CHAPTER 8: ONTOGENY AND PHYLOGENY

Von Baer's Rule

suggests that the earlier a trait appears in ontogeny, the higher the taxonomic rank it should signify.

Because the nature of a species includes its ontogeny, and because many changes in shape are accomplished by changes in the programs controlling growth, we say that

"phylogenies are sequences of ontogenies." Thus, ontogenetic studies have implications for the understanding of phylogeny.

Haeckel's Law

(Ontogeny Recapitulates Phylogeny) suggests that we may study ontogenetic development in order to reconstruct phyletic relationships. Organisms that share the same early developmental stages should be closely related.

Adaptation

(functional fitness) must be maintained at every stage of development. Shape changes during ontogeny signal either that the function of a structure is changing or that it must change in order to continue to perform the same function.

Parallel evolution is a pattern in which two closely related lineages tend to show the same morphological evolutionary trends through time.

For example, many of the parallel evolutionary trends that characterize lineages of Cenozoic horses are controlled by allometric growth: elongation of the distal part of the leg, reduction of peripheral toes, elongation and deepening of the jaws to accommodate high-crowned molars with infolded enamel.

Heterochrony is a common expression of allometry, often as a change in timing of the switch from one allometric coefficient to another.

For example, the foraminifer Clavulina starts out triserial in early ontogeny, becoming biserial and finally uniserial. In some lineages of Clavulina, it has been noted that the uniserial portion of the test becomes shorter and may even disappear, as if some descendants just never quite get around to making this change (retardation).

The adults of these populations show many juvenile characters and appear to be retarded in their morphological development relative to those of normal populations. In fact, they deviate less from the logarithmic spiral.

In species populations, individuals vary slightly in many characters, and every population contains a few individuals with slight tendencies toward developmental retardation. In times of stress, such tendencies could be favored by natural selection.

Because each paedomorphic population (race, subspecies) has the same shape trends, this is an example of parallel evolution.

It is also iterative evolution, because the same morphological form is developed over and over again.

For example, many foraminifera have alternating generations in the life cycle. The microspheric phase has a test with a small proloculus (initial chamber), but the test grows larger overall. It reproduces assexually by fission. The megalospheric phase has a test that is smaller overall but has a large proloculus.

It reproduces sexually, producing gametes. In such species we find that the juvenile portion of the megalospheric test shows a much condensed, abbreviated ontogeny as compared to the juvenile portion of the microspheric test (acceleration).

Parallel evolution is a pattern in which two closely related lineages tend to show the same morphological evolutionary trends through time.

Parallel evolution is a pattern in which two closely related lineages tend to show the same morphological evolutionary trends through time.

Because allometric solutions are so simple, they are likely to be independently discovered in different lineages and at different times.

Probably for this reason, parallel and iterative evolutionary patterns are very

HETEROCHRONY

Significant change may also be introduced without any novelty, simply by modifing the developmental timing. A great deal of the morphologic change that we see in phyletic lineages is accomplished by changes in developmental timing.

For example, at many times during the Pleistocene Bermuda land snails have developed local populations (races, subspecies) that show paedomorphosis.

The adults of these populations show many juvenile characters and appear to be retarded in their morphological development relative to those of normal populations. In fact, they deviate less from the logarithmic spiral.

Because allometric solutions to problems of growth and function are very simple mathematically, genetically and physiologically, they may be independently discovered by separate lineages.

Thus, parallel evolution is common.

A species is a population including individuals of all ages and sizes, reproducing itself generation after generation. The ______ of a species includes the programs for growth through all stages, juveniles as well as adults

genotype

Heterochrony

is a shift in the time when a feature appears in ontogeny. The time of its appearance may be shifted earlier or later than the time in the ancestor.

Ontogeny

is the development and growth of the individual. The study of ontogeny can help to explain form. By studying how a structure grows, we may learn its adaptive significance and make conjectures about the evolutionary processes responsible for its production.

Phylogeny

is the origin and evolution of lineages. It is the ancestor-to-descendent component of evolution.

If it is moved later, this is

paedomorphosis, or retardation or bradygenesis or neoteny.

If it is moved earlier, this is

recapitulation, also called acceleration or tachygenesis