Species
Species
While discussing Darwin’s book, The Origin of Species, in 1859, Thomas Henry Huxley asked, “In the first place, what is a species? The question is a simple one, but the right answer to it is hard to find, even if we appeal to those who should know most about it.” We have used the term “species” so far as if it had a simple and unambiguous meaning. Actually, Huxley’s commentary is as valid today as it was 135 years ago. Our concepts of species have become more sophisticated, but the diversity of different concepts and the disagreements surrounding their use are as evident now as they were in Darwin’s time.
Criteria for Recognition of Species
Despite widespread disagreement about the nature of species, biologists have repeatedly designated certain criteria as being important to their identification of species. First, the criterion of common descent is central to nearly all modern concepts of species. Members of a species must trace their ancestry to a common ancestral population although not necessarily to a single pair of parents. Species are thus historical entities. A second criterion is that species must be the smallest distinct groupings of organisms sharing patterns of ancestry and descent; otherwise, it would be difficult to separate species from higher taxa whose members also share common descent. Morphological characters traditionally have been important in identifying such groupings, but chromosomal and molecular characters increasingly are being used for this purpose. A third important criterion is that of reproductive community, which applies only to sexually reproducing organisms; members of a species must form a reproductive community that excludes members of other species. This criterion is very important to many modern species concepts.
Typological Species Concept
Before Darwin, a species was considered a distinct and immutable entity. Species were defined by fixed, essential features (usually morphological) that were interpreted as a divinely created pattern or archetype. This practice constitutes the typological (or morphological) species concept. Scientists recognized species formally by designating a type specimen that was labeled and deposited in a museum to represent the ideal form or morphology for the species (Figure 10-9). When scientists obtained additional specimens and wanted to assign them to a species, the type specimens of described species were consulted. The new specimens were assigned to a previously described species if they possessed the essential features of its type specimen. Small differences from the type specimen were considered accidental imperfections. Large differences from existing type specimens would lead the scientist to describe a new species with the designation of its own type specimen. In this manner, the living world was categorized into species.
Evolutionists discarded the typological species concept, but some of its traditions remain. Scientists still name species by describing type specimens deposited in museums. Organismal morphology is likewise still important in recognizing species; however, species are no longer viewed as classes defined by possession of certain morphological features. The basis of the evolutionary world view is that species are historical entities whose properties are subject always to change. Variation that we observe among organisms within a species is not the imperfect manifestation of an eternal “type”; the type itself is only an abstraction taken from the very real and important variation present within the species. The type is at best an average form that will change as organismal variation is sorted through time by natural selection. The type specimen serves only as a guide to the general kinds of morphological features that we may expect to find in the species as we observe it today.
Biological Species Concept
The most influential concept of species inspired by Darwinian evolutionary theory is the biological species concept formulated by Theodosius Dobzhansky and Ernst Mayr. This concept solidified during the evolutionary synthesis of the 1930s and 1940s from earlier ideas, and it has been refined and reworded several times since then. In 1982, Mayr stated the biological species concept as follows: “A species is a reproductive community of populations (reproductively isolated from others) that occupies a specific niche in nature.” Note that the species is identified here according to reproductive properties of populations, not according to possession of any specific organismal characteristics. The species is an interbreeding population of individuals having common descent and sharing intergrading characteristics. The study of populational variation in organismal morphology, chromosomal structure, and molecular genetic features will be very useful for evaluating the geographical boundaries of interbreeding populations in nature. The criterion of the “niche” (Animal Ecology) recognizes that members of a reproductive community are expected also to have common ecological properties.
Because a reproductive community should maintain genetic cohesiveness, we expect that organismal variation will be relatively smooth and continuous within species and discontinuous between them. Although the biological species is based on reproductive properties of populations rather than organismal morphology, morphology nonetheless can help us to diagnose biological species. Sometimes species status can be evaluated directly by conducting breeding experiments. Controlled breeding is practical only in a minority of cases, however, and our decisions regarding species membership are usually made by studying character variation. Variation in molecular characters is very useful for identifying geographical boundaries of reproductive communities. Molecular studies have revealed the presence of cryptic or sibling species that are too similar in morphology to be diagnosed as separate species by morphological characters alone.
