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If you have a sibling, you can probably note down some differences between yourselves, one of you have your father's eyes, while the other have you mother's. If you then compare yourself with your first cousins, the differences maybe more pronounced.
Now, how far do we have to go back the family tree to be completely unrecognizable? And how did this came about? |
"I do not want to discuss evolution in such depth, however, only touch on it from my own perspective: from the moment when I stood on the Serengeti plains holding the fossilized bones of ancient creatures in my hands to the moment when, staring into the eyes of a chimpanzee, I saw a thinking, reasoning personality looking back. You may not believe in evolution, and that is all right. How we humans came to be the way we are is far less important than how we should act now to get out of the mess we have made for ourselves."
Dr. Jane Goodall, Reason for Hope: a Spiritual Journey (2000)
Most people can look at a dog and a horse and guess that they are two different species. That is, unless it's a Great Dane. Those must be some kind of dog-horse hybrid, right? Just kidding. The species game becomes a little more difficult when things start looking more similar. For instance, are toy poodles and Great Danes in the same species? Yep. They're in the species Canis lupis familiaris. What about broccoli and cabbage? Yep. They're in the species Brassica oleracea.
There are many cases in biology where defining species gets a little fuzzy. The trouble is that there are so many exceptions. What if horses and dogs could mate and have horse-dog offspring? Aside from making what would probably resemble a Great Dane, it would really confuse our idea of horses and dogs as separate species. Maybe horse-dogs belong to the sci-fi world, but there are tons of real examples of hybridization between what we think of as distinct species.
What about organisms that don't mate at all? Lots of microbes and plants reproduce clonally—are they each their own species? Even weirder, some organisms can pick up random bits of DNA from other organisms and make it their own. Are these critters the original species, or a new species altogether? This is called horizontal gene transfer (HGT), and strange as it might sound it happens all the time, even among organisms that mate. Bacteria are notorious DNA collectors, and there's even evidence of shared genes between old world monkeys and housecats because of HGT. Imagine if you ate a peach and a little bit of stray peach DNA escaped and joined up with your own. Weird. While HGT might sound peachy, it makes defining a species really hard.
If these exceptions are making your head spin, join the club. People have been arguing about species definitions for a long time, and not just about the details. The debate actually starts with a pretty basic question: are species even real?
If we're talking about the difference between a dog, a horse, and a cabbage, then the answer seems obvious. If we're talking about the difference between the hybridizing oak species Quercus robur, Q. petraea, Q. pubescens, and Q. frainetto, then the answer isn't as easy. Are species actually distinct biological entities, or are they just a way that biologists categorize the world around them with no real scientific basis?
Even if scientists can't always define species, most will try to convince you that they're more than mere human constructs. Yes, there are lots of exceptions, problems, and unanswered questions when it comes to making a universal species definition. Even so, there are a number of compelling reasons to believe in species.
First off, species recognition is intuitive. Most people can distinguish between groups of organisms that look similar, as long as there are a few subtle differences. The distinctions that we make would probably be similar to the distinctions Bob, the body builder would make. In biology we would say that the distinctions are congruent.
Congruence goes further than just friends and family. There have been several studies that demonstrate congruence between the species definitions made by western scientists and those made by indigenous cultures. There is a famous story about Ernst Mayr, the fancy pants Harvard biologist. Ernst visited the island of New Guinea and asked the local tribesmen to identify the bird species of the island. Amazingly, the tribe distinguished 136 species of birds, while classic Linnaean taxonomy knew of 137. That's an incredible amount of congruence among species delineations, and it just goes to show that we don't need formal taxonomy training to be great biologists. There is just something intuitively recognizable and distinguishable about some groups of species, they must be more than figments of our scientific imaginations.
Second off, even if organisms can hybridize, or move, or otherwise thwart our convenient species definitions, they still tend to cluster into more or less distinct groups. It's not like there's a gradual continuum of birds ranging from eagles to hummingbirds—they tend to come in different types, each more or less distinguishable from the next. Sure, there might be mixing among some groups, but the mixing among them is smaller than the mixing within them, so they form clusters. Individuals from one group tend to mate with other individuals from their own group more often that with individuals from a different group. Notice the vocabulary here: groups, types, clusters, or maybe species.
