upon species as fixed natural entities.

A section of scientists feels that many biologists still don’t understand that there is a “species" problem. In Kunz’s words, what is it that makes a group of organisms a species? If you try to find the answer to the question, what a species is, you will notice that the answer is very difficult, if not impossible, to find. Try and answer, What is a tiger? And you will end up describing the characteristics of the animal.

Out In The Wild is a column on the good, bad and ugly of nature conservation.

The writer tweets at @protectwildlife

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