Speciation, Reproductive Isolation, and Extinction
Speciation is the evolution of new species from existing ones, usually when populations become reproductively isolated so that they can no longer interbreed, and extinction is the permanent disappearance of a species. In brief: in allopatric speciation, a geographic barrier splits a population, and the separated groups diverge by mutation, natural selection and genetic drift until they cannot interbreed even if they meet again. Over 99 percent of all species that have ever lived are estimated to be extinct, which shows that extinction is a normal part of the history of life.
Speciation explains the diversity of species, and extinction explains why most species have disappeared. This guide defines species, explains reproductive barriers, describes allopatric and sympatric speciation, discusses extinction and mass extinctions, uses Darwin’s finches as a worked example and corrects common mistakes.
What is a species?
The most widely used definition is the biological species concept of Ernst Mayr: a species is a group of populations whose members can interbreed in nature and produce fertile offspring, and which is reproductively isolated from other such groups. Horses (Equus caballus) and donkeys (Equus asinus) are different species, because they produce mules, which are almost always sterile.
The concept has limits. It cannot be applied to fossils, to organisms that reproduce only asexually (such as many bacteria), or to species that sometimes hybridize, as some wolves and coyotes do. Biologists therefore also use other concepts, based on morphology, ecology or DNA similarity.
What is reproductive isolation?
Reproductive isolation is the set of barriers that prevent members of two populations from producing fertile offspring. Barriers fall into two groups.
Prezygotic barriers (before the zygote forms)
- Habitat isolation: the groups live in different places or habitats, so they rarely meet.
- Temporal isolation: they breed at different times of day or season.
- Behavioral isolation: courtship signals differ, such as songs, dances or light flashes in fireflies, which are not recognized.
- Mechanical isolation: the reproductive structures do not fit.
- Gametic isolation: sperm and egg do not recognize each other or cannot fuse.
Postzygotic barriers (after the zygote forms)
- Hybrid inviability: the hybrid embryo does not develop properly or dies.
- Hybrid sterility: the hybrid survives but is infertile, as with the mule.
- Hybrid breakdown: the first-generation hybrids are fertile, but their offspring are weak or sterile.
How does speciation happen?
Allopatric speciation
The word allopatric means “in other homelands”. A geographic barrier, such as a mountain range, river, glacier or sea, separates a population into two. There is no gene flow between them. Each population is exposed to different selection pressures, different mutations arise, and genetic drift acts independently. After enough time, they differ so much that they can no longer interbreed if they meet again. This is the most common form of speciation. Examples include the many species of finches on the Galápagos Islands and the squirrels on either side of the Grand Canyon, which are related, but are distinct species or subspecies.
Sympatric speciation
“Sympatric” means “in the same homeland”. New species form within the same area, without a geographic barrier. It is less common, but well documented. In plants, it often occurs by polyploidy, a doubling of chromosome sets, which can instantly create a new species that is reproductively isolated from its parent. Many crop plants, such as wheat and cotton, are polyploid. In animals, it can occur if groups specialize on different food or mates, as in apple maggot flies (Rhagoletis), some of which have shifted from hawthorn fruit to apples, and in African cichlid fish in lakes.
Adaptive radiation
Adaptive radiation is the rapid diversification of a lineage into many species that occupy different ecological niches. It happens when new habitats are colonized or after mass extinctions, which leave empty niches. Mammals diversified rapidly after the extinction of the non-avian dinosaurs about 66 million years ago.
Types of speciation compared
| Feature | Allopatric | Sympatric |
|---|---|---|
| Geographic barrier | Yes | No |
| Gene flow during divergence | Stopped by barrier | Possible, then reduced |
| Frequency | More common | Less common |
| Typical mechanism | Selection and drift in isolation | Polyploidy, niche specialization |
| Example | Galápagos finches | Polyploid wheat, apple maggot flies |
What is extinction?
A species is extinct when the last individual has died. Species go extinct when they cannot adapt quickly enough to changes, such as climate change, loss of habitat, new predators, competition or disease. The background rate of extinction is around one to five species per year over Earth’s history in some estimates, with large uncertainty. Paleontologists have identified five mass extinctions, when many species vanished in a geologically short time.
- End-Ordovician, about 445 million years ago.
- Late Devonian, about 370 million years ago.
- End-Permian, about 252 million years ago, the most severe, in which an estimated 90 percent or more of marine species disappeared.
- End-Triassic, about 201 million years ago.
- End-Cretaceous, about 66 million years ago, caused in large part by an asteroid impact, and ended the non-avian dinosaurs.
After each mass extinction, biodiversity recovered over millions of years, with new groups taking the empty niches. Many scientists argue that human activity is now causing a sixth mass extinction, with current extinction rates estimated to be many times higher than the background rate.
What does common ancestry mean?
The idea of common ancestry is that all life on Earth descends from one or a few earlier forms, and that species are connected by branching lines of descent, like branches on a tree. Two species that share a recent common ancestor are closely related, and those that share an ancestor only in the very distant past are distantly related. The evidence includes the universal genetic code, the similarity of basic cell biology, homologous structures and DNA comparisons. The relationships can be drawn as phylogenetic trees, as described in the next article.
Worked example: Darwin’s finches
Darwin’s finches are a group of about 15 species on the Galápagos Islands. Explain how a single ancestral species could have produced them.
- Colonization: a small group of finches from South America arrived on the islands about 2 to 3 million years ago.
- Isolation: the islands are separated by sea, so populations on different islands rarely interbred (allopatric speciation).
- Different selection pressures: islands differ in vegetation and food. On islands with hard seeds, finches with big, strong beaks survived better. On islands with insects or cactus flowers, thinner beaks were favored.
- Divergence: beak shape is inherited, so over many generations the populations became different. Peter and Rosemary Grant showed on the island of Daphne Major that during a drought in 1977 finches with larger beaks survived better, as they could crack the remaining large, hard seeds, and the average beak depth increased in the next generation by about 4 to 5 percent.
- Reproductive isolation: differences in song and beak shape lead the birds to choose mates of their own type. The populations became separate species.
The case shows speciation in progress, and the Grants’ long-term study demonstrated that natural selection can produce measurable changes within a few years. DNA studies confirm that all species are closely related and derive from one ancestor.
How did the ideas develop?
Darwin collected finches and mockingbirds on the Galápagos in 1835, and later realized that they were related species that had diverged. Ernst Mayr defined the biological species concept in 1942, and Theodosius Dobzhansky studied how reproductive isolation arises. Georges Cuvier established the reality of extinction in the 1790s and early 1800s by showing that fossil elephants differed from living ones. The asteroid impact hypothesis for the end-Cretaceous extinction was proposed by Luis and Walter Alvarez and colleagues in 1980 based on a layer of iridium-rich clay.
Which mistakes are common?
Mistake 1: Saying that species change because they want to
Isolated populations diverge by mutation, selection and drift. There is no goal.
Mistake 2: Thinking that populations are different species as soon as they differ in appearance
Dog breeds differ greatly in appearance, but they can interbreed. The key test is reproductive isolation.
Mistake 3: Saying that extinction is always caused by humans
Extinction is a natural process, and most species that have lived are extinct. Human activities have greatly increased the rate in recent times.
Mistake 4: Mixing up allopatric and sympatric
Allopatric speciation has a geographic barrier. Sympatric speciation occurs in the same area.