Evolution Basics

Lecture 01

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History of Evolution is told in 3 parts: Before Darwin, Darwin and Russell, After Darwin

Biological Evolution is change in organisms measured by comparing among generations

• which organisms to compare and what sort of changes to measure are large questions that we will cover in some depth

Before Darwin

Evolution not defined as it is today

• Progress important part of western intellectual thought
• Debate whether or not species change and the degree of change possible

Lamarck documented change but left the nature of the change still open to debate

• Proposed a kind of Transformism based on progress
• Species created at some time in past and then changed because of 2 mechanisms
  • one internal, progression so that the species changed along a pre-determined path over time
  • second external, not progressive, species react to environment and changes developed in adult passed to the offspring (famous "acquired characteristics"
• no extinction or splitting of lineages in original version of Lamarck's "Zoological Philosophy" but later accepted extinction
  • fossils simply earlier versions of current species

Cuvier established extinction as a fact through the use of fossil data and further concluded that a catastrophe was responsible

• opposed evolution and felt that species complexity too great to allow change without failure (version of intelligent design)
• Catastrophism vs Uniformitarianism (Hutton and Lyell)

Darwin and Russell

• Documented variation and proposed a mechanism of evolution (not only mechanism!)
• both extinction and origination of species part of evolution
• viewed evolution as a branching process
• Natural selection tied change to environment, so not progressive in the Lamarckian sense

After Darwin

• Darwin and the roles of gradualism and saltation in evolution
  • Thomas H. Huxley supported the role of natural selection but favored the large change over the gradual change, an area of disagreement between him and Darwin
  • Huxley felt that Darwinism (gradual change) would produce a fossil record of gradual change, but the record favored gaps between species (Lyell agreed with Huxley)
  • Huxley did what Darwin was reluctant to do - he directly confronted those who dismissed Darwin's work
• Francis Galton (Darwin's cousin) also differed from Darwin on importance of "discontinuous" variation
  • Galton's Law of Ancestral Heredity (also known as Galton's Law of Regression or as Galton's Law)
    • Phenotype determined by separate contributions of the parents, grandparents, great grandparents, great,great... and so on
    • This "ancestral contribution" meant that, no matter what the parents happened to be like, the overall effect was to produce offspring that were closer to the species mean than were the parents
    • To Galton, this law meant that continuous variation "regressed" to the species mean and could not produce evolutionary novelty
  • Galton thought that a separate genetic mechanism produced large phenotypic leaps in some offspring and that evolution was driven by these leaps
• A Great Historical Oddity
  • Galton's work was the basis of two schools of genetics with very different views of Darwinism (= evolution by means of small, gradual change in this sense - not evolution by means of natural selection, which is how it is usually meant today!)
  • Biometric School (Karl Pearson, WFR Weldon) felt that Galton's law was the basis of evolution by gradual change
  • Mendelian School (William Bateson) felt that discontinuous genetic variation was the basis of evolutionary change
  • The school adopted Mendel's work as it was based on discontinuous variation and championed Mendelism as a result
  • The founders and leading lights of these schools committed a seemingly inevitable but still sad error
    • They failed to see the worth of the opponents arguments and never asked the question, what if both of us are partially right and partially wrong?
    • Collaboration would have produced great results
  • The argument dominated the field of evolution from 1900 to the 1920's
• Modern Synthesis of Darwinism and Mendelism
  • RA Fisher (theory of evolution in large populations), JBS Haldane (evolution at a single locus), Sewall Wright (evolution in subdivided populations) - remember, these characterizations are more like caricatures than portraits - responsible for reconciling genetics and evolution
  • Reconciliation is now called the Modern Synthesis
  • Fisher began it with his demonstration that complex traits could be described by Mendelism if more than one locus affected the outcome (a notion proposed by Yule 10 years previously but not pursued at that time)
    • Fisher devoted much time to developing methods for conducting experiments and analyzing the results
  • Haldane and Fisher worked to incorporate genetic phenomena, like linkage and dominance, into a mathematical description of evolution within populations
  • Wright began his studies with an examination of inbreeding's effects on populations

Evolutionary Genetics

• DNA and Inheritance
  • Levels of expression (Genome [Sequence, Chromosomal], Transcriptome [Alternative Splicing, Sequestration], Proteome [Domains, Polymeric Proteins, Post-Tranlational Processing], Phenotype [Epistasis, Development])
  • Duplication, Relaxation, and Saltation
• Mutational Rate (damage repair, transcription error)
  • Missing lethals
  • Germ Line
  • Per Generation, per nucleotide, per  gene, per genome
• Diploidism
  • Dominance
• Particulate Mendelism vs. Blending Inheritance

Evidence for evolution

• Evidence only evidence when it supports one set of contentions and is inconsistent with another set of contentions, so we need to define these sets of contentions
  • Ridley likes  three possible general theories for evolution
    • Special Creation - each species created separately from other species and do not change
    • Transformism - species created separately but can change
    • Evolution - new species arise from pre-existing species, species have ancestors and descendents
• Process of change within species is easy to observe (drug resistance, bottlenecks, Peppered Moth, changes in species with documented histories (House Sparrow), artificial selection
• Ring Species
• Hybrid Species
• Homology (Uracil??)
  • Frozen Accident
  • Common descent vs. common function (Analogy)
• Fossil Record

Natural Selection

• Conditions needed for natural selection
  • Variation in Characters that affect Fitness
  • Biased Inheritance of Characters that affect Fitness
• Fitness
  • Not all organisms born live to have offspring or the same number of offspring
  • Success is relative
• Adaptation (the thing, not the process)
• Basal Role of variation
  • How much variation is present?
  • Origins of variation
  • Is there variation in success?
• Nature of Selection
  • Directional, Stabilizing, and Disruptive (Diversifying)
• Sufficiency of natural selection as the mechanism of evolution
• Natural Selection and Adaptation

Last modified on January 19, 2008