Index to all names of insect genera, families and higher taxa
Higher Categories and Higher Taxa
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Transcript of Higher Categories and Higher Taxa
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Higher Categories and Taxa
University of the Punjab
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Zainab Saeed
Z08-13
7thsemester
Bsc (Hons.) Zoology
University of Punjab
Submitted To:
Mrs Dr. Abida Butt
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Table of Contents
Higher Categories and Higher Taxa......................................................................................................... 4
The Genus: .......................................................................................................................................... 4
Generic Characters: ......................................................................................................................... 5
Meaning of a genus: ........................................................................................................................ 5
The Family: .......................................................................................................................................... 6
Orders, Classes and Phyla: .................................................................................................................. 6
The Process of Ranking ........................................................................................................................... 7
Relationship and Similarity ..................................................................................................................... 7
Homology: ........................................................................................................................................... 8
Serial Homology: ............................................................................................................................. 9
Analogy: .......................................................................................................................................... 9
Homoplasy: ..................................................................................................................................... 9
Convergence in characters:................................................................................................................. 9
Parallel Characters: ....................................................................................................................... 10
Reversed characters: ..................................................................................................................... 11
Difficulties Encountered in Macrotaxonomy ........................................................................................ 11
Mosaic Evolution: .............................................................................................................................. 11
Fossils: ............................................................................................................................................... 12
Fossils & Converging Evidence ...................................................................................................... 13
Fossils & Scientific Predictions ...................................................................................................... 14
The Improvement of Existing Classification .......................................................................................... 14
Stability: ............................................................................................................................................ 15
The Printed Sequence: .................................................................................................................. 16
Graphical Representation ............................................................................................................. 16
Phylogenetic Trees: ........................................................................................................................... 16
Importance of Sound Classification: ................................................................................................. 17
References: ........................................................................................................................................... 18
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Higher Categories and Higher Taxa
A higher category is a class into which all the higher taxa that are ranked at the same level in
a hierarchic classification are placed
This means that the taxa which are on the same level in the classification are given some rank
and the category in which they are placed indicated their rank in the hierarchical
classification. When we say that categories are based on concepts and taxa are based on
zoological realities, it means that organisms are placed in taxa while the categories are some
concept on which the taxa are arranged in categories.
In Linnaean hierarchy there is no difference between species category and higher categories
but in other respects it is quite different.
The species category actually is signifies singularity, distinctness and differences but theother higher categories are based on the comparison among each other. In species categories
all the taxa are placed according to their individual characters and their uniqueness but in the
other higher categories, the taxa are compared and those which have similarities are placed
together. They are arranged according to affinities among group of species.
The taxon is given a certain limit and according to its characters it is placed in a higher
category, as long as it is consistent with the theory of common decent. The higher taxa are
themselves separated by a certain gap from other taxa of same rank. Meaning there are
different taxa in a category and these taxa are separated from one another by some limit or
boundary in characteristics.
Another difference in higher categories and species is that in higher categories differentiation
is through comparative studies which delimit the taxa and placed into higher categories but in
species we use the concept of interbreeding and producing fertile offspring.
Darwin stated the matter of hierarchy as;
The natural system is genealogical in the arrangement, like a pedigree; but the degree of
modification which the different groups have undergone has to be expressed by ranking them
under different so-called genera, sub-families, families, sections, orders, and classes.
It must be noted that the higher categorical rank evolve from a lower rank not the other way
round. Because when we classify, we only have the organism and on that basis, we make all
the ranks of classification.
Different criteria and operations for ranking are employed by different schools of
macrotaxonomy.
The Genus:
Thegenus is the lowest obligatory higher category and the lowest of all categories
established strictly by comparative data (Cain, 1956).
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A modern pragmatic definition of genus is as follows.
The genus is the obligatory taxonomic category directly above that of the species in the
Linnaean hierarchy.
The generic taxon or genus is a monophyletic group containing one or more species and areseparated from other generic taxa by a decided gap.
