Macroevolution:
Investigating the Origin of Species
*Adapted from Macroevolution lecture at ccbcmd.edu.
Learning Targets: Distinguish between
anagenesis and cladogenesis. Define biological
species (Mayr) and describe limitations of the
concept. Distinguish between prezygotic and
postzygotic mechanisms of reproductive
isolation.
Speciation
• Anagenesis
– One species replaces
another
– No net change in number of
species
• Cladogenesis
– One species branches off
from an existing species
– A net increase in number of
species (diversity)
Biological Species Concept
• A species is a group of individuals whose
members have the potential to interbreed
with one another in nature to produce viable,
fertile offspring but cannot successfully
interbreed with members of other species
• Reproductive isolation prevents breeding with
other species
Problems with the biological
species concept…
• Not helpful in determining species in fossil
record
• Doesn’t account for asexual organisms
• Scientists can’t observe all potential
interbreeding (in nature or in the lab)
Reproductive Isolation:
Prezygotic Barriers
Impede mating or fertilization:
1. Geographical isolation
2. Behavioral isolation
3. Temporal isolation
4. Hybrids
Geographical Isolation: Species are
separated by physical barriers.
Example: Spotted Owl Species
Scientists believe that
glaciers during the ice
age separated the two
populations of owls.
• Behavioral isolation
– Species choose mates based on different
behaviors
– Examples: bird songs or
mating dances
Temporal Isolation
1. Species mate at
different times of year
2. Species mate at
different times of day
Hybrid Inviability
• Example: embryos of sheep and goat hybrids
usually die in early development, seeds of
some hybrids of peanut plant species will not
develop
Hybrid Sterility
Horse (2N=64) X Donkey (2N=62) = Mule
- Mule (hybrid of the two species) is sterile
Hybrid Breakdown
• Offspring of hybrid (F2
generation) not able to
survive or reproduce
• Example: sunflower
hybrids
– 80% of F2 generation
are defective in some
way and cannot
reproduce
successfully
Allopatric Speciation
• Allo = other, patric =
homeland
• Speciation in
geographically separate
populations
• Example: speciation of
squirrels after separation
by the Grand Canyon,
speciation of reef fish
separated by a land
bridge
Adaptive Radiation
• Evolution of many diversely adapted species
from a common ancestor – common on island
chains
•Example:
Hawaiian
honeycreepers
evolved from
one common
ancestor
Sympatric Speciation
• Sym = same, patric =
homeland
• Speciation in over-lapping
populations
• Example: speciation of
stickleback species in one
same lake due to food
specialization, speciation
of cichlids due to mate
choice
Polyploidy in Plants
• Speciation of plants
with extra
chromosomes due to
abnormal cell division,
or hybridization of two
different species
• Polyploids may be able
self-fertilize or cross
with other polyploids
Gradualism vs Punctuated
Equilibrium
Which is which?
• Small continuous changes over time
• Rapid change followed by periods of stasis
(little or no change)
Gradualism vs Punctuated
Equilibrium
Which is which?
• Small continuous changes over time = Gradualism
• Rapid change followed by periods of stasis (little or
no change) = Punctuated Equilibrium