The Historical Development of Evolutionary Theory

LIFS 691

Advanced Evolution

What is evolution?

• change in the distribution of phenotypes

• change in genotypic frequencies

• change in allelic frequences

• to unfold, open, or unfold (Webster’s Dictionary)

Mechanisms of Evolution

• Natural Selection (including sexual selection)

• Genetic Drift

• Mutation

• Gen Flow

Natural Selection

• defined as the differential survival and reproduction of heritable phenotypic variants

• without heritability of phenotypes, the process is merely “selection”

Lamarckism was alive and wellin the late 19th century.

Jean-Baptiste Lamarck

(1744-1829)

• Darwin’s theory could not overcome problems created by the theory of Blending Inheritance

• Only about 1/3 of biologists at the time subscribed to Darwin’s theory of Evolution by Natural Selection; many evolutionary biologists still agreed with Lamarck’s ideas.

Gregor Johann Mendel(1822-1884)

Mendel’s garden

“Discovery” of Mendel’s Work

Mendel’s peas

• Provided a theory of particulate inheritance, which seemingly overcame problems imposed by the theory of blending inheritance.

• Unfortunately, many traits are not controlled by a single polymorphic locus; thus, Mendel’s laws did not have immediate generality. Many still doubted natural selection was the primary mechanism of evolution.

The Birth of Mutation Theory

Hugo de Vries (1848-1935)

primroses

• Noted sudden origins of seemingly novel phenotypes after many generations of stability

• Hypothesized that biochemical changes were occurring, which became known as mutations.

(Actually, these changes were the product of hybridization between species of plants, but this was unknown at the time.)

Beginning of NeoDarwinian Theory

Sewell Wright Sir Ronald Fisher(or Bakken’s lost twin?)

An Alternative Synthesis

Richard Goldschmidt

• Some approached evolution theory from an understanding of genetics and development.

• Goldschmidt’s ideas were made unpopular to protect selectionist theories from the threat of mutationist theories. Today, he is most remembered for the concept of a “Hopeful monster,” which was a only a very small part of his contribution to evolutionary biology.

“Competing” Views in Evolution

• Neutral Theory vs. Selectionist Theories

• Adaptationist vs. Anti-adaptationist

• Optimization vs. Quantitative Genetic Theories

Methods for Examining Genes

• Candidate Loci

• Electrophoresis (mid-1960’s)

• Sequencing (mid-1970’s)

• Genomics (mid-1990’s)

“In ‘nothing in biology makes sense except in the light of evolution’, nothing in evolution makes sense except in the light of population genetics.”

Richard Lewontin, Harvard University

“The science of population genetics is the auto mechanics of

evolutionary biology.” Rama S. Singh, McMaster University

Singh (2003), Genome 46: 938-942

Hardy-Weinberg Equilibrium

• No selection• Infinite population size• Random mating• No mutation• Simple genetic system (one locus, two alleles)• Non-overlapping generations

Assumptions:

Hardy (1908), Science

Hardy-Weinberg Equilibrium

p = frequency of allele Aq = (1-p) = frequency of allele a

At equilibrium the frequencies of genotypes is as follows:

AA Aa aap2 2p(1-p) (1-p)2

Hardy (1908), Science