Alternatives to the Biological Species Concept
The biological species concept has received strong criticism. To understand why, we must keep in mind several important facts about species. First, a species has dimensions in space and time, which usually creates problems for locating discrete boundaries between species. Second, we view the species both as a unit of evolution and as a rank in the taxonomic hierarchy. We will see that these roles sometimes conflict. A third problem is that according to the biological species concept, species do not exist in groups of organisms that reproduce only asexually. It is common systematic practice, however, to describe species in all groups of organisms, regardless of whether reproduction is sexual or asexual.
The Species in Space and Time
Any species has a distribution through space, known as its geographic range, and a distribution through time, known as its evolutionary duration. Species differ greatly from each other in both of these dimensions. Species having very large geographic ranges or worldwide distributions are called cosmopolitan, whereas those with very restricted geographic distributions are called endemic. If a species were restricted to a single point in space and time, we would have little difficulty recognizing it, and nearly every species concept would lead us to the same decision. We have little difficulty distinguishing from each other the different species of animals that we can find living in our local park or woods. However, when we compare a local population of a species to similar but not identical populations located hundreds of miles away, it may be hard to determine whether these populations represent parts of a single species or different species (Figure 10-10).
Throughout the evolutionary duration of a species, its geographic range may change many times. A geographic range may be either continuous or disjunct, the latter having breaks within it where the species is absent. Suppose that we find two similar but not identical populations living 300 miles apart with no related populations between them. Are we observing a single species with a disjunct distribution or two different but closely related species? Suppose that these populations have been separated historically for 50,000 years. Is this enough time for them to have evolved separate reproductive communities, or can we still view them as being part of the same reproductive community? The answers to such questions are very hard to find. Much of the disagreement among different species concepts relates to solving these problems.
Evolutionary Species Concept
The time dimension described above creates obvious problems for the biological species concept. How do we assign fossil specimens to biological species that are recognized today? If we trace a lineage backward through time, how far must we go before we have crossed a species boundary? If we could follow the unbroken genealogical chain of populations backward through time to the point where two sister species converge on their common ancestor, we would need to cross at least one species boundary somewhere. It would be very hard to decide, however, where to draw a sharp line between the two species.
To address this problem, the evolutionary species concept was proposed by Simpson in the 1940s to add an evolutionary time dimension to the biological species concept. This concept persists in a modified form today. A current definition of the evolutionary species is a single lineage of ancestordescendant populations that maintains its identity from other such lineages and that has its own evolutionary tendencies and historical fate. Note that the criterion of common descent is retained here in the need for a lineage to have a distinct historical identity. Reproductive cohesion is the means by which a species maintains its identity from other such lineages and keeps its evolutionary fate separate from other species. The same kinds of diagnostic features discussed previously will be relevant for identifying evolutionary species, although in most cases only morphological features will be available from fossils. Unlike the biological species concept, the evolutionary species concept applies both to sexually and asexually reproducing forms. As long as continuity of diagnostic features is maintained by the evolving lineage, it will be recognized as a species. Abrupt changes in diagnostic features will mark the boundaries of different species in evolutionary time.
Phylogenetic Species Concept
The last concept that we present is the phylogenetic species concept. The phylogenetic species concept is defined as an irreducible (basal) grouping of organisms diagnosably distinct from other such groupings and within which there is a parental pattern of ancestry and descent. This concept emphasizes most strongly the criterion of common descent. Both asexual and sexual groups are covered.