It is recommended that the size of the gap should be the inverse of the side of the taxon. The
more the species in a group, easier it is to recognise it as a separate genus i.e. less gap is
required. Smaller the species group more the gap required. Delimiting the species group as
genera required a lot of experience, good judgement, and common sense.
Generic Characters:
It is the genus that gives characters not the characters that makes the genus.
This is generally valid. The species included in the genus have many characters in common
and the recognition of higher presence of correlated complex. This may include some minute
and inconspicuous characters but as Darwin said;
The importance, for classification, of trifling characters, mainly depends on their being
correlated with several other characters of more or less importance. The value indeed of an
aggregate of characters is very evident in natural history.
This principle led to the many generic splitting. Whenever a new character was discovered it
often led to the formation of new genera. Many genera cantbe diagnosed on the basis of a
single character.
Meaning of a genus:
Whenever we assign a generic rank to group of species, we always try to describe the
characters of all the species in that genus. Genus is a phylogenetic unit. This means that all
the species in genus have been descended from near ancestors.
Sometimes the genus is an ecological unit, consisting of species which have been adapted for
same kind of environment.
Species of the same genus also possess genetic identity. It is also possible that the species ofthe same genus can produce hybrids. Dubois has gone so far as to demand that all the species
that produce hybrids, be placed in the same genus.
For the recognition of generic taxon,
Where alternative are available, we stand by the concept or theory that are more useful-the
one that generalizes the most observation and permits the most reliable predictions (Inger
1958:383).
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The Family:
A non-arbitrary definition of the family category is not possible. We can say that a kind of
animal is often a family. To determine how distinctive a group of genera be to be classified as
family differ from one group of thought to another. There is no definite criterion which
indicated the rules for the family classification.
Family can be defined as taxa;
The family is a taxonomic category for a taxon composed of a single genus or a group of
related genera. It is separated from other families by a decided gap.
As in the case of genus the size of the gap is inverse ratio to the size of the family.
The family is distinguished by certain adaptive character in a much greater extent than
genera. The more distinct the character, the greater is the gap between the families. The
families are older than genera and have a worldwide distribution. If an entomologist has 422families of British insects and goes to Africa he will see almost all the families there two. The
characters of a family are especially important for a general zoologist as the each family
presents some general characters that can be recognized at a glance, so one can easily
recognize the members of that family easily. For example in spiders each family has some
general characters which separate it from other families. The family Oxyopidae has large
front eyes which is its distinguishing character.
At a given locality the various families are generally distinct. They have their gaps which
separate them. It is sometimes the case that when broadening the spectrum of families, we
encounter some difficulties. Families are known to form some distinctive group in eachcontinent. So we have to make a larger group also known as a super family. Some families
were based on homoplasy. These members were to be placed in the different families or
separate families were to be made. Linnaeus did not recognise the family as a category but
many of his genera have been elevated to the family rank. This shoes that there was a little
difference between Linnaean genera and our families. With only 312 genera of animals,
Linnaeus had no need for an intermediate category between genus and order. Now many new
animals have been discovered, so new families have been formed. Nowadays 5600 families
of Metazoa and 580 families of protozoa, totalling approximately 6200 families, have been
formed.
Orders, Classes and Phyla:
These highest categories above the family are, on the whole, very well defined. The taxa
ranked in these highest categories represent the main branches of the phylogenetic tree. They
are characterized by a basic structural pattern laid down early in evolutionary history.
Taxa in higher categories are definable in terms of a basic structural pattern, but except for
certain highly specialized groups, the higher taxa are not primarily or even predominantly
distinguished by special adaptations. The taxa included in higher categories are widely
distributed in space and time.
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According to the recent tabulations, there are approximately 29 phyla, 144 classes, and 722
orders of recent animals.
The Process of Ranking
The process of ranking is not complete when species are placed in genera, because these
genera have to be placed in family and these families into higher taxa until Linnaean
hierarchy is complete.
The reason for the hierarchy was clear to Darwin as he saw that diversity must be originated
after speciation and that chance and adaptive processes were responsible for the gradual
evolution of higher taxa and still higher taxa separated by the gap of divergent evolution and
extinction.
The three major schools of macrotaxonomy differ in the matter of classification.
