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NATURE
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Scientific classification or biological classification is how biologists group and categorize extinct and living species of organisms. Scientific classification can also be called scientific taxonomy, but should be distinguished from folk taxonomy, which lacks scientific basis. Modern classification has its root in the work of Carolus Linnaeus, who grouped species according to shared physical characteristics. These groupings have been revised since Linnaeus to improve consistency with the Darwinian principle of common descent. Molecular systematics, which uses DNA sequences as data, has driven many recent revisions and is likely to continue to do so. Scientific classification belongs to the science of taxonomy or biological systematics. Early systems The earliest known system of classifying forms of life comes from the Greek philosopher Aristotle, who classified all living organisms known at that time as either a plant or an animal. He further classified animals based on their means of transportation (air, land, or water). In 1172 Ibn Rushd (Averroes), who was a judge (Qadi) in Seville, translated and abridged Aristotle's book de Anima (On the Soul) into Arabic. His original commentary is now lost, but its translation into Latin by Michael Scot survives. An important advance was made by the Swiss professor, Conrad von Gesner (1516–1565). Gesner's work was a critical compilation of life known at the time.The exploration of parts of the New World next brought to hand descriptions and specimens of many novel forms of animal life. In the latter part of the 16th century and the beginning of the 17th, careful study of animals commenced, which, directed first to familiar kinds, was gradually extended until it formed a sufficient body of knowledge to serve as an anatomical basis for classification. Advances in using this knowledge to classify living beings bear a debt to the research of medical anatomists, such as Fabricius (1537–1619), Petrus Severinus (1580–1656), William Harvey (1578–1657), and Edward Tyson (1649–1708). Advances in classification due to the work of entomologists and the first microscopists is due to the research of people like Marcello Malpighi (1628–1694), Jan Swammerdam (1637–1680), and Robert Hooke (1635–1702). Lord Monboddo (1714-1799) was one of the early abstract thinkers whose works illustrate knowledge of species relationships and who foreshadowed the theory of evolution. Successive developments in the history of insect classification may be followed on the website[1] by clicking on succeeding works in chronological order. Early methodists Since late in the 15th century, a number of authors had become concerned with what they called methodus, or method. By method they meant an arrangement of minerals, plants, and animals according to the principles of logical division. The term methodists was coined by Carolus Linnaeus in his Bibliotheca Botanica to denote the authors who care about the principles of classification (in contrast to the mere collectors who are concerned primarily with the description of plants paying little or no attention to their arrangement into genera, etc). Important early methodists were an Italian philosopher, physician, and botanist Andrea Caesalpino, an English naturalist John Ray, a German physician and botanist Augustus Quirinus Rivinus, and a French physician, botanist, and traveller Joseph Pitton de Tournefort. Andrea Caesalpino (1519–1603) in his De plantis libri XVI (1583) proposed the first methodical arrangement of plants. On the basis of the structure of trunk and fructification he divided plants into fifteen "higher genera".John Ray (1627–1705) was an English naturalist who published important works on plants, animals, and natural theology. The approach he took to the classification of plants in his Historia Plantarum was an important step towards modern taxonomy. Ray rejected the system of dichotomous division by which species were classified according to a pre-conceived, either/or type system, and instead classified plants according to similarities and differences that emerged from observation.Both Caesalpino and Ray used traditional plant names and thus, the name of a plant did not reflect its taxonomic position (e.g. even though the apple and the peach belonged to different "higher genera" of John Ray's methodus, both retained their traditional names Malus and Malus Persica respectively). A further step was taken by Rivinus and Pitton de Tournefort who made genus a distinct rank within taxonomic hierarchy and introduced the practice of naming the plants according to their genera. Augustus Quirinus Rivinus (1652–1723), in his classification of plants based on the characters of the flower, introduced the category of order (corresponding to the "higher" genera of John Ray and Andrea Caesalpino). He was the first to abolish the ancient division of plants into herbs and trees and insisted that the true method of division should be based on the parts of the fructification alone. Rivinus extensively used dichotomous keys to define both orders and genera. His method of naming plant species resembled that of Joseph Pitton de