A phylogenetic species is a strictly monophyletic unit. The main difference in practice between the evolutionary and phylogenetic species concepts is that the latter emphasizes recognizing as separate species the smallest groupings of organisms that have undergone independent evolutionary change. The evolutionary species concept would group into a single species geographically disjunct populations that demonstrate some genetic divergence but are judged similar in their “evolutionary tendencies,” whereas the phylogenetic species concept would treat them as separate species. In general, a greater number of species would be described using the phylogenetic species concept than any other species concept, and many taxonomists consider it impractical for this reason. For strict adherence to cladistic systematics, however, the phylogenetic species concept is ideal because only this concept guarantees strictly monophyletic units at the species level.
The phylogenetic species concept intentionally disregards details of evolutionary process and gives us a criterion that allows us to describe species without first needing to conduct detailed studies on evolutionary processes. Advocates of the phylogenetic species concept do not necessarily disregard the importance of studying evolutionary process. They argue, however, that the first step in studying evolutionary process is to have a clear picture of life’s history. To accomplish this task, the pattern of common descent must be reconstructed in the greatest detail possible by starting with the smallest taxonomic units that have a history of common descent.
Dynamism of Species Concepts
Current disagreements concerning concepts of species should not be considered discouraging. Whenever a field of scientific investigation enters a phase of dynamic growth, old concepts will be reevaluated and either refined or replaced with newer, more progressive ones. The active debate occurring within systematics shows that this field has acquired unprecedented activity and importance in biology. Just as Thomas Henry Huxley’s time was one of enormous advances in biology, so is the present time. Both times are marked by fundamental reconsiderations of the meaning of species. We cannot predict which concept of species will be dominant 10 years from now, or even whether any of the concepts of species currently being advocated will survive until then. The conflicts between the current concepts, however, will lead us into the future. Understanding the conflicting perspectives, rather than learning a single species concept, is therefore of greatest importance for people now entering the study of zoology.
While discussing Darwin’s book, The Origin of Species, in 1859, Thomas Henry Huxley asked, “In the first place, what is a species? The question is a simple one, but the right answer to it is hard to find, even if we appeal to those who should know most about it.” We have used the term “species” so far as if it had a simple and unambiguous meaning. Actually, Huxley’s commentary is as valid today as it was 135 years ago. Our concepts of species have become more sophisticated, but the diversity of different concepts and the disagreements surrounding their use are as evident now as they were in Darwin’s time.
Criteria for Recognition of Species
Despite widespread disagreement about the nature of species, biologists have repeatedly designated certain criteria as being important to their identification of species. First, the criterion of common descent is central to nearly all modern concepts of species. Members of a species must trace their ancestry to a common ancestral population although not necessarily to a single pair of parents. Species are thus historical entities. A second criterion is that species must be the smallest distinct groupings of organisms sharing patterns of ancestry and descent; otherwise, it would be difficult to separate species from higher taxa whose members also share common descent. Morphological characters traditionally have been important in identifying such groupings, but chromosomal and molecular characters increasingly are being used for this purpose. A third important criterion is that of reproductive community, which applies only to sexually reproducing organisms; members of a species must form a reproductive community that excludes members of other species. This criterion is very important to many modern species concepts.
Typological Species Concept
Before Darwin, a species was considered a distinct and immutable entity. Species were defined by fixed, essential features (usually morphological) that were interpreted as a divinely created pattern or archetype. This practice constitutes the typological (or morphological) species concept. Scientists recognized species formally by designating a type specimen that was labeled and deposited in a museum to represent the ideal form or morphology for the species (Figure 10-9). When scientists obtained additional specimens and wanted to assign them to a species, the type specimens of described species were consulted. The new specimens were assigned to a previously described species if they possessed the essential features of its type specimen. Small differences from the type specimen were considered accidental imperfections. Large differences from existing type specimens would lead the scientist to describe a new species with the designation of its own type specimen. In this manner, the living world was categorized into species.
 |
| Figure 10-9 Specimens of birds from the Smithsonian Institution (Washington D.C.), including birds originally collected by John J. Audubon, Theodore Roosevelt, John Gould, and Charles Darwin. |
Evolutionists discarded the typological species concept, but some of its traditions remain. Scientists still name species by describing type specimens deposited in museums. Organismal morphology is likewise still important in recognizing species; however, species are no longer viewed as classes defined by possession of certain morphological features. The basis of the evolutionary world view is that species are historical entities whose properties are subject always to change. Variation that we observe among organisms within a species is not the imperfect manifestation of an eternal “type”; the type itself is only an abstraction taken from the very real and important variation present within the species. The type is at best an average form that will change as organismal variation is sorted through time by natural selection. The type specimen serves only as a guide to the general kinds of morphological features that we may expect to find in the species as we observe it today.
