Our definition of evolution is any change in allele frequencies over generations has given us five mechanisms for how species evolve, mutation, natural selection, sexual selection, gene flow, and genetic drift. These are the mechanisms that cause evolutionary change within a species. But how do new species come into existence? That was the ultimate question that Darwin wanted to address and based on the title of his most famous publication on the Origin of Species, you would think it would have been the primary focus of his work. But ironically, the Origin of Species focused more on how natural selection leads to changes within a species, but doesn't reveal as much about the process by which new species evolve. Darwin used the example of Galapagos Finches to show how natural selection can cause species to change from generation to generation and suggested that this could eventually give rise to new species. But he didn't give a very clear explanation for how new species would arise. Researchers, since Darwin had been able to learn much more about how new species come into existence, a process known as speciation. But before we dive into how speciation works, we need to address a more fundamental question. What is a species? Surprisingly, there isn't one definition that is universally agreed upon and used by all biologists. Each definition that has been proposed either isn't useful under certain circumstances or it doesn't apply to all species. For example, one concept of species define species based on their ecological niche or the role they play in their environment. According to this definition, all members of a species will occupy a particular ecological niche, meaning it will live in a particular climate and habitat and use particular resources like food, and that other species will occupy different niches. This may work fine for biologists studying living species. But what if you're a paleontologist trying to sort out extinct species based on fossils. You might not have enough information to sort out the ecological niches. You'd have a hard time applying the ecological species concept. One commonly used definition of species is known as the biological species concept. According to this view, first suggested by evolutionary biologists Theodosius Dobzhansky in 1935 and popularized by biologist Ernst Mayr in 1942, members of a species are able to interbreed with one another and produce viable, fertile offspring. Well, this definition is often used in textbooks. It doesn't apply to species that reproduce asexually like bacteria. Well, we may not have a universal definition of what a species is. Most biologists agree that species exist as real entities. In practice, biologists often use a combination of definitions when describing a new species or sorting out where to draw the line between closely-related species. For example, in my work on ants, I've often used a combination of morphological, genetic, and ecological information to distinguish between species. So how do new species, however you define them, come into existence? Let's begin by using the biological species concept that species are defined by their ability to interbreed. Dobzhansky pointed out that under this definition, speciation must involve barriers to reproduction. One simple way for barriers to reproduction to come about is if a few individuals of a species become physically isolated from the others. When speciation involves geographical separation, it's called allopatric speciation. This can happen, for example, when a few individuals happened to find their way to a new place like an island. If they become isolated from all other members of their species, then eventually they'll develop new mutations and natural and sexual selection will cause some traits become more or less common. Eventually, the isolated population begins to accumulate enough differences for it to be considered a distinct species. Here's another scenario for allopatric speciation. Let's say we have a single population of some species consisting of just five individuals. Within the population, all the mechanisms of evolution we've already discussed should be happening. There'll be mutations, natural selection, sexual selection, and genetic drift. If there are other populations nearby, individuals may occasionally move from one population to another, leading to gene flow. We call this system of discrete populations connected to one another through dispersal, a metapopulation. There isn't much opportunity for a new species to come into existence because any new mutation that pops up can easily spread within the population and can even be spread to other populations through gene flow. But what happens if one of the populations gets cut off from the others? Now, the isolated population can accumulate differences and evolve into a new species. This seems to be exactly the scenario that has played out in archipelagos or clusters of islands. One example of an archipelago is the Galapagos Islands. The reason that the Galapagos Islands have become such a useful place for studying evolution, is that their geography is ideal for generating new species. As a group they're isolated from the mainland where most of the plants and animals originated. Within Galapagos, the individual islands are located far enough apart for most plants and animals to be unable to easily move from one to the other. Yet they're close enough that this still happens on rare occasions. The key ingredient for allopatric speciation is isolation. The Galapagos Islands have just the right amount. The Galapagos finches are a great example of how speciation can play out in allopatry. The common ancestor of the 14 living species of Galapagos finches arrived from mainland South or Central America some 2-3 million years ago. Being isolated from other members of its species, it began to accumulate differences through mutation, natural selection and sexual selection. The habitats in Galapagos are quite different from mainland ecosystems. Natural selection would help the birds become better adapted to the new environment. Since there were probably only a small number of individuals at first, genetic drift may have also played a role in causing some traits to randomly become more or less common. Eventually, through some combination of selection and drift, it evolved into a new species distinct from the original founders and from its relatives back on the mainland. Sometime later, a few individuals of this new species might be able to make their way to another island in the Galapagos. Isolated once again, they would experience a similar sequence of events causing them to diverge from the species on the island. This process would be enhanced by natural selection if the second island contains habitats that are different from the first island. At some point, the birds from the second island might make it back to the first island. If they do, what might happen next?. The two populations of finches have become somewhat different from one another after having been isolated on different islands and perhaps adapting to different environments. However, if they are still capable of interbreeding, their offspring would be hybrids that would likely be different from either parent. If the hybrids are able to survive just fine and can themselves reproduce, then they may not continue to diverge. Hybridization between recently diverged populations can prevent speciation from becoming complete. In this scenario, the finches don't evolve into a second distinct species. Another possibility is that after the recently diverged populations come back together and mate the hybrid offspring are at a disadvantage. Perhaps one population has evolved deep beaks that are good for crushing seeds, while the other evolved narrower beaks that are good for extracting insects from inside a cactus. If their hybrid offspring have beaks that aren't particularly good at either crushing seeds or extracting insects, then the offspring might starve. Under this scenario, it would be better for all the finches if the birds from each population never mate, or if they do, if mating doesn't lead to the birth of an offspring. In other words, natural selection should favor any trait or behavior that prevents the birth of hybrid offspring. There are many different ways in which this can be accomplished. One or both populations might evolve a new type of mating call that isn't recognized by the other populations, so they don't come together to breed. Perhaps one population shifts its breeding season to be a little earlier or later than the other population. Alternatively, if mating does occur, one population might develop genetic differences that prevent the formation of a viable hybrid embryo. These examples all involve preventing the formation of a fertilized embryo or zygote. They're known as pre-zygotic reproductive barriers. Other barriers to the formation of hybrids can exist after the formation of a zygote, such as if the hybrid embryo fails to develop properly, or if the hybrid offspring can survive but is infertile. These are known as post-zygotic reproductive barriers. These situations in which the initial differences between two recently diverged populations become enhanced by reproductive barriers is called reinforcement. When reinforcement takes place, it makes speciation much more likely to take place because the populations that had already begun diverging can continue to do so without any differences they accumulated, getting erased by gene flow. This sequence of events, colonization of a new island, accumulation of differences through mutation selection and drift, migration to another island, migration back to the original island and then reinforcement can play out again and again in an archipelago. The fact that each island has somewhat different environments means that adaptation can take place in a slightly different way on each island. Put it all together and you have the recipe for adaptive radiation. The emergence of a variety of new species, like the 14 Galapagos finches from a single common ancestor.
