Sunday, July 19, 2009

Darwins Correlations

First Correlation:

Observation: All natural populations have the reproductive capacity to exceed the resources required to sustain them.

Observation: Despite their enormous reproductive potential, populations do not keep increasing indefinitely in size over time. (To give a simple example, a single sea star can release 2,500,000 every year, but oceans obviously do not fill with sea stars.)

Observation: In natural environments, food supplies and other resources do not increase explosively. In fact, they remain much the same over time.

Observation: The limited availability of resources puts limits on population growth. (There is only so much water, nutrients, and growing space for a plant population; only so many plants to feed an elephant population; and so on.)

Inference: When a population outstrips the supplies of necessary resources, there must be competition among its members for the resources that are available. Because of this competition, not all of the individuals who were born will themselves survive and reproduce.

Second Correlation:

Observation: The members of a natural population show great variation in their traits, and much of the variation is passed on through generations (it has a heritable, or genetic, basis.)

Inference: Some heritable traits are more adaptive than others. They give the individual a competitive edge in surviving and reproducing.

Inference: Over the generations, there is natural selection - a measurable difference in survival and reproduction among individuals that differ from one another in one or more traits.

Inference: Thus the character of the population changes over time - it evolves - as some forms of traits increase in frequency and others decrease or disappear.