Making Sense of Biology
The Evidence for Development by Design
by Jonathan Wells
A developing embryo is an amazing thing to watch. First, a single cell divides into many cells; then, with uncanny precision, those cells rearrange themselves into the basic shape of the animal, forming a gut in the process; and finally, small groups of cells form specific structures such as eyes and limbs.
As a developmental biologist, I study embryos primarily by perturbing their development. I am constantly impressed by their resilience: despite my interference, a surprising number of them develop to adulthood. Remarkably, although my interference may introduce various deformities, the basic endpoint of development never changes. If they survive, fruit fly eggs always become fruit flies, frog eggs always become frogs, and mouse eggs always become mice. Not even the species changes.
So every embryo is somehow programmed to develop into a particular species of animal. For centuries, embryologists have been trying to understand the nature of this developmental program. Now, finally, many think they have succeeded.
In this essay I will describe the understanding of genetic programs that is currently most popular, and present evidence against it. I will then argue that its popularity depends largely on its logical connection with the neo-Darwinian theory of evolution. Neo-Darwinism itself, however, is based less on scientific evidence than on a philosophically motivated desire to exclude the idea that living things are designed. I will conclude by arguing that this exclusion is unwarranted and that acknowledging the possibility of design would contribute to future progress in the study of embryonic development.
Genetic Programs: Theory & Counterevidence
In 1865, Gregor Mendel attributed patterns of inheritance in pea plants to factors that were later called genes. When Watson and Crick discovered the structure of DNA in the 1950s, Mendelian genetics found a molecular basis, and in 1973 Sydney Brenner suggested that embryonic development might be directed by a genetic “program” encoded in DNA.
During the 1970s and 1980s, developmental geneticists made significant progress in understanding the role of genes in fruit fly development. Some of the genes they identified are involved in patterning the embryo from head to rear, others are involved in patterning it from back to front, and still others are involved in specifying the identities of particular cells. Since 1990, developmental geneticists have made similar progress with other organisms, including mammals, and the idea that development is directed by a genetic program is now widely held.
Widely, but not universally. According to a small but growing number of biologists, there is considerable evidence that genes do not control development. For example, when an egg’s genes are removed and replaced with genes from another type of animal, development follows the pattern of the original egg until the embryo dies from lack of the right proteins. The “Jurassic Park” approach of putting dinosaur DNA into ostrich eggs to produce a Tyrannosaurus rex makes exciting fiction, but it ignores scientific fact.
What about mutating the DNA instead of replacing it completely? Using a technique called “saturation mutagenesis,” biologists have found that mutations in developmental genes often lead to death or deformity, but they never produce changes that benefit the organism. Furthermore, DNA mutations never alter the endpoint of embryonic development; they cannot even change the species. An embryo needs the right genes to make new proteins, and its development suffers without them; but being dependent on genes is not the same as being controlled by them. A house under construction needs suitable building materials, but they do not determine its floor plan.
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