Inheritance and DNA · Classify
What makes a species
A great dane and a chihuahua are one species. A donkey and a horse are two, even though they can have offspring together. Looking similar is not the test, and neither is being able to breed.
Start here
A mule has a horse for a mother and a donkey for a father.
Mules are strong, healthy, long-lived and have been bred deliberately for four thousand years. They are also, almost without exception, unable to have offspring of their own. Horses and donkeys are classed as two species, and the mule is the reason.
So what is the test for two organisms being the same species?
Whether they can produce offspring that can themselves reproduce. Not whether they look alike, and not merely whether they can have young — the word doing the work is fertile. A mule is a living animal and a genetic dead end, which is why its parents belong to different species.
At the bench · seven hard cases
Same species or not?
0 of 7 settled
Every one of these is chosen because the obvious answer is wrong, or because the test itself struggles. Commit before you check — and read the last two carefully, because they are the ones biologists still argue about.
The pair
A great dane and a chihuahua
A mass difference of about forty times, and skulls so different that a vet can tell the breeds apart from a single bone. Mating them naturally is impractical; with veterinary help the puppies are healthy and can themselves have puppies.
Same species?
That is the answer
Not quite
Same species
One species. Every dog breed is a variety of the same species, and the offspring are fertile — which is the only thing that counts. The pair also demonstrates how far appearance can differ within one species: these two look less alike than a wolf and a coyote, which are two.
A horse and a donkey
They mate readily and produce mules, which are healthy, strong and long-lived. A horse has 64 chromosomes, a donkey 62, and a mule 63. Mules are almost always unable to reproduce.
Same species?
That is the answer
Not quite
Different species
Two species. The offspring exists but is infertile, and the reason is visible in the numbers: 63 chromosomes cannot be sorted into pairs when gametes are made. This is the case the definition was written around.
A lion and a tiger
They do not meet in the wild, but in captivity they have produced ligers and tigons. The hybrids are large, and the males are infertile; a few females have had offspring.
Same species?
That is the answer
Not quite
Different species
Two species. The male hybrids are infertile and the animals never breed together in nature. The occasional fertile female is a genuine complication and is the kind of exception that makes biologists talk about how strongly two populations are separated rather than whether they are.
Two bats that nobody could tell apart
Common pipistrelles in Britain were treated as one species for a century. In the 1990s researchers noticed two groups echolocating at clearly different frequencies, roosting separately and not interbreeding.
Same species?
That is the answer
Not quite
Different species
Two species, and they look effectively identical — the reverse of the dog case. These are called cryptic species, and DNA work has since found a great many of them. Appearance failed completely here, and the breeding test settled it.
Two bacteria in a petri dish
Bacteria reproduce by dividing in two, not by breeding. They also swap sections of DNA with unrelated bacteria, including ones that would never be called the same species.
Same species?
That is the answer
Not quite
The test does not settle it
The test does not settle it, because it asks about breeding and these organisms do not breed. Biologists classify bacteria by comparing DNA sequences and drawing a line at a chosen level of similarity — a line that is useful, agreed, and openly somewhat arbitrary.
Two dandelions on a school field
Most British dandelions produce seed without fertilisation, so each plant produces clones of itself. Botanists have named over two hundred British microspecies.
Same species?
That is the answer
Not quite
The test does not settle it
The test does not settle it. With no interbreeding there is nothing for the definition to test, so botanists fall back on consistent inherited differences — and end up with a number of species that depends on how finely they choose to divide.
Herring gulls and lesser black-backed gulls
A chain of gull populations circles the Arctic. Each interbreeds with its neighbours all the way round, but where the two ends meet in Britain, the birds behave as two separate species.
Same species?
That is the answer
Not quite
The test does not settle it
The test gives different answers depending on where you apply it, which is not a fault in the gulls. It is what you should expect if species form gradually as populations drift apart — catch the process halfway and there is no sharp line to find. The gull example is also messier than the textbook version, and honest sources say so.
The test, and where it runs out
A good definition that does not cover everything.
The definition
Fertile offspring
Two organisms are the same species if they can breed to produce offspring that can themselves reproduce. Both words matter: offspring, and fertile.