Biological Species Concept
The most influential concept of species inspired by Darwinian evolutionary theory is the biological species concept formulated by Theodosius Dobzhansky and Ernst Mayr. This concept solidified during the evolutionary synthesis of the 1930s and 1940s from earlier ideas, and it has been refined and reworded several times since then. In 1982, Mayr stated the biological species concept as follows: “A species is a reproductive community of populations (reproductively isolated from others) that occupies a specific niche in nature.” Note that the species is identified here according to reproductive properties of populations, not according to possession of any specific organismal characteristics. The species is an interbreeding population of individuals having common descent and sharing intergrading characteristics. The study of populational variation in organismal morphology, chromosomal structure, and molecular genetic features will be very useful for evaluating the geographical boundaries of interbreeding populations in nature. The criterion of the “niche” (Animal Ecology) recognizes that members of a reproductive community are expected also to have common ecological properties.
Because a reproductive community should maintain genetic cohesiveness, we expect that organismal variation will be relatively smooth and continuous within species and discontinuous between them. Although the biological species is based on reproductive properties of populations rather than organismal morphology, morphology nonetheless can help us to diagnose biological species. Sometimes species status can be evaluated directly by conducting breeding experiments. Controlled breeding is practical only in a minority of cases, however, and our decisions regarding species membership are usually made by studying character variation. Variation in molecular characters is very useful for identifying geographical boundaries of reproductive communities. Molecular studies have revealed the presence of cryptic or sibling species that are too similar in morphology to be diagnosed as separate species by morphological characters alone.
Alternatives to the Biological Species Concept
The biological species concept has received strong criticism. To understand why, we must keep in mind several important facts about species. First, a species has dimensions in space and time, which usually creates problems for locating discrete boundaries between species. Second, we view the species both as a unit of evolution and as a rank in the taxonomic hierarchy. We will see that these roles sometimes conflict. A third problem is that according to the biological species concept, species do not exist in groups of organisms that reproduce only asexually. It is common systematic practice, however, to describe species in all groups of organisms, regardless of whether reproduction is sexual or asexual.
The Species in Space and Time
Any species has a distribution through space, known as its geographic range, and a distribution through time, known as its evolutionary duration. Species differ greatly from each other in both of these dimensions. Species having very large geographic ranges or worldwide distributions are called cosmopolitan, whereas those with very restricted geographic distributions are called endemic. If a species were restricted to a single point in space and time, we would have little difficulty recognizing it, and nearly every species concept would lead us to the same decision. We have little difficulty distinguishing from each other the different species of animals that we can find living in our local park or woods. However, when we compare a local population of a species to similar but not identical populations located hundreds of miles away, it may be hard to determine whether these populations represent parts of a single species or different species (Figure 10-10).
 |
| Figure 10-10 Geographic variation of color patterns in the salamander genus Ensatina. The species status of these populations has puzzled taxonomists for generations and continues to do so. Current taxonomy recognizes only a single species (Ensatina eschscholtzii) divided into subspecies as shown. Hybridization is evident between most adjacent populations, but studies of variation in proteins and DNA show large amounts of genetic divergence among populations. Furthermore, populations of the subspecies E. e. eschscholtzii and E. e. klauberi can overlap eographi cally without interbreeding. |
Throughout the evolutionary duration of a species, its geographic range may change many times. A geographic range may be either continuous or disjunct, the latter having breaks within it where the species is absent. Suppose that we find two similar but not identical populations living 300 miles apart with no related populations between them. Are we observing a single species with a disjunct distribution or two different but closely related species? Suppose that these populations have been separated historically for 50,000 years. Is this enough time for them to have evolved separate reproductive communities, or can we still view them as being part of the same reproductive community? The answers to such questions are very hard to find. Much of the disagreement among different species concepts relates to solving these problems.