Tournefort. The names of all plants belonging to the same genus should begin with the same word (generic name). In the genera containing more than one species the first species was named with generic name only, while the second, etc were named with a combination of the generic name and a modifier (differentia specifica).Joseph Pitton de Tournefort (1656–1708) introduced an even more sophisticated hierarchy of class, section, genus, and species. He was the first to use consistently the uniformly composed species names which consisted of a generic name and a many-worded diagnostic phrase differentia specifica. Unlike Rivinus, he used differentiae with all species of polytipic genera. Linnaeus Two years after John Ray's death, Carolus Linnaeus (1707–1778) was born. His great work, the Systema Naturae, ran through twelve editions during his lifetime (1st ed. 1735). In this work, nature was divided into three kingdoms: mineral, vegetable and animal. Linnaeus used five ranks: class, order, genus, species, and variety.He abandoned long descriptive names of classes and orders and two-word generic names (e. g. Bursa pastoris) still used by his immediate predecessors (Rivinus and Pitton de Tournefort) and replaced them with single-word names, provided genera with detailed diagnoses (characteres naturales), and reduced numerous varieties to their species, thus saving botany from the chaos of new forms produced by horticulturalists. Linnaeus is best known for his introduction of the method still used to formulate the scientific name of every species. Before Linnaeus, long many-worded names (composed of a generic name and a differentia specifica) had been used, but as these names gave a description of the species, they were not fixed. In his Philosophia Botanica (1751) Linnaeus took every effort to improve the composition and reduce the length of the many-worded names by abolishing unnecessary rhetorics, introducing new descriptive terms and defining their meaning with an unprecedented precision. In the late 1740s Linnaeus began to use a parallel system of naming species with nomina trivialia. Nomen triviale, a trivial name, was a single- or two-word epithet placed on the margin of the page next to the many-worded "scientific" name. The only rules Linnaeus applied to them was that the trivial names should be short, unique within a given genus, and that they should not be changed. Linnaeus consistently applied nomina trivialia to the species of plants in Species Plantarum (1st edn. 1753) and to the species of animals in the 10th edition of Systema Naturae (1758). By consistently using these specific epithets, Linnaeus separated nomenclature from taxonomy. Even though the parallel use of nomina trivialia and many-worded descriptive names continued until late in the eighteenth century, it was gradually replaced by the practice of using shorter proper names combined of the generic name and the trivial name of the species. In the nineteenth century, this new practice was codified in the first Rules and Laws of Nomenclature, and the 1st edn. of Species Plantarum and the 10th edn. of Systema Naturae were chosen as starting points for the Botanical and Zoological Nomenclature respectively. This convention for naming species is referred to as binomial nomenclature. Today, nomenclature is regulated by Nomenclature Codes, which allows names divided into ranks; see rank (botany) and rank (zoology). Modern developments Whereas Linnaeus classified for ease of identification, it is now generally accepted that classification should reflect the Darwinian principle of common descent. Since the 1960s a trend called cladistic taxonomy (or cladistics or cladism) has emerged, arranging taxa in an evolutionary tree. If a taxon includes all the descendants of some ancestral form, it is called monophyletic, as opposed to paraphyletic. Other groups are called polyphyletic. A new formal code of nomenclature, the PhyloCode, is currently under development, intended to deal with clades rather than taxa. It is unclear, should this be implemented, how the different codes will coexist. Domains are a relatively new grouping. The three-domain system was first invented in 1990, but not generally accepted until later. Now, the majority of biologists accept the domain system, but a large minority use the five-kingdom method. One main characteristic of the three-domain method is the separation of Archaea and Bacteria, previously grouped into the single kingdom Bacteria (sometimes Monera). A small minority of scientists add Archaea as a sixth kingdom but do not accept the domain method. Examples The usual classifications of five species follow: the fruit fly so familiar in genetics laboratories (Drosophila melanogaster), humans (Homo sapiens), the peas used by Gregor Mendel in his discovery of genetics (Pisum sativum), the "fly agaric" mushroom Amanita muscaria, and the bacterium Escherichia coli. The eight major ranks are given in bold; a selection of minor ranks are given as well. Notes:
Terminations of namesTaxa above the genus level are often given names based on the type genus, with a standard termination. The terminations used in forming these names depend on the kingdom, and sometimes the phylum and class, as set out in the table below.
