In class, we've discussed evolution (the change in allele frequencies in a population over time). So far, our class has focused on microevolution (the various ways that a population or species changes over time, often to better fit their environment -- at least, if natural selection is the driving force of change). During the next section of this course, we will focus on macroevolution, which is the formation of new species. First, we will look at the evolutionary history of the primates (animals that fall within our own order, our closest relatives the monkeys and apes), and where humans fall within the spectrum of primate adaptations. Then, we will focus on the evolutionary history of our own species. But before we can cover that material, we must know cover the definition of species, speciation (the formation of new species) and how it comes about.

What is a Species?

We consider two individuals or two populations to be members of the same species if the males and females are able to produce viable, fertile offspring. For example, horses are all one species, no matter where they come from or what their breed. Any healthy horse, when put together with a healthy member of the opposite sex, will produce baby horses that are viable (meaning they will survive, they don't automatically die in the embryonic stage) and fertile (when they grow up, the offspring are capable of producing offspring of their own).

Horses and donkeys, on the other hand, are not members of the same species. Although they can produce viable offspring together (mules), their offspring are sterile (unable to reproduce). Therefore, they are not members of the same species.

This definition of a species is the most straight-forward and easily tested. A number of other definitions exist to deal with the complexities of the plant and animal world. For the purposes of this class, however, we're going to use the simplest definition. If you're interested in exploring the issue farther, I recommend the Evolution 101 webpage at UC Berkeley.

How do New Species Form?

In general, new species emerge when one population of a species ceases to reproduce with other populations of the species. Once reproductive isolation has occurred (that is, once there is little or no gene flow between one population and other populations), then the isolated population may evolve in a different direction (either through natural selection or genetic drift) and become quite different from its parent species. A new species is born!

Allopatric Speciation

Allopatric speciation is speciation that occurs when two populations are geographically isolated, so that there is little or no gene flow between them. For example, populations of squirrels might be divided by a new river that developed after a glacier melt. Over generations, the populations on either side of the river develop into new species.

Peripatric Speciation

Peripatric speciation is a type of allopatric speciation that occurs when one of the isolated populations is very small. Founders effect (a type of genetic drift) will have a big impact on that small population, which may have rare genes, or very different frequencies of genes than the larger population. This will lead to that small, isolated population becoming a different species after a some generations of interbreeding and adaptation to their isolated environment.

Sympatric Speciation

Sympatric speciation occurs when two populations have reduced gene flow, even though they are not physically isolated. The two populations may then evolve in different directions and become two different species.

An example would be monkeys that wash up on the shore of an island where no other monkeys live. Over time, they produce a fairly large population of monkeys. As the populations get large, they start to run out of resources. If all the monkeys focus on eating fruit in trees, then there won't be enough fruit for all of them to eat. Some monkeys start to eat leaves, instead. Eating leaves allows them to get food without having to compete with all the other monkeys. If groups of leaf-eating monkeys get together as a troop, then they will be mating mostly with other leaf-eating monkeys. The fruit-eating monkeys will mostly mate with other fruit-eating monkeys. Over time, the fruit-eating monkeys and the leaf-eating monkeys may evolve into separate species, each with the physical adaptations that best fit their particular food.

Specialization to avoid competition is common when populations find themselves in new environments. Sympatric speciation can lead to an adaptive radiation, where a number of different species develop in order to fill all the available niches of the new environment. In the example above, fruit-eating and leaf-eating monkeys might eventually evolve into a large number of monkey species that all eat different foods.

Theories of Speciation

Gradualism

When Darwin first proposed his theory of evolution, he assumed that natural selection was the primary way that new species were created. Through a process called gradualism, small changes in the gene pool would occur through natural selection, eventually leading to new species. This process was slow, gradual, and based on the accumulation of microevolutionary changes.

Gradualism fit well with the Victorian-era cultural perspective. Darwin's world was the world of the British Elite at the height of the Empire. His contemporaries - the men who made his views popular and wide-spread - were a little scared of change. They rather liked things the way they were. They took comfort in knowing that natural change was slow and ponderous, rather like the British Empire. So they focused on the role of natural selection in creating new species.

Subsequent research has shown that natural selection is critical for microevolution, but can it really cause speciation? That claim has been contentious in modern biology. Many biologists and anthropologists have turned to an alternative theory of speciation:

Punctuated Equilibrium

Punctuated Equilibrium is a theory championed by paleontologist Steven Jay Gould. He argued that speciation occurs mostly through genetic drift, rather than natural selection. In his view, catastrophic or chance events are more important for explaining the emergence of new species than the slow, gradual accumulation of changes caused by natural selection. He viewed the normal history of a species as a long periods of equilibrium (during which natural selection worked to fine-tune the species' adaptation to its environment), periodically punctuated by catastrophic events that led to periods of strong selective pressure and/or major changes that occurred through chance alone.

Gould's theory makes a great deal of sense if you look back at the previous section on types of speciation. Allopatric speciation and parapatric speciation both require some kind of external, catastrophic event, such as the splitting of a land mass, or the flooding of a new river channel. Even parapatric speciation requires the opening of a new habitat, either through physically moving individuals or through climate change. Natural selection is important after those events occur, for creating differences between isolated populations as they adapt to their environments, but each of the speciation events was originally triggered by a chance event. That is genetic drift.