Evolutionary Species Concept
The time dimension described above creates obvious problems for the biological species concept. How do we assign fossil specimens to biological species that are recognized today? If we trace a lineage backward through time, how far must we go before we have crossed a species boundary? If we could follow the unbroken genealogical chain of populations backward through time to the point where two sister species converge on their common ancestor, we would need to cross at least one species boundary somewhere. It would be very hard to decide, however, where to draw a sharp line between the two species.
To address this problem, the evolutionary species concept was proposed by Simpson in the 1940s to add an evolutionary time dimension to the biological species concept. This concept persists in a modified form today. A current definition of the evolutionary species is a single lineage of ancestordescendant populations that maintains its identity from other such lineages and that has its own evolutionary tendencies and historical fate. Note that the criterion of common descent is retained here in the need for a lineage to have a distinct historical identity. Reproductive cohesion is the means by which a species maintains its identity from other such lineages and keeps its evolutionary fate separate from other species. The same kinds of diagnostic features discussed previously will be relevant for identifying evolutionary species, although in most cases only morphological features will be available from fossils. Unlike the biological species concept, the evolutionary species concept applies both to sexually and asexually reproducing forms. As long as continuity of diagnostic features is maintained by the evolving lineage, it will be recognized as a species. Abrupt changes in diagnostic features will mark the boundaries of different species in evolutionary time.
Phylogenetic Species Concept
The last concept that we present is the phylogenetic species concept. The phylogenetic species concept is defined as an irreducible (basal) grouping of organisms diagnosably distinct from other such groupings and within which there is a parental pattern of ancestry and descent. This concept emphasizes most strongly the criterion of common descent. Both asexual and sexual groups are covered.
A phylogenetic species is a strictly monophyletic unit. The main difference in practice between the evolutionary and phylogenetic species concepts is that the latter emphasizes recognizing as separate species the smallest groupings of organisms that have undergone independent evolutionary change. The evolutionary species concept would group into a single species geographically disjunct populations that demonstrate some genetic divergence but are judged similar in their “evolutionary tendencies,” whereas the phylogenetic species concept would treat them as separate species. In general, a greater number of species would be described using the phylogenetic species concept than any other species concept, and many taxonomists consider it impractical for this reason. For strict adherence to cladistic systematics, however, the phylogenetic species concept is ideal because only this concept guarantees strictly monophyletic units at the species level.
The phylogenetic species concept intentionally disregards details of evolutionary process and gives us a criterion that allows us to describe species without first needing to conduct detailed studies on evolutionary processes. Advocates of the phylogenetic species concept do not necessarily disregard the importance of studying evolutionary process. They argue, however, that the first step in studying evolutionary process is to have a clear picture of life’s history. To accomplish this task, the pattern of common descent must be reconstructed in the greatest detail possible by starting with the smallest taxonomic units that have a history of common descent.
Dynamism of Species Concepts
Current disagreements concerning concepts of species should not be considered discouraging. Whenever a field of scientific investigation enters a phase of dynamic growth, old concepts will be reevaluated and either refined or replaced with newer, more progressive ones. The active debate occurring within systematics shows that this field has acquired unprecedented activity and importance in biology. Just as Thomas Henry Huxley’s time was one of enormous advances in biology, so is the present time. Both times are marked by fundamental reconsiderations of the meaning of species. We cannot predict which concept of species will be dominant 10 years from now, or even whether any of the concepts of species currently being advocated will survive until then. The conflicts between the current concepts, however, will lead us into the future. Understanding the conflicting perspectives, rather than learning a single species concept, is therefore of greatest importance for people now entering the study of zoology.
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