But what are species, exactly? There are many definitions for the concept of species. There is no universal species concept—one that applies to all organisms. Instead, scientists have been proposing different species concepts for years, and each is based on slightly different biological reasoning. There's the genealogical species concept, the cohesion species concept, genotypic cluster species concept, and more. Here are a few of our faves:
The ecological species concept claims that species are groups or populations that share the exact same ecological niche. The definition gets a little tricky with organisms that change niches over their development. For examples, some organisms live in the water as larvae, and on land as adults. It's sometimes expanded to include a set of niches.
The phenetic species concept defines species as groups of organisms that are similar in some defining trait(s) chosen by taxonomists, like eyebrow density, tastiness, or affinity for staring off into the distance. They'd probably pick something more scientific, but we'd be all for running a phenetic species taste test.
The phylogenetic species concept is similar, but with a slight twist: it states that a species is the smallest set of lineages or populations that can be recognized by a unique combination of different traits.
Is it just us, or do none of these concepts seem right? In a world of imperfect species concepts, allow us to introduce the victor—the biological species concept, or BSC. Is it foolproof? No. Does it allow for horse-dogs? No. Do we love it anyway? You'd better believe it.
Championed by Ernst Mayr, the BSC puts the emphasis where it belongs: on reproduction. In 1963, Mayr defined a species as follows, "species are groups of interbreeding natural populations that are reproductively isolated from other such groups."
The fact the a species must be some 'natural' designation means that, regardless of what sort of transgenic weirdness scientists can conjure up in a lab, we cannot claim to make new species. Even when we're creating organisms that share genes with distantly-related organisms (ahem, Fish Tomato anyone?).
Species are limited to reproduction in natural populations, not lab mice. The 'reproductively isolated' bit is taken to mean the following: if there is a chance that genes from one population could end up in the offspring of another population, then they belong to the same reproductive group. The two populations are reproductively isolated if there is no way they can share genes without alien intervention. We added in that last part, but it seems important somehow.
The BSC is useful because it emphasizes interbreeding, and so a species becomes synonymous with its gene pool. As long as two organisms belong to the same gene pool, they're the same species. This makes things easier, because scientists have a lot of tools for tracking genes in a population, measuring gene flow, and testing genetic similarity between related species. Seems simple. Almost too simple. As much as we love the BSC, there are still lots of cases where it starts to fall apart.
One of the more interesting scenarios in which the BSC doesn't quite work is ring species. Ring species are groups of closely related species whose ranges partially overlap to form an imperfect ring. Each species can hybridize and pass on genes with the species on either side of it, but the kicker is that the last species in line is so different from the first species, that they cannot hybridize.
This is like evolution's version of telephone. Usually it comes out totally distorted and hilarious. Ring species are less hilarious, but fascinating from the perspective of the BSC.
Here's why they're so fascinating: the first and last species in the ring cannot hybridize with each other. According to the BSC, they're separate species…right? Nope. Because of the hybridization that occurs in all the in-between species, it is theoretically possible that a gene from the first species could get passed on through all the others and ends up in the last species. That's a pretty successful game of telephone. Under the BSC, it makes them the same species.
In fact, any time there's hybridization between two species—even if it's only the tiniest little smidgeon—the BSC says they're the same species. This gets to be a little troublesome, as there are lots of examples of two populations that can hybridize, but rarely do. Maybe they have totally different geographic ranges or seasonality, or slightly different mating preferences. If we look at their population genetics we see that they belong to separate gene pools. The pools spring a rare leak every once in a while.
What do we do if the BSC says two entities are the same species, but our intuition says it just isn't so? We invent a new taxonomic level. Enter the subspecies. A subspecies is a population or group of organisms that can interbreed with individuals from another group or population, but usually don't. Now we have a way of designating potentially interbreeding groups from actually interbreeding groups, and we don't have to abandon the BSC. Have you thanked a taxonomist today?
The truth is, there are plenty of weird situations where the BSC isn't foolproof. For the time being it's the best we've got, and it's a pretty decent working definition. By focusing on interbreeding and gene flow, we can shift our focus from what a species is and get to the good stuff: why are there so many different species, and how did they arise?
