Chapter 24
The Origin of Species
Macroevolution is the origin of new taxonomic groups, as
opposed to microevolution, which is genetic variation
between generations within a species.
A. What is a species?
1. Biological species concept
- A species is a population or group of populations whose
members have the potential to interbreed with one another
and produce viable offspring, but who cannot produce viable
offspring with other species.
The biological
species concept is
based on interfertility, rather than
physical similarity.
Speciation - process by which a new species originates.
- Involves the creation of a population of organisms that are
novel enough to be classified in their own group.
- Two processes by which this can occur:
- Anagenesis is the accumulation of heritable traits in a
population, that transforms that population into a new species.
- Cladogenesis is branching evolution, in which a new species
arises as a branch of from the evolutionary tree. The original
species still exists. This process is the source of biological
diversity.
- For a new species to form, there needs to be isolation of
some members of a species as a separate population.
- Forms of isolation, that interfere with breeding include
both…
2. Prezygotic and postzygotic barriers
 Prezygotic barriers prevent mating or egg fertilization if
members of different species try to mate. Examples:
a. Habitat isolation - Two species that live in the same area,
but occupy different habitats rarely encounter each other.
b. Behavioral isolation - Signals that attract mates are often
unique to a species. (e.g., different species of fireflies flash
different patterns)
Courtship ritual as a
behavioral barrier between species.
c. Temporal isolation
- Two species breed at different times of the day or during
different seasons.
d. Mechanical isolation
- Closely related species attempt to mate, but are anatomically
incompatable. (Example: flowering plants with pollination
barriers; some plants are specific with respect to the insect
pollinator, often occurs with butterflies/moths)
e. Gametic isolation
- Gametes must recognize each other. (Example: fertilization of
fish eggs, chemical signals between sperm and egg allows
sperm to “recognize” the correct egg)
- Postzygotic barriers
a. Reduced hybrid viability
- Abort development of hybrid at some embryonic stage.
b. Reduced hybrid fertility
- Meiosis doesn’t produce fertile gametes in vigorous hybrids.
 donkey + horse = mule (sterile hybrid)
c. Hybrid breakdown
- First-generation hybrids are fertile, but they cannot produce
fertile offspring in the next generation (e.g. different species of
cotton).
A summary of
reproductive barriers
between closely related
species
The problem with the idea of biological species concept -->
How do you get organisms to breed to see whether viable
offspring are produced? There are…
3. Alternative concepts of species
a. Ecological species concept
- Species are defined by their use of environmental resources;
their ecological niche (e.g. species that are defined by their
food source such as butterflies with certain flowers).
b. Pluralistic species concept
- Factors that are most important for the maintenance of
individuals as a species vary. These factors include
reproductive isolation, i.e. not being in the same area so
mating is precluded.
c. Morphological species concept
- Species are characterized according to a unique set of
structural features. This is still the most widely used
concept by taxonomists.
d. Genealogical species concept
- Recognizes species are sets of organisms with unique
genetic histories. This idea is based often on molecular
analyses such as DNA sequences.
B. Modes of speciation
1. Allopatric speciation
- Allopatric speciation describes speciation that takes place
in populations with geographically separate ranges. Gene
flow is interrupted and new species evolve.
2. Sympatric speciation
- Sympatric speciation describes speciation that takes place
in geographically overlapping populations. Chromosomal
changes and nonrandom mating reduce gene flow.
Remember: Species arise when individuals in a population
become isolated one from the other.
Examples of Allopatric speciation:
Allopatric speciation of squirrels in the Grand
Canyon. Animals like birds do not show speciation like
those animals that are barred from breeding by the
canyon.
Key question about allopatric species is whether they are
indeed different enough that viable, fertile offspring would
not be produced by mating. This can be tested sometimes
as in…
Another place where adaptive radiation is apparent is on
island chains. This example is illustrative of what happened
on the Hawaiian islands.
Would this example be allopatric or sympatric speciation?
Remember  Once geographic isolation has occurred, there
still must be changes that reproductively isolate populations
of individuals. If the populations evolve so that they are now
new species, they cannot interbreed to produce fertile, viable
offspring.
2. Sympatric speciation
- Speciation that takes place in geographically overlapping
populations.
- This can occur by chromosomal changes and nonrandom
mating. Both can reduce gene flow between organisms and
cause populations to evolve to new species.
Example: Polyploidy (greater than 2 sets of chromosomes) most common in plants.
Sympatric speciation by autopolyploidy in plants.
-Animals diverge mostly due to reproductive isolation.
-Reproductive isolation is a result of genetic factors that cause
offspring to rely upon resources not used by previous
generations. (Example: switch to a new food source)
-Example: Lake Victoria has 200 closely related species of
Cichlids (fish) which probably all arose from one ancestor with
the driving force for speciation being:
Competition for a limited resource (food) within the lake, and
adaptation to new food sources. This gave rise to different
species that are kept from breeding with each other by
distinctive coloration pattern.
C. From speciation to macroevolution
How then do we get from the mechanism of speciation to
evolution on a grand scale; i.e., macroevolution?
Two models:
-The Gradualism model suggests that change is gradual with
the accumulation of unique morphological adaptation.
- The Punctuated Equilibrium model suggests that rapid
change occurs, with a new species “erupting” from the
ancestral lineage and then staying the same thereafter.
However it does occur, we need to remember that 
Speciation occurs when divergence leads to reproductive
barriers between the new and the ancestral population.
And this probably takes vast amounts of time to occur.
 But how do evolutional novelties emerge? For
example, how did something as complex as the eye first
evolve? We need to remember that:
 Most evolutionary novelties are modified versions of
older structures.
A range of eye
complexity among
mollusks.
Lesson from the eye example:
 Existing structures can be modified for brand new
functions. These are called Exaptations: structures that
evolve for one purpose but become useful for another
function.
Finally, we should…
Remember that evolution is not goal oriented. Differential
reproduction is only a reaction of individuals to their
environment.
Example: The branched evolution of horses. This figure can
give the illusion of goal-oriented evolution of the horse, but it
is only an illusion.