Probabilistic Outcomes

We can treat evolution as a statistical process. Namely, two species meet, exchange genes, produce offspring that may or may not survive, depending upon the survival rules of the ecosystem (we will simulate this in a while).

These survival rules are generally unknown to the external observer. All the external observer can measure is the form of the distribuition of outcomes. From that, one has to infer survival rules.

The key to understanding this is that Populations evolve , not individuals. In order to understand evolution, it is necessary to view populations as a collection of individuals, each harboring a slightly different sets of traits and abilities.

No one single organism is ever typical of the entire population unless there is no variation within that population.

The overall process of evolution can be summarized in three steps:

• Genes mutate (via random means).
• Individuals are selected.
• Populations evolve (mostly randomly).

Evolution requires genetic variation. In order for continuing evolution there must be mechanisms to increase or create genetic variation and mechanisms to decrease it. Mutation is a change in a gene. These changes are the source of new genetic variation. Natural selection operates on this variation.

These variations are random on short timescales, and modified by catastrophe (e.g. asteroid impacts) on long timescales.

The point here is that random variations modify the distribution of population traits.

Selection:

Some types of organisms within a population leave more offspring than others. Over time, the frequency of the more prolific type will increase. The difference in reproductive capability is called natural selection.

The most common action of natural selection is to remove unfit variants as they arise via mutation. In other words, natural selection usually prevents unfit genes from increasing in frequency.

Natural selection can be broken down into many components, of which survival is only one. Many other factors can also come into play. Overall, one can identify three major forms for population evolution:

Three examples of selection are shown before stabilizing, disruptive and directional. The black dots are individuals that die out before passing on their genes.

Stabilization: Removes the extreme ends of the distribution. Moves everything closer to the middle.

Directional: Depopulates preferentially one extreme side of the distribution (usually the disfavorable side)

Disruptive: Occurs when individuals at both ends of the curve have a higher survival probablity than those in the middle. Ultimately drives mutation and new, isolated species. Argueably this is what happens on Earth.