Now that we're armed with the BSC, we can finally explore Speciation—the evolutionary process that leads to the generation of multiple species from a single common ancestor. We have speciation to thank for the Earth's incredible biodiversity: one million species of mites (yes, we know you’re drafting speciation a thank you note right now), up to 30 million insect species (putting a stamp on the envelope), more bacteria than we can imagine, 270,000 plant species, and about 56,000 species of vertebrates. The numbers are staggering and growing all the time.
The study of speciation starts with the question: how does one species split to become two? As we approach this question, it's important to think about how lineages change in general. Even in the absence of speciation, species don't just sit around, staying the same and waiting for some evolutionary action to pick up. They're usually gradually changing and adapting to their environment, which is also changing. For example, a frog head might respond to some mating cue or environmental pressure over time to slowly change color. This frog species isn't speciation—it's still all one species—but it is evolving.
Now it's helpful to look at a phylogenetic tree like the one below. There are two important parts we want to point out: the branches themselves (at the ends of which we list species), and the nodes where one branch splits into two. Throughout the entire tree, organisms are changing and evolving. The two different parts—branches and nodes—represent two different types of evolution: anagenesis and cladogenesis.
The evolutionary change that occurs within a branch is called anagenesis, demonstrated by our first frog head example. The changes could be new mutations, lost alleles, and changes in gene frequencies, but all the organisms in that branch still belong to the same species. At the nodes, cladogenesis is occurring. In cladogenesis something happens that results in the splitting of two new species. The two branches now represent distinct species that will undergo their own separate evolutionary paths (via anagenesis), until another speciation event occurs (cladogenesis).
There are many cases in biology where defining species gets a little fuzzy. The trouble is that there are so many exceptions. What if horses and dogs could mate and have horse-dog offspring? Aside from making what would probably resemble a Great Dane, it would really confuse our idea of horses and dogs as separate species. Maybe horse-dogs belong to the sci-fi world, but there are tons of real examples of hybridization between what we think of as distinct species.
What about organisms that don't mate at all? Lots of microbes and plants reproduce clonally—are they each their own species? Even weirder, some organisms can pick up random bits of DNA from other organisms and make it their own. Are these critters the original species, or a new species altogether? This is called horizontal gene transfer (HGT), and strange as it might sound it happens all the time, even among organisms that mate. Bacteria are notorious DNA collectors, and there's even evidence of shared genes between old world monkeys and housecats because of HGT. Imagine if you ate a peach and a little bit of stray peach DNA escaped and joined up with your own. Weird. While HGT might sound peachy, it makes defining a species really hard.
If these exceptions are making your head spin, join the club. People have been arguing about species definitions for a long time, and not just about the details. The debate actually starts with a pretty basic question: are species even real?
If we're talking about the difference between a dog, a horse, and a cabbage, then the answer seems obvious. If we're talking about the difference between the hybridizing oak species Quercus robur, Q. petraea, Q. pubescens, and Q. frainetto, then the answer isn't as easy. Are species actually distinct biological entities, or are they just a way that biologists categorize the world around them with no real scientific basis?
Even if scientists can't always define species, most will try to convince you that they're more than mere human constructs. Yes, there are lots of exceptions, problems, and unanswered questions when it comes to making a universal species definition. Even so, there are a number of compelling reasons to believe in species.
First off, species recognition is intuitive. Most people can distinguish between groups of organisms that look similar, as long as there are a few subtle differences. The distinctions that we make would probably be similar to the distinctions Bob, the body builder would make. In biology we would say that the distinctions are congruent.
Congruence goes further than just friends and family. There have been several studies that demonstrate congruence between the species definitions made by western scientists and those made by indigenous cultures. There is a famous story about Ernst Mayr, the fancy pants Harvard biologist. Ernst visited the island of New Guinea and asked the local tribesmen to identify the bird species of the island. Amazingly, the tribe distinguished 136 species of birds, while classic Linnaean taxonomy knew of 137. That's an incredible amount of congruence among species delineations, and it just goes to show that we don't need formal taxonomy training to be great biologists. There is just something intuitively recognizable and distinguishable about some groups of species, they must be more than figments of our scientific imaginations.