TAXONOMY
Taxonomy
(from Greek verb τασσεῖν or tassein = "to classify" and νόμος or nomos =
law, science, cf "economy") was once only the science of classifying
living organisms (alpha taxonomy)
Taxonomy, sometimes alpha taxonomy, is the science of finding, describing and naming organisms, thus giving rise to taxa.
* In today's usage, Taxonomy (as a science) deals with finding, describing and naming organisms. This science is supported by institutions holding collections of these organisms, with relevant data, carefully curated: such institutes include Natural History Museums, Herbaria and Botanical Gardens. * Systematics (as a science) deals with the relationships between taxa, especially at the higher levels. These days systematics is greatly influenced by data derived from DNA from mitochondria and chloroplasts. This is sometimes known as molecular systematics and is doing well, likely at the expense of taxonomy (Wheeler, 2004). Cladistics is a branch of biology that determines the evolutionary relationships between organisms based on derived similarities. It is the most prominent of several forms of phylogenetic systematics, which study the evolutionary relationships between organisms. Cladistics is a method of rigorous analysis, using "shared derived traits" (synapomorphies: see below) of the organisms being studied. Cladistic analysis forms the basis for most modern systems of biological classification, which seek to group organisms by evolutionary relationships. In contrast, phenetics groups organisms based on their overall similarity, while approaches that are more traditional tend to rely on key characters (morphology). The word cladistics is derived from the ancient Greek ??????, klados, "branch."
As the end result of a cladistic analysis, treelike relationship-diagrams called "cladograms" are drawn up to show different hypotheses of relationships. A cladistic analysis can be based on as much or as little information as the researcher selects. Modern systematic research is likely to be based on a wide variety of information, including DNA-sequences (so called "molecular data"), biochemical data and morphological data.
In a cladogram, all organisms lie at the leaves, and each inner node is ideally binary (two-way). The two taxa on either side of a split are called sister taxa or sister groups. Each subtree, whether it contains one item or a hundred thousand items, is called a clade. A natural group has all the organisms contained in any one clade that share a unique ancestor (one which they do not share with any other organisms on the diagram) for that clade. Each clade is set off by a series of characteristics that appear in its members, but not in the other forms from which it diverged. These identifying characteristics of a clade are called synapomorphies (shared, derived characters). For instance, hardened front wings (elytra) are a synapomorphy of beetles, while circinate vernation, or the unrolling of new fronds, is a synapomorphy of ferns.
Willi Hennig (1913-1976) is widely regarded as the founder of cladistics.
Biological
systematics is the study of the diversity of life on the planet earth,
both past and present, and the relationships among living things through
time. Systematics, in other words, is used to understand the
evolutionary history of life on earth.
The term "systematics" is sometimes used synonymously with "taxonomy" and may be confused with "scientific classification." However, taxonomy is properly the describing, identifying, classifying, and naming of organisms, while "classification" is focused on placing organisms within groups that show their relationships to other organisms. All of these biological disciplines can be involved with extinct and extant organisms. However, systematics alone deals specifically with relationships through time, requiring recognition of the fossil record when dealing with the systematics of organisms. Systematics uses taxonomy as a primary tool in understanding organisms, as nothing about an organism's relationships with other living things can be understood without it first being properly studied and described in sufficient detail to identify and classify it correctly. Scientific classifications are aids in recording and reporting information to other scientists and to laymen. The systematist, a scientist who specializes in systematics, must, therefore, be able to use existing classification systems, or at least know them well enough to skillfully justify not using them. Phenetic systematics involves clarifying the biodiversity of earth through time by using the morphology and physiology of the organisms, while phylogenetic systematics, also called cladistics, uses apomorphies, or evolutionarily novel characteristics, to group earth's various organisms and their relationships through time. Today systematists generally make extensive use of molecular genetics and computer programs to study organisms. Systematics is fundamental to biology because it is the foundation for all studies of organisms, by showing how any organism relates to other living things. Systematics is also of major importance in understanding conservation issues because it attempts to explain the earth's biodiversity and could be used to assist in allocating limited means to preserve and protect endangered species, by looking at, for example, the genetic diversity among various taxa of plants or animals and deciding how much of that it is necessary to preserve. The living world consists of millions of species of organisms. These present enormous diversity ranging from micro-organisms to the highest evolved plants and animals. The knowledge about all these organisms will be highly confusing, meaningless and useless if they are not properly identified and arranged systematically. The systematic arrangement of properly identified and named organisms is called classification, systematics or taxonomy. (taxis = arrangement, nomos = order or law)
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Cladistics
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This cladogram shows the relationship among various insect groups.
