The Mutation Theory and
the Species-Concept
Hongyu Guo
March, 2007
Background

Darwin’s Origin of Species and Darwinism
In 1859, Darwin published his pivotal work On the
Origin of Species by Means of Natural Selection.
 All organisms have descended with modification
from a common ancestor.
 He introduced the theory that populations evolve
over the course of generations through a process of
natural selection
 In the view of Darwinism, hereditary variation
arises by continuous “fluctuation”, and
evolutionary change accumulates automatically in
infinitesimal increments as selection preserves
fluctuations in the favorable direction.
Background

Mutationism (Mendelism)
In 1900, Mendelian inheritance was
“rediscovered”.
 Geneticists learned that discontinuous
variations could arise by mutation and be
transmitted to offspring by Mendel’s laws.
 In the view of Mutationism, the infinitesimal
heritable variation could not be taken for granted.
As a result, evolution was seen as a two-step
process: the chance occurrence of a mutation,
followed by its persistence or elimination, it is
also called “jumping” evolution.

The Mutation Theory and the Species-Concept
-R. R Gates, 1917
Maianthemum Canadense
Smilacina (Maianthemum)
Maianthemum dilatatum
Maianthemum
Maianthemum bifolium (M. dilatatum)
The Mutation Theory and the Species-Concept
-R. R Gates, 1917
Kruhsea strepotoides
(Streptopus streptopoides)
Streptopus brevipes
(Streptopus streptopoides)
Different in flower size
Platystemon
Platystigma
Different in pistils
The Mutation Theory and the Species-Concept
-R. R Gates, 1917
paler and
smaller
Distribution of North American screech owl
The Mutation Theory and the Species-Concept
-R. R Gates, 1917
Distribution of Colaptes auratus
Distribution of Colaptes cafer
Gates suggested that these species differ remarkably in their color
makings, they can not be reasonable supposed to have developed
through gradual adaptation.
The Mutation Theory and the Species-Concept
-R. R Gates, 1917
Conclusion
In plant and animal species there are two
distinct types of variability:
• Discontinuous, independent of environmental
or functional influence, and has given rise to
many specific and generic characters.
• Continuous and apparently represents the
results of the stress of the environment of the
species in its dispersal.
The Mutation Theory and the Species-Concept
-R. R Gates, 1917
Darwinism
Mendelism
Gates
Modern Evolutionary Synthesis
The Correlation Between Relatives on the Supposition
of Mendelian Inheritance
-R. A. Fisher, 1918
The starting point of Modern Evolutionary Synthesis
 Fisher provide a rigorous statistical model for
Mendelian inheritance.
 He showed using the model how continuous variation
could be the result of the action of many discrete loci,
satisfying both Mendelism and Darwinism.

Modern Evolutionary Synthesis
A Mathematical Theory of Natural and Artificial Selection
-J.B.S. Haldane, 1924
Haldane outlined the first mathematical models for
many cases of evolution due to selection.

He Showed that selection of a given intensity is most
effective when amphimixis does not affect the character
selected, e.g. in complete inbreeding or homogamy.
Selection is very ineffective on autosomal recessive
characters so long as they are rare.

Modern Evolutionary Synthesis
Evolution in Mendelian Populations
-Sewall Wright, 1930
Wright using gene frequency model
showed that:
The differing statistical situations to be
expected among natural species are
adequate to account for the different sorts
of evolutionary processes.

Conditions in nature are often such as to
bring about the state of poise among
opposing tendencies on which an
indefinitely continuing evolutionary
process depends.

Modern Evolutionary Synthesis
Genetics and the Origin of Species
-Theodosius Dobzhansky, 1937
A landmark book in Modern
Evolutionary Synthesis

He suggested that mutations crop up
naturally all the time. Some mutations
are harmful in certain circumstances, but
a surprising number have no effect one
way or the other. These neutral changes
appear in different populations and linger,
creating variability that is far greater than
anyone had previously imagined.

Modern Evolutionary Synthesis
Systematics and the Origin of Species
-Ernst Mayr, 1942
New species definition: A species is
not just a group of morphologically
similar individuals, but a group that can
breed only among themselves, excluding
all others. (Biological Species)

 Allopatric
speciation theory: When
populations of organisms get isolated,
the sub-populations will start to differ by
genetic drift and natural selection over a
period of time, and thereby evolve into
new species.
Modern Evolutionary Synthesis
Modern Evolutionary Synthesis described by D. J. Futuyma, 1986
“The major tenets of the evolutionary synthesis, then, were
that populations contain genetic variation that arises by random (i.e.. not
adaptively directed) mutation and recombination;

that populations evolve by changes in gene frequency brought about by
random genetic drift, gene flow, and especially natural selection;

that most adaptive genetic variants have individually slight phenotypic
effects so that phenotypic changes are gradual (although some alleles with
discrete effects may be advantageous, as in certain color polymorphisms);

that diversification comes about by speciation, which normally entails
the gradual evolution of reproductive isolation among populations;

and that these processes, continued for sufficiently long, give rise to
changes of such great magnitude as to warrant the designation of higher
taxonomic levels (genera, families, and so forth).” - D. J. Futuyma

Molecular era of Speciation
Molecular Origin of Species – M. Nei & J. Zhang, 1998
Reproductive isolation between
different species appears to be caused
by the incompatibility of alleles at
two or more loci that control mating,
spermiogenesis and development.

Prezygotic
isolation
If so, what is the evolutionary
mechanism of reproductive isolation?
Ai
Bi
Aim
Bim
Ai+1
Bi+1
Postzygotic
isolation
Making new species, two ways
Epigenetics and Evolution
Timescales of Genetic and Epigenetic Inheritance
–O. J. Rando & K. J. Verstrepen, 2007
Organisms have evolved epigenetic mechanisms to influence the timing or
genomic location of heritable variability.
Hypervariable contingency loci and epigenetic switches increase the
variability of specific phenotypes.
 Error-prone DNA replicases produce bursts of variability in times of stress.
 Interestingly, these mechanisms seem to tune the variability of a given
phenotype to match the variability of the acting selective pressure.

These suggest that selection and variability are less independent than
once thought. ( Nonrandom evolution? )
Species Problem
How best to define "species" ?
Mayr’ s species definition: a species is a group of
actually or potentially interbreeding populations that
are reproductively isolated from other such groups.
(assuming sexual reproduction)

Diversity of species definition:
Typological species
Ecological species
Morphological species
Phenetic species
Biological / Isolation species
Recognition species
Biological / reproductive species
Microspecies
Mate-recognition species
Cohesion species
Phylogenetic (Cladistic)/ Evolutionary / Darwinian species
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