Second off, even if organisms can hybridize, or move, or otherwise thwart our convenient species definitions, they still tend to cluster into more or less distinct groups. It's not like there's a gradual continuum of birds ranging from eagles to hummingbirds—they tend to come in different types, each more or less distinguishable from the next. Sure, there might be mixing among some groups, but the mixing among them is smaller than the mixing within them, so they form clusters. Individuals from one group tend to mate with other individuals from their own group more often that with individuals from a different group. Notice the vocabulary here: groups, types, clusters, or maybe species.
But what are species, exactly? There are many definitions for the concept of species. There is no universal species concept—one that applies to all organisms. Instead, scientists have been proposing different species concepts for years, and each is based on slightly different biological reasoning. There's the genealogical species concept, the cohesion species concept, genotypic cluster species concept, and more. Here are a few of our faves:
The ecological species concept claims that species are groups or populations that share the exact same ecological niche. The definition gets a little tricky with organisms that change niches over their development. For examples, some organisms live in the water as larvae, and on land as adults. It's sometimes expanded to include a set of niches.
The phenetic species concept defines species as groups of organisms that are similar in some defining trait(s) chosen by taxonomists, like eyebrow density, tastiness, or affinity for staring off into the distance. They'd probably pick something more scientific, but we'd be all for running a phenetic species taste test.
The phylogenetic species concept is similar, but with a slight twist: it states that a species is the smallest set of lineages or populations that can be recognized by a unique combination of different traits.
Is it just us, or do none of these concepts seem right? In a world of imperfect species concepts, allow us to introduce the victor—the biological species concept, or BSC. Is it foolproof? No. Does it allow for horse-dogs? No. Do we love it anyway? You'd better believe it.
Championed by Ernst Mayr, the BSC puts the emphasis where it belongs: on reproduction. In 1963, Mayr defined a species as follows, "species are groups of interbreeding natural populations that are reproductively isolated from other such groups."
The fact the a species must be some 'natural' designation means that, regardless of what sort of transgenic weirdness scientists can conjure up in a lab, we cannot claim to make new species. Even when we're creating organisms that share genes with distantly-related organisms (ahem, Fish Tomato anyone?).
Species are limited to reproduction in natural populations, not lab mice. The 'reproductively isolated' bit is taken to mean the following: if there is a chance that genes from one population could end up in the offspring of another population, then they belong to the same reproductive group. The two populations are reproductively isolated if there is no way they can share genes without alien intervention. We added in that last part, but it seems important somehow.
The BSC is useful because it emphasizes interbreeding, and so a species becomes synonymous with its gene pool. As long as two organisms belong to the same gene pool, they're the same species. This makes things easier, because scientists have a lot of tools for tracking genes in a population, measuring gene flow, and testing genetic similarity between related species. Seems simple. Almost too simple. As much as we love the BSC, there are still lots of cases where it starts to fall apart.
One of the more interesting scenarios in which the BSC doesn't quite work is ring species. Ring species are groups of closely related species whose ranges partially overlap to form an imperfect ring. Each species can hybridize and pass on genes with the species on either side of it, but the kicker is that the last species in line is so different from the first species, that they cannot hybridize.
This is like evolution's version of telephone. Usually it comes out totally distorted and hilarious. Ring species are less hilarious, but fascinating from the perspective of the BSC.
Here's why they're so fascinating: the first and last species in the ring cannot hybridize with each other. According to the BSC, they're separate species…right? Nope. Because of the hybridization that occurs in all the in-between species, it is theoretically possible that a gene from the first species could get passed on through all the others and ends up in the last species. That's a pretty successful game of telephone. Under the BSC, it makes them the same species.
In fact, any time there's hybridization between two species—even if it's only the tiniest little smidgeon—the BSC says they're the same species. This gets to be a little troublesome, as there are lots of examples of two populations that can hybridize, but rarely do. Maybe they have totally different geographic ranges or seasonality, or slightly different mating preferences. If we look at their population genetics we see that they belong to separate gene pools. The pools spring a rare leak every once in a while.
What do we do if the BSC says two entities are the same species, but our intuition says it just isn't so? We invent a new taxonomic level. Enter the subspecies. A subspecies is a population or group of organisms that can interbreed with individuals from another group or population, but usually don't. Now we have a way of designating potentially interbreeding groups from actually interbreeding groups, and we don't have to abandon the BSC. Have you thanked a taxonomist today?