Enlarge
This cladogram shows the relationship among various insect groups.
This representation emphasises that cladograms are trees.
Enlarge
This representation emphasises that cladograms are trees.
Cladistics is a branch of biology that determines the evolutionary relationships between organisms based on derived similarities. It is the most prominent of several forms of phylogenetic systematics, which study the evolutionary relationships between organisms. Cladistics is a method of rigorous analysis, using "shared derived traits" (synapomorphies: see below) of the organisms being studied. Cladistic analysis forms the basis for most modern systems of biological classification, which seek to group organisms by evolutionary relationships. In contrast, phenetics groups organisms based on their overall similarity, while approaches that are more traditional tend to rely on key characters (morphology). The word cladistics is derived from the ancient Greek ??????, klados, "branch."
As the end result of a cladistic analysis, treelike relationship-diagrams called "cladograms" are drawn up to show different hypotheses of relationships. A cladistic analysis can be based on as much or as little information as the researcher selects. Modern systematic research is likely to be based on a wide variety of information, including DNA-sequences (so called "molecular data"), biochemical data and morphological data.
In a cladogram, all organisms lie at the leaves, and each inner node is ideally binary (two-way). The two taxa on either side of a split are called sister taxa or sister groups. Each subtree, whether it contains one item or a hundred thousand items, is called a clade. A natural group has all the organisms contained in any one clade that share a unique ancestor (one which they do not share with any other organisms on the diagram) for that clade. Each clade is set off by a series of characteristics that appear in its members, but not in the other forms from which it diverged. These identifying characteristics of a clade are called synapomorphies (shared, derived characters). For instance, hardened front wings (elytra) are a synapomorphy of beetles, while circinate vernation, or the unrolling of new fronds, is a synapomorphy of ferns.
Willi Hennig (1913-1976) is widely regarded as the founder of cladistics.
Contents
[show]
* 1 Definitions
* 2 Cladistic methods
* 3 Cladistic classification
* 4 See also
* 5 References
* 6 External links
[edit] Definitions
A character state (see below) that is present in both the outgroups (the nearest relatives of the group, that are not part of the group itself) and in the ancestors is called a plesiomorphy (meaning "close form", also called ancestral state). A character state that occurs only in later descendants is called an apomorphy (meaning "separate form", also called the "derived" state) for that group. The adjectives plesiomorphic and apomorphic are used instead of "primitive" and "advanced" to avoid placing value-judgments on the evolution of the character states, since both may be advantageous in different circumstances. It is not uncommon to informally refer to a collective set of plesiomorphies as a ground plan for the clade or clades they refer to.
Several more terms are defined for the description of cladograms and the positions of items within them. A species or clade is basal to another clade if it holds more plesiomorphic characters than that other clade. Usually a basal group is very species-poor as compared to a more derived group. It is not a requirement that a basal group is present. For example when considering birds and mammals together, neither is basal to the other: both have many derived characters.
A clade or species located within another clade can be described as nested within that clade.
Cladistic methods
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SUMMARY - CLASSIFICATION
(1) Classification is essential for the proper study
and easy reference to the immense variety of life forms.
(2) Systematics deals with identification,
nomenclature and taxonomic classification of organisms.
(3) Species has a great significance as a taxonomic
unit.
(4) Recent taxonomy gives more importance to
sub-species and populations.
