Biology 101A
Life on Planet Earth
25 February, 1998

Dawkins' River Out of Eden, part 2
DNA & Evolution

"The purpose of this chapter is to destroy the argument that complicated contrivances have to be perfect if they are to work at all."

- Dawkins, page 61

1. Principle of Variation.

2. Principle of Heredity.

3. Principle of Selection.

• Observations of Variation
• Morphological variation

• Chromosomal Polymorphism

• Immunological Polymorphism

• Protein Polymorphism

• DNA Sequence Polymorphism
• Restriction-Site Variation
• Tandem Repeats (including VNTRs)
• Complete Sequence Variation

• Quantitative Variation

2b. The Sources of Variation

• Variation from Mutations

Figure 26-9. The change over generations in the frequency of a gene A due to mutation from A to a at a constant mutation rate (m = 10^-5)

• Variation from Recombination

This is by far the quickest source of variation. Two chromosomes with "normal" survival from Drosophila can account for as much as 75% of the genetic variation seen in the entire population in the wild.

• Variation from Migration

• The Origin of New Functions

Figure 26-10. Frequency distribution of haploid chromosome numbers in dicotyledonous plants.

• Inbreeding and Assortive Mating

Two deviations of random mating:

1. Inbreeding - when mating between relatives occurs more commonly than would occur by pure chance.

2. Assortive mating - when individuals tend to choose each other as mates because of their degree of resemblance to each other (note that this does not necessarily mean they are related).

3. Heredity
DNA is the common vehicle used to store genetic information. In nearly all cases, the DNA alone (not RNA, proteins, or any other polymer or chemical) is what is passed on from generation to generation. The DNA is Dawkins' "River Out of Eden". Of course, the information in this "digital river" is stored by the nucleotide sequence. The human chromosome # 1 contains perhaps 100,000,000 nucleotides, in a single, very long double-stranded DNA molecule.

4a. Selection

• Fitness and the Struggle for Existence


• Two Forms of the Struggle for Existence


1. Frequency independent - the organism struggles with the environment directly.
2. Frequency dependent fitness - the organism struggles with other organisms within the same population, as well as with the environment; this is a type of "selection to blend in with the crowd". Figure 26-14. (a). A blue jay eating a monarch butterfly, which (b) induces vomiting in the jay. Because of this experience, the jay later will refuse to eat a viceroy butterfly that is similar in appearance to the monarch, although jays that have never tried monarchs will eat the viceroys.

• How Selection Works

Figure 26-20. Differences in horn morphology in two geographically separated species of rhinoceros: (a) the African rhinoceros; (b) the Indian rhinoceros.

4b. Artificial Selection

Figure 26-22. Changes in average egg production in a chicken population selected for its increase in egg laying rate over a period of 30 years.

Random Events

Figure 26-24. The appearance, loss, and eventual incorporation of new mutations during the life of a population. If random genetic drift does not cause the loss of a new mutation, then it must eventually cause the entire population to become homozygous for the mutation (in the absence of selection). If the figure, 10 mutations have arisen, of which 9 increased slightly in frequency and then died out. Only the fourth mutation eventually spread into the population.

5. The Origin of Species
Speciation - the origin of a new species - is the origin of a group of individuals capable of making a living in a new way and at the same time acquiring some barrier to genetic exchange with the species from which it arose.

References:

Richard Dawkins, "River Out of Eden - A Darwinian View of Life", (Weidenfeld & Nicolson, London, 1995).

Griffiths, J.F., Miller,J.H., Suzuki,D.T., Lewontin,R.C., Gelbart,W.N. "An Introduction to Genetic Analysis", (sixth edition, W.H. Freeman & Company, New York, NY; USA 1996). (Chapter 26 is the source of many of the Figures - especially anything that has "Figure 26-xx".)

Niles Eldredge, "FOSSILS - The Evolution and Extinction of Species", (Princeton University Press, Princeton, New Jersey, 1991).

Stephen Jay Gould, "FULL HOUSE - The Spread of Excellence from Plato to Darwin", (Three Rivers Press, New York, 1996).

Richard Dawkins, "Climbing Mount Improbable",
(W.W. Norton & Co., New York, 1997).