Where it works
Animals that breed sexually
Clean and decisive for most animals and many plants, and it explains hybrids like the mule without needing an extra rule.
Where it strains
Organisms that do not breed
Bacteria divide, many dandelions clone themselves, and fossils cannot be bred at all. For these, biologists compare DNA or inherited features instead.
What it is not
Looking alike
One species can look wildly different individual to individual, and two species can be indistinguishable. Appearance is where you start, not where you finish.
Key fact
Two organisms belong to the same species if they can breed together to produce fertile offspring. Members of a species vary enormously in appearance; organisms of different species may look almost identical. Appearance is a clue, not the test.
Think again
“Organisms that look alike are the same species.”
Appearance fails in both directions at once, which is why it cannot be the test. In one direction, a great dane and a chihuahua differ in mass by a factor of forty and are unquestionably one species — every breed of dog is Canis familiaris, and a great dane and a chihuahua are more different in appearance than a wolf and a coyote, which are two species. In the other direction, biologists regularly find cryptic species: populations that are visually identical, that entomologists happily filed under one name for a century, and that turn out on closer study not to interbreed at all. There are pipistrelle bats in Britain that were recognised as two species only when someone noticed they echolocated at different frequencies and never bred with each other. Looking at an organism tells you what to guess. Only breeding tells you the answer.
“If two animals can have a baby together, they are the same species.”
The definition has a second word in it, and it is the one people drop. A lion and a tiger can produce a liger; a horse and a donkey produce a mule; a false killer whale and a bottlenose dolphin have produced offspring in captivity. In each case the hybrid is a real, living, often healthy animal, and in almost every case it cannot reproduce. The reason is usually chromosome number: a horse has 64 chromosomes and a donkey 62, so a mule has 63 — an odd number that cannot be sorted into matching pairs when gametes are made, and the process stalls. So the test is fertile offspring, and hybrids are precisely the evidence that two populations have separated far enough to count as two species. They are not a loophole in the definition; they are how it is applied.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Apply the definition
Two birds mate and produce healthy chicks. The chicks grow up and are unable to have young of their own. Same species?
Rung 2 · The one that catches people
A great dane and a chihuahua look nothing alike. Why are they the same species?
Rung 3 · Explain the mule
Explain why horses and donkeys are classed as two species even though they readily produce mules, and use the chromosome numbers in your answer.
Rung 4 · Take it somewhere new
A biologist finds two populations of beetle on either side of a mountain range. They look identical. Describe what she would need to find out to decide whether they are one species or two, what she should do if they cannot be brought together, and why the answer might reasonably be "not yet decided".
Key note
A species is a group of organisms that can breed together to produce fertile offspring. Appearance is not the test: one species can contain wildly different-looking individuals, and two species can look identical. Hybrids such as the mule show two populations have separated far enough to count as different species. The definition does not work for organisms that never breed sexually, and for those biologists compare DNA instead.
Going further
Ring species are the case that shows the definition bending in real time. Around the Arctic, herring gulls and lesser black-backed gulls form a chain of populations that circles the pole. Each population interbreeds happily with its neighbours all the way round — but where the two ends of the chain meet again, in Britain, they behave as two clearly separate species and do not interbreed. There is no point along the ring where you could draw a line and say a species ends here. The example has been picked over by biologists and the gull case in particular turns out to be messier than the textbook version, which is worth knowing too. What it demonstrates is that species boundaries are real without being sharp: they are the product of populations drifting apart over time, and if you catch one in the middle of that process you should expect the definition to strain — which is exactly what natural selection predicts.
Before this lesson
Connects to
At GCSE this becomes
- Speciation, isolation mechanisms, classification systems and the use of DNA sequencing to work out relationships.
Where to next
- Next: Variation and competitive success
Evolution, extinction and biodiversity
- Previous: Passing it on: heredity
Ask Mr Badmus AI
Want to test the definition on a pair the bench does not cover?
The seven cases are chosen to stress the definition rather than to illustrate it comfortably, and several are genuinely contested among biologists. Chromosome numbers are as usually given; hybrid fertility is described as "almost always infertile" rather than "always" because rare exceptions are recorded, including a small number of fertile female mules.
Lesson content © MrBadmusAI.