10.7: How do new species arise?

Biologists don’t really have a clue about exactly how many species there are on earth. Estimates vary tremendously, from 5 million to 100 million. Biologists do know, however, the process by which all species arose.

The process of speciation, in which one species splits into two distinct species, occurs in two phases and requires more than just evolutionary change in a population. The first phase of speciation is reproductive isolation, through which two populations are separated from one another and so come to have independent evolutionary fates. The second phase is genetic divergence, in which two populations evolving separately accumulate physical and behavioral differences over time. These differences may arise as random neutral mutations, or through each population adapting differently to features of their separate environments, including to different predators and different types of food. When differences have accumulated that prevent members of the two populations from interbreeding, we say that speciation has occurred.

Often, the initial reproductive isolation necessary for speciation comes about when two populations are geographically separated. Although this is an effective and common way for speciation to occur, speciation can also occur without it.

Speciation with geographic isolation: allopatric speciation. Suppose one population of squirrels is split into two separate populations because the local climate grows wetter and a river forms that splits the habitat in two. Because the squirrels cannot cross the river, the populations on either side are reproductively isolated from each other. Over time, the two populations have different evolutionary paths as they accumulate different mutations and adapt to particular features of their separate habitats, which may differ. Eventually, the two populations might genetically diverge so much that even if the river separating them disappeared and the populations came back into contact, squirrels from the two groups could no longer interbreed. In fact, two species of antelope ground squirrels formed on the north and south rims of the Grand Canyon as a result of this type of speciation. Speciation that occurs as a result of geographic isolation is known as allopatric speciation (FIGURE 10-11).

Another example of allopatric speciation is seen in the various finch species of the Galápagos Islands, the same finches that Darwin observed and collected while on his ‘round-the-world trip on HMS Beagle. Individual finches from the nearest mainland, now Ecuador, originally colonized one or more of the islands. (And later, additional islands may have been colonized by birds from the mainland or from previously colonized islands.) But because the islands are far apart, the finches tended not to travel between them, and the populations remained reproductively isolated from one another. Consequently, 14 different finch species have evolved in the Galápagos Islands, each species adapting to the predominant food source on its particular island and now having features that allow it to specialize in eating certain of the wide range of insects, buds, and seeds found on the islands (FIGURE 10-12). Only one species of finch is found in mainland Ecuador.

The barrier doesn’t have to be an expanse of water. A forming glacier could split a population into two or more isolated populations. Or a drop in the water level of a lake might expose strips of land that divide the lake into detached, smaller bodies of water, separating one large population of fish into two distinct populations. In each case, the result is the same: geographic isolation that enforces reproductive isolation.

Researchers can easily create new species in the lab, using an analogous strategy. In one experiment, a single population of fruit flies (Drosophila pseudoobscura) was divided into two. The two populations were then maintained on different diets—one on the sugar maltose and the other on a starch-based food. After only eight generations of enforced reproductive isolation and adaptation to their differing nutritional environments, the populations had diverged sufficiently to form separate species; when the populations were mixed, the fruit flies from one population would no longer interbreed with flies from the other population.

Speciation without geographic isolation: sympatric speciation. Speciation can also occur among populations that overlap geographically. This is called sympatric speciation. Among vertebrates, it is rare for populations of the same animal to become reproductively isolated when they coexist in the same area, so this method of speciation is relatively uncommon. But it is common among plants, and it occurs in one of two ways.

During cell division in plants (both in reproductive cells and in other cells of the plant body), an error sometimes occurs in which the chromosomes are duplicated but a cell does not divide. This creates a new cell that can then grow into an individual with twice as many sets of chromosomes as the parent from which it came. The new individual may have four sets of chromosomes, for example, while the original individual had two. This doubling of the number of sets of chromosomes is called polyploidy (FIGURE 10-13).

The individual with four sets of chromosomes can no longer interbreed with individuals having only two sets, because their offspring would have three sets (two sets from the parent that had four, and one set from the parent that had two), which could not divide evenly during cell division. The individual with four sets can, however, propagate through self-fertilization or by mating with other individuals that have four sets. As a consequence, the individuals with four sets of chromosomes have achieved instant reproductive isolation from the original population and are therefore considered a new species.

Although rare in animals, speciation by polyploidy has occurred several times among some species of tree frogs.

A much more common method of sympatric speciation occurs when plants of different (but closely related) species interbreed, forming a hybrid. The hybrid may not be able to interbreed with either of the parental species, but it may be able to propagate asexually—as many plants can. And meiosis often is disrupted in hybrids, causing a chromosome doubling (polyploidy). When this occurs, the hybrids with a doubled number of chromosome sets can interbreed with each other (see Figure 10-13). This method of speciation has led to the production of a large number of important crop plants, including wheat, bananas, potatoes, and coffee.

Whether populations are separated from each other allopatrically or sympatrically, speciation is not considered complete until sufficient differences have evolved in the two populations that they could no longer interbreed even if they did come in contact.

Separation of two populations of a species can occur relatively quickly—as long as it takes for a new river (or freeway) to divide one large population into two—but the genetic divergence that causes true reproductive isolation can take a very long time, sometimes thousands of years. For this reason, speciation can be difficult to study and observe.