Note Conflict Fossils Histograms

Skewed Histograms

Paleontologists have done many computer simulations in order to gain a feel for the range of outcomes of random walks in biodiversity, with both equal and unequal rates of speciation and extinction. Some simulated groups expand to the point of swamping the computer's memory; other groups go extinct rather soon. Extinction of a group is most common when the group starts out small, just as a casino gambler is most likely to go broke quickly if he starts with a small stake, close to the absorbing boundary.

In evolution, a group such as a genus of family must, by definition, start with a singles species. For the fledgling group to survive, the founding species must speciate before it goes extinct. Because new evolutionary groups start small, they usually don't last long. This, in turn, yields an important facet of the history of life: most groups of species have life spans shorter than the average of all groups. Figure 3-2 shows a histogram of life spans of fossil genera. It has a skewed shape, with many short durations and only a few long ones.

The skewed (asymmetrical) shape of variation is typical of important biological properties germane to the extinction question. These include
- number of species in a genus
- life spans of species
- number of individuals in a species
- geographic ranges of species


http://sepkoski.com/sepvv02.cfm?XRPF=VVALEN

Distribution of Length of Lifespans of Genera
    Y-Axis: Number of Genera
    X-Axis: Lifespan of Genus