Phenetic system of classification is that, that does not try to reflect evolutionary relationships;
instead it is based on physical similarities among organisms (phenotype); organisms are
placed in the same category because they look alike. The pheneticist, only consider the
similarities in characters in classification.
Cladistic system of classification is based on the phylogenetic relationships and evolutionary
history of groups of organisms. Cladists who follow Henning introduce a new rank at each
branching point of cladogram and give sister groups identical categorical rank.
For classical taxonomist, ranking results from the degrees of difference found among taxa;
much divergence from the ancestral condition requires that a taxon be given a higher rank.
Classifications proposed by the Cladists are on the whole rather more elaborate than those of
evolutionary taxonomists, because Cladists want their classifications to reflect as minutely as
possible the actual branching pattern of the genealogy (Wiley 1981:199-238); gaps are
consciously ignored. Evolutionary taxonomist tends to emphasize major groupings and the
existence of major gaps.
Relationship and Similarity
Relationship is used in different terms. Pheneticists take this relationship in only similarities
of characters while Cladists take this relationship as only in genealogy. The evolutionary
taxonomist consider both ancestor-descendent relationship and collateral relationship among
sister lineages, while Cladists consider only holophyletic lineages. Developed mostly on the
basic
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Relationship between taxa is generally according to the similarities but sometimes similarities
can lead to false grouping. This can be solved by the careful analysis of taxonomic
characters. In this case one must distinguish between different potential causes of similarities.
Homology:
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen
defined homology as "the same organ in different animals under every variety of form and
function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand
have a common underlying structure of bones and muscles. Owen reasoned that there must be
a common structural plan for all vertebrates, as well as for each class of vertebrates.
Homologous traits of organisms are due to sharing a common ancestor, and such traits often
have similar embryological origins and development. This is contrasted with analogous traits:
similarities between organisms that were not present in the last common ancestor of the taxa
being considered but rather evolved separately. An example of analogous traits would be thewings of bats and birds, which evolved separately but both of which evolved from the
vertebrate forelimb and therefore have similar early embryology.
Whether or not a trait is homologous depends on both the taxonomic and anatomical levels at
which the trait is examined. For example, the bird and bat wings are homologous as forearms
in tetrapods. However, they are not homologous as wings, because the organ served as a
forearm (not a wing) in the last common ancestor of tetrapods. By definition, any
homologous trait defines a cladea monophyletic taxon in which all the members have the
trait (or have lost it secondarily); and all non-members lack it.
A homologous trait may be homoplasious that is, it has evolved independently, but from
the same ancestral structure plesiomorphic that is, present in a common ancestor but
secondarily lost in some of its descendants or (syn)apomorphicpresent in an ancestor
and all of its descendants.
The word homology, coined in about 1656, derives from the Greek homologos, where homo
= agreeing, equivalent, same + logos = relation. In biology, two things are homologous if
they bear the same relationship to one another, such as a certain bone in various forms of the
"hand."
Ray Lankester defined the terms "homogeny", meaning homology due to inheritance from a
common ancestor, and "homoplasty", meaning homology due to other factors.
As most problems in science, obvious hypothesis are accepted provisionally unless they lead
to logical contradictions. The establishment of homologies ranges from simple comparison of
features of closely related species, where the matter need hardly be given a second thought, to
the frustratingly difficult comparison of dissimilar features in higher taxa.
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Serial Homology:
Serial homology is the representative or repetitive relation in the segments of the sameorganism, as in the lobster, where the parts follow each other in a straight line or series.
This was coined by Owen (1866).
Serial homology is the concept that initially existing structures were gradually modified viadiscrete intermediary steps until such time as an evolutionary novelty (e.g., jaws) appeared.
Many examples of serial homology, e.g. the body segments of many animals (vertebrates,
arthropods etc.), are examples of gene duplication on regulatory genes such as homeobox
genes, followed by evolution differentiating the duplicated genes.
Analogy:
In biology, an analogy is a trait or an organ that appears similar in two unrelated organisms.
The cladistic term for the same phenomenon is homoplasy, from Greek for same form.Biological anologies are often the result of convergent evolution.
