Scott Gilbert’s Developmental Biology is a standard textbook used in college courses everywhere. A beautiful book with great illustrations and clear synopses of classic experiments, it provides an overall summary of what is known about how organisms develop. Yet Gilbert says on page 46 of the eighth edition:

The life cycle can be considered a central unit in biology. The adult form need not be paramount. In a sense, the life cycle is the organism. [Emphasis added.]

It’s true that the life cycle is the central unit of biology, and that in a manner of speaking, the life cycle is the organism. An oak is an oak whether acorn or tree. A human is human whether fetus, child, or adult.

But the adult is paramount in one very important evolutionary respect. It is the adult stage that reproduces. Eggs, embryos, tadpoles, acorns, caterpillars, puppies, and babies do not reproduce. The adult’s ability to successfully reproduce and pass on its genes determines the organism’s evolutionary success or failure.

The reason Gilbert says that adults need not be paramount is because he wants it to be true. He wants embryos and not adults to show how to solve the evolutionary puzzle. Gilbert is of the school called evolutionary developmental biology — evo-devo for short. He believes that if we study how organisms develop, using genetics and other allied tools, we will gain insight into how they evolved. This idea is powered by the observation that many important molecules involved in early development are shared by organisms of all kinds, and that these molecules often have similar functions. I’ve written about that before; in fact I did so just yesterday.

At the end of his book Gilbert writes that the standard evolutionary model of random mutation and natural selection cannot account for the origin of animal form, but that evo-devo can. He puts it this way (p. 749): “While the population genetics approach focuses on the survival of the fittest, the developmental genetic approach to evolution is more concerned with the arrival of the fittest.”

This play on words is based on a statement in a famous book, Species and Varieties: Their Origin by Mutation (1904), by Dutch botanist Hugo de Vries: “Natural selection may explain the survival of the fittest, but it cannot explain the arrival of the fittest.” That I can definitely agree with.

But it is not clear that evolutionary developmental biology can answer any questions about the origin of body plans or organisms either. The same fundamental problem remains. Each stage in an animal’s evolution must be both viable and reproductively successful, and it must confer a selective advantage if it is to persist. Furthermore, each stage can only occur one very small step at a time, one mutation at a time.

The limit on how many mutations can accumulate before a new advantageous function or form “arrives” is four (including any gene duplications or overexpression that is necessary for the mutations to accumulate and have a selective benefit). That won’t pay for much in the way of evolutionary change.

Image: Oak forest, by Margus6 (Own work) [CC BY-SA 4.0], via Wikimedia Commons.