Extended reading: what is a species?

One question, three tests

The main chapter says that a species is a group of individuals with fundamental similarities. Now we ask a harder question: how similar is similar enough? A Great Dane and a pug look very different, and both are dogs. A horse and a donkey look much alike, and they are two species.

Biologists use three tests together.

1. Looks. Members of a species have the same body plan and features.
2. Breeding. Members of a species can mate and have young that can also have young.
3. DNA. The genes of members of one species are more alike than the genes of different species.

The biological species concept

The second test is the biological species concept, put forward by Ernst Mayr. A species is a group of natural populations whose members can interbreed (actually or potentially) and which are reproductively isolated from other such groups.

Reproductive isolation can work in two ways.

• Before mating. The animals never meet or never mate. They may live apart, breed in different seasons, or answer different calls.
• After mating. The animals mate, but the young do not live, or they are sterile.

The mule is the second kind. A horse and a donkey do mate, and the mule is strong but nearly always sterile. So the parents are two species, and their genes do not mix freely into one pool.

The numbers behind the mule

A horse cell has $64$ chromosomes, and a donkey cell has $62$. A sperm or an egg has half of its parent's number: $32$ in the horse and $31$ in the donkey. The mule gets one set from each parent:

The number $63$ is odd. To make sperm or eggs, the chromosomes must pair up two by two. In the mule the horse and donkey chromosomes do not match well, and one is left without a partner. (You will study this in the chapter on cell division.)

You should find that a lion and a tiger, with $38$ each, give a hybrid with $38$: an even number. Even so, the two sets come from two species and may not match, which is one reason such hybrids are often sterile.

Where the concept fails

No test works for every organism.

• Bacteria and other organisms that reproduce only without a partner do not mate at all.
• Fossils are dead. We cannot see whether two fossil animals would have mated.
• Populations that never meet cannot be tested in the field.
• Some species do cross in nature, so the line between them is not sharp.

So biologists put looks, breeding and DNA together. A species is a useful and honest idea, not a perfect one.

Subspecies: a name with three words

Sometimes members of one species in different regions differ a little, but they can still breed together. Such a regional form is a subspecies. Its name has three words: the genus, the species and the subspecies. All three are in italics, and only the genus has a capital letter.

• Panthera tigris tigris is the Bengal tiger.
• Panthera tigris altaica is the Siberian tiger.

Both are one species, Panthera tigris. After the full name has been written once, it may be shortened: P. t. altaica.

Older books write the dog as Canis familiaris. Newer books write Canis lupus familiaris, which says that the dog is a subspecies of the wolf.

Try it in Python

This program finds the number of chromosomes in the mule from the numbers of the parents, and tests whether they can pair up evenly.

Practise with new numbers

Common mistakes

• Calling two animals one species because they can mate. Their young must also be able to have young.
• Adding the two parents' full numbers. The young gets a half set from each: $32 + 31$, not $64 + 62$.
• Using the breeding test for bacteria or fossils. It has nothing to say about them.
• Writing a subspecies with a capital. Only the genus starts with a capital: Panthera tigris altaica.

Remember these

• A species is judged by looks, by breeding and by DNA, together.
• The biological species concept: interbreeding populations, reproductively isolated from other groups.
• A mule has $63$ chromosomes, an odd number, and it is nearly always sterile.
• The concept fails for bacteria and for fossils.
• A subspecies has a name of three words, for example Panthera tigris tigris.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. How many chromosomes are in a donkey sperm?

2. Which word of Panthera tigris altaica is the subspecies?

3. Why can we not use the breeding test for a bacterium?

4. Two bird populations look different but mate and have fertile young when they meet. One species or two, by this concept?