(5) In the systematic classification of organisms,
various taxa are arranged in the descending order of their taxonomic
categories as per the taxonomic hierarchy.
(6) Modern taxonomy makes use of the data from all
branches of botany, including genetics, cytology, ecology,
chemotaxonomy, numerical taxonomy, etc. in order to develop a
phylogenetic system of classification of plants.
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The Integrated Taxonomic
Information System (ITIS) is a partnership designed to provide
consistent and reliable information on the taxonomy of biological
species. ITIS was originally formed in 1996 as an interagency group
within the U.S. federal government, involving agencies from the
Department of Commerce to the Smithsonian Institution. It has now become
an international body, with Canadian and Mexican government agencies
participating. The primary focus of ITIS is North American species, but
many groups are worldwide and ITIS continues to collaborate with other
international agencies to increase its global coverage.
ITIS provides an automated reference database of scientific and common names for species. As of December 2005, it contains over 500,000 scientific names, synonyms, and common names for terrestrial, marine, and freshwater taxa from all biological kingdoms (animals, plants, fungi, and microbes). While the system does focus on North American species, it also includes many species not found in North America, especially among birds, fishes, amphibians, mammals, many reptiles, and several invertebrate animal groups. ITIS couples each scientific name with a stable and unique taxonomic serial number TSN as the “common denominator” for accessing information on such issues as invasive species, declining amphibians, migratory birds, fishery stocks, pollinators, agricultural pests, and emerging diseases. It presents the names in a standard classification that contains author, date, distributional, and bibliographic information related to the names. In addition, common names are available through ITIS in the major official languages of the Americas (English, French, Spanish, and Portuguese). ITIS and its international partner, Species 2000, cooperate to annually produce the Catalogue of Life, a checklist and index of the world’s species. The Catalogue of Life goal is to complete the global checklist of 1.8 million species by 2011.
Of the nearly 415,660 (April 2006) scientific names in the current database, approximately 210,000 were inherited from the database formerly maintained by the National Oceanographic Data Center (NODC) of the US National Oceanic and Atmospheric Administration (NOAA). The newer material has been checked to higher standards of taxonomic credibility, and over half of the original material has been checked and improved to the same standard. For further background on ITIS and for information to help interpret what can be found therein, please see the ITIS' Data Development History and Data Quality page [1], and the Glossary of Terms Used in ITIS [2].
Biological taxonomy is not fixed, and opinions about the correct status of taxa at all levels, and their correct placement, are constantly revised as a result of new research. Many aspects of classification will always remain a matter of scientific judgement. The ITIS database is updated to take account of new research as it becomes available, and the information it yields is likely to represent a fair consensus of modern taxonomic opinion. Records within ITIS include information about how far it has been possible to check and verify them. Its information should be checked against other sources where these are available, and against the primary research scientific literature where possible.
ITIS provides an automated reference database of scientific and common names for species. As of December 2005, it contains over 500,000 scientific names, synonyms, and common names for terrestrial, marine, and freshwater taxa from all biological kingdoms (animals, plants, fungi, and microbes). While the system does focus on North American species, it also includes many species not found in North America, especially among birds, fishes, amphibians, mammals, many reptiles, and several invertebrate animal groups. ITIS couples each scientific name with a stable and unique taxonomic serial number TSN as the “common denominator” for accessing information on such issues as invasive species, declining amphibians, migratory birds, fishery stocks, pollinators, agricultural pests, and emerging diseases. It presents the names in a standard classification that contains author, date, distributional, and bibliographic information related to the names. In addition, common names are available through ITIS in the major official languages of the Americas (English, French, Spanish, and Portuguese). ITIS and its international partner, Species 2000, cooperate to annually produce the Catalogue of Life, a checklist and index of the world’s species. The Catalogue of Life goal is to complete the global checklist of 1.8 million species by 2011.
Of the nearly 415,660 (April 2006) scientific names in the current database, approximately 210,000 were inherited from the database formerly maintained by the National Oceanographic Data Center (NODC) of the US National Oceanic and Atmospheric Administration (NOAA). The newer material has been checked to higher standards of taxonomic credibility, and over half of the original material has been checked and improved to the same standard. For further background on ITIS and for information to help interpret what can be found therein, please see the ITIS' Data Development History and Data Quality page [1], and the Glossary of Terms Used in ITIS [2].
