How Does Speciation Drive Evolution?

In his 1859 book On the Origin of Species, Charles Darwin introduced the idea that a single species could split apart into two or more separate species. But despite giving this process a name, “specification,” Darwin was surprisingly imprecise about the mechanisms through which one type of organism separates into several.

Today, biologists are much more knowledgeable about this process, which they call “speciation.” Here’s what you need to know about speciation, and about the primary pathways of this process, known as “allopatric speciation” and “sympatric speciation.”

Before jumping into the specifics of speciation, it is beneficial to first understand the biological definition of a species. A species is a group of organisms whose members share similar traits, but biologists don't always agree on what those traits should be. “No one definition has as yet satisfied all naturalists,” Darwin wrote all the way back in 1859, “yet every naturalist knows vaguely what he means when he speaks of a species.”

The first generations of taxonomists thought that organisms needed to share the same morphological traits to be members of the same species. Later generations thought otherwise, proposing that organisms needed to be capable of interbreeding to share the same specification.

Today, there are as many as 26 different definitions that have been used to define species. However, most modern biologists maintain that the most accurate description is a group of genetically distinctive organisms that mate amongst themselves. As such, an animal’s species can be determined by observing its breeding behavior and its genes, the latter of which can also reveal important information about an animal’s ancestry.

Read More: How Do We Define A Species?

With that in mind, speciation is the process through which a “parent species” separates into two or more “progeny species.” It occurs when a group of organisms diverges from the other organisms of the same species — geographically or otherwise — and starts to develop its own distinctive traits. In time, these traits become so distinctive that the progeny group becomes a separate species, unable to breed with the parent group.

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There are several types of speciation, some of which require physical separation and some of which do not. Among the types of speciation that do, the most important is called allopatric. And among the types of speciation that do not, the most important is called sympatric.

Allopatric speciation occurs when a species is separated into groups by physical barriers. Prevented from mating with one another due to the presence of some sort of physical boundary — such as a sea or a waterway — the groups develop into different species, with traits that are adapted to their distinctive surroundings.

The Galápagos is famous for fueling this type of speciation, due to the fact that its islands are all so secluded from one another. In fact, scattered across this archipelago are around 18 separate species of finch, each inhabiting its own island and entirely isolated from the others.

“I do not doubt that isolation is of considerable importance to the production of new species,” Darwin wrote of the islands. And modern genetic measures only confirm this point, demonstrating that the Galápagos finches (also known as “Darwin’s finches”) are all descended from the same small ancestor.

Splitting off from a type of tanager called a grassquit, the Galápagos finches developed their own morphologies over the course of millions of years of separation. While some acquired blunt beaks for breaking seeds, others acquired pointy beaks for poking into pollen-filled flowers. Intensified through natural selection, these distinct traits adapted the birds to their distinct surroundings and distinguished each as its own unique species.

Sympatric speciation, on the other hand, occurs when a species is separated into groups by behavioral barriers. Prevented from mating with one another due to the seemingly spontaneous development of some sort of behavioral boundary — such as a preference for an alternative type of food or shelter or for a specific type of mate — the groups develop into different species, with traits that are adapted to their distinctive tastes.

Evidence of this type of speciation is rather rare. In fact, it is so difficult to prove that a species experienced sympatric speciation that some scientists aren’t even sure that it exists. But several solid examples do exist.

Most notable among these examples are the African cichlids. Inhabiting the lakes of East Africa, including Lake Victoria, Lake Malawi (also known as “Darwin’s pond”), and Lake Massoko (also known as “Darwin’s puddle”), these fish demonstrate a wide variety of shapes, sizes, colors and behaviors. And though biologists have already identified over 2,000 species in these waters, some suspect that there are many more to discover.

Biologists believe that these fish developed their distinctiveness over the course of millions of years, not through the power of physical separation but through the process of sexual selection, instead. In fact, because female cichlids choose to mate with male cichlids whose colors correspond to their own, these fish have successfully separated themselves into separate mating pools, and thus species, while still living in close proximity.

Read More: What Animals Are Going Extinct?

For what it's worth, Charles Darwin never found the project of defining species to be particularly pressing. To him, sorting organisms into specific species and pinpointing the specific moment that they transitioned from one species to another was far less important than understanding the way that they adapted to their surroundings (by developing broader or pointier beaks on specific islands, for instance).

“It is really laughable to see what different ideas are prominent in various naturalists’ minds, when they speak of ‘species,’” Darwin wrote in 1856. “It all comes, I believe, from trying to define the indefinable.”

To Darwin, the constant evolution of organisms meant that they weren't easy or even necessary to separate into species. All that was important was the fact that they were constantly changing to fit to their surroundings, eventually transforming into something entirely different from what they once were.

Today, many biologists take a more positive stance. They see the sorting of organisms into species as an essential part of understanding and protecting the planet's biological diversity. And understanding their speciation — their transformation from one species into another — is equally essential.

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