The classical example of an analogy is the ability to fly in birds and bats. Both groups can
move by powered flight, but flight has evolved independently in the two groups. The ability
to fly does not make birds and bats close relatives. The opposite of analogy is homology,
where the ability or organ in question has been inherited from a common ancestor. The
British anatomist Richard Owen was the first scientist to recognise the fundamental
difference between analogies and homologies, and named them.
Analogous traits will often arise due to convergence, where different species live in similar
ways and/or similar environment, and thus face the same environmental factors. Both
herrings and dolphins are streamlined. Both are active predators in a high drag environment,but the herring is a bony fish, the dolphin a mammal. In the Mesozoic, similarly streamlined
ichthyosaurs navigated the worlds oceans, yet another example of a group evolving a
similar shape due to the same environmental factors. A similar phenomenon is earless seals
and eared seals. It was long debated whether the two groups are a single marine group, or
represent two separate episodes of carnivores turning to a marine environment.
Homoplasy:
Homology means the similarity due to the common ancestor. Homoplasy, on the other
hand, means similarity due to convergent evolution, but independent origins. For instance,
take the fin and the caudal fin of tuna and of dolphin; they are similar but have independenthistories, and their similarity comes from adaptation to similar environments and functions.
This is homoplasy. However, the fin of tuna and bonito are similar because of the common
ancestor, and that's homology.
The attempt to determine whether an observed similarity is a genuine homology or a
homoplasy ought to be an indispensable component of every taxonomic analysis.
Unfortunately, it is altogether ignored in unweighted phenetic procedures and often
insufficiently considered in the construction of shortest trees.
Convergence in characters:
Convergent evolution describes the acquisition of the same biological trait in unrelatedlineages.
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The wing is a classic example of convergent evolution in action. Although their last
common ancestor did not have wings, both birds and bats do, and are capable of powered
flight. The wings are similar in construction, due to the physical constraints imposed upon
wing shape. Similarity can also be explained by shared ancestry. Wings were modified from
limbs, as evidenced by their bone structure.
Traits arising through convergent evolution are termed analogous structures, in contrast to
homologous structures, which have a common origin. Bat and pterosaur wings are an
example of analogous structures, while the bat wing is homologous to human and other
mammal forearms, sharing an ancestral state despite serving different functions. Similarity
in species of different ancestry that is the result of convergent evolution is called
homoplasy. The opposite of convergent evolution is divergent evolution, whereby related
species evolve different traits. On a molecular level, this can happen due to random
mutation unrelated to adaptive changes. Convergent evolution is similar to, but
distinguishable from, the phenomena of evolutionary relay and parallel evolution.
Evolutionary relay describes how independent species acquire similar characteristics
through their evolution in similar ecosystems at different timesfor example the dorsal finsof extinct ichthyosaurs and sharks. Parallel evolution occurs when two independent species
evolve together at the same time in the same ecospace and acquire similar characteristics for
instance extinct browsing-horses and paleotheres.
Similarity can also result if organisms occupy similar ecological niches that is, a distinctive
way of life. A classic comparison is between the marsupial fauna of Australia and the
placental mammals of the Old World. The two lineages are clades that is, they each share a
common ancestor that belongs to their own group, and are more closely related to one
another than to any other cladebut very similar forms evolved in each isolated population.
Many body plans, for instance sabre-toothed cats and flying squirrels, evolved
independently in both populations.
In some cases, it is difficult to tell whether a trait has been lost then re-evolved
convergently, or whether a gene has simply been 'switched off' and then re-enabled later.
Such a re-emerged trait is called an atavism. From a mathematical standpoint, an unused
gene has a reasonable probability of remaining in the genome in a functional state for
around 6 million years, but after 10 million years it is almost certain that the gene will no
longer function.
Convergent characters are mostly found when different animals become adapted to similar
niches. For example, loons and grebes, which are both diving birds agree in numerous
structural characters, particularly of legs, yet are only very distantly related to each other.
Many marsupial adaptive types (wolves, mice, moles. badgers etc.) are remarkably similarto analogous placental types; the similarity is due to selection for similar modes of life.