Biological taxonomy is not fixed, and opinions about the correct status of taxa at all levels, and their correct placement, are constantly revised as a result of new research. Many aspects of classification will always remain a matter of scientific judgement. The ITIS database is updated to take account of new research as it becomes available, and the information it yields is likely to represent a fair consensus of modern taxonomic opinion. Records within ITIS include information about how far it has been possible to check and verify them. Its information should be checked against other sources where these are available, and against the primary research scientific literature where possible.
The nature of plant species
Loren H. Rieseberg, Troy E. Wood1 and Eric J. Baack
Many botanists doubt the existence of plant species viewing them as arbitrary constructs of the human mind, as opposed to discrete, objective entities that represent reproductively independent lineages or 'units of evolution'. However, the discreteness of plant species and their correspondence with reproductive communities have not been tested quantitatively, allowing zoologists to argue that botanists have been overly influenced by a few 'botanical horror stories', such as dandelions, blackberries and oaks6, 7. Here we analyse phenetic and/or crossing relationships in over 400 genera of plants and animals. We show that although discrete phenotypic clusters exist in most genera (> 80%), the correspondence of taxonomic species to these clusters is poor (< 60%) and no different between plants and animals. Lack of congruence is caused by polyploidy, asexual reproduction and over-differentiation by taxonomists, but not by contemporary hybridization. Nonetheless, crossability data indicate that 70% of taxonomic species and 75% of phenotypic clusters in plants correspond to reproductively independent lineages (as measured by postmating isolation), and thus represent biologically real entities. Contrary to conventional wisdom8, plant species are more likely than animal species to represent reproductively independent lineages.
Loren H. Rieseberg, Troy E. Wood1 and Eric J. Baack
Many botanists doubt the existence of plant species viewing them as arbitrary constructs of the human mind, as opposed to discrete, objective entities that represent reproductively independent lineages or 'units of evolution'. However, the discreteness of plant species and their correspondence with reproductive communities have not been tested quantitatively, allowing zoologists to argue that botanists have been overly influenced by a few 'botanical horror stories', such as dandelions, blackberries and oaks6, 7. Here we analyse phenetic and/or crossing relationships in over 400 genera of plants and animals. We show that although discrete phenotypic clusters exist in most genera (> 80%), the correspondence of taxonomic species to these clusters is poor (< 60%) and no different between plants and animals. Lack of congruence is caused by polyploidy, asexual reproduction and over-differentiation by taxonomists, but not by contemporary hybridization. Nonetheless, crossability data indicate that 70% of taxonomic species and 75% of phenotypic clusters in plants correspond to reproductively independent lineages (as measured by postmating isolation), and thus represent biologically real entities. Contrary to conventional wisdom8, plant species are more likely than animal species to represent reproductively independent lineages.
Do plant species really exist? Yes, scientists say
FOR IMMEDIATE RELEASE
March 22, 2006
BLOOMINGTON, Ind. -- Notoriously "promiscuous" plants like oaks and dandelions have led some biologists to conclude plants cannot be divided into species the same way animals are.
That perception is wrong, say Indiana University Bloomington scientists in this week's Nature. Their analysis of 882 plant and animal species and 1,347 inter-species crossings -- the first large-scale comparison of species barriers in plants and animals -- showed that plant species are just as easily categorized as animal species.
The study also yielded a surprise. The hybrid offspring of different animal species are more likely to be fertile than the hybrid offspring of plant species.
Plant fantasy
Photo by: Robert Czarny
Are plant species real or imagined? Many botanists have argued that grouping plants into species is merely an exercise of convenience.
Print-Quality Photo
"We found that not only are plants just as easily subdivided into species as animals when analyzed statistically, but plants are more likely to be reproductively isolated due to hybrid sterility," said evolutionary biologist Loren Rieseberg, who led the study. "Most plant species are indeed 'real.' The problem has been that botanists have been way over-attracted to the plant species that readily hybridize and where the hybrids perpetuate themselves asexually. While it's true that dandelions and blackberries pose problems, these horror stories only make up 1 percent of the whole."
