The Structure of Scientific Thought

The structure of evolutionary theory

1. Pattern versus process in evolutionary theory

   1. Two aspects to study of evolution

      1. Pattern: Analysis and reconstruction of the actual historical course of events producing the diversity of life; analysis and reconstruction of genealogical relationships among living things (including those now extinct)

      2. Process: Analysis of the mechanisms responsible for producing evolutionary change, e.g., natural selection, genetic drift, mutation

   2. Theories of pattern

      1. Jean Baptiste de Lamarck

         1. Two aspects of evolutionary change:

            1. Evolution within a lineage

            2. Splitting of lineages

         2. Continual spontaneous generation of “lower forms” (single-celled micro-organisms is necessary to account for the great diversity of bodily organization in animals

      2. Charles Darwin

         1. All living things are ultimately descended from a single common ancestor

         2. All of the diversity of living things is accounted for by descent with modification, i.e. the combination of

            1. Evolution within a lineage

            2. Splitting of lineages

         3. This aspect of Darwin’s evolutionary thinking was accepted very rapidly

2. Theories of process

   1. Lamarck

      1. Intrinsic drive to perfections (complexity) is responsible for evolutionary change

   2. Darwin

      1. Natural selection is responsible for evolutionary change

      2. This aspect of Darwin’s evolutionary thinking was not fully accepted until more than 70 years after the Origin was published, i.e., in the 1930’s.

   3. Confusion in popular press about the nature of debates about evolution: primarily about which process is important, not whether Darwin’s description of the pattern is correct

3. The theory of natural selection

   1. Requirements for evolution by natural selection

      1. Differences among individuals in traits that cause differences in ability to survive and reproduce

      2. These differences are hereditarily transmissible

   2. The sickle-cell anemia example

4. Essentialism versus population thinking

   1. Essentialism: Aristotle

      1. Every object has a natural state. Its natural state is associated with its essence or form.

      2. Natural change consists of change toward the natural state.

      3. Any other kind of change is violent change.

      4. Natural change is good.

      5. Variation is the result of disturbing forces, i.e., the result of violent change.

      6. Scientific investigations should focus on uncovering the unchanging, eternal essence.

   2. Population thinking: Darwin

      1. The natural state of a population is variability.

      2. Homogeneity of a population requires the operation of some force.

      3. No particular state of an individual can be regarded as the “natural” state; all states found are equally natural.

   3. Human nature

      1. Aristotelian view: corresponds to the essential properties associated with being human, e.g., rational thought

      2. Darwinian view: Does it make sense to think that there is such a thing as human nature if no particular features can be regarded as more natural than others?