Animal Systematics:
The goal of animal systematics is to arrange animals into group, that reflect evolutionary relationship.
Monophyletic group: When groups include single ancestral species and all of its descendents such a group is called monophyletic group, in searching out monophyletic groups, taxonomists look for animal attributes called characters that indicate relatedness.
Character: A character is virtually anything that has a genetic basis and can be measured from an anatomical feature to a sequence of nitrogenous bases in DNA or RNA.
Polyphyletic groups: They have members that can be traced to separate ancestors. Since each group should have single ancestor, a polyphyletic group reflects insufficient knowledge of the group.
Paraphyletic group: It includes some, but not all, members of a lineage. Paraphyletic groups result when knowledge of the group is insufficient.
Evolutionary Systematics: It is the oldest of three approaches. It is also called traditional approach. A basic assumption of evolutionary systematics is that organisms closely related to an ancestor will resemble that ancestor more closely than they resemble distantly related organisms. Two kinds of similarities between organisms are recognized.
(1) Homologies are resemblances that result from common ancestry and are useful in classifying animals. Example is the similar arrangement of bones in the wing of a bird and the arm of human.
(2) Analogies are resemblances that result from organisms adapting under similar evolutionary pressures. This process is sometimes called convergent evolution. Analogies do not reflect common ancestry and are not used in animal taxonomy e.g. similarity between wings of birds and insects in an analogy. Evolutionary systematics often portray the results of their work on phylogenetic trees where organisms are grouped according to their evolutionary relationships.
Numerical Taxonomy: It emerged during 1950s and 1960s and represents opposite end of the spectrum from evolutionary systematics. The founders of numerical taxonomy believed that the criteria for grouping taxa had become too arbitrary. They tried to make taxonomy more objective.
Taxonomists use mathematical models and compute aided techniques to group samples of organisms according to overall similarity. They do not attempt to distinguish between homologies and analogies. Difference between evolutionary systematic and numerical taxonomy is that numerical taxonomists limit discussion of evolutionary relationships to closely related taxa. Numerical taxonomy is the least popular of three taxonomic schools. However; all taxonomists use compute programme that numerical taxonomists developed.
Phylogenetic Systematics: It is third approach to animal systematic. The goal of cladistics is similar to evolutionary systematics the generation of hypotheses of genealogical relationships among monophyletic groups of organisms. Cladists contend that their methods are more open to analysis and testing and thus are more scientific, than those of evolutionary systematics.
Like evolutionary systematics, cladists differentiate between homologous and analogies. They believe that homologies of recent origin are most useful in phylogenetic studies.
Symplesiomorphies: Characters that all members of a group share are referred to as symplesiomorphies. (sym=together+plesio=near+morphe=form).
These characters are homologues that may indicate a shared ancestry but they are useless in describing relationships within the group.
Out group: To decide what character is ancestral from a group of organisms. Cladists look for a related group of organisms called an out group that are included in the study group.
Synapomorphies: Characters that have arisen since common ancestry with the out group are called derived characters or synopomorphies (syn=together, apo=away and morphe=form).
Cladogram: A cladogram is interpreted as a family tree depicting a hypothesis regarding monphyletic lineages. New data is the form of newly investigated characters or reinterpretation of old data are used to test the hypothesis, the cladogram describes. The tumicates and cephalochordates are an out group for the entire vertebrate lineage. Derived characters are
(1) Extra embryonic membrane is a synapomorphy used to define the clade containing the reptiles, birds and mammals and absent in any of the fish taxa or the amphibians.
(2) Derived characters, the shell, distinguishes turtles, from all other members of the clade
(3) Skill characters distinguish the lizard/crocodile bird lineage from the mammal lineage
(4) and haris, mammary glands and endothermy is a unique mammalian character combination. A synapomorphy at one level of taxonomy may be a symplesiomorphy at different levels of taxonomy. Extra embryonic membrane is synapomorphic for reptiles, birds and mammals because it is ancestral for the clade and cannot be used to distinguish among members of these three groups. Zoologists widely accept cladistcs. This acceptance has resulted in some non traditional interpretations of animal phylogeny. Generations of taxonomists assigned class level status (Aves) to birds and reptilian to reptiles.
Cladistic analysis: It has show that birds are more closely tried by common ancestry to alligators and crocodiles than to any other group.
According to cladistics interpretation birds and crocodiles should be assigned to a group that reflects this close common ancestry. Birds would become a subgroup within larger group that included both birds and reptiles. Crocodiles would be depicted more closely related to birds than they would be to snakes and lizards.
Traditional evolutionary systematics maintain that the traditional interpretation is still correct because it takes into account the greater importance of key characters of birds (e.g. feathers and endothermy) that make the group unique. Cladists support their position by pointing out that the designation of key characters involves value judgments that cannot be tested. Debates are the fuel that force scientists to examine old assumptions. Animal systematics is certain to be lively and exciting field in future years.
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