The scientists did find categorization problems with nearly half of the plant and animal species they surveyed.
Rieseberg and his co-authors, doctoral student Troy Wood and postdoctoral research associate Eric Baack, examined hundreds of peer-reviewed papers reporting the measurement of various plant and animal characteristics, or reporting on the success or failure of hybridization of plant and animal species with similar species. The scientists culled the papers for information, grouped and combined data for each given species, and then looked at how often characteristics clustered in accordance with named species.
The scientists found that while real, quantifiable clusters did exist in most groups of plants and animals, the one-to-one correspondence of species names and character clusters was quite low -- about 54 percent. One explanation for this, Rieseberg said, is that too many taxonomists are "splitters" -- they give too many species names to a single group of related organisms.
After analyzing the hybridization data, the scientists found that only 30 percent of the approximately 500 plant species they surveyed are able to produce fertile hybrids when mated with other species. By stark contrast, 61 percent of animal species surveyed are able to reproduce successfully with other species.
The hybridization of animal species is often portrayed as rare and strange, or else the result of human-forced matings, as is the case with ligers (lion-tiger hybrids) and mules (horse-ass hybrids). It is not common knowledge that many bird and fish species successfully hybridize in the wild. The scientists found that birds were most likely to produce fertile hybrids when crossed with other bird species. Ferns, of all things, were least likely to generate fertile hybrids.
Many of the hybridization papers that Rieseberg, Wood and Baack looked at reported crossings under laboratory conditions, and therefore the crossings may not accurately represent what happens in nature. For example, two species that can hybridize may not actually do so, perhaps because they exist on different continents or because they prefer to mate with members of their own species. For that reason, the percentages of plant and animal species that hybridize in the wild are likely to be lower than those reported by the scientists.
The Nature study is meant to address gaps in scientists' knowledge in two areas: the fundamental nature of species and the divisibility of plant and animal species using commonly accepted definitions of species. Debates in both areas began with the 1859 edition of Darwin's Origin of Species, and they have not yet been settled.
"These discussions should have been settled earlier, but no one bothered to summarize the relevant literature, perhaps because it is so vast," Rieseberg said.
Over the past 50 years, numerous scientific papers have been published in which species are categorized by statistical analysis of observable traits (i.e., numerical taxonomy) and/or by the ease with which species can be hybridized (i.e., breeding studies). "After going through all this literature, we realized someone just needed to compile and analyze it all," Rieseberg said.
The scientists decided to use the mass of data to see whether taxonomists were doing a good job, and whether cross-species mating in the plant kingdom is especially likely to be successful.
"The species concept debate has devolved from an empirical discussion into a philosophical one," Rieseberg said. "But this is fundamentally an empirical question. These data support the notion that species can be both units of evolution and products of evolution."
Rieseberg holds the Class of '54 Chair and is a distinguished professor of biology at IU Bloomington. Troy Wood and Eric Baack also contributed to the report. It was funded primarily by a Guggenheim Fellowship grant. Supplementary support came from the MacArthur Foundation, the National Institutes of Health and the National Science Foundation.
Paper coauthors Troy Wood and Eric Baack are available for comment. Rieseberg is away and unavailable. To speak with Wood, call 812-855-5873 or e-mail trowood@indiana.edu. To speak with Baack, call 812-855-9018 or e-mail ebaack@indiana.edu.
"The Nature of Plant Species," Nature, v. 440 (7081)
FOR IMMEDIATE RELEASE
March 22, 2006
BLOOMINGTON, Ind. -- Notoriously "promiscuous" plants like oaks and dandelions have led some biologists to conclude plants cannot be divided into species the same way animals are.
That perception is wrong, say Indiana University Bloomington scientists in this week's Nature. Their analysis of 882 plant and animal species and 1,347 inter-species crossings -- the first large-scale comparison of species barriers in plants and animals -- showed that plant species are just as easily categorized as animal species.