Parallel Characters:
Similar characters derived independently by related taxa with a similar genetic background
cause systematists the most trouble. These characters range from distinctive to rather simple
characters.
Characters that evolve from parallelism are not homologous because they are not derived
from the same phenotypic feature of their nearest common ancestor. This interpretation is
most congenial to taxonomists who are simply concerned with the construction of a
character matrix.
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Some evolutionary taxonomist consider characters due to parallelism to be homologous, the
synamorphy is the potential to develop the character.
Reversed characters:
Phylogenists have tended to consider morphological change, an inexorably advancing
process. Character analysis, however, remarkably often shows that what appears to beprimitive character are actually reversals (psuedoprimitiveness). There is much evolutionary
reversal owing to the loss of specialization or other derived characters. Recent cladistic
analysis shows that these reversals are very much common. They generally affect a single
character of a character complex and can be discovered by character analysis. However,
Dolos rule, according to which a more or less complex structure that has been lost is not
reacquired in the same complexity, has few if any exceptions.
Difficulties Encountered in Macrotaxonomy
No matter what school of macrotaxonomy do the scientists belong to, they encounter manyproblems while classification and speciation. Some of the difficult situations that are not
always considered by taxonomists require careful analysis.
Mosaic Evolution:
Mosaic evolution (or modular evolution) is the concept that evolutionary change takes place
in some body parts or systems without simultaneous changes in other parts. Another
definition is the "evolution of characters at various rates both within and between species".
Its place in evolutionary theory comes under long-term trends or macroevolution.
In the Neo-Darwinist theory of evolution, as postulated by Stephen Jay Gould, there is room
for differing development, when a life form matures earlier or later, in shape and size. This
is due to allomorphism. Organs develop at differing rhythms, as a creature grows and
matures. Thus a "heterochronic clock" has three variants:
1) Time, as a straight line;
2) General size, as a curved line;
3) Shape, as another curved line.
When a creature is advanced in size, it may develop at a smaller size; alternatively, it may
maintain its original size or, if delayed, it may result in a larger sized creature. That is
insufficient to understand heterochronic mechanism. Size must be combined with shape, soa creature may retain paedomorphic features if advanced in shape or present recapitulatory
appearance when retarded in shape. These names are not very indicative, as past theories of
development were very confusing.
A creature in its ontogeny may combine heterochronic features in six vectors, although
Gould considers that there is some binding with growth and sexual maturation. A creature
may, for example, present some neotenic features and retarded development, resulting in
new features derived from an original creature only by regulatory genes. Most novel human
features (compared to closely related apes) were of this nature, not implying major change
in structural genes, as was classically considered.
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By its very nature, the evidence for this idea comes mainly from palaeontology. It is not
claimed that this pattern is universal, but there are now a wide range of examples from
many different taxa. Some examples:
The early evolution of bipedalism in Australopithecines, and its modification of thepelvic girdle took place well before there was any significant change in the skull, orbrain size.
Archaeopteryx. Nearly 150 years ago Thomas Henry Huxley comparedArchaeopteryx with a small theropod dinosaur, Compsognathus. These two fossils
came from the Solnhofen limestone in Bavaria. He showed that the two were very
similar, except for the front limbs and feathers of Archaeopteryx. Huxley's interest
was in the basic affinity of birds and reptiles, which he united as the Sauropsida. The
interest here is that the rest of the skeleton had not changed.
Meadow voles during the last 500,000 years. The pterosaur Darwinopterus. The type species, D. modularis was the first known
pterosaur to display features of both long-tailed (rhamphorhynchoid) and short-
tailed (pterodactyloid) pterosaurs. Evolution of the horse, in which the major changes took place at different times, not
all simultaneously.
Mammalian evolution, especially during the Mesozoic is undoubtedly one of thebest examples.
Fossils:
The fossil record has one important, unique characteristic: it is our only actual glimpse intothe past where common descent is proposed to have taken place. As such it provides
invaluable evidence for common descent. The fossil record is not "complete" (fossilization
is a rare event, so this is to be expected), but there is still a wealth of fossil information.
