Variation and Fossil Individuals


Sources of Variation within Populations:

The ultimate goal of taxonomy is the description of the diversity of living things. Without this knowledge, we can barely begin to understand the fossil record, however before attempting to do taxonomy, we must consider the mechanisms that generate diversity among organisms.



A small sample of human variation

Growth Strategies:

After individual variation - an essential driver of evolution, ontogenetic variation is arguably the most significant source of diversity, so its interpretation merits detailed attention. Interpreting ontogenetic status requires first that the growth mode of hard parts be identified. The major growth strategies:

But note that many creatures combine strategies. E.G.:


Isometric scaling of a cube

Allometry:

A distinct ontogenetic issues is that of scaling. Typically, different parts of the organism grow at different rates - i.e. they experience allometric scaling. This results from a simple fact of geometry:

Comparative allometry:


White-tailed deer and cape buffalo - scaling with mechanical similarity

Allometric scaling is also evident in comparisons across taxa.


The "evolution" of Babylon 5's Jason Ironheart from Not Entirely Stable

Growth and evolution:

Remember, ontogeny pertains to the individual, evolution to the lineage. (TV sci-fi plots notwithstanding.) That doesn't mean they aren't connected in any way. Now that we know something about how organisms grow, let's consider how alterations in ontogeny over evolutionary time can drive evolutionary change through the process of heterochrony.

Heterchrony:

Subtle changes over evolutionary time in an organism's developmental timetable are a potent source of overall evolutionary change. This is an idea with a history as long as the study of evolution.

General patterns of heterochrony:


Gould added clarity to these terms by identifying the developmental parameters that could be tweaked to produce either major pattern, and naming the different pathways that resulted. In Gould's system, heterochronic change could result from changes in:

Combining these developmental parameters gives us:

Varieties of Paedomorphosis:


  • Varieties of Peramorphosis:

    Cultural "evolution": Note that paedomorphosis and peramorphosis occur in popular cultural figures.

    All of this has huge consequences for Morphometrics, the quantified statistical comparisions of shape.

    To Syllabus.