The study also yielded a surprise. The hybrid offspring of different animal species are more likely to be fertile than the hybrid offspring of plant species.
Plant fantasy
Photo by: Robert Czarny
Are plant species real or imagined? Many botanists have argued that grouping plants into species is merely an exercise of convenience.
Print-Quality Photo
"We found that not only are plants just as easily subdivided into species as animals when analyzed statistically, but plants are more likely to be reproductively isolated due to hybrid sterility," said evolutionary biologist Loren Rieseberg, who led the study. "Most plant species are indeed 'real.' The problem has been that botanists have been way over-attracted to the plant species that readily hybridize and where the hybrids perpetuate themselves asexually. While it's true that dandelions and blackberries pose problems, these horror stories only make up 1 percent of the whole."
The scientists did find categorization problems with nearly half of the plant and animal species they surveyed.
Rieseberg and his co-authors, doctoral student Troy Wood and postdoctoral research associate Eric Baack, examined hundreds of peer-reviewed papers reporting the measurement of various plant and animal characteristics, or reporting on the success or failure of hybridization of plant and animal species with similar species. The scientists culled the papers for information, grouped and combined data for each given species, and then looked at how often characteristics clustered in accordance with named species.
The scientists found that while real, quantifiable clusters did exist in most groups of plants and animals, the one-to-one correspondence of species names and character clusters was quite low -- about 54 percent. One explanation for this, Rieseberg said, is that too many taxonomists are "splitters" -- they give too many species names to a single group of related organisms.
After analyzing the hybridization data, the scientists found that only 30 percent of the approximately 500 plant species they surveyed are able to produce fertile hybrids when mated with other species. By stark contrast, 61 percent of animal species surveyed are able to reproduce successfully with other species.
The hybridization of animal species is often portrayed as rare and strange, or else the result of human-forced matings, as is the case with ligers (lion-tiger hybrids) and mules (horse-ass hybrids). It is not common knowledge that many bird and fish species successfully hybridize in the wild. The scientists found that birds were most likely to produce fertile hybrids when crossed with other bird species. Ferns, of all things, were least likely to generate fertile hybrids.
Many of the hybridization papers that Rieseberg, Wood and Baack looked at reported crossings under laboratory conditions, and therefore the crossings may not accurately represent what happens in nature. For example, two species that can hybridize may not actually do so, perhaps because they exist on different continents or because they prefer to mate with members of their own species. For that reason, the percentages of plant and animal species that hybridize in the wild are likely to be lower than those reported by the scientists.
The Nature study is meant to address gaps in scientists' knowledge in two areas: the fundamental nature of species and the divisibility of plant and animal species using commonly accepted definitions of species. Debates in both areas began with the 1859 edition of Darwin's Origin of Species, and they have not yet been settled.
"These discussions should have been settled earlier, but no one bothered to summarize the relevant literature, perhaps because it is so vast," Rieseberg said.
Over the past 50 years, numerous scientific papers have been published in which species are categorized by statistical analysis of observable traits (i.e., numerical taxonomy) and/or by the ease with which species can be hybridized (i.e., breeding studies). "After going through all this literature, we realized someone just needed to compile and analyze it all," Rieseberg said.
The scientists decided to use the mass of data to see whether taxonomists were doing a good job, and whether cross-species mating in the plant kingdom is especially likely to be successful.
"The species concept debate has devolved from an empirical discussion into a philosophical one," Rieseberg said. "But this is fundamentally an empirical question. These data support the notion that species can be both units of evolution and products of evolution."
Rieseberg holds the Class of '54 Chair and is a distinguished professor of biology at IU Bloomington. Troy Wood and Eric Baack also contributed to the report. It was funded primarily by a Guggenheim Fellowship grant. Supplementary support came from the MacArthur Foundation, the National Institutes of Health and the National Science Foundation.
Paper coauthors Troy Wood and Eric Baack are available for comment. Rieseberg is away and unavailable. To speak with Wood, call 812-855-5873 or e-mail trowood@indiana.edu. To speak with Baack, call 812-855-9018 or e-mail ebaack@indiana.edu.
"The Nature of Plant Species," Nature, v. 440 (7081)
