Timing is Everything: Morphogenesis, Heterochrony and Evolution

Morphogenesis is the origin of form. (From the Greek morph meaning "shape" (NOT change) and genesis, meaning "origin"). It is the process by which embryos grow into their proper form.

As an embryo grows from a zygote through its various forms...

Genes in each cell turn on and off, changing the identity and fate of each cell.
If the timing of these changes is modified, even slightly, profound effects on the finished product can arise.
In short, if ontogenies diverge, then adult morphologies may also diverge.

Heterochrony (from the Greek hetero meaning "other" and chronos meaning "time") describes a change in the timing of ontogenetic events between two taxa. These can be the result of relatively small genetic changes that may not even be alterations in DNA sequence, but in the timing of particular genes being expressed during development.

Isometric vs. Allometric Growth

Many animals undergo isometric growth as they mature from new hatchling to adult. This means that all the body parts grow at approximately the same rate, and the adult proportions are not significantly different from those of the juvenile. For example, see our pal Batrachoseps, one of the few salamanders that has a terrestrial (not a gilled, aquatic) larva:

A heterochronic change can result from a mutation that causes the rate of one cell line of the body to develop at a rate different from that of other cell lines in the body. This can result in allometric growth (from the Greek allo meaning "different" and metr meaning "measure" (and also, interestingly "womb")).

In a species that exhibits allometric growth, different cell lines/body parts grow at different rates (relative to an ancestral, isometrically growing form) during development from juvenile to adult.

Humans are a good example of a species that undergoes allometric growth. The head, limbs, and body grow at different rates, resulting in a human adult with proportions completely different from those of the newborn baby:

Hold that thought.

Paedomorphy as a result of Heterochrony

In animals, there are two major classes of cells:

A somatic cell is defined as one that makes up part of the body. It will not become a gamete (sperm or egg).
A germline cell is one that has the potential to undergo meiosis to become a haploid gamete.

In animals, the body becomes reproductively mature at a very specific stage of somatic development.
In some species, a heterochronic mutation can cause the organism to become reproductive relatively sooner than an ancestral species.
This can happen in one of two ways:

Neoteny: Germ line cell development proceeds at the same rate as in an ancestral species, but somatic cell development is retarded as compared to the rate in that same ancestor.
Progenesis: Somatic development proceeds at the same rate as in an ancestral species, but germ line cell development is accelerated as compared to the rate in that same ancestor.

Here's an analogy that might help you visualize this.

The difference between these two processes is subtle, but important to note when comparing a taxon's development to that of a closely related taxon, or to that of a hypothetical ancestor.

In either case, the body is still in its juvenile form when the animal becomes sexually mature, and body development then STOPS.

The resulting condition is known as paedomorphy (from the Greek paed, meaning "juvenile" and morph, meaning "form"): a reproductive adult that has a more juvenile form of the ancestral species.

Examples of paedomorphic organisms:
1. The Common Mudpuppy (Necturus maculosus) is a salamander that retains its juvenile gills as an adult
Most salamander species have aquatic larvae that lose their external gills when they reach adulthood: Juvenile Tiger Salamander (Ambystoma mabeei):

Adult Tiger Salamander:

The mudpuppy is paedomorphic with respect to other salamander species: It retains its external gills as a reproductive adult due to either neoteny or progenesis:

2. Many domestic dog breeds (Canis lupus familiaris), derived from wolves (Canis lupus), exhibit paedomorphy with respect to adult wolves.

3. Homo sapiens, whose prolonged brain development period and relatively flat face reflect a prolonged juvenile period, relative to that of our closest relatives, the chimpanzees (Pan paniscus and P. troglodytes)

Let's face it:

Although some humans tend to look more like our ancestors than others do.

Remember:

Paedomorphy/osis: the condition of an adult organism retaining juvenile features as an adult

Progenesis and Neoteny are two processes (two different examples of heterochrony) by which this state can occur.

Paedomorphy isn't the only possible result of Heterochrony

Other phenotypic differences between closely related species also can be a result of differences in developmental timing. In any species:

A characteristic/feature may appear relatively early in embryo development compared to an ancestor's embryo development or
A characteristic/feature may appear relatively late in embryo development compared to an ancestor's embryo development.

Color pattern differences in animals can be a result of heterochronic changes that affect pigment deposition during ontogeny, which is one of several hypothetical explanations for...