Macroevolution
Biological Species Concept
• Species are groups of actually or
potentially interbreeding natural
populations which are reproductively
isolated from other such groups.
– Breeding behavior can be real or potential, if
two species come together and breed then
there was only one species.
– Breeding behavior in nature can be different
than in captivity.
How do new species arise?
• Speciation is the development of a new species
through evolution.
• Evolution within a species means a change in
that population’s allele frequency.
• When two populations are separated their allele
frequency changes. Since they no longer have
migration between the populations, two separate
species develop.
• There are many ways to separate two
populations besides geographically.
Allopatric Speciation
• When geographical barriers divide a
population, followed by the development
of mechanisms in the separated
populations that prevent interbreeding.
– *Allopatric means ‘of other countries’
– Geographical isolation is the most important
factor in starting speciation.
– Physical or behavioral changes develop that
will keep the two species isolated from
interbreeding.
The isthmus of Panama came into being 3 million years ago. The
species of shrimp on either side are now separate species.
• Allopatric Speciation in shrimp
Geographic isolation leads to
allopatric speciation.
The Abert squirrel lives on the south rim, the Kaiba squirrel on the north rim
Reproductive Isolation
• Any factor in nature that prevents
interbreeding between individuals of the
same or closely related species.
– Extrinsic isolating mechanism – outside of the
organisms in question
• Geographic isolation is extrinsic
– Intrinsic isolating mechanism – internal
characteristics that prevent interbreeding
• Differences in anatomy, physiology and behavior
Six Intrinsic Reproductive
Isolating Mechanisms
•
•
•
•
•
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Ecological
Temporal
Behavioral
Mechanical
Gametic
Hybrid inviability
Ecological Isolation
• When two species have different habitats
they will rarely have contact.
– Lions prefer open grassland, tigers prefer
forest
Temporal Isolation
• Two species that share the same habitat
but do not mate within the same time
frame.
– Two populations of the same species of plant
release their pollen at different times of the
year. If their reproductive periods did not
overlap they would have reduced gene flow.
Behavioral Isolation
• Even if populations are in contact and
breeding can occur, they must choose to
mate.
• Such a choice is based on specific
courtship and mating displays.
Behavioral isolation
These albatrosses are behaviorally isolated from
other bird species by their elaborate, speciesspecific courtship behavior.
Great-tailed Grackle Mating
Display
Damsel fly mating behavior
Marine flatworms mating.
Sting rays mating behavior.
Mechanical Isolation
• Reproductive organs
differ in size or shape
or other feature.
– Different species of
Alpine Butterfly look
similar but have
different reproductive
organs.
Gametic Isolation
• Even if mating occurs, offspring may not
result if there are incompatibilities
between sperm and egg, or between
sperm and the female reproductive tract.
Hybrid Inviability or Infertility
• Even if offspring
develop, they are
most often malformed
or sterile
Real Cross-breed Animals
A zebroid is a cross between
a horse and a zebra.
Sympatric Speciation
• Intrinsic isolating mechanisms can
develop between two populations in the
absence of geographical separation.
• Sympatric* speciation - any speciation
that does not involve geographic
separation.
– Sympatric means ‘of the same country’
– Two types include polyploidy and speciation
through hybridization
Prior to colonization
•Hawthorn trees + Haw fruit
flies
Colonization by Europeans
•Apple trees + fruit flies
(called Apple flies)
Sympatric Speciation in fruit flies
• Only 6% of flies interbreed
• Conduct courting, mating, laying eggs in
their specific type of tree
• In transition to being two separate
species
• Mutation or combination of rare alleles
allowed flies to smell the apple trees
Apples mature earlier
than Hawthorne trees.
Fruit flies winter
underground as
larvae.
The flies that mate on
the Apple tree emerge
earlier that the Haw
flies. They share their
variant alleles among
themselves.
Temporal Isolation
Speciation through Hybridization
• Where the egg and sperm come together
to produce offspring.
• Rarely a healthy hybrid is produced.
Polyploidy
• 100,000 species of plants exist today
because of polyploidy
• A multiplication of the normal number of
chromosomes in an organism.
• Most often happens in plants that self
pollinate
• Rarely happens in animals because they
do not self pollinate.
Gametes are formed in separate
species, A and B. Each has two
chromosomes.
Gametes fuse and form a new,
sterile plant.
The zygote doubles its
chromosomes for cell division
then fails to divide.
Gamete formation takes place
Gametes from the same plant
fuse, forming a new plant with 4
chromosomes.
When does speciation occur?
• Consider the
horseshoe crab which
has not changed
much in 300 million
years.
• Consider Darwin’s
finches, they
developed into 13
separate species in
100,000 years.
Generalists vs Specialists
• The horseshoe crabs are generalists –
their diet is diverse
• The finches are specialists – their diet is
very specific, especially when food is
scarce.
• When the food source changes, the
finches must adapt or they will not
survive. The horseshoe crab just moves
on and eats something else.
Adaptive Radiation
• Rapid emergence of species from a single
species that has been introduced into a new
environment.
• When the finches arrived on the Galapagos
there were no other birds of their kind living in
the islands.
• It was easy for them to specialize and fill the
available niches. This is especially true on the
Galapagos because there are 25 separate
islands.
– Water between the islands is a barrier = allopatric
speciation
Two conditions that
contribute to speciation
• Specialization of food source and
environment
• Migration to a new environment,
especially if there is no competition.
Is speciation smooth or jerky?
• Gradualism model
• Punctuated equilibria
model