The concept of "species" is one which is vital to our understanding of biology, but which remains elusive. This term also has strong emotional baggage, as evidenced by at least one recent thread. I believe it would be useful to provide a discussion of this topic in a bit more depth than is generally possible in threads not dedicated to this topic.
First, we must dismiss the idea that "species" is a term with a single definition. This is the view presented in high-school biology textbooks, but it is not the actual view of researchers. Nor do researchers widely consider the term to be easily defined. It's easy enough to understand--until you actually deal with it, at which point it becomes nearly incomprehensible. In reality, researchers use multiple definitions of the term, each with its own benefits and limitations.
The most commonly used species definition in biology is the biological species concept. Essentially, this concept states that if two organisms (or members from two populations) can successfully interbreed (meaning that the coupling produces fertile offspring), they are members of the same species. This definition is useful because it provides a very concrete test, always a good thing in science. Second, it directly ties the concept of "species" to evolution--the division of populations into distinct species is the establishment of genetic barriers, which has important implications for evolutionary history (essentially, it's the division of evolutionary history between these two species). However, there are serious problems. First, it's very animal-centric. The view requires sexually reproducing organisms--those which produce asexually cannot be defined using this definition. Self-fertilizing organisms, such as some parasites, are also undefinable via this concept. Mor damningly, no one actually runs these tests. Sometimes you get a biologist who does so, but the overwhelming majority of them do not. This is no trivial issue; many areas (such as New York's finger lakes, mountain ranges, or some ocean environments) consist of isolated areas of particular habitats, and if organisms cannot cross the gaps without assistance by humans the only way to truly know if they are the same species is to run breeding experiments. There are areas of the United States with thousands of lakes; breeding experiments for fish in these lakes would require more fish than exist. Hybrids are not always sterile in animals, either--the most famous example, the mule, can rarely be fertile, and it is hypothesized that some members of our own species are actually hybrids with Neanderthals (I am among those races). Finally, this concept is impossible to apply to anything but living organisms, for obvious reasons.
In paleontology, we by necessity utilize the morphospecies concept. Essentially, this concept states that organisms more similar to each other than to anything else are considered to be the same species. In practice, this involves experts in the taxa in question analyzing key features (and arguing endlessly over what constitute key features). In a stricly taxonomical sense, those key features define the species--so, for example, any canine with a different dental formula is a different species. This has a number of advantages over the biological species concept. First, it requires far fewer resources. An expert can often identify a taxa by simply glancing at it (anyone who has done bird watching is using this concept, for example). More detailed examinations require more resources, obviously, but do not require extensive laboratory set-ups and decades-long research programs like the biological species concept would require. In addition, the morphospecies concept can be used with dead organisms, asexual organisms, self-fertilizing organisms, and others that are left out of the biological species concept. The main drawback is that there's really no way to test these ideas. Different experts can have different opinions on what constitutes a key feature. Cladistics and other statistical methods help, but there's an inherent subjectivity to them (someone has to choose what traits to look at in the beginning), meaning that there's still a potential that the bias of an expert will seep its way into the results. Also, the morphospecies concept cannot differentiate between identical but genetically distinct species. We simply can never know if all of the members of a Sillurian brachiopod species were, in fact, members of the same species.
The rise of genetics has offered us a new species concept: the genetic species concept. Essentially, if two organisms share similar enough DNA, we call them a single species. This is a powerful tool, because it examines the blueprints of the organism itself, and thus only addresses traits that can potentially be passed on (whereas morphology is a combination of genetic and environmental factors, and many traits once considered important have been relegated to the dustbin of mere accidents). However, it suffers from serious flaws as well. First, it can only be used when we have DNA to work with, and significant amounts of it. We cannot use this to classify the Vendian Fauna, for example. Second, the same conceptual flaws with the morphospecies concept (ie, who decides what's important?) are applicable here. No one examines the full DNA strand, and different samples fo DNA can yield different results. Plus, it doesn't address bacteria that exchange plasmids--an individual bacteria may have multiple genetic codes in its lifetime.