If you look at the fossil record, you find a succession of organisms that suggest a history of
incremental development from one species to another. You see very simple organisms at
first and then new, more complex organisms appearing over time. The characteristics of
newer organisms frequently appear to be modified forms of characteristics of older
organisms.
This succession of life forms, from simpler to more complex, showing relationships
between new life forms and those that preceded them is strong inferential evidence of
evolution. There are gaps in the fossil record and some unusual occurrences, such as what is
commonly called the Cambrian explosion, but the overall picture created by the fossil
record is one of consistent, incremental development.
At the same time, the fossil record is not in any way, shape, or form suggestive of the idea
of sudden generation of all life as it appears now, nor does it support transformationism.
There is no way to look at the fossil record and interpret the evidence as pointing towards
anything other than evolution despite all the gaps in record and in our understanding,
evolution and common descent are the only conclusions that are supported by the full
spectrum of evidence.
This is very important when considering inferential evidence because inferential evidence
can always, in theory, be challenged on its interpretation: why interpret the evidence as
inferring one thing rather than another? Such a challenge is only reasonable, though, when
one has stronger alternativean alternative that not only explains the evidence better than
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what's being challenged, but which preferably also explains other evidence that the first
explanation does not. If you look at the fossil record, you find a succession of organisms
that suggest a history of incremental development from one species to another. You see
very simple organisms at first and then new, more complex organisms appearing over time.
The characteristics of newer organisms frequently appear to be modified forms of
characteristics of older organisms.
This succession of life forms, from simpler to more complex, showing relationships
between new life forms and those that preceded them is strong inferential evidence of
evolution. There are gaps in the fossil record and some unusual occurrences, such as what is
commonly called the Cambrian explosion, but the overall picture created by the fossil
record is one of consistent, incremental development.
At the same time, the fossil record is not in any way, shape, or form suggestive of the idea
of sudden generation of all life as it appears now, nor does it support transformationism.
There is no way to look at the fossil record and interpret the evidence as pointing towards
anything other than evolution despite all the gaps in record and in our understanding,
evolution and common descent are the only conclusions that are supported by the fullspectrum of evidence.
This is very important when considering inferential evidence because inferential evidence
can always, in theory, be challenged on its interpretation: why interpret the evidence as
inferring one thing rather than another? Such a challenge is only reasonable, though, when
one has stronger alternative an alternative that not only explains the evidence better than
what's being challenged, but which preferably also explains other evidence that the first
explanation does not.
We don't have this when with any form of creationism. For all their insistence that evolution
is only a "faith" because so much evidence is "merely" inferential, they are unable topresent an alternative that explains all that inferential evidence better than evolution or
even anywhere close to evolution. Inferential evidence isn't as strong as direct evidence, but
it's treated as sufficient in most cases when enough evidence exists and especially when
there are no reasonable alternatives.
Fossils & Converging Evidence
That the fossil record in general suggests evolution is certainly an important piece ofevidence, but it becomes even more telling when it is combined with other evidence for
evolution. For example, the fossil record is consistent in terms of biogeography and if
evolution is true, we would expect that the fossil record would be in harmony with current
biogeography, the phylogenetic tree, and the knowledge of ancient geography suggested byplate tectonics. In fact, some finds, such as fossil remains of marsupials in Antarctica are
strongly supportive of evolution, given that Antarctica, South America and Australia were
once part of the same continent.
If evolution did happen, then you would expect not just that the fossil record would show a
succession of organisms as described above, but that the succession seen in the record
would be compatible with that derived by looking at currently living creatures. For
example, when examining the anatomy and biochemistry of living species, it appears that
the general order of development for the major types of vertebrate animals was fish to
amphibians to reptiles to mammals. If current species developed as a result of common
descent then the fossil record should show the same order of development.
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In fact, the fossil record does show the same order of development. In general, the fossil
record is consistent with the developmental order suggested by looking at the characteristics
of living species. As such it represents another independent piece of evidence for common
descent, and a very significant one since the fossil record is a window to the past.
