Taxonomy: Classifying Life

At least 1.7 million species of living organisms have been discovered, and the list grows longer every year (especially of insects in the tropical rain forest). How are they to be classified?

Ideally, classification should be based on homology; that is, shared characteristics that have been inherited from a common ancestor. The more recently two species have shared a common ancestor,

• the more homologies they share, and
• the more similar these homologies are.

Until recent decades, the study of homologies was limited to

• anatomical structures and
• pattern of embryonic development.

However, since the birth of molecular biology, homologies can now also be studied at the level of

• proteins and
• DNA
  • DNA-DNA Hybridization
  • Chromosome Painting
  • Comparing DNA Sequences

Anatomical homology: an example

The figure shows the bones in the forelimbs of three mammals: human, whale, and bat (obviously not drawn to the same scale!). Although used for such different functions as throwing, swimming, and flying, the same basic structural plan is evident in them all. In each case, the bone shown in color is the radius.

Body parts are considered homologous if they have

• the same basic structure
• the same relationship to other body parts, and, as it turns out,
• develop in a similar manner in the embryo.

It seems unlikely that a single pattern of bones represents the best possible structure to accomplish the functions to which these forelimbs are put. However, if we interpret the persistence of the basic pattern as evidence of inheritance from a common ancestor, we see that the various modifications are adaptations of the plan to the special needs of the organism. It tells us that evolution is opportunistic, working with materials that have been handed down by inheritance.

Embryonic Development

The embryonic development of all vertebrates shows remarkable similarities as you can see from these drawings (supplied by Open Court Publishing Company). The drawings in the top row are of the embryonic stage called the pharyngula. At this stage ("I") they all contain a:

• notochord
• dorsal hollow nerve cord
• post-anal tail, and
• a series of paired branchial grooves.

The branchial grooves are matched on the inside by a series of paired gill pouches. In fishes, the pouches and grooves eventually meet and form the gill slits, which allow water to pass from the pharynx over the gills and out the body.

In the other vertebrates shown here, the grooves and pouches disappear. In humans, the chief trace of their existence is the eustachian tube and auditory canal which (interrupted only by the eardrum) connect the pharynx with the outside of the head.

Recapitulation

The idea that embryonic development repeats that of one's ancestors is called recapitulation. It is often expressed as "ontogeny recapitulates phylogeny"; that is, embryonic development (ontogeny) repeats phylogeny (the genealogy of the species).

This is a distortion of the truth. It implies, for example, that early in our embryonic development we go through a fishlike stage. We do not. Rather, we pass through some (not all) of the embryonic stages that our ancestors passed through. Therefore, we find that the more distantly related two vertebrates are, the shorter the period during which they pass through similar embryonic stages (fish and human) and vice versa (fish and salamander).

We should also keep in mind that embryonic development prior to the pharyngula (stage I) may also be very different in the different groups. For example, while the pharyngulas of the human and the salamander look quite similar, their earlier development, starting with their fertilized eggs, are very different.

The idea that "ontogeny recapitulates phylogeny" was proposed over a century ago by the biologist Ernst Haeckel. He also made the drawings on which the drawings above are based. Periodically, people rediscover that in making them, he altered certain details to emphasize his theory. Though they are schematic, the story they illustrate here has stood the test of time.