Download Chapter 17 Speciation

What is Speciation?
Speciation is the formation of
new biological species,
usually by the division of a
single species into two or
more genetically distinct
one.
 Natural selection and chance events can
change the relative frequencies of alleles in a
population and lead to speciation.
 Speciation is the formation of new species.
 A species is a group of organisms that breed
with one another and produce fertile
offspring.
KEY CONCEPT
New species can arise when populations are isolated.
Four Types of Isolating Mechanisms
Reproductive Isolation
Behavioral Isolation
Temporal Isolation
Geographic Isolation
Reproductive Isolation
 Reproductive Isolation is the separation of a
species or population so that they no longer
interbreed and evolve into two separate
species.
 Members of a certain species share a
common gene pool. Over time, genes are
shared by interbreeding.
Reproductive Isolation
 Over time, isolated gene pools diverge
into separate species.
Behavioral Isolation
 Behavioral Isolation is a form of reproductive isolation
in which two populations develop differences in
courtship rituals or other behaviors that prevent them
from breeding.
 Example: Eastern and Western Meadowlark
 The populations of the two subspecies overlap in the
midwest.
Behavioral Isolation
 Male birds sing a mating song that females like,
East and West have different songs. Females only
respond to their subspecies song.

Behavioral isolation occurs when two
populations are capable of interbreeding but
have differences in courtship rituals or other
reproductive strategies that involve behavior.
Temporal Isolation
 a form of reproductive isolation in which two or more
species reproduce at two separate times.
 Example:
Northern Leopard Frog & North American Bullfrog
 Mates in April
Mates in July

Temporal isolation or seasonal isolation
occurs when two or more species
reproduce at different times of the year.
Geographic Isolation
 Geographic Isolation is a form of reproductive
isolation in which two populations are separated by
geographic barriers such as rivers, mountains or
bodies of water, leading to the foundation of two
separate subspecies.
 Example: 10,000 years ago the Colorado River
separated two squirrel populations.
Geographic Isolation

Kaibab Squirrel Albert Squirrel
 This resulted in a subspecies, but did not result in
speciation because the two can still mate if
brought together
 Geographic barriers do
not guarantee the
formation of new species.
Speciation in Darwin's Finches
 Speciation of Galápagos finches occurred by:





founding of a new population
geographic isolation
changes in new population's gene pool
reproductive isolation
ecological competition
Speciation in Darwin's Finches
Founders Arrive
A few finches—
species A—travel
from South
America to one of
the Galápagos
Islands.
There, they survive
and reproduce.
Speciation in Darwin's Finches
Geographic Isolation
Some birds from
species A cross to
a second island.
The two
populations no
longer share a
gene pool.
Speciation in Darwin's Finches
Changes in the Gene Pool
Seed sizes on the
second island
favor birds with
large beaks.
The population on
the second island
evolves into
population B, with
larger beaks.
Speciation in Darwin's Finches
 Reproductive Isolation
If population B birds cross back to the
first island, they will not mate with birds
from population A.
 Populations A and B have become
separate species.

Speciation in Darwin's Finches
 Ecological Competition
As species A and B
compete for available
seeds on the first island,
they continue to evolve in
a way that increases the
differences between them.
 A new species—C—may
evolve.

Speciation in Darwin's Finches
Continued Evolution


This process of isolation, genetic change,
and reproductive isolation probably
repeated itself often across the entire
Galápagos island chain.
Birds that are most different from each
other have the highest fitness. More
specialized birds have less competition for
food. Over time, species evolve in a way that
increases the differences between them,
and new species may evolve (C, D, and E).
 When two species do not reproduce
because of differences in mating
rituals, the situation is referred to as
A.
B.
C.
D.
temporal isolation.
geographic isolation.
behavioral isolation.
reproductive isolation.
 All of the following played a role in
speciation of Galápagos finches
EXCEPT
A.
B.
C.
D.
no changes in the gene pool.
separation of populations.
reproductive isolation.
natural selection.
 Beak size in the various groups of
Galápagos finches changed primarily
in response to
A.
B.
C.
D.
climate.
mating preference.
food source.
availability of water.
KEY CONCEPT
Molecular clocks provide clues to evolutionary history.
Molecular Clocks
 Uses mutation rates in DNA to estimate the
time that two species have been evolving
independently.
Molecular clocks use mutations to
estimate evolutionary time.
 Mutations add up at a constant rate in related species.
 This rate is the ticking of the molecular clock.
 As more time passes, there will be more mutations.
Mutations add up at a fairly
constant rate in the DNA of
species that evolved from a
common ancestor.
DNA sequence from a
hypothetical ancestor
Ten million years later—
one mutation in each lineage
Another ten million years later—
one more mutation in each lineage
The DNA sequences from two
descendant species show mutations
that have accumulated (black).
The mutation rate of this
sequence equals one mutation
per ten million years.
Calibrating the Clock
 Genes accumulate mutations at different rates
 Compare the number of mutations in a particular
gene in species whose age has been determined by
other methods.
 Scientists estimate mutation rates by linking molecular
data and real time.
– an event known to separate species
– the first appearance of a species in fossil record
Hox Genes and Evolution
 Small changes in Hox
gene activity during
embryological
development can
produce large changes
in adult animals.
Change in a Hox Gene
 Mutation in a single
Hox gene (Ubx) “turns
off” the growth of
some pairs of legs.
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