Fossils & Scientific Predictions
We should also be able to make some predictions and retrodictions as to what we would
expect to see in the fossil record. If common descent occurred, then the organisms found in
the fossil record should generally conform to the phylogenetic tree the nodes on the tree
at which a split occurs represent common ancestors of the organisms on the new branches
of the tree.
We would predict that we could find organisms in the fossil record showing characteristics
that are intermediate in nature between the different organisms that evolved from it and
from the organisms from which it evolved. For example, the standard tree suggests thatbirds are most closely related to reptiles, so we would predict that we could find fossils
which show a mix of bird and reptile characteristics. Fossilized organisms that possess
intermediate characteristics are called transitional fossils.
Exactly these sorts of fossils have been found.
We would also expect that we would not find fossils showing intermediate characteristics
between organisms that are not closely related. For example, we would not expect to see
fossils that appear to be intermediates between birds and mammals or between fish and
mammals. Again, the record is consistent.
The Improvement of Existing Classification
The complete reclassification of higher taxa may be the greatest achievement of a
taxonomist, but the taxonomists daily routine consists of minor additions to or modification
of existing classifications. The following are the most frequent activities of taxonomists.
1. The assignment of the newly discovered species into the proper genus by answeringthese questions,
a) Can it be included in an established genus?b) Does it require a new genus and possibly a new higher taxon?
2. The transfer of an incorrectly placed taxon to its proper position.3. The splitting of a taxon into several taxa of the same rank either by cleaving a
heterogeneous assemblage of species into several smaller and more homogenous
ones or by removing an alien element from an otherwise homogenous taxon. When
one breaks up too large a taxon, certain rules must be observed in the naming and
rankling of the resulting new taxa.
a) The rank of the original taxon is to be maintained if all possible. Finerdiscrimination can be achieved by means of the elaboration of subtaxa. For
instance, it is usually less desirable to raise a heterogeneous family to the
rank of superfamily and then to raise the previously recognized subfamilies
to the rank of the families than it is to develop a finer subdivision of the
subfamilies into tribes and genus groups.
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b) In ranking no taxon should fall out of step with its sister groups. Theclassification of fossil humans by certain anthropologists who recognizes
more than 30 genera of fossil hominids is an illustration of an unbalanced
classification.
c) A minimal number of names are desirable. If one adopts informal groupingssuch as species group (instead of a new gens or subgenus) and genus group(instead of a new family, subfamily, or tribe), the same information can be
conveyed without burdening the memory and disturbing the balance of the
hierarchy of categories.
d) An inconveniently large taxon should be subdivided only if it can becleaved that is, if it can be divided into taxa of approximately equal size.
Splitting off a number of monotype genera from a genus with 500 species
would only impede information retrieval.
4. The raising in rank of an existing taxon, e.g., a genus to a subfamily or a subfamilyto a family.
5. The fusion of a several taxa of the same rank and the synonymizing of the taxa withjunior names.
6. The reduction in rank of taxon, for instance that of genus to a sub genus or that of afamily to a subfamily. Such a reduction in rank may lead to a considerable
simplification of a classification.
Such a reduction is necessary in many groups of animals. For instance, there is little doubt
that both birds and fishes are badly oversplit and that natural taxa in these groups are ranked
in categories higher than necessary. Even the specialists concerned admit that there is little
justification for having 412 families of fishes and 171 families of birds. What which of
these families could be reduced to subfamilies? There is no easy answer.
7. The creation of new higher taxon not by raising the rank of taxon but making anentirely new grouping of taxa of the next lower rank. The proposal of a new super
family for a number of existing families or a new order for a series of families
illustrates this procedure.
8. This search for the nearest relative of an isolated taxon and, if this is successful, thestudy the question whether a new taxon of higher rank should be created for the
newly established group of relatives.