Cladistics presents a unique constraint on the term: a species, in as much as it is a taxon, must share a common ancestor. This doesn't seem like much of a constraint, but it provides a way to differentiate identical but genetically isolated species. The obvious issue here is demonstrating that organisms share a common ancestor. Also, by itself this does not constitute a species concept; it is merely a constraint upon species concepts.
Therer are numerous others (a decent discussion can be found herre), but they are all minor variations on these main themes. The whole issue is that there is a smooth continuum between species (illustrated by ring species and chronospecies), and the term "species" by its nature an attempt to create a discrete classification on that gradational system. Whenever htat is attempted, the edges become highly problematic.
This is no trivial issue. We are in a mass extinction, and understanding what will happen requires us to know what's going on and what has happened before. The use of different species concepts in different diciplines, however, renders comparisons between their datasets difficult if not impossible. One cannot directly compare paleontological data with genetic data--ignoring the biases in collecting the data, the simple fact is that these two datasets are not examining the same thing. This difficulty can be avoided to a certain extent by discussing guilds (ie, niches) instead of species, but that only pushes the issue back a step (how do you define what goes into a guild? After all, an organism may occupy multiple niches during its lifespan). There are also implications for astrobiology: the species is the basic unit of evolutionary thought, and of ecology, and of multiple biological sub-fields. How we define the term dictates how we look for life on other planets to a certain extent, in that it sets our expectations for what to find. Those who advocate the biological species concept tend to favor Earth-like biomes, and animal-like organisms; however, animals are the result of historical contingency. There is no reason to assume that sexual reproduction and heterotrophy are conjoined on other planets; we see ample examples of photosynethetic heterotrophs (for example, pitcher plants) on our own planet!
Hopefully this illustrates at least some of the problems inherent in defining the term "species". Any of htese definitions is viable, so long as the limitations of the chosen definition are acknowledged. None are wrong, but none are The One True Definition either. The term should really be treated with a great deal more caution than people generally use. I've toyed with the idea of abandoning the concept entirely, and discussing populations (a group of organisms that interbreeds), but that obviously hasn't caught on. :)
First, we must dismiss the idea that "species" is a term with a single definition. This is the view presented in high-school biology textbooks, but it is not the actual view of researchers. Nor do researchers widely consider the term to be easily defined. It's easy enough to understand--until you actually deal with it, at which point it becomes nearly incomprehensible. In reality, researchers use multiple definitions of the term, each with its own benefits and limitations.
The most commonly used species definition in biology is the biological species concept. Essentially, this concept states that if two organisms (or members from two populations) can successfully interbreed (meaning that the coupling produces fertile offspring), they are members of the same species. This definition is useful because it provides a very concrete test, always a good thing in science. Second, it directly ties the concept of "species" to evolution--the division of populations into distinct species is the establishment of genetic barriers, which has important implications for evolutionary history (essentially, it's the division of evolutionary history between these two species). However, there are serious problems. First, it's very animal-centric. The view requires sexually reproducing organisms--those which produce asexually cannot be defined using this definition. Self-fertilizing organisms, such as some parasites, are also undefinable via this concept. Mor damningly, no one actually runs these tests. Sometimes you get a biologist who does so, but the overwhelming majority of them do not. This is no trivial issue; many areas (such as New York's finger lakes, mountain ranges, or some ocean environments) consist of isolated areas of particular habitats, and if organisms cannot cross the gaps without assistance by humans the only way to truly know if they are the same species is to run breeding experiments. There are areas of the United States with thousands of lakes; breeding experiments for fish in these lakes would require more fish than exist. Hybrids are not always sterile in animals, either--the most famous example, the mule, can rarely be fertile, and it is hypothesized that some members of our own species are actually hybrids with Neanderthals (I am among those races). Finally, this concept is impossible to apply to anything but living organisms, for obvious reasons.
