Chapter 18: Speciation
Heather Marg- 11-16-09
What is a Species?
-
Creationist definition: God created each animal “after its kind” once and forever
Fundamentalist definition: a fixed, unchanging “unit of creation”
Book definition: in the sense of biological species, the members of a group of populations
that interbreed or potentially interbreed with one another under natural conditions
Speciation
-
According to Ernst Mayr the quest to answer the question about evolution and speciation
emerged very slowly, greats like Darwin and geneticists in 1900 like Haldane couldn’t figure
it out.
It took over 60 years after 1859 that a way to solve the problem of speciation was found.
The idea of allopatric( geographical) speciation was adopted
Modes of Speciation:
1. Allopatric speciation- evolution of reproductive barriers in populations that are prevented by a
geographic barrier from exchanging genes at more than a negligible rate.
2. Peripatric Speciation (Founder Effect) - divergence of small population from a widely
distributed ancestral form.
3. Parapatric Speciation- spatially distinct populations, between which there is some gene flow,
diverge and become reproductively isolated.
4. Sympatric Speciation- the evolution of reproductive barriers within a single, initially randomly
mating population.
Vocab:
*Vicariance- Separation of a continuously distributed ancestral population or species into separate
populations due to the development of a geographic or ecological barrier.
*Ecological Speciation- evolution of barriers to gene flow caused by divergent ecologically based
selection.
* Reinforcement- the enhancement of prezygotic barriers.
* Character Displacement- a pattern whereby characters differ more where two taxa are sympatric then
when they are allopatric.
* Founder Effect Speciation- Peripatric speciation
Allopatric Speciation
-
A physical barrier reduces gene flow to such a degree that if they were to come into contact
again, no interbreeding would occur.
Can occur “microgeographically” in species that disperse very little or favor a certain habitat
Allopatric populations can expand their range and come into contact again and become
sympatric without exchanging genes.
The new population that evolves from the parent population is called an incipient species
Evidence for Allopatric Speciation
-Dusky Salamander- experiment conducted by Stephen Tilly- They brought male and female
salamanders from different and same populations and mated them. They found that the further
apart the males and females in the pairs came from, the more genetically different they were, and
the less likely they were to breed.
- Speciation often occurs with the appearances of geographical barriers, for example
mountains.
Role of Natural Selection
-
Causes the evolution of genetic differences that create incompatibility between species.
Role of Ecological Selection
-
Ecologically divergent characters are thought to play a role in reproductive isolation.
o Ex) Two species of Monkeyflowers that have become adapted to different pollinators.
Role of Sexual Selection
-
Closely related species are often sexually isolated by a females preference in males, there are
two hypotheses for this:
o This enables the females to recognize conspecific mates and reduce hybridization which
would reduce the fitness of potential offspring
o Divergent sexual selection in different geographic populations of species result in
different male traits and females preferences for them
Alternatives to Allopatric Speciation
- Peripatric
o
o
One of Ernst Mayr’s most controversial hypotheses
Mandy birds and animals have populations in areas very close to that of the “parent”
species, but similar but are classified as different species; species evolve as localized
“buds” from a widespread parent species
- Parapatric
o
Can happen when two populations share a border and some hybridization at that point
by some individuals. Steady genetic divergence could eventually result in reproductive
isolation
- Sympatric
o
o
o
Highly controversial subject. Ernst Mayr was the most influential critic of this model.
When two populations share the same space and were initially a randomly mating
population. A biological barrier to gene exchange arises from within the population and
they still share the same space
Has been seen in a few laboratory cases of Drosophila
Polyploidy and Recombinational Speciation
Polyploidy
*usually occurs because of a failure during reproductive division in meiosis.
*The diploid hybrids of species are mostly sterile (Ligers, Mules)
*Tetraploid offspring of these hybrids happen to be highly fertile
Recombinational

Hybridization sometimes gives rise to distinct species with the same number of
chromosomes as their parents. Certain genotypes may be fertile but reproductively
isolated from the parent species, once this happens in a population that genotype
can increase in occurrence and a new species can arise
Rate of Speciation
-
The rate that speciation occurs at is primarily determined by ecological factors
When a species is separated by a geographical barrier, speciation is rapid
There are species that speciate rarely or very slowly, this gives rise to “living fossils” (Ginkgo,
Aardvark, Tuatara, Coelacanth)
Bibliography
Futuyma, Douglas J. Evolution 2nd ed. Sunderland, MA: Sinauer Associates Inc., 2009. Print.
Mayr, Ernst. What Evolution Is. New York: Basic Books, 2002. Print.
Milner, Richard. Encyclopedia of evolution humanity's search for its origins. New York: Facts on File,
1990. Print.
Pictures:
http://bio.research.ucsc.edu/~barrylab/classes/animal_behavior/IMAGES4.DIR/SPECMODE.GIF
http://bio100.nicerweb.com/Locked/media/ch21/allopatric_speciation.jpg
http://www.uwyo.edu/dbmcd/molmark/Speciation_modes.png