Download Paedomorphosis

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
Why Use Drosophila ?
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
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 = 1
where frequency (or proportion) of dominant allele = p
frequency (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.
Integration
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
Putting It Into Practice
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
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:
Phosphate
Sugar
Nucleotide Bases
Adenine
Guanine
Thymine
Cytosine
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 way
Natural Selection is the force that directs such change
Extrapolationism
Small genetic changes in genotype can be extrapolated
indefinitely upward to explain all evolutionary changes
Gradualism
Gradual changes in phenotype over hundreds of generations
Produce new anatomical features and eventually new species
Punctuated Equilibrium
Niles Eldredge
Stephen Jay Gould
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”
stasis
stasis
stasis
Phyletic Gradualism vs. Punctuated Equilibrium
speciation
speciation
Speciation gradual
and ongoing
Speciation rapid and
followed by no
change (stasis)
A Bit of A Diversion: Heterochrony
Meaning: Different Time
Definition: 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
Paedomorphosis
Paedomorphosis: trend in development
(example: a lineage of trilobites)
Note that the shape of the last species on the
right resembles the larval stage of the first
species on the left
An Nifty Example
Significance of Heterochrony to Human Evolution
END OF LECTURE