In paleontology, we by necessity utilize the morphospecies concept. Essentially, this concept states that organisms more similar to each other than to anything else are considered to be the same species. In practice, this involves experts in the taxa in question analyzing key features (and arguing endlessly over what constitute key features). In a stricly taxonomical sense, those key features define the species--so, for example, any canine with a different dental formula is a different species. This has a number of advantages over the biological species concept. First, it requires far fewer resources. An expert can often identify a taxa by simply glancing at it (anyone who has done bird watching is using this concept, for example). More detailed examinations require more resources, obviously, but do not require extensive laboratory set-ups and decades-long research programs like the biological species concept would require. In addition, the morphospecies concept can be used with dead organisms, asexual organisms, self-fertilizing organisms, and others that are left out of the biological species concept. The main drawback is that there's really no way to test these ideas. Different experts can have different opinions on what constitutes a key feature. Cladistics and other statistical methods help, but there's an inherent subjectivity to them (someone has to choose what traits to look at in the beginning), meaning that there's still a potential that the bias of an expert will seep its way into the results. Also, the morphospecies concept cannot differentiate between identical but genetically distinct species. We simply can never know if all of the members of a Sillurian brachiopod species were, in fact, members of the same species.
The rise of genetics has offered us a new species concept: the genetic species concept. Essentially, if two organisms share similar enough DNA, we call them a single species. This is a powerful tool, because it examines the blueprints of the organism itself, and thus only addresses traits that can potentially be passed on (whereas morphology is a combination of genetic and environmental factors, and many traits once considered important have been relegated to the dustbin of mere accidents). However, it suffers from serious flaws as well. First, it can only be used when we have DNA to work with, and significant amounts of it. We cannot use this to classify the Vendian Fauna, for example. Second, the same conceptual flaws with the morphospecies concept (ie, who decides what's important?) are applicable here. No one examines the full DNA strand, and different samples fo DNA can yield different results. Plus, it doesn't address bacteria that exchange plasmids--an individual bacteria may have multiple genetic codes in its lifetime.
Cladistics presents a unique constraint on the term: a species, in as much as it is a taxon, must share a common ancestor. This doesn't seem like much of a constraint, but it provides a way to differentiate identical but genetically isolated species. The obvious issue here is demonstrating that organisms share a common ancestor. Also, by itself this does not constitute a species concept; it is merely a constraint upon species concepts.
Therer are numerous others (a decent discussion can be found herre), but they are all minor variations on these main themes. The whole issue is that there is a smooth continuum between species (illustrated by ring species and chronospecies), and the term "species" by its nature an attempt to create a discrete classification on that gradational system. Whenever htat is attempted, the edges become highly problematic.
This is no trivial issue. We are in a mass extinction, and understanding what will happen requires us to know what's going on and what has happened before. The use of different species concepts in different diciplines, however, renders comparisons between their datasets difficult if not impossible. One cannot directly compare paleontological data with genetic data--ignoring the biases in collecting the data, the simple fact is that these two datasets are not examining the same thing. This difficulty can be avoided to a certain extent by discussing guilds (ie, niches) instead of species, but that only pushes the issue back a step (how do you define what goes into a guild? After all, an organism may occupy multiple niches during its lifespan). There are also implications for astrobiology: the species is the basic unit of evolutionary thought, and of ecology, and of multiple biological sub-fields. How we define the term dictates how we look for life on other planets to a certain extent, in that it sets our expectations for what to find. Those who advocate the biological species concept tend to favor Earth-like biomes, and animal-like organisms; however, animals are the result of historical contingency. There is no reason to assume that sexual reproduction and heterotrophy are conjoined on other planets; we see ample examples of photosynethetic heterotrophs (for example, pitcher plants) on our own planet!
Hopefully this illustrates at least some of the problems inherent in defining the term "species". Any of htese definitions is viable, so long as the limitations of the chosen definition are acknowledged. None are wrong, but none are The One True Definition either. The term should really be treated with a great deal more caution than people generally use. I've toyed with the idea of abandoning the concept entirely, and discussing populations (a group of organisms that interbreeds), but that obviously hasn't caught on. :)
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