Developing the Theory of Evolution

Evolution is the core theme of biology

“Nothing in biology makes sense except in the light of evolution”

Theodosius Dobzhansky

Pre-Darwin

Naturalists of the 1700swere pre-occupied with naming new

speciesaccepted a limited time scaleassumed the fixity of speciesused newly invented microscopes and

telescope to look at embryos and starsrecognized that fossils existed of

animals unknown in their explorations

Plato (427-347 BC)

Stated that variations in plant and animal populations as being imperfect representations of ideal forms

Only the perfect forms of organisms were real: idealism, essentialism philosophy

Aristotle (384-322 BC) - Plato’s student

Believed all living forms could be arranged on a scale of increasing complexity “scala naturae” (scale of nature), with no vacancies and no means to improve on this ladder of life.

species are fixed, permanent, and do not evolve (Stability of Species)

Natural Theology: based on Judeo-Christian culture (old

testament) Argued that adaptations in organisms

were planned by the creator.- each and every species designed for a particular purpose

Forms today’s Intelligent design theory

Thinkers of the 19th Century

European colonialism leads to discovery that the number of species is very large.

Maybe species can change after all? Although this was discussed in numerous

circles, it was contrary to religious teachings and dismissed as heresy.

Georges Cuvier (1769-1832):

Founder of Paleontology (the study of fossils)

Opposed evolution

Thought that boundaries between fossil layers corresponded to catastrophic events such as Noah’s flood or droughts

Developed the theory of catastrophism

The oldest fossils are in the oldest strata

James Hutton:(1726 - 1797) Scottish Geologist

In 1795 suggested Gradualism - profound change is the cumulative product of a slow but continuous process.

He looked at a present day process like erosion by water and believed that these processes operatingover millions of years could have createdthe geologic features we see today.

Charles Lyell (1797-1875):

Uniformitarianism –An embellishment on Hutton’s gradualism, geological processes are so uniform that their rates and effects must balance out through time

Suggested the Earth was hundreds of millions of years old.

Darwin took only three books with him on the Beagle.One was Lyell’s book Principles of Geology

•Jean Baptiste Lamarck (1744-1829)

- The Inheritance of Acquired Characteristics

- environmental pressures require animals to strive(unconsciously) toward higher branching pathways of perfection - Believed that evolution responded to organisms “felt needs” i.e. ‘use and disuse’: examples: biceps of blacksmith, giraffes neck

- inheritance of acquired characteristics: modifications acquired during a lifetime can be passed on to offspring- Although wrong, his thinking was visionary. (Environment)

Erasmus Darwin (1731-1802)

Charles Darwin’s grandfather observed adaptations of all

kinds including protective coloration

noted intricate web of ecological relationships among different forms of life

estimated the antiquity of the earth at “millions of ages”

recognized that through “a certain similitude on the features of nature … that the whole is one family of one parent”

believed in acquired heritable characteristics

Natural Selection

Charles Darwin (1809-1882) originally intended to

study medicine at Edinburgh; later went to Cambridge

left to go as a naturalist on the voyage of the Beagle in 1831 to South America

DARWIN Darwin used 2 ideas from Hutton and Lyell:

If geological changes results from slow continuous changes then earth must be older than 6000 years (natural theologian calculation)

Very slow and subtle processes persisting over a great length of time can cause substantial change

•In 1831 he left on a 5 year voyage on board the Beagle to survey the coast of South America.

•He stopped off at the Galapagos Islands for five weeks where the diversity of the plants and animals amazed him.

•In 1836 he returned to England

•In 1859 published On the Origin of Species

•What ideas influenced Darwin?

Darwin saw variation within species on the different Islands.

But didn’t fully understand what he saw till he returned to England

Darwin’s Finches

Thomas Malthus published his Essay on the Principles of Population - “survival of the fittest” shows the tendency of life to multiply

faster than its food supply, which leads to a struggle for existence

The Economist Malthus explained population growth to Darwin Populations breed rapidly But populations don’t grow unchecked

(Limiting Factors) Organisms need to breed to maintain

their species This with his observations, he could

see now how variations could arise within species.

Comparison of Lamarck’s Theory with that of Darwin

Darwin’s Theory of Natural Selection

All organisms exhibit variabilityAll organisms reproduce more offspring than

surviveTherefore, it must be that:Those individual variants best fitted to their

environments surviveThose less well fitted fail to reproduceThe characteristics thus favored by selective

pressure are passed on to the next generation

The End