All Along the Ivory Tower: The Modern Synthesis

On The Origin of Species did two things:

1. summarized all of the evidence in favor of the idea that all organisms have descended with modification from a common ancestor, and thus built a strong case for evolution

2. Suggested natural selection as a mechanism of evolution

The Modern Synthesis attempts to explain how evolution works at the level of genes, phenotypes, and populations whereas Darwinism was concerned mainly with organisms, speciation and individuals

1899-1903

Hugo de Vries, William Bateson and Thomas Hunt Morgan

- rediscover and elaborate upon Mendel's findings on inheritance

- establish the concept of genes and the role of mutation in altering genes.

Note: Morgan later works on the genetics of the fruit fly, Drosophila melanogaster, which remains an extremely important research animal today

1. Small (about 3 mm long, allowing for the housing of large populations)

2. Has only 4 pairs of chromosomes (so relatively “simple”)

Note: Drosophila has about 14,000 genes in its chromosomes, whereas humans have about 30,000 genes

3. Has short life cycle (so variations in traits can be observed over many generations on short time-scales)

4. Shows striking variations in features such as eye colour

Why Use Drosophila ?

1908 Hardy-Weinberg Equation

G. H. Hardy (British Mathemetician), W. Weinberg (German physician)

-expresses an ideal distribution of genotypes within a population, assuming that the gene frequencies are known.

Note: gene frequency is the frequency of occurrence or proportions of different alleles of a particular gene in a given population).

Assumptions: 1.no mutation

2.infinitely large population

3.random mating

4.no migration

5.no genetic drift

If all requirements are met, the genotype frequencies should occur in constant

proportions based on the individual gene frequencies:

p2 + 2pq + q2 = 1where frequency (or proportion) of dominant allele = pfrequency (or proportion) of recessive allele = q

What is Genetic Drift ?

Formal Definition: Random changes in the frequencies of alleles or genotypes in a populations due to sampling error of gametes/genotypes in their contribution to next generation

Cam’s Definition: Changes in overall genetic makeup of population due to chance, rather than natural selection.

Abridged Version: Shit happens

The Major Players

R. A. Fisher: statistics of gene flow(mixing of genes between populations)

J. S. Haldane: mathematical models of gene frequency

Sewall Wright: 'genetic drift': evolution not directed by natural selection only

Fisher, Haldane and Wright (1918-1932) use studies of the distribution of genes in populations to recognize that natural selection is the only evolutionary mechanism compatible with the new discoveries in genetics.

Ronald A. Fisher, J. B. S. Haldane, and Sewall Wright:

the integration of:

1. Theory of inheritance

2. Genetics the cause of continuous variation

3. Genetics as a factor that allows variation to be maintained.

Bottom Line: Darwin’s “natural selection” and Mendel’s concept of “particulate inheritance” are compatible

Integration

Theodosius Dobzhansky, Ernst Mayr and George Gaylord Simpson (1937-1944) apply theoretical genetics, population studies, selection theory to real populations

- also incorporate comparative biology, biogeography and paleontology into evolutionary theory

T. Dobzhansky

Putting It Into Practice

E. Mayr G.G. Simpson

Francis Crick and James Watson (1953) outline the coiled, helical structure of DNA, which provides a molecular basis for inheritance and evolution.

Beginning of Reductionist Evolutionary Philosophy:Living Things are Their Genes

Structure of DNA

Key components:

PhosphateSugarNucleotide Bases

AdenineGuanineThymineCytosine

Basic Tenets of Modern Synthesis:

Biological species concept: a species is group of interbreeding organisms

Genotype and Phenotype are separate: genetic makeup and outward physical appearance are related, but are separate entities

Mendel’s Laws of Inheritance: no blending of characters

Random mutations and recombinations are the sources of variation (the raw material for natural selection).

Natural populations are gene pools that contain variation

Natural Selection and Random Genetic Drift both play role (evolution = change in gene frequencies)

Reproductive isolation leads to speciation

The Four Great “Isms”Reductionism:All that shapes an organism is its genes. Populations are simply gene pools.

Panselectionism:All changes are adaptive in some wayNatural Selection is the force that directs such change

ExtrapolationismSmall genetic changes in genotype can be extrapolated indefinitely upward to explain all evolutionary changes

GradualismGradual changes in phenotype over hundreds of generationsProduce new anatomical features and eventually new species

Niles Eldredge Stephen Jay Gould

Punctuated Equilibrium

Boldly stated that speciation is not slow, gradual and ongoing,but rather, occurs in rapid bursts. Long periods of no change (“stasis”) separate bursts of speciation

In other words, “equilibrium” is “punctuated by rapid change”

Phyletic Gradualism vs. Punctuated Equilibrium

speciation

Speciation gradual and ongoing

Speciation rapid and followed by no change (stasis)

speciation

speciation

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A Bit of A Diversion: Heterochrony

Meaning: Different TimeDefinition: a deviation from the typical sequence in the formation of organs or parts.

Popularized by Ernst Haeckel (1834-1919)

Haeckel stated that “ontogeny (life history of an organism)recapitulates phylogeny (evolutionary history of an organism)

You gotta admit, there is a similarity in the embryos !

But he was wrong ! Organisms don’t truly repeat their ancestral stages !Has to do with timing and rate of development

Peramorphosis

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Paedomorphosis

Note that the shape of the last species on the right resembles the larval stage of the first species on the left

Paedomorphosis: trend in development(example: a lineage of trilobites)

An Nifty Example

Significance of Heterochrony to Human Evolution

END OF LECTURE