Download Evolution

Speciation
Proposing Mechanisms for
Species Formation and
Identifying Ways to Maintain
Reproductive Isolation
Two Types of Evolution
Microevolution
Macroevolution
Change within a
population or species
Change to a new
species
Species Formation
• Species = Group of actually or potentially
interbreeding natural populations which
are reproductively isolated from other
such groups
• Speciation depends on
– isolation (lack of gene flow)
– genetic divergence
Mechanisms for Speciation
• Allopatric Speciation
– Occurs as a result of geographical isolation
– Most common mechanism
• Sympatric Speciation
– Occurs in the same location
– Can be due to Ecological isolation
– Can be due to Polyploidy
• Occurs for plants that have a sudden change
in numbers of chromosome sets
Allopatric Speciation
Single species
(white mice);
homogeneous habitat
(a)
Geographical barrier
(impassable river);
isolated populations
(b)
(c)
(d)
Genetic drift;
genetic divergence;
tan vs. white mice
Barrier removed
(river dries up);
Mice mix but don’t interbreed.
Summary of Allopatric Speciation
• One group separates from the population.
• Separate evolutionary pressures cause
different genetic changes in both groups.
(Is this (1) microevolution or (2) macroevolution?)
• Sufficient genetic changes accumulate so
that interbreeding cannot occur if groups
are rejoined.
(Is this (1) microevolution or (2) macroevolution?)
Genetic Divergence After
Geographical Isolation