Speciation: How New Species Form

Two fish, one lake

In a few small lakes in southwestern British Columbia, two kinds of threespine stickleback live side by side. One is big-bodied and feeds on the lake bottom; the other is slim and feeds on plankton in open water. They rarely interbreed in nature, and they mostly prefer mates of their own kind. Yet they evolved only since the end of the last ice age, roughly the last 10 000 to 12 000 years.

Canadian biologist Dolph Schluter and his colleagues at the University of British Columbia have studied these fish for decades, as a window on one of the most important events in evolution: the birth of a new species. This lesson explains how that happens.

What is a species?

The most widely used definition is the biological species concept, proposed by the biologist Ernst Mayr: a species is a group of populations whose members can interbreed in nature and produce fertile offspring, and that are reproductively isolated from other such groups.

This definition works well for most animals, but it has limits:

• It cannot be applied to fossils, because we cannot test whether extinct organisms could interbreed. Palaeontologists use body form instead (the morphological species concept).
• It does not fit organisms that reproduce asexually, such as bacteria.
• Some distinct species hybridize in nature. For example, coyotes in eastern North America carry some wolf DNA from past hybridization.

So biologists also use other species concepts, including ones based on DNA and phylogeny (the evolutionary relationships you studied in the Diversity of Living Things unit).

Speciation needs reproductive isolation

Speciation is the formation of a new species. The key step is reproductive isolation: once two populations can no longer exchange genes, mutation, natural selection and drift can make them diverge independently.

Prezygotic barriers stop mating or fertilization from happening:

• Habitat isolation: they live in different habitats in the same area (bottom-feeding and open-water sticklebacks).
• Temporal isolation: they breed at different times. American toads usually breed earlier in spring than Fowler's toads.
• Behavioural isolation: different courtship signals. Eastern and western meadowlarks look almost identical but sing different songs, and females respond to their own species' song.
• Mechanical isolation: body parts do not fit. Many insects and flowers have shapes that only match their own species.
• Gametic isolation: egg and sperm cannot fuse. Many sea urchins release eggs and sperm into the water, but sperm can only fertilize eggs of their own species.

Postzygotic barriers act after a hybrid zygote forms:

• Hybrid inviability: the hybrid embryo does not develop or the hybrid dies young.
• Hybrid sterility: the hybrid lives but cannot reproduce. A mule (horse × donkey) is strong and healthy but almost always sterile.
• Hybrid breakdown: the first-generation hybrids are fertile, but their offspring are weak or sterile.

Allopatric speciation: separated by geography

Allopatric means "other homeland." In allopatric speciation, a geographic barrier splits a population:

1. A barrier forms, such as a mountain range rising, a river changing course, a glacier advancing, or sea level changing. Or a few individuals cross to an island.
2. Gene flow stops between the separated groups.
3. Each group experiences different selective pressures (climate, food, predators) and different mutations and genetic drift. Their gene pools diverge.
4. Over time, they build up so many differences that, even if the barrier disappears, they can no longer interbreed. They are now separate species.

Examples: When the Isthmus of Panama rose about 3 million years ago, it separated the Caribbean Sea from the Pacific Ocean. Pairs of snapping shrimp species, one on each side, now look very similar, but when brought together most pairs will not mate. On the Galápagos, finches that colonized different islands diverged into separate species.

Sympatric speciation: same place, new species

Sympatric means "same homeland." Sympatric speciation happens without a geographic barrier. It is less common in animals but very important in plants.

• Polyploidy. Sometimes an error in cell division produces offspring with extra complete sets of chromosomes. A polyploid plant usually cannot produce fertile offspring with its parent species (their chromosome sets do not match), but it can often self-fertilize or breed with other polyploids. A new species can form in a single generation. Many crop plants are polyploid, including bread wheat, potatoes and strawberries.
• Shifting to a new habitat or food. The apple maggot fly in North America originally laid eggs only on hawthorn fruit. After apple trees were brought from Europe, some flies began to use apples. Flies tend to mate on the fruit they grew up on, and apples and hawthorns ripen at different times, so the two groups are becoming isolated even in the same area. Biologists see this as speciation in progress.
• Disruptive selection plus mate preference, as in the BC sticklebacks, can also split one population in the same lake. (Biologists debate whether the sticklebacks count as fully sympatric, since they may have arrived in two waves.)