Natural selection, drift and gene flow change populations, but how does one species become two? A horse and a donkey can mate, yet their offspring, the mule, is sterile, so the two species stay separate. Understanding speciation, the formation of new species, means understanding what stops genes from flowing between groups. The other side of the story is extinction, the permanent loss of a species. Both processes shape the diversity of life you studied in the previous unit.
What is a species?
The biological species concept, formulated by Ernst Mayr, defines a species as a group of populations whose members can interbreed in nature and produce fertile offspring, and that are reproductively isolated from other such groups. It is powerful because it explains why species stay distinct: if genes cannot flow between two groups, each group evolves independently. It has limits, though. It cannot be applied to organisms that reproduce asexually (such as bacteria), to extinct species known only from fossils, or neatly to species that occasionally hybridize. In those cases biologists group organisms by shared body features (a morphological species concept) or by DNA and evolutionary history (a phylogenetic species concept).
Reproductive isolating mechanisms
| Barrier | Type | How it works | Example |
|---|---|---|---|
| Habitat isolation | Prezygotic | Species live in different habitats in the same area, so they rarely meet | One snake species lives mainly in water, a related species mainly on land |
| Temporal isolation | Prezygotic | Species breed at different times of day or year | Two related plants in the same meadow flower in different months |
| Behavioural isolation | Prezygotic | Courtship signals are not recognized by the other species | Each firefly species has its own pattern of light flashes |
| Mechanical isolation | Prezygotic | Reproductive structures do not fit together | A flower's shape allows only one kind of pollinator to reach its pollen |
| Gametic isolation | Prezygotic | Sperm and egg cannot fuse | Sea urchins release gametes into the water, but sperm bind only to eggs of their own species |
| Reduced hybrid viability | Postzygotic | The hybrid zygote fails to develop or the hybrid dies young | Hybrid embryos of some related frog species stop developing |
| Reduced hybrid fertility | Postzygotic | The hybrid survives but is sterile | A mule (horse × donkey) is healthy but sterile |
| Hybrid breakdown | Postzygotic | First-generation hybrids are fertile, but later generations are weak or sterile | Some crosses between rice varieties |
How new species form
- A geographic barrier, such as a mountain range, river, glacier or ocean strait, splits a population.
- Gene flow between the two groups stops.
- Mutation, natural selection and genetic drift act differently on each side.
- Over many generations, the groups become so different that they could no longer interbreed if they met again.
- The most common way new animal species form.
- A new species forms within the range of the parent species, with no geographic barrier.
- Common in plants through polyploidy: an error in cell division produces offspring with extra complete sets of chromosomes.
- A polyploid plant cannot produce fertile offspring with its parent species, so it is isolated in a single generation.
- Can also happen when part of a population shifts to a new food source or mating site.
An allopatric example: when the Isthmus of Panama rose from the sea, it separated the Caribbean Sea from the Pacific Ocean. Pairs of closely related snapping shrimp species now live on either side, and members of a pair mate far less successfully with each other than with shrimp from their own side. A sympatric example: many crop plants are polyploid, including bread wheat, which carries six sets of chromosomes. In North America, some apple maggot flies have shifted from laying eggs on native hawthorn fruit to laying them on apples. Because the flies mate on the fruit they grew up on, and apples and hawthorns ripen at different times, the two groups may be in the process of splitting into separate species.
Adaptive radiation and Darwin's finches
Adaptive radiation is the rapid evolution of many species from one ancestor, each adapted to a different niche, meaning the particular role a species plays in its ecosystem: what it eats, where it lives and how it interacts with other species. It often follows the colonization of new islands or the extinction of competitors. Darwin's finches are the classic example. More than a dozen finch species on the Galápagos Islands descended from an ancestral species that arrived from the mainland. Their beaks became specialized for different foods: thick, deep beaks for cracking hard seeds, slender beaks for catching insects, and long beaks for probing cactus flowers. On the island of Daphne Major, biologists Peter and Rosemary Grant measured finches for decades. After a severe drought in 1977 left mostly large, hard seeds, medium ground finches with deeper beaks survived better, and the next generation had deeper beaks on average: evolution measured within a human lifetime. This branching of one ancestral form into many different forms is divergent evolution. Its opposite, convergent evolution, is when unrelated species evolve similar traits in similar environments, like the streamlined bodies of sharks and dolphins.
Extinction and endangered species
Extinction is a normal part of evolution: most species that have ever lived are now extinct. Species disappear at a low, steady background rate as environments change and competitors evolve. At least five times in Earth's history, mass extinctions wiped out a large fraction of all species in a geologically short time. The largest ended the Permian period about 252 million years ago and eliminated most marine species. The most famous, about 66 million years ago, followed an asteroid impact and ended the non-avian dinosaurs, after which mammals underwent a major adaptive radiation. Today, human activity is driving extinctions far above the background rate through habitat loss, overexploitation, invasive species, pollution and climate change. The passenger pigeon was once one of the most abundant birds in North America; hunting and forest clearing drove it to extinction, and the last known individual died in a zoo in 1914.
- In Canada, a committee of experts called COSEWIC assesses the status of wild species, and the federal Species at Risk Act (SARA) protects listed species. The main categories of risk are:
- Extinct: no longer exists anywhere in the world.
- Extirpated: no longer exists in the wild in Canada, but still exists elsewhere.
- Endangered: facing imminent extirpation or extinction.
- Threatened: likely to become endangered if nothing is done to reverse the factors threatening it.
- Special concern: may become threatened or endangered because of a combination of biological characteristics and identified threats.