Evolution and Natural Selection

Goals

1. To understand the difference between evolution and natural selection.

2. To have some appreciation for the influences on Darwin's thinking as he was developing his ideas on evolution.

3. To understand how natural selection can result in the evolution of populations of organisms.

Outline

Evolution

• How living things change genetically over time
• How living things become better adapted to their environments
• How living things have changed since life first appeared about 4 billion years ago
• The mechanism by which change occurs

  The strength of the Theory lies in its ability to explain both the Unity and Diversity of life

The Classical Tradition

• Species are arranged on a scala naturae, or Great Chain of Being, in which all organisms are placed in order of increasing perfection (humans at the top, of course).
• Species are fixed and unchanging.
• The earth is assumed to be quite young (seveal thousands of years)

  This idea began to break down with new discoveries that didn't fit easily into the scala:

• microscopic organisms
• new lands, new creatures
• fossils of extinct plants and animals
• investigations into geology suggesting a very old earth

Lamarck

The first to propose a coherent theory of evolution:

1. Living things modify themselves to function better in their environment (e.g., short-necked giraffes stretch their necks to reach high leaves)
2. These modifications are passed on to their offspring (e.g., these newly longer-necked giraffes have offspring with longer necks)
3. The result over time is a gradual adaptation of an organism to its environment (e.g., after many generations we have modern, long-necked giraffes)

   This mechanism for change depends on the inheritence of acquired characteristics: individuals evolve.

Darwin

Influences

• Lamarck
• Natural theology
• the ideas of Thomas Malthus
• Beetle collections
• Gradualism (Lyell's Principles of Geology)
• The Voyage of the Beagle

Darwin made two points:

1. Evolution has occurred

• all living things derive from a common ancestor
• there is descent with modification
• this descent has resulted in a branching "tree of life" istead of a linear scala naturae

2. Natural selection is the mechanism by which evolution has occurred

The 5 tenets of Darwinism:

1. there is variation among the individuals within a population
2. at least some of this variation can be inherited
3. more individuals are born than will live to reproduce
4. individuals with certain traits have a better chance of surviving and reproducing than do individuals with other traits
5. enormous spans of time are availible for slow, gradual change

Implications:

• The result of natural selection is organisms that are better adapted to their environments (i.e., they FIT better)
• Individuals don't evolve -- populations do!

Evidence for evolution 

biological phenomena that support the idea of descent with modification from a common ancestor

fossil evidience

when arranged in chronological sequence, support a change in living things

• from simple to more complex
• from life-forms different from today's life to similar life-forms

radioisotopes and dating of rocks/fossils

biogeography

evolution on drifting continents

anatomy

homology

homoplasy

vestigial organs

comparative embryology

molecular biology

 molecular similarities confirm evolutionary relationships

Vocabulary

click here to go to evolution vocabulary

For a cool evolutionary link, try the Tree of Life. It's a website at the University of Arizona about the evolutionary relationships among all living things. I suggest you bypass the bacteria and go straight to the eukaryota to get to some of the species you're more familiar with.

Another good link is the Talk Origin Archives -- a good info source for information about evolution, including an intelligent discussion about "Creation Science"