# Introduction to Non-Genetic Darwinism/Darwin's Theory of Natural Selection
## Aims

* To learn the fundamental principles of Darwin's theory of natural selection.
* To understand the historical context of this theory.
* To provide a window into Darwin's thought processes.
* To understand that natural selection is by no means the only cause of evolution.

## Lesson

Darwin was one of many early contributors to evolutionary biology. His observations on the natural world, ranging over a span of decades, have been preserved in his books about subjects ranging from barnacles to island erosion. It is generally agreed that his most important work was the development of the theory of natural selection, at around the same time as fellow naturalist Alfred Russell Wallace was developing a similar theory.
The theory of natural selection arose within a rich historical context. The concept of evolution, i.e. that species are not fixed but change over time, predated Darwin. In fact, Charles Darwin's grandfather Erasmus Darwin was a prominent advocate of the theory of evolution. The mechanism of how evolution occurred, however, remained unknown. Darwin's theory of natural selection was one of many hypotheses about this mechanism, and was not widely accepted by evolutionary biologists until the 20th century.
Darwin's theory of natural selection is:
* There is variation in traits.
* Some specific variations in traits cause individuals to produce a larger number of surviving offspring.
* These variations are heritable.
* The end result is that specific trait variations increase in prevalence in a population.
* Eventually, most or all individuals in a population may have the more successful variations. As a result, the physical characteristics of the population may change.
A competing theory, of the inheritance of acquired characters, had been proposed in ancient times. It is commonly associated with another naturalist named Lamarck, a contemporary of Erasmus Darwin and a prominent champion of the theory. Inheritance of acquired characteristics was discredited much later, by the discovery of the mechanisms of heredity (these were not known until the 20th century!) and by laboratory studies showing that acquired characteristics were not inherited.
Natural selection is by no means the only cause of evolution. For example, migration can lead to new characteristics being introduced into a population. Drift is the chance inheritance of a particular characteristic by a disproportionate number of individuals in a given generation. (As you might imagine, the effect of drift is particularly strong in small populations.) These factors were little appreciated until the Modern Evolutionary Synthesis of the 20th century, a scientific movement that did not begin until decades after Darwin's death.
However, in the end of the twentieth century, several scientists, one of them Steven Jay Gould, proposed that the Modern Evolutionary Synthesis and its unification of Mendelian Genetics and Darwinian Natural selection was not enough to account for several phenomena of biology, like niche construction, epigenetic inheritance and particularly, the deep homology which several phyla of animals exhibit. Gould, along with other evolutionary biologists called for an "Extended Synthesis," a theoretical framework that would again unite Ecology with Embryology and the previous Neodarwinist Modern Synthesis. This has been met by criticisms by several biologists like Richard Dawkins, and even among philosophers of science like Daniel Dennett.