http://en.wikipedia.org/wiki/Transitional_fossil
OK, heres a simple (if you can call it that) explanation of a "missing Link"
"The Missing Link" is a popular term used for transitional forms. The term is especially used in the regular media, many times implies that there was a single link missing to complete adjoining species, The term arose in the 19th century where the awaited discovery of a "missing link" between humans and "lower" animals such as chimps was considered to be the final proof of evolution.
Transitional fossils are the remains of a life form that illustrates an evolutionary transition. It can be identified by having certain primitive (plesiomorphic) traits in comparison with its more derived relatives, such as defined in the study of cladistics.
http://www.palaeos.com/ http://www.answers.com/ http://www.biology-online.org/dictionary
Plesiomorphic means widely shared Features more in a particular group. These primitive features for a group cannot provide evidence for the group. An evolutionary trait that is homologous (Biologically speaking is similar in structure and evolutionary origin, though not necessarily in function, as the flippers of a seal and the hands of a human.) within a particular group of organisms but is not unique to members of that group and not used as a defining character for the group. For example, vertebrae are found in zebras, cheetahs, and orangutans, but the common ancestor in which this trait first evolved is so distant that the trait is shared by many other animals. Therefore, possession of vertebrae sheds no light on the phylogenetic relations of these three species.
Phylogenetic relates to phylogenesis, or the race history of a type of organism.
According to modern evolutionary theory, all populations of organisms are in transition. Therefore, a "transitional form" is a human construct that represents a particular evolutionary stage, as recognized by hindsight.
Charles Darwin in The Origin of Species claims that there was a lack of transitional fossils. Darwin also stressed that the lack of it was the most formidable obstacle to his theory. Yet, the discovery of Archaeopteryx two years later was hailed as a triumph for supposition of common descent. Significant gaps remain in the fossil record, which is a considerable problem for evolutionists. Most scientists accept the fact that the rarity of many extinct animals fossils will always remain unknown.
Various Sources
Cladistics is a philosophy of classification that arranges organisms only by their order of branching in an evolutionary tree and not by their morphological similarity. As the end result of a cladistic analysis, tree-like relationship-diagrams are made with an approach to the classification of living things in which organisms are defined and grouped by the possession of one or more shared characteristics that are derived from a common ancestor and that were not present in any ancestral group (as envisioned by Charles Darwin's idea of “descent with modificationâ€Â). For example, all vertebrates make up a clade; all tetrapods (vertebrates that have four limbs with wrists, ankles, toes, and fingers) form their own clade within the vertebrate clade. In this example the vertebrate clade would be considered “primitive†and the tetrapod clade “derived.†In paleontology the characters are primarily skeletal. Cladistics is especially significant in paleontology, as it points out gaps in the fossil evidence. It is also felt to be more objective than fossil study, which of necessity extrapolates from a limited number of finds that may or may not be representative of the whole. The different so-called 'natural' or 'monophyletic' groups form nested units that do not overlap. Within cladistics there is thus no longer a
transition between groups, but a differentiation within groups. In this context, transitionals can be defined as the different branches of a cladogram between one particular branch and the group that is placed at the top of a lineage.
http://www.answers.com/
Morphology in biology, the study of the size, shape, and structure of organisms in relation to some principle or generalization. Whereas anatomy describes the structure of organisms, morphology explains the shapes and arrangement of parts of organisms in terms of such general principles as evolutionary relations, function, and development.
http://en.wikipedia.org/wiki/Transitional_fossil
Transitional forms vs. intermediate forms
The terms 'transitional' and 'intermediate' are for the most part used as synonyms to each other. However, a distinction between the two can be madein two ways, they are: "Transitional" can be used for those forms that do not have a significant amount of unique derived traits that the derived relative does not possess as well. In other words, a transitional is morphologically close to the actual common ancestor of itself and the derived relative. "Intermediate" can be used for those forms that do have a large number of uniquely derived traits not connected to its derived relative. According to this definition, Archaeopteryx, which does not show any derived traits that more derived birds do not possess as well, is transitional. In contrast, the Duck-billed Platypus is intermediate because it retains certain reptilian traits no longer found in modern mammals and at the same time possesses a lot of derived traits of its own, as a highly specialized aquatic animal. Following this definition, all living organisms are to be regarded as intermediate forms to some other related life-forms. There are many species alive today that can be considered to be transitional between two or more groups.
Critics of evolution often state that there are no known transitional fossils per say. What does represents a transitional feature? Some creationist argue that is no fossils are found with partially functional features. Scientists say it is plausible, that a complex feature with one function can adapt a wholly different function through evolution. For example, a wing, might originally have only been meant for gliding, trapping flying prey, and or mating display. Nowadays, wings can still have all of these functions, but they are also used in active flight. This explanation seems a bit redundant, and really doesn't address the question.
Although transitional fossils elucidate the evolutionary transition of one life form to another, they only exemplify snapshots of this process. Due to the special circumstances required for preservation of living beings, only a very small percentage of all life-forms that ever have existed can be expected to be discovered.Thus, the transition itself can only be by fossils, but it will never be known in detail. Critics of evolution often cite this argument as being a convenient way to explain off the lack of 'snapshot' fossils that show crucial steps between species.
The theory of punctuated equilibrium developed by Stephen Jay Gould and Niles Eldredge is often drawn into the discussion of transitional fossils. The theory, pertains only to well-documented transitions within taxa or between closely related taxa over a geologically short period of time. These transitions, usually traceable in the same geological outcrop, often show small jumps in morphology between periods of morphological stability. To explain these jumps, Gould and Eldredge envisaged comparatively long periods of genetic stability separated by periods of rapid evolution.
en.wikipedia.org/wiki/Taxon
A taxon (plural taxa), or taxonomic unit, is a name designating an organism or group of organisms assigned a rank and placed at a particular level in a systematic hierarchy reflecting evolutionary relationships. Distinction's are to be made between taxa/taxonomy, refering to biological names and the rules of naming and classification/systematics, the rank ordering of taxa according to presumptive evolutionary (phylogenetic) relationships.
