Welcome to week 6 of Science Friday! Before reading this week’s post, it is important that you first read week 5 (click the sciencefriday tag), as this post is a continuation of the topic of evolution. Last week, I talked about how the principal mechanism of evolution, natural selection, changes the composition of populations over time. This week, I am going to use this concept as a springboard to discuss the concept of [i]speciation[/i], the rise of new species from common ancestors. Before we talk about how new species arise, we should first define exactly what a species [i]is[/i]. For the longest time, we defined a species as a group of similar looking organisms. While this method was useful, and still is in certain contexts, it is not too reliable overall. The modern day definition of a species is the following: “A group of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups.” The use of the words “actually” and “potentially” clarifies that, even if two individuals [i]choose[/i] not to mate, they are still members of the same species if they have the ability. Reproductive isolation is the inability to produce offspring with members of another species. Now that we have set a precise definition for what a species is, let’s talk about how they form. There are two principal types of speciation: allopatric and sympatric. For brevity’s sake, I will only discuss allopatric speciation this week because a) it is the more common type of speciation event, and b) it is easier to explain more succinctly. Allopatric comes from the roots “allo-,” which means “different,” and “patris,” which means “country.” Allopatric speciation is the formation of new species because of a geographic barrier that fragmented a larger population. The best way to relay this concept is to provide a concrete example. Darwin’s finches are a taxon (group) of birds that reside on the Galápagos Islands off the coast of South America. These birds actually consist of several different species. Since these different species share morphological characteristics, it has been theorized that they actually descended from a recent common ancestor. At some point in the past, there was likely one species of bird that lived on the islands. As there are many islands off the coast of South America, and these islands are fairly distant from one another, it is likely that once a subpopulation (smaller group of the larger population) settled a particular island, they would not migrate to other islands. Different islands, of course, will not share identical environmental conditions, which means they will harbor differing [i]selection pressures[/i]. Differing selection pressures will lead to the evolution of different adaptations on different islands. The evolution of different adaptations must mean that the genetics of populations on different islands are diverging. Genetic divergence, if persistent, may lead to reproductive isolation. It is very important to understand two things, however. Genetic divergence may not necessarily lead to reproductive isolation. However, if divergence persists, it may also become an inevitability. The process whereby genetic divergence leads to reproductive isolation can be a very, very slow process, manifesting itself only after millions of years. Once the different populations become reproductively isolated from one another, they are considered different species, as per the definition we established. This process of speciation highlights a very important fact. The rise of new species is not a linear progression. One of the most common “arguments” against the theory of evolution by natural selection is along the lines of, “If humans evolved from monkeys, why are there still monkeys?” This question is predicated on a completely false premise—that evolution works down a continuous line. Speciation should be thought of as a branching of the tree of life. It is not that humans evolved from monkeys, it is that humans and monkeys share a relatively recent common ancestor. Chimpanzees and humans share an even more recent common ancestor. The relatedness of different species is viewed by phylogenetic trees, a topic which warrants, at the very minimum, its own post. Perhaps I will continue this miniseries another time. For now, I hope you enjoyed this two-part Science Friday on evolution. If you are unclear on the nature of selection pressures, review week 5 or leave a comment. Next week, I will likely move away from biology to generate more diversity in my lineup. As always, feel free to leave questions, comments, articles, and new insights. Responses to these threads motivate me to continue Science Friday. Also, don’t forget to check out my other postings by clicking the #sciencefriday tag and changing the filter settings to view all threads.