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4.5 Global species richness results from a balance between speciation and extinction

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Just as the number of species on islands is largely determined by rates of immigration and extinction, the number of species on Earth as a whole is determined by the relative rates at which new species form and existing ones become extinct. In Chapter 3, we discussed patterns of extinction over time, including how human activity is increasing extinction rates to levels suggesting a mass extinction. Here, we examine extinction’s opposite: the process by which new species arise.

Allopatric Speciation

What factors could keep two geographically isolated populations from diverging genetically?

speciation An evolutionary process by which new species arise.

allopatric (geographic) speciation A process by which new species are formed that occurs as the result of the division of a population into two geographically separate populations; over time, genetic differences arise and accumulate in the two separate populations, eventually leading to reproductive isolation.

The evolutionary process by which new species arise is called speciation. Evolutionary biologists have proposed several ways in which new species may form. The most common form of speciation, however, appears to occur when a geographic barrier, such as a river, canyon, or mountain range, divides a single population into separate subpopulations in a process called allopatric, or geographic, speciation. Because the separated organisms cannot cross the geographic barrier to mate, genetic differences between the two populations can gradually accumulate either because the separated populations adapt to different environmental conditions or because of random mutations. When the populations become so different genetically or behaviorally that they no longer interbreed, they have become two separate species (Figure 4.17).

ALLOPATRIC SPECIATION: ITS FOUR BASIC STEPS

FIGURE 4.17 Allopatric speciation, also known as geographic speciation, is thought to be one of the main ways in which new species arise.

Gorillas provide a good example of allopatric speciation. The two recognized species—the western gorilla (Gorilla gorilla) and the eastern gorilla (Gorilla beringei)—live on opposite sides of the African continent and are today separated by over 1,000 kilometers (620 miles) (Figure 4.18). Eastern gorillas are heftier, live in lager groups, and are more strictly vegetarian compared with western gorillas. However, these two gorilla species once consisted of a single continuous population, with individuals mating and exchanging genes across their entire range. But approximately 1 million years ago, a large-scale climatic or geological event separated the single gorilla population into two allopatric populations. Through successive generations, the separate populations began to accumulate genetic differences, eventually producing the distinctive eastern and western gorillas of today. Yet evolutionary differentiation did not stop there. Both gorilla species eventually evolved into two separate subspecies.

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GEOGRAPHIC SEPARATION OF GORILLA SPECIES

FIGURE 4.18 The western gorilla (Gorilla gorilla) and the eastern gorilla (Gorilla beringei) are separated by more than 1,000 kilometers (620 miles). As shown, both gorilla species have further differentiated into two subspecies.

(Ronald van der Beek/Shutterstock) (Ryan M. Bolton/Shutterstock) (Nickolay Khoroshov/Shutterstock) (© Orokiet/Alamy)

Sympatric Speciation

People all around the world are concerned about survival of gorillas. What are the possible sources of this concern?

New species can also arise without geographic isolation, in a process called sympatric speciation. Sympatric speciation may follow ecological or behavioral separation of two subpopulations of a species that use different habitats or foods. If two subpopulations of a species use different criteria for choosing mates, they may also diverge without geographic isolation. Sympatric speciation can also occur through polyploidy, an increase in the number of sets of chromosomes, either by doubling the number of sets within a species lineage or by doubling the number of sets of chromosomes following hybridization between two closely related species. Polyploidy has been an important mechanism for sympatric speciation in plants. The evidence for sympatric speciation among animals is weaker. However, this is an area of active current research.

Think About It

Some birds, originally described as separate species because they differ greatly in color pattern and some aspects of behavior, have since been combined under a single species name because members of the two populations have been found to interbreed in nature and produce fully fertile offspring. What is the scientific justification for considering these physically distinct populations as members of the same species?
If extinction has been a natural process throughout the history of life on Earth, why are environmental scientists working to reduce the present threats of extinction?

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4.1–4.5 Science: Summary

Species diversity is a function of the number of species—species richness—and their relative abundances—species evenness. Ecosystem diversity consists of the number and kinds of ecosystems in an area. Species richness generally increases from the poles to the equator. Biodiversity hotspots, inside and outside the tropics, support especially high numbers of species.

Diversity is shaped by a number of factors. Species richness on islands, determined by species immigration and extinction, is usually lower compared with similar-sized areas on continents. Keystone species, foundation species, and ecosystem engineers disproportionately influence the biodiversity of their communities. Diversity at any given point in time depends on a region’s history of a disturbance. Following severe disturbance, organisms will begin to colonize the area, setting in motion an ecological process called primary succession. Succession after less severe disturbances is called secondary succession. Species richness generally peaks during the middle stages of a successional process, in which the community commonly consists of a mixture of early (r-selected) and late (K-selected) successional species.

Species diversity is also the product of evolution. During allopatric speciation, a population is divided into two geographically separate populations. The two separated populations then accumulate differences over time, becoming separate species when they no longer interbreed. During sympatric speciation, new species form without geographic isolation.