The Evolution of Morphology

As we said above, evolution occurs when individuals carrying some particular genetic variation have more offspring than others, and over the course of many generations, out-compete their fellows. We also said that the source of genetic variation is mutation--changes in DNA. The three sections, "Genes and Morphology," "Limb Development," and "Internal Controls in Developing Systems," provide a brief summary of how genes can control the shapes of organs, limbs, and the whole organism. By linking these different concepts, we can understand how evolution of morphology can occur.

A genetic mutation--a change in DNA--can alter the function of a protein if it occurs within the part of the gene that is the code for the protein. A mutation can also change the time or place that the gene is activated, if it occurs in a gene's control elements. Any of these kinds of mutations can alter the morphology (shape) of an organism. The internal controls of development ensure that an altered body part still integrates properly with the rest of the organism.

This means that evolution of morphology can occur through the mutation of single genes. It is not necessary for all of the genes to mutate at once.

Thus, it requires only minor alterations in the developmental controls in early limb development to cause differences in overall limb morphology that we see as seemingly quite dramatic, such as those illustrated below.

These considerations of the limbs of existing vertebrates provides us with insights into the evolution of limbs. Fossil evidence suggests that land animals are the descendents of lobe-finned fish similar to the coelacanth. Unlike most fish today, these fish (both the living ones and the fossilized ones) display bones in their fins that are remarkably similar to the bones in modern animal limbs. We can understand, based on the molecular and developmental biology of limb development, how mutations in the genes that control fin (limb) development could, over the course of millions of years, result in limbs such as we would find on an amphibian.

As described by Carl Zimmer in At the Water's Edge--how life came ashore and then went back to sea again, fossils of animals from Greenland give us insights into this transition. Plant fossils associated with the fossils of these animals indicate that they lived in swampy environments, somewhat similar to Mangrove swamps today. In this environment, large fish would have difficulty swimming among the tangled vegetation. However, pushing with their fins would be very effective. Pushing provides the selective pressure, which would enable the occasional individual with slightly stronger fins to catch prey more easily, and escape predators more readily. Over the course of numerous generations, stronger fins would become the norm in the population.

This example of limb evolution, under-pinned by an understanding of limb development, provides insight into several principles of evolution.

- Evolution results from the increase in frequency of particular genetic variants in a population, at the expense of other genetic variants.

- Genetic variation results from mutation of DNA.

- Mutations in DNA affect the activity or expression pattern of proteins--micromachines that carry out the mechanics of life.

- Evolution usually occurs by modification of pre-existing structures, rather than the appearance of altogether new structures. In the case of limb evolution, the pre-existing structures were the fins of lobe-finned fish.

- Changes to one part of a complex structure, resulting from mutation, can often be accommodated by compensatory changes in the rest of the complex structure, not by additional mutation, but through internal control mechanisms that operate during embryo development.

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Last updated: 31 December 2005
Comments: Jose Bonner, OSO
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