all the perennial miracles [Nature] offers, ... perhaps the most worthy
of admiration is the development of a plant or of an animal from its
of Algorithmic Growth in Plants
that emulate Leaf Growth and Tree Branching
Algorithmic Tree and Root Branching
|Example of Topological Growth
Note: For the articulated trilobite legs see the Wiki article
on the Arthropod Leg.
|Humans contain 39 Hox genes in four clusters|
|HOXA||chromosome 7||HOXA1, HOXA2, HOXA3, HOXA4, HOXA5, HOXA6, HOXA7, HOXA9, HOXA10, HOXA11, HOXA13|
||chromosome 17||HOXB1, HOXB2, HOXB3, HOXB4, HOXB5, HOXB6, HOXB7, HOXB8, HOXB9, HOXB13|
|HOXC||chromosome 12||HOXC4, HOXC5, HOXC6, HOXC8, HOXC9, HOXC10, HOXC11, HOXC12, HOXC13|
||chromosome 2||HOXD1, HOXD3, HOXD4, HOXD8, HOXD9, HOXD10, HOXD11, HOXD12, HOXD13|
Hox Gene Mapping
(Anterior to Posterior)
Favier & Dollé, Development Functions of mammalian Hox genes
Molecular Human Reproduction 3 #2 (1997)
[Non?]Evolution in Gene Expression 1.1
Evo-Devo is the biology of evolutionary development, a branch of experimental biology that has come into being within the past few decades.
The remarkable fact that has been uncovered in evo-devo is that (virtually) the same small packages of genes, linked to specific body parts, occur across broad swaths of species. It is how these genes are expressed during regulation that determines the physical form of end product. Thus, (virtually) the same small package of "eye genes" underlies both the human's simple eye and a fly's compound eye -- the different end result comes about by how these genes are expressed.
Regulatory genes control the basic defining features of animal body shapes in particular (as distinguished from plants), through the homeobox (hox) genes. Such things as bilateral symmetry are controlled by these genes. It is gene regulation that results in the left side growing as the mirror-image (more or less) of the right side, even though the mirror image objects (human hands, for example) are physically separated as they grow. It is gene regulation that forms a fly's compound eye and a human's simple eye, and both grow in mirror-image pairs: the underlying genes are the same.
Is commonality "proof" of descent? Not for a creationist -- any more than the physical similarity of appendages, for example. The interesting issue is how gradual changes can be passed on to other generations? What is the mechanism? How much variability is built in, and how expressed? etc. The parameters of change are interesting1.2.
The question of where the packages came from in the first place? How are they made? They first appear in the fossil record suddenly and fully formed, in the Cambrian Explosion.
Body Plans. Both plants and animals grow in a building-block fashion. The body plans involve algorithmic growth (typical of plants), modular construction, and the use of repeated parts with modification (typical of animals).
The remarkable insight of evo-devo is that behind the modules and repeated parts are small packages of genes that are essentially the same for all animals that have the corresponding body part. For example, virtually all animals that have eyes, whether simple or compound, have the same small package of eye genes; all animals that have jointed appendages (legs, antennae, etc.) have the same small package of appendage genes, and so on for other sensor systems: for nerves, muscles, digestive and circulatory systems, etc. The well-known homologous systems (Figure 4) have at root a shared gene package: the differences between the species are not so much the result of new genes, but of how those genes are expressed.
This observation has radically changed the former understanding of Convergent Evolution.
Evo-devo concerns not the creation of new genes, but of new ways to express a highly conserved set of genes. The question of how the genes got there in the first place is thus separate from the question of how the gene expression changed over time, just as the question of how life first arose is separate from the question of how life evolved.
The essential features of evo-devo -- the "simple rules that shape animal form and evolution?" are:
(1) Each specialized animal body structure and system is developed from a small and highly conserved gene package. The differing morphologies are determined by gene expression controlled through the homeobox development genes.
(2) The gene package for a body structure anticipates a broad range of end morphologies.
(3) The development genes include a built-in ability to vary the gene expression within certain limits;
(4) There is some mechanism to pass on variations in gene expression to succeeding generations -- in part this is through Mendelian genetics, but it is also through expression gene parameters that are preserved in the genetic information (DNA and the "cloud" of accompanying information).
The net effect of Evo-Devo is to replace the concept of convergent evolution (repeated appearance of the same organ in widely different species) with the concept of a common ancestral package of homeobox genes01.03. As one scientist expressed it, "Long before animals with limbs (tetrapods) came onto the scene about 365 million years ago, fish already possessed the genes associated with helping to grow hands and feet."01.04
• Animal species across many phyla use the same gene packages to code for eyes, appendages, and other body parts and systems, even though the appearance of those body parts in the adult may be radically different. The different end products are determined by development rules -- how, when and where genes are turned on and off -- not by the genes themselves. Homologous body parts arise because of the use of similar genes.
• The development (homeobox or hox) genes that control gene expression, are likewise essentially the same across many animal phyla.
• The identification of the gene packages and hox genes is unequivocal, and has been determined by extensive laboratory experimentation and DNA sequencing.
- To a natural evolutionist, these common gene packages prove descent from a common ancestor.
- To a creationist, they are the expected result of a common creator who re-uses the genetic material.
The origin of the hox genes and the gene packages is unknown to science, although the first sudden appearance of the body parts in the Cambrian era (about 540 Ma) is well attested -- trilobites, for example, had all of them -- and so the existence of the gene packages and the hox genes at that time can be inferred by analogy to modern species. Information that determines exactly how a given species expresses the gene packages is located in portions of the DNA that do not code for genes. Additional information may reside with the accompanying molecules that are present in the fertilized egg when development begins.