19th Century Theories

Jean-Baptiste Lamarck proposed a theory of inheritance of acquired characteristics. Environmental pressures and internal "desires" bring about changes in the individual's body. Offspring can inherit these changes. This provided a force for perfection up the Chain of Being.

Charles Lyell's book Principles of Geology proposed that subtle, repetitive processes of change, had shaped the Earth. He challenged the view that Earth was only 6,000 years old.

Charles Darwin and the Voyage of the Beagle

For example, Darwin noted examples of organisms whose existence challenged the concept of creation. In Argentina, the fossils of extinct glyptodonts were similar to the living armadillos he observed.

Thomas Malthus, a clergyman and economist, wrote an essay that Darwin read on his return to England. It argued that as population size increases, resources dwindle, the struggle to live intensifies and conflict increases. Darwin suspected that any population has the capacity to produce more individuals than the environment can support. He reasoned that the variations in traits might affect an individual's ability to secure resources, and to survive and reproduce.

On the Galapagos Islands, he had observed that the finches varied slightly from each other but all resembled the mainland finches to some extent. He attempted to correlate variations in their traits with environmental challenges.

Darwin reasoned that those animals now present could have descended from those living in the past, but with changes. Darwin gradually developed a theory of natural selection to explain how such changes could come about.

Alfred Wallace was a naturalist who arrived at the same conclusions Darwin did. He wrote to Darwin describing his views and prompted Darwin to finally present his ideas in a formal paper.

Darwin and Alfred Wallace published their ideas jointly in 1858. Darwin published his theory in book form in 1859. The book laid out in great detail his evidence in support of the theory of evolution by natural selection. If one species can evolve into another, there should be transitional forms. When Darwin published his work, no such forms were known. He was hoping that "transitional" animals would be found to strengthen his ideas. Almost seventy years later the study of genetics led to substantiation of Darwin's ideas.

This animation (Audio - Important) describes the Galapagos Islands.

This animation (Audio - Important) describes Darwin's finches.

Populations

Biological evolution does not change individuals. It changes a population. Traits in a population vary among individuals. Evolution is change in the frequency of traits.

The "Gene Pool"

This animation (no audio) describes alleles.

Sources of variation (caused by genes, manifested in phenotype) include:

• Gene mutations that create new alleles.
• Crossing over at meiosis I leads to new combinations of alleles.
• Independent assortment in meiosis I mixes paternal and maternal chromosomes in the gametes.
• Fertilization mixes alleles from two parents.
• Changes in chromosome structure or number can lead to the loss, duplication, or alteration of alleles.

REVIEW: The place Darwin visited on his trip around the world that had the greatest impact on his thinking was
Answer the Galápagos Islands.