3.4.5 - Species and taxonomy

What is a species?

Two organisms can look almost identical and be different species, so biologists test breeding, not looks.

Appearance misleads in both directions. A peacock and a peahen look very different but belong to one species, while the chiffchaff and the willow warbler look almost identical but are two. The biological species concept therefore uses a different criterion: reproductive compatibility.

Fertile is the key word. A living offspring is not enough, because it must also be able to reproduce.

The horse and the donkey show why. They can mate and produce a mule, but the mule is almost always infertile, so horse and donkey are different species even though they produce a living hybrid.

The test applies to the species as a group: two males produce no offspring together, yet they can belong to one species.

Breeding can't always be tested, so the definition has limits:

• Some organisms reproduce asexually, so they do not interbreed.
• Some populations are geographically isolated and may never meet.
• Extinct organisms can't be bred.
• Species change over time, so boundaries are not always sharp.

Naming species with the binomial system

A robin in London and a robin in New York are different birds, which is the confusion scientific names prevent.

Common names vary between countries and even between regions. Scientific names do not: the European robin is Erithacus rubecula and the American robin is Turdus migratorius.

The binomial system gives each species one internationally recognised name, such as Homo sapiens or Canis lupus. It has two parts:

• The genus name comes first and is capitalised.
• The species name, formally the specific epithet, comes second and starts with a lower-case letter.

The second word alone is not enough because the same word can be used in different genera. The red squirrel is Sciurus vulgaris and garden thyme is Thymus vulgaris.

The first word adds information. Species in the same genus are usually closely related, which is why the tiger, Panthera tigris, and the lion, Panthera leo, share a first name.

Courtship behaviour and species recognition

Many animals mate only after a sequence of signals and replies, and a wrong reply can end it there.

In species with courtship, the sequence is a necessary precursor to successful mating because it works as a chain:

1. One partner gives a signal, such as a call, a posture or a movement.
2. A suitable partner recognises the signal and responds with its own.
3. That response triggers the next signal, and the sequence continues.

If a partner responds wrongly or not at all, the sequence stops and mating does not happen.

Signals that differ between species

Courtship displays are usually species specific, so the correct signal triggers the correct response in members of the same species but not in others. Signals can separate species when looks do not: the chiffchaff and the willow warbler sing very different songs.

So less time and energy go on mating attempts that would not produce fertile offspring.

What else courtship does

The same exchange of signals can also:

• identify a mate that is sexually mature and receptive, because immature or unreceptive individuals usually do not respond correctly
• synchronise mating, so gametes are more likely to meet
• form a pair bond, in species where cooperation improves survival of offspring

The great crested grebe's sequence includes head shaking, mutual display and the weed dance, in which both birds dive for weed and rise breast to breast. It helps both birds recognise that the other is an appropriate mate.

With courtship data, link each change to its effect on mating. In one AQA question on frog calls, “fewer advertisement calls” alone earned no mark, but “fewer advertisement calls, so fewer females are attracted” did.

Phylogenetic classification and taxonomy

A phylogenetic classification makes a claim about ancestry; it is not just a convenient way to sort organisms.