Stability:
During such minor improvement activities a determined effort must be made to disturbed
the stability of the currently prevailing classification as little as possible and to maintain, if
nor improve its information retrieval qualities. The successfulness of a classification ascommunication system stands in direct relation to its stability, which is one of the basic
prerequisites of any such systems. The names for the higher taxa serve as convenient labels
for the purpose of information retrieval. Terms such as Coleoptera and Papilionidae must
mean the same thing to zoologist all over the world to have maximum usefulness. This is
even truer for the genus, which is included in the scientific name. The overriding need forstability, dictates that accepted taxa and their names is maintained in all cases except when
they are strongly contradicted by the evidence.
In publishing the classification that has resulted from ones taxonomic studies, one must
present it either as a printed list, a diagram, or both. Both methods of presentation raise
problems.
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The Printed Sequence:
The technology of printing requires a linear one-dimensional sequence for any printedclassification. One species will have to come first and another species last, while all others
will have to be listed sequentially between the first and the last.
An alphabetical sequence is often most useful for information retrieval. Themultidimensional phylogenetic tree with the dimensions of time, space and adaptational
divergence must be converted into a single linear sequence. To do this, the taxonomists
must make some inevitable compromises between various considerations. Most important
among these considerations are the following three;
1. Continuity: Each species is to be listed as near as possible to its closest relatives.2. Progression: Each series of species or higher taxa should begin with the one closest
to the ancestral condition (the most primitive one), to follower by derived taxa
deviate increasingly from the ancestral state.
3. Stability: one should not change previously accepted sequences unless they areproved unequivocally wrong. A classification is a reference system and adoptingundocumented experimental changes can drastically reduce its usefulness,
particularly in a comparison of faunal lists.
Graphical Representation
The deficiencies in printed sequence have led to scientist to represent the information in
diagram form. They are mostly in tree like form with emphasis on the age and prevalence of
each taxon.
Each three schools of macrotaxonomy use different diagrams. Pheneticists use phenogramwhich is the representation of degree of phenetic differences. The cladogram of the Cladists
is a branching diagram of taxa as inferred from synapomophies. It reflects the cladogenesis.
The taxa are delimited by holophyly. The phylogram of evolutionary taxonomists is a
phylogenetic dendrogram in which an attempt is made to represent the taxa by the totality of
their characters, not only their diagnostic ones, and by changing the lengths and angles of
internodes to reflect differing rates of evolution.
Phylogenetic Trees:
A phylogenetic tree or evolutionary tree is a branching diagram or "tree" showing the
inferred evolutionary relationships among various biological species or other entities based
upon similarities and differences in their physical and/or genetic characteristics. The taxajoined together in the tree are implied to have descended from a common ancestor.
In a rooted phylogenetic tree, each node with descendants represents the inferred most
recent common ancestor of the descendants and the edge lengths in some trees may be
interpreted as time estimates. Each node is called a taxonomic unit. Internal nodes are
generally called hypothetical taxonomic units (HTUs) as they cannot be directly observed.
Trees are useful in fields of biology such as bioinformatics, systematics and comparative
phylogenetics.
The idea of a "tree of life" arose from ancient notions of a ladder-like progression from
lower to higher forms of life (such as in the Great Chain of Being). Early representations ofbranching phylogenetic trees include a "Paleontological chart" showing the geological
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relationships among plants and animals in the book Elementary Geology, by Edward
Hitchcock (first edition: 1840).
Charles Darwin (1859) also produced one of the first illustrations and crucially popularized
the notion of an evolutionary "tree" in his seminal book The Origin of Species. Over a
century later, evolutionary biologists still use tree diagrams to depict evolution because such
diagrams effectively convey the concept that speciation occurs through the adaptive and
random splitting of lineages. Over time, species classification has become less static and
more dynamic.
Importance of Sound Classification:
A sound classification is the indispensable basis of much biological research. It is aprerequisite for the application of the comparative methods. Consistent with Simpsons
(1961:7) definition of systematics as the scientific study of the kinds and diversity of
organisms and of any and all relationship among them, the systematist studies all aspects
of living organism. Such studies are often meaningless without a sound classification.
Studies of species formation, the factors of evolution, and the history of faunas areunthinkable unless they are based on sound classifications. Classifications are particularly
important in applied biology. The recognition of this importance explains why even today
so many biologists are dedicated to the task of improving the classification of animals.
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