Speciation and
Population
Evolution
Leopard Frog
Pickerel Frog
What is a
species?
But what is a species?
• An individual belonging to a group of organisms,
having common characteristics, and are capable
of mating with one another in nature.
• A species is given a two-part name:
– the generic name
– specific name (or specific epithet).
Allium cepa (commonly known as onion)
Homo sapien (commonly known as us!)
Speciation is a lineage-splitting event that
produces two or more separate species.
How do Species Form—
Speciation!
• Remember: natural selection favors
changes that increase reproductive
success.
• The accumulation of differences between
groups is called divergence.
• Divergence leads to the formation of a
new species.
• Speciation is the process by which new
species form.
Divergent
evolution
Speciation—A Story
of Lost Love…
The scene: a population of wild fruit
flies minding its own business on
several bunches of rotting bananas,
cheerfully laying their eggs in the
mushy fruit...
Disaster strikes: A hurricane washes the bananas and the
immature fruit flies they contain out to sea. The banana bunch
eventually washes up on an island off the coast of the
mainland. The fruit flies mature and emerge from their slimy
nursery onto the lonely island. The two portions of the
population, mainland and island, are now too far apart for
gene flow to unite them. At this point, speciation has not
occurred — any fruit flies that got back to the mainland could
mate and produce healthy offspring with the mainland flies.
The populations diverge: Ecological conditions are slightly
different on the island, and the island population evolves
under different selective pressures and experiences different
random events than the mainland population does.
Morphology, food preferences, and courtship displays
change over the course of many generations of natural
selection.
So we meet again: When another storm reintroduces the
island flies to the mainland, they will not readily mate with
the mainland flies since they've evolved different courtship
behaviors. The few that do mate with the mainland flies,
produce inviable (unable to survive or develop normally ) eggs
because of other genetic differences between the two
populations. The lineage has split now that genes cannot
flow between the populations.
Ensatina eschscholtzi –subspecies on the way to
becoming new species
Which is the Transitional Species?
Speciation cont.
• Separate populations of a single
species often live in several different
environments and become
reproductively isolated.
• If the environments are different
enough, separate populations of the
same population can become very
dissimilar.
Kaibab
Squirrel
Albert
Squirrel
Maintaining New Species
• There are several kinds of barriers that
prevent different groups from breeding
with one another:
– Temporal isolation (seasonal)
– Geographic isolation
– Behavioral .
• For example, organisms may be
geographically isolated from one
another or may reproduce at different
times.
Geographic
isolation
Different mating times prevents the Pickerel frog and
the Leopard frog from mating.
Reproductive Isolation
The differences shown in the beaks of the ten species of
Galapagos finches in the chart are most likely the result of –
a.
b.
c.
d.
a genetic equilibrium of the finch population
beak changes acquired during the life of the finch
natural selection due to competition for food
parasitic relationships among the finch species