How new species form

A horse has 64 chromosomes and a donkey 62, so their mule gets 63, and that odd set is why mules almost never have foals.

What counts as a species? For animals and plants that reproduce sexually, biologists usually use the biological definition: a species is a group of individuals that actually or potentially interbreed in nature and produce fertile, viable offspring. Looks can mislead in both directions. Poodles and cocker spaniels look nothing alike, yet dogs of different breeds can mate and have puppies that grow up and breed, so they are one species. The bald eagle and the African fish eagle look very similar but are two species; if a hybrid chick were made in a lab, it would probably be infertile.

The mule is the classic case. A mule is the offspring of a female horse and a male donkey. It inherits 32 chromosomes from the horse and 31 from the donkey, 63 in all. To make eggs or sperm, chromosomes have to line up in matching pairs, and horse and donkey chromosomes do not match well; one is left with no partner at all. So the sex cells usually end up incomplete and mules are sterile. A rare female mule has been recorded giving birth, but horses and donkeys stay two separate species.

Separated by geography

For one species to become two, its populations have to stop exchanging genes and then drift apart until they can no longer interbreed. The commonest route is allopatric speciation, from Greek roots meaning "other homeland". A river changes course, a valley opens, or a few animals cross the sea to an island and cannot get back. Once separated, each population gets its own mutations, its own chance events and its own selection from a different climate, food supply and set of predators. OpenStax describes the northern spotted owl and the Mexican spotted owl in the western United States as populations that have diverged this way. How big a barrier has to be depends on the animal: a valley that stops a mouse is nothing to a flying insect.

Many species from one

When a single species spreads into many empty niches, it can split again and again. This is adaptive radiation. Island chains are ideal places for it because the water between islands keeps populations apart. The Hawaiian honeycreepers come from one founding species and now include seed eaters with thick, strong beaks, nectar feeders with long beaks and insect eaters with pointed beaks. Darwin's finches on the Galápagos are another radiation. On Daphne Major, scientists followed a male finch that flew in from Santa Cruz Island, and his descendants, for 31 years and six generations. From the second generation onward, the family behaved as a species separate from the other finches on the island.

Splitting in the same place

Sympatric speciation happens with no physical barrier at all. In plants it often starts with polyploidy, an error in cell division that leaves an individual with extra whole sets of chromosomes. A plant with 2n = 6 that makes diploid gametes by mistake can produce tetraploid offspring with 4n = 12. Those can breed with each other but not with the parent species, so a new species can appear in one or two generations. Some cultivated forms of wheat, cotton and tobacco are allopolyploids, formed through hybridization between species and chromosome doubling. In animals, groups can also split by feeding or living in different parts of one place. In Lake Apoyeque in Nicaragua, a crater lake about 1,800 years old, a cichlid fish population arrived only about 100 years ago, and two forms with different bodies and diets already live there, possibly at an early stage of becoming two species.

What keeps species apart

Biologists sort the barriers into two groups, before and after a zygote, the fertilised egg, forms. Prezygotic barriers stop mating or fertilisation. Two frog species may breed at different times of year, one from January to March and the other from March to May. Two cricket species may prefer different soils. Fireflies of one species do not answer the light pattern of another. Body parts may not fit, and some flowers suit only one kind of pollinator. Postzygotic barriers act afterwards: the hybrid embryo may not develop, which is hybrid inviability, or the hybrid may grow up sterile like the mule, which is hybrid sterility.