DARWINIAN NATURAL SELECTION

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3.1  NATURAL SELECTION: DARWIN'S FOUR POSTULATES

1. individuals within species vary
2. some of these variations are passed on to offspring
3. individuals vary in their ability to survive and reproduce
4. individuals with the most favorable adaptations are more likely to survive and reproduce.

• natural selection produces descent with modification
• Darwinian fitness: ability to survive and reproduce in a particular environment
• adaptation: a trait that increases the fitness of an individual.

3.2  THE EVOLUTION OF BEAK SHAPE IN GALAPAGOS FINCHES

• 13 species of Galapagos and one Cocos finches [Figs. 3.1, 3.2]
• Geospiza fortis on Daphne Major is a seed eater.
• beak size and shape correlate with seed size
• size and shape of the beak has an important consequence for fitness.

Testing Postulate 1: Are Populations Variable?

• beak depth (and many other characteristics) are variable
• 1976 measurements form a bell shaped curve [Fig. 3.3]

Testing Postulate 2: Is Some of the Variation Among Individuals Heritable?

• is the variation caused by environment or genetics?
• heritability of beak depth in Geospiza fortis [Fig. 3.4].
• issues that complicate how heritabilities are measured [Box 3.1]

Testing Postulate 3: Is There an Excess of Offspring So That Only Some Individuals Survive to Reproduce?

• A drought in 1977 produced a dramatic selection event [Fig. 3.5]
• even in normal times--89% of Geospiza conirostris die before breeding
• in stable populations, each parent leaves on average of one offspring in spite of astonishing reproductive capacity (biotic potential) [Table 3.1].

Testing Postulate 4: Are Survival and Reproduction Nonrandom?

• a distinct subset of the population survived better [Fig. 3.6]
• change in average beak depth [Fig. 3.7a] is correlated to change in seed hardness [Fig. 3.7b]
• following heavy rains selection was for smaller body and beak size.

Did Evolution Occur

• evolution is a change in traits between generations
• Figure 3.6 shows selection within a generation
• Table 3.2 shows evolution: increased size between generations.
• increase in weight, wing length, bill width, bill depth

3.3  THE NATURE OF NATURAL SELECTION

1. Natural Selection Acts on Individuals, but Its Consequences Occur in Populations
   1. individuals and their genes are not modified; selection determines which genes will survive
2. Natural Selection Acts on Phenotypes, but Evolution Consists of Changes in Gene Frequencies
   1. unless the genotype frequency changes there is no evolution 
3. Natural Selection is Backward, Not Forward Looking
   1. each generation is the result of selection on the previous generation
4. Natural Selection Can Produce New Traits, Even Though It Acts on Existing Traits
   1. selection works on variation already present.
   2. tripling the oil content in corn by artificial selection over 60 generations [Fig. 3.8]
   3. selection can lead to new characteristics by changing functions of preexisting traits, genes, etc. preadaptations, exaptions---the panda's thumb [Fig 3.9]
5. Natural Selection is Not Perfect
   1. Geospiza fortis individuals with narrow bills were also favored in the 1977 drought; can better concentrate seed cracking forces.
   2. however body size, bill depth, and bill width are all controlled by the same genes and correlate with each other
   3. therefore selection for large bills results in larger body size and wider bills, even if narrower bills should have been favored
6.  Natural Selection is Nonrandom, but It is not Progressive
   1. selection is not teleological, there is no conscious intent (either by the organism or a creator), but it does increase adaptation to the environment.
   2. evolution has tended to increase complexity, but some organisms have changed very little (e.g., bacteria) or become less complex (e.g., tapeworms).
   3. all life has evolved for the same amount of time--not higher or lower organisms
7. Fitness is Not Circular
   1. fitness is testable, not a tautology (circular reasoning--those who survive)
   2. research can determine why certain nonrandom groups are favored
   3. fitness can be measured by counting offspring, observing which individuals survive selection events.
8. Natural Selection Act on Individuals, Not Groups
   1. altruism does not occur in nature.
   2. behavior maximizes individual fitness

3.4   THE EVOLUTION OF DARWINISM

There were three serious problems with Darwin's original proposals.

1. Darwin was unaware of mutation, and did not give a correct interpretation of the source of variation.
2. Darwin did not know the nature of heredity and how variation were passed on
   1. blending inheritance was the accepted mode of inheritance. 
   2. traits which appear to show blending (e.g., human skin color) are controlled by many genes and can be subject to selection [Fig. 3.10]
   3. Darwin adopted Lamarckian explanations in later editions of the Origin;
3. Lord Kelvin determined that the earth was 30 million years old
   1. Kelvin's calculations did not take radioactivity into account

The Modern Synthesis

1. variation is created by mutation and recombination
2. genes are passed on independent of other genes
3. excess reproductive capacity and competition for resources
4. individuals with advantageous alleles survive and reproduce.

3.5  THE DEBATE OVER SCIENTIFIC CREATIONISM

• History of the Controversy
• Perfection and Complexity in Nature
• natural theology versus the blind watchmaker
• evolution of the mollusk eye [Fig. 3.11]

Other Objections (that are untrue)

1. evolution is not science because it can not be tested
2. the Earth is too young for natural selection to operate
3. evolution violates the second law of thermodynamics
4. no one has ever seen a new species evolve, therefore evolution is based on faith

RESOURCES

1. Evolutionary Theory  from the Evolutionary Psychology site at the University of California at Santa Barbara.
2. Science and Creationism - From the National Academy of Sciences.
3. Evolution and Natural Selection from the University of Michigan Global Change web page
4. Evolution--UCMP, Berkeley
5. The panda's thumb--Athro, Limited. All Rights Reserved. Written by Paul J. Morris mole@morris.net and Susan F. Morris

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