The Darwinian Revolution

Archbishop James Ussher of Ireland (1581-1656): The earth was created on October 22, 4004 BC.

Usher’s “History of the World”

 " ... we find no vestige of a beginning,— no prospect of an end." 

James Hutton, Theory of the Earth (1795)

Table Mountain

Cape Town, South Africa

Conclusions:

1. The earth is much older than we thought.

2. Different creatures have inhabited the earth at different times.

Problem: How did this happen?

Two Theories:

•Catastrophism

•Descent with Modification (Evolution)

Darwin’s Three Observations:The geometrical increase of populations. If left unchecked, the size of the population of a single animal or plant species would increase until the world is overrun.

Variation. Individuals within the same species are not always exactly alike—they differ in their particular characteristics.

Inheritance. Individuals tend to pass on their own particular characteristics to their descendants.

Darwin’s Argument(Created from Animals, pp. 35-6)

(1) Organisms tend to reproduce in such numbers that, if all survived to reproduce again, they would soon overrun the earth.

(2)This does not (and could not) happen. No species can continue to multiply unchecked.

(3) It follows that a high percentage of organisms must die before they are able to reproduce.

(4)Therefore, there will be a "struggle for existence" to determine which individuals live and which die. What determines the outcome of this struggle? What determines which live and which die?

There are two possibilities: it could be the result of random causes; or the reason could be related to the differences between particular individuals.

(5) Darwin admits that sometimes it is random; that is, the reason one organism survives to reproduce, while another does not, may sometimes be attributable to causes that have nothing to do with their particular characteristics. One may be struck by lightning, while another is not, and this may be mere luck.

(6) But sometimes it is a matter of differences between individual organisms. Consider:

•There are differences ("variations") between members of species.

•Some of these differences will affect the organism's relation to its environment, in ways that are helpful or harmful to its chances for survival.

•Therefore, because of their particular characteristics, some individuals will be more likely to survive (and reproduce) than others.

(7) Organisms pass on their characteristics to their descendants.

(8) Therefore, the characteristics that have "survival value" are passed on, and tend to be more widely represented in future generations, while other characteristics tend to be eliminated from the species.

(9) In this way, a species will be

modified--the descendants of the

original stock will come to have

different characteristics than their

forebears.

(10) When enough of these

modifications have accumulated,

we call the result a new species.

Alfred Russel Wallace (1823-1913)