Evolution: Palaeontology

Basic Ideas: Definition of mosaic and transitional forms

Content

In the following text, we define the terms mosaic form and transitional form.  Examples then illustrate why it is necessary to differentiate these two terms.

Opposing contentions

“Mosaic form” as a descriptive term

“Transitional form” as an interpretive term

When can a form be called transitional?

Transitional forms and the phylogenetic taxonomy system

Opposing contentions

If a general evolution of all living things is assumed, then throughout history there must have been many transitional life forms which display a blueprint that makes a bridge between one life form and another.  Contentions as to whether any such forms have ever been found vary widely. Some say hundreds of evolutionary links existed; others are of the opinion that not a single one has been found. These differing opinions can be partly explained by the fact that there is no consensus about what features an animal has to have, in order to be interpreted as a connecting link. The requirements of a transitional species are often not being defined in detail. Such a definition is not easy after all. In the following text, we make some proposals for such a definition.  To make the discussion mostly factual, descriptive and interpretive terms will be distinguished as well as possible.

“Mosaic form” as a descriptive term

“Mosaic form”: a creature has a combination of physical characteristics which each generally belong to different groups of creatures. An appropriate example is the well-known Archaeopteryx (fig. 17): The Archaeopteryx has characteristics of reptiles such as the long vertebrate tail and the toothed mandible (which present day’s birds do not have), as well as characteristics of birds like having feathers. Thus we have a mosaic of characteristics or a mosaic form presented by this creature. One can call this an “intermediate form” as well because its composition of characteristics stands somewhere in-between two other forms or grouped forms (although not exactly in between the two groups).

An even more impressive mosaic is exhibited by the present day duck-billed platypus: It has characteristics of reptiles (combined orifice for excrement and genitals, lays eggs), characteristics of mammals (hair, lactiferous glands), duck-like characteristics such as a beak webbing and a flat tail. Since it has hair and lactiferous glands, in the broadest sense, this animal is classified as a mammal, in its own branch of so-called “monotremes”.

Both the terms “mosaic form” and “intermediate form”, should be understood in a purely descriptive manner. They do not describe the actual transition or link. Whether an intermediate form might be interpreted as an evolutionary connecting link, a stopover in an evolutionary transition needs its own justification. A mosaic-like distribution of characteristics within an intermediate form can be initially considered under certain circumstances merely as a possible indication of an evolutionary transition.

Mosaic forms exist in abundance both as fossils and as living creatures.

“Transitional form” as an interpretive term

“Transitional form” or “connecting link”: a form situated on its way from one basic type (or more generally from one typical type) to another. Interpreting Archaeopteryx as a transitional form means that its ancestors belonged to the group of reptiles and its followers to “modern” birds. Whether a species is a mosaic form can be determined immediately by combining its characteristics. This is the result of direct observation. Whether a species represents a transitional form is not a result of observation, but of interpretation based on the theory of common descent. How mosaic forms can be interpreted within the context of creation theory is shown in the chapter: basic / general types of mosaic forms.

When can a form be called transitional?

That mosaic forms cannot be interpreted as transitional forms without having been examined can be shown especially well by the example of the duck-billed platypus (fig. 18). Between which two evolutionary groups does it belong? If it would be on its way from a reptile to a mammal with a placenta, then the bird like features of a horned beak and a flattened tail would not fit. That is why theoretically, the duck-billed platypus can not be seen as a transitional species, rather it is placed on a side branch. Generally we can conclude that a transitional form within a phylogenetic tree must fit into the space which lies between the two groups which need to be connected. Obviously this is not the case with the duck-billed platypus as most evolutionists agree.  The Archaeopteryx, is commonly interpreted as a transitional form, however, it also has several characteristics that do not fit (LINK to Archaeopteryx). 

A further criterion for the question whether a mosaic form can be interpreted as a transitional form must also be considered. The evolutionary view expects that intricate organs would evolve step by step. There should be examples of transitional forms with the details of an organ or form changing into another form For instance, predecessors and transitional forms of bird feathers should exist. Some think that feathers are derived from Reptilian scales. The difference between these two structures is enormous. Based on what can be recognized from fossil Archaeopteryx feathers, their feathers are the same as modern day bird feathers. In this case, the second criterion is not met. Some dinosaurs do have feather-like structures that are possible candidates for transition of individual characteristics. These discoveries are dealt with later. This page focuses on the fundamental aspects.

We can summarize as follows:

Mosaic forms                          Transitional forms

Intermediate form                    connecting link

descriptive                               interpretive

combines characteristics of      fits without contradiction into a
different groups                       phylogenic taxonomy

                                                have transitions in single characteristics

The problem is illustrated in fig. 20.

Why are transitional forms missing? Missing suitable evolutionary transitional forms are often justified by the claim that transitions take place relatively fast, within small populations and within a small geographic area. That is why there are so few transitional examples found in the fossil record.  This is the case of the so-called “punctuated equilibrium” (punctualism). The plausibility of these arguments is discussed later.

Transitional forms and the phylogenetic taxonomy system

In the modern phylogenetic system (reconstruction of the genealogy of all species) the terms “mosaic form” and “transitional form” have no meaning (Cladistics or biological systematic classification). This is not surprising because phylogeny presumes a general evolution of all living beings. On this assumption, it establishes its procedure of building up the historical genealogic tree of life forms.

Mosaic forms which are not suitable as transitional forms can always be integrated into a genealogical reconstruction. This is expressed by the appearance of convergences as discussed in the article Similarities in Morphology and Anatomy. For example the form “M” of fig. 19 can naturally be integrated into the scheme. Nevertheless it does not change the fact that “M” cannot be interpreted as a transitional form between “A” and “B”. In addition, in this case one has to assume that certain (quite complex) characteristics have evolved at least twice autonomously in different evolutionary lines (fig. 21). When cases like that appear, it is an evolutionary problem. This issue is raised in the chapter “morphology/anatomy”.

Comment to fig. 21: The cladistical approach would put “Ü” at the end of its own branch, not on the branch. However, this is not about cladistics, but about the clarification of the terms “mosaic form” and “transitional form”. An alternative to the dual autonomous development of a characteristic would be a loss of a characteristic that was present in the base form.

Translator: Olivier Muff, 27.04.2010

Author: Studiengemeinschaft Wort und Wissen

 
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