Introduction to Speciation - Online Tutor, Practice Problems & Exam Prep

Speciation is the process where one species diverges into two distinct species, often illustrated through evolutionary trees. Nodes on these trees represent points where populations split, influenced by factors like microevolutionary changes. While defining a species remains complex, understanding speciation bridges microevolution and macroevolution, highlighting how small changes can lead to significant differences over time. This process emphasizes the importance of evolutionary dynamics in biodiversity and species classification.

Introduction to Speciation

When we think of evolution, sometimes we call it Darwinian evolution, but of course, Darwin's book was called On the Origin of Species. So a major question in evolution is just where do new species come from? That's what we're talking about here, and we're going to define that process as speciation. And we have a very simple definition for it here. It's just when one species breaks into two species.

Now to think about this, we've brought back this evolutionary or phylogenetic tree here of the big cats. As I look at it, right, it's very obvious to me that lions and leopards are different species, that leopards and tigers are different species. But what we want to focus in on here are the nodes. Right? On a branch like this, when we look at lions and leopards, this branch here, well, it shows me that in the past they were one species, and then it split at this node into two different branches that we call different species.

What are the things that can happen at that node that split a population into two? And when we're looking at populations, how do we make that decision that, well, are these two populations of the same species? Or no. These are definitely two different species. So going forward, I want you to sort of think of this as what happens at the nodes in an evolutionary tree.

Alright. Now remember, we've talked before about microevolution and macroevolution. Microevolution are those small changes that happen in a short period of time, things like changes in allele frequency, where macroevolution were those sort of larger changes that take a longer time, kind of like what are the differences between lions and tigers. We can think of speciation as the link or, oh, right here, the bridge between micro and macroevolution. Right?

Here we're going to take more of a microevolution approach. We're going to really be thinking in sort of shorter time periods, how do these populations split? But once that happens, it's really easy to imagine how these different species accumulate big changes over long periods of time and become very different from each other. Now before we go on, I just want to note that defining a species well, we're going to call it here a fuzzy art. There really is not one way to do it.

Right? And remember, Charles Darwin's book was called On the Origin of Species. You'd think that in it he had a good definition of species. In fact, he doesn't. He kind of avoids defining species.

In the book, he says, no one definition of species has yet satisfied all naturalists. Yet every naturalist knows vaguely what he means when he speaks of a species. In some ways, we're still in that boat today. Right? Now, we're going to have some very clear rules onto what makes two species or what makes things different species.

But not all rules apply in all cases. And also remember, this is going to be a process. So when you're looking at something, making that judgment call of whether it's in the process of speciation or whether it's actually two species, sometimes just a judgment call. What we want to do is figure out how we apply those rules to make those judgments. Alright.

With that, let's get to it. I'll see you in the next video.

Here’s what students ask on this topic:

What is speciation and how does it relate to microevolution and macroevolution?

Speciation is the process by which an ancestral species diverges into two or more distinct species. It serves as a bridge between microevolution and macroevolution. Microevolution involves small changes within a population over a short period, such as changes in allele frequencies. Macroevolution refers to larger evolutionary changes that occur over long periods, leading to the formation of new species. Speciation links these two processes by explaining how small genetic changes can accumulate over time, eventually resulting in the formation of new species. This process can be driven by factors like geographic isolation (allopatric speciation) or reproductive barriers within the same area (sympatric speciation).

What are the main types of reproductive isolation that contribute to speciation?

Reproductive isolation is a key factor in speciation and can be categorized into prezygotic and postzygotic barriers. Prezygotic barriers prevent the formation of a zygote and include mechanisms like temporal isolation (different mating times), behavioral isolation (different mating behaviors), mechanical isolation (incompatible reproductive structures), and gametic isolation (incompatibility of sperm and egg). Postzygotic barriers occur after zygote formation and affect the viability or fertility of hybrids. These include hybrid inviability (hybrids do not develop properly), hybrid sterility (hybrids are sterile), and hybrid breakdown (hybrids are fertile but their offspring are inviable or sterile).

How does allopatric speciation differ from sympatric speciation?

Allopatric speciation occurs when a population is geographically isolated, leading to the formation of new species. Geographic barriers such as mountains, rivers, or distances prevent gene flow between separated populations, allowing them to evolve independently. Over time, genetic differences accumulate, resulting in speciation. In contrast, sympatric speciation happens within the same geographic area. It often involves mechanisms like disruptive selection, where different environmental pressures favor different traits within the same population, or polyploidy, where an organism has more than two sets of chromosomes, leading to reproductive isolation even without physical separation.

What role do hybrid zones play in the process of speciation?

Hybrid zones are regions where two distinct species meet and interbreed, producing hybrid offspring. These zones provide valuable insights into the dynamics of speciation. Depending on the fitness of the hybrids, hybrid zones can lead to three possible outcomes: reinforcement, fusion, or stability. Reinforcement occurs when hybrids are less fit, leading to stronger reproductive barriers and further divergence of the species. Fusion happens when hybrids are as fit as the parent species, potentially merging the two species back into one. Stability occurs when hybrids continue to be produced, maintaining a stable hybrid zone without significant changes in the parent species.

Why is defining a species considered a 'fuzzy art'?

Defining a species is considered a 'fuzzy art' because there is no single, universally accepted definition that applies to all organisms. The biological species concept, which defines species based on reproductive isolation, works well for sexually reproducing organisms but not for asexual organisms or those with extensive hybridization. Other concepts, like the morphological species concept (based on physical traits) and the phylogenetic species concept (based on evolutionary history), also have limitations. As a result, determining whether two populations are distinct species often involves a combination of criteria and can be subjective, making it a complex and sometimes ambiguous process.