The truth is, there are plenty of weird situations where the BSC isn't foolproof. For the time being it's the best we've got, and it's a pretty decent working definition. By focusing on interbreeding and gene flow, we can shift our focus from what a species is and get to the good stuff: why are there so many different species, and how did they arise?
Now that we're armed with the BSC, we can finally explore Speciation—the evolutionary process that leads to the generation of multiple species from a single common ancestor. We have speciation to thank for the Earth's incredible biodiversity: one million species of mites (yes, we know you’re drafting speciation a thank you note right now), up to 30 million insect species (putting a stamp on the envelope), more bacteria than we can imagine, 270,000 plant species, and about 56,000 species of vertebrates. The numbers are staggering and growing all the time.
The study of speciation starts with the question: how does one species split to become two? As we approach this question, it's important to think about how lineages change in general. Even in the absence of speciation, species don't just sit around, staying the same and waiting for some evolutionary action to pick up. They're usually gradually changing and adapting to their environment, which is also changing. For example, a frog head might respond to some mating cue or environmental pressure over time to slowly change color. This frog species isn't speciation—it's still all one species—but it is evolving.
Now it's helpful to look at a phylogenetic tree like the one below. There are two important parts we want to point out: the branches themselves (at the ends of which we list species), and the nodes where one branch splits into two. Throughout the entire tree, organisms are changing and evolving. The two different parts—branches and nodes—represent two different types of evolution: anagenesis and cladogenesis.
The evolutionary change that occurs within a branch is called anagenesis, demonstrated by our first frog head example. The changes could be new mutations, lost alleles, and changes in gene frequencies, but all the organisms in that branch still belong to the same species. At the nodes, cladogenesis is occurring. In cladogenesis something happens that results in the splitting of two new species. The two branches now represent distinct species that will undergo their own separate evolutionary paths (via anagenesis), until another speciation event occurs (cladogenesis).
That's cool, but…what's going on in cladogenesis that makes it so wildly different from anagenesis? It all comes down to gene flow, and a better way to look at the tree above would be to zoom in on the actual genes that are being passed down, because after all, species are just big packages of genes. The following image shows the actual genes of a species that is polymorphic for a given allele, and the two alleles are depicted by orange and blue. In generations 1-3, genes are shared among individuals and the species remains one. However, in generation 4-5 gene flow is disrupted. Gene flow is no longer possible among all members of the species, but only among members belonging to two distinct subgroups. With drift and selection acting on these groups independently, and with no sharing of genes between them, what was once one gene pool now branches into two.
During periods of anagenesis, gene flow is unrestricted. One individual might have very different genes from another individual, but they belong to the same gene pool and could theoretically mate to produce an offspring that shared both their genes.
In periods of cladogeneis, though, everything's fine and dandy in the gene pool. Then something happens that disrupts gene flow. Cannonball? This is the beginning of speciation. What used to be one big happy gene pool begins to split into two kiddie pools, each with different allele frequencies and very little genetic exchange between them. Voilà. What was once one species becomes two.
During periods of anagenesis, gene flow is unrestricted. One individual might have very different genes from another individual, but they belong to the same gene pool and could theoretically mate to produce an offspring that shared both their genes.
In periods of cladogeneis, though, everything's fine and dandy in the gene pool. Then something happens that disrupts gene flow. Cannonball? This is the beginning of speciation. What used to be one big happy gene pool begins to split into two kiddie pools, each with different allele frequencies and very little genetic exchange between them. Voilà. What was once one species becomes two.
Ponder this
What determines how long it takes for distinct species to appear? The number of generations in isolation? The level of evolutionary pressure by natural selection? The propensity for generational genetic change in those particular species?
If two isolated populations of the same species were to endure the same conditions, will the evolve into different species, or will it be a considered convergent evolution?
Discuss
Remember our first question about going back in the family tree? Pick a few random species, and try to determine how far back in the family tree did that species and yourself have the same ancestor. What is your evidence? Discuss the possible lineage of descent.
Further readings
"Biologists Watch Speciation in a Laboratory Flask", biologists of the UCSD observe the process of speciation in a simple laboratory flask.
Speciation, from the University of California Berkeley's Understanding Evolution.
"Speciation: The Origin of New Species", from Nature 's Knowledge Project
