The development of any complex organism reveals a weave of relationships between the organism's phenotype and its genotype. I will focus on four of these relationships and argue for a new interpretation of the genetic data:
First, although the skin, liver, muscle, brain and other cell types that compose a body are morphologically and functionally diverse, they are not genetically diverse. They all inherit the same genotype from their common ancestor, a zygote. That is, during the descent with modification from a zygote to its descendant cell types, DNA is conserved. Diverse phenotypes do not require diverse genotypes.
Second, Because all cells in a body (excluding parasites and symbionts) inherit the same genotype, they necessarily inherit many genes that they do not need. Skin cells don't express genes specific to the functioning of liver cells, for example. Neither do muscle cells express genes specific to the functioning of brain cells. And so on. The excess DNA in each cell type includes genes needed to create and operate all the other types. But from the point of view of a given type of cell, the DNA for the other types is junk.
Third, the expression of genes in any particular cell type, and the timing of their expression, is controlled by other genes that act as on/off switches. This is how a single genotype expresses multiple phenotypes (skin, liver, muscle, etc.) in a single body -- by turning various genes on and off here and there at various times.
Fourth, a zygote carries genes that will be required by its descendants. The zygote anticipates the needs of the skin, liver, muscle, and other descendant cell types and carries their genes, even if the zygote itself does not express them.
These, then, are some of the characteristics of development. They include conservation of DNA, junk DNA, switches that conrol the expression of cellular phenotypes, and genes that anticipate the needs of descendants.
Now, when we look at the genetics of evolution, we find all the same hallmarks. Since genetic sequencing and analysis have come online, the parallels between development and evolution--between ontogeny and phylogeny--have come sharply into focus. A new discipline within evolutionary biology, called evolutionary developmental biology, or Evo Devo, is trying to shoehorn the new genetic data into the old, Darwinian, paradigm. But comparative genomics is rewriting the book of evolution into something that readers of the first edition might not recognize as the same work.
Consider:
First, insects, fish, birds and primates are morphologically and behaviorally dissimilar, but not because their genotypes are to any comparable degree dissimilar. Genetic sequencing and analysis tell us that these creatures all inherited same basic genetic toolkit from a common ancestor. That is, despite all the phenotypic variation, DNA is conserved across species during evolutionary descent.
Genes for limbs are pretty much the same from limbed species to limbed species, whether the wings are on a dragonfly, a bat, or a bald eagle. The underlying genes are about the same. Evolution, like development, conserves DNA. Researcher Sean Carroll, an architect of Evo Devo, comments, “Comparison of genomes tells us that not only do flies and humans share a large set of developmental genes, but that mice and humans have nearly identical sets of about 25,000 genes, and that chimps and humans are almost 99 percent identical at the DNA level. The common tool kit and the great similarities among different species genomes present an apparent paradox.” (All Carroll quotes are from his book, Endless Forms Most Beautiful, The New Science of Evo Devo.) Yes, the great similarities do present a paradox. Because they make evolution look like a large-scale development.
