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Unit 6 – Evolutionary Biology
Part 3: Modes of Speciation
(segue between evolution and taxonomy)
In part 2, we learned about microevolution – how a population's gene pool can change from
generation to generation. This part is about macroevolution, or the long term result – the origin
of new taxonomic groups.
Definition of species according to the biological species concept:
Prezygotic and postzygotic barriers isolate gene pools of biological species
Any factor that impedes two species from producing viable, fertile hybrids contributes to
reproductive isolation. The various reproductive barriers which isolate the gene pools of
species are classified as either prezygotic or postzygotic depending on whether they
function before or after the formation of zygotes.
Prezygotic barriers impede mating between species or hinder the fertilization of the
ova, should member of different species attempt to mate.
In the event that fertilization does occur, postzygotic barriers prevent the hybrid zygote
from developing into a viable, fertile adult.
A. Prezygotic Barriers
1) Habitat Isolation
Two species living in different habitats within the same area may encounter each other rarely
if at all, even though they are not technically geographically isolated.
2) Temporal Isolation
Two species that breed at different times of the day, seasons, or years cannot mix their
3) Behavioral Isolation
Species – specific signals and elaborate behavior to attract mates are important reproductive
barriers among closely related species
4) Mechanical Isolation
Anatomical incompatibility may prevent sperm transfer when closely related species attempt to
5) Gametic Isolation
Gametes of different species that meet rarely fuse to form a zygote.
B. Postzygotic Barriers
When prezygotic barriers are crossed and a hybrid zygote forms, one of several
postzygotic barriers may prevent development of a viable, fertile hybrid.
1) Reduced Hybrid Viability
Genetic incompatibility between the two species may abort development of the hybrid at some
embryonic stage.
2) Reduced Hybrid Fertility
If two species mate and produce hybrid offspring that are viable, reproductive isolation is intact if
the hybrids are sterile because genes cannot flow from one species' gene pool to the other.
3) Hybrid Breakdown
In some cases when species crossmate, the first generation hybrids are viable and fertile, but
when these hybrids mate with one another, offspring of the next generation are feeble or sterile.
There are two general modes of speciation based on how gene flow among populations is
initially interrupted.
A. Geographical isolation = allopatric speciation
Allopatric speciation = Speciation that occurs when the initial block to gene flow is a
geographical barrier that physically isolates the population.
Caused by the physical separation of members of population.
Such occurrences include emergence of mountain ranges, movement of glaciers,
formation of land bridges, etc...
Conditions favoring allopatric speciation
The geographical isolation of a small population usually occurs at the fringe of the parent
population's range.
As long as the gene pools are isolated from the parental population, the smaller “cut-off”
population is a good candidate for speciation because:
1. The gene pool of the small population probably differs from that of the parent
population. So think of this as founder effect.
2. Genetic drift will continue to cause chance changes in the gene pool of the small
population until a large population is formed.
3. Evolution caused by natural selection is likely to take a different direction in the new
population, as they have different gene pools and are exposed to different selection
Main Example: Darwin's finches on Galapagos (adaptive radiation)
B. Sympatric speciation
Sympatric speciation = formation of new species within the range of parent populations.
Reproductive isolation = when organisms become genetically isolated without being
physically isolated. This occurs when successful mating is prevented.
** Polyploidy = Many plant species have originated from improper cell division
that results in extra sets of chromosomes.
Occurs from nondisjunction that results in diploid gametes.
From this, self-fertilization results in tetraploids (4n). These cannot interbreed with the
diploid parent population because the 3n triploid would be sterile. (because unpaired
Very important in plant evolution: oats, cotton, potatoes, tobacco, wheat
IV. Two models for the tempo of speciation
Speciation rates can vary, especially when adaptive radiation occurs when new habitats
become available.
A. Gradualism
Species descended from a common ancestor gradually diverge more and more in morphology
as they obtain unique adaptations.
B. Punctuated Equilibrium
A new species changes most as it buds from a parent species, and then changes little for the
rest of its existence.
V. Species extinctions have occurred throughout history, and are rapid at times of ecological stress.
Scientific evidence supports the idea that both speciation and extinction have occurred throughout
Earth’s history and that life continues to evolve within a changing environment, thus explaining the
diversity of life.
A) There have been five major mass extinctions in Earth's history - Mass extinctions are
periods in Earth's history when abnormally large numbers of species die out
simultaneously or within a limited time frame.
Cretaceous–Paleogene extinction event: 65 mya - About 17% of all families, 50% of all genera and 75% of
all species became extinct. The majority of non-avian dinosaurs became extinct during that time. Mammals
and birds emerged as dominant land vertebrates in the age of new life.
Triassic–Jurassic extinction event: 200 mya - About 23% of all families, 48% of all genera and 70% to 75%
of all species went extinct. Most of the large amphibians were eliminated, leaving dinosaurs with little
terrestrial competition.
Permian–Triassic extinction event: 251 mya - Earth's largest extinction killed 57% of all families, 83% of
all genera and 90% to 96% of all species. The "Great Dying" had enormous evolutionary significance: on
land, it ended the primacy of mammal-like reptiles. The recovery of vertebrates took 30 million years.
Late Devonian extinction: 375–360 mya - a prolonged series of extinctions eliminated about 19% of all
families, 50% of all genera and 70% of all species.
Ordovician–Silurian extinction: 450–440 mya - Two events occurred that killed off 27% of all families,
57% of all genera and 60% to 70% of all species. Together they are ranked by many scientists as the second
largest of the five major extinctions in Earth's history in terms of percentage of genera that went extinct.
B) ?? The Anthropocene Extinction?? = the hypothesized, current high extinction rates
caused by human activity (anthropogenic)
12% of known species of birds and 21% of known species of mammals are threatened
nearly 200 of the 20,000 known plant species in the U.S. have become extinct since such records
have been kept (730 more are endangered or threatened)
In North America, at least 123 freshwater animal species have become extinct since 1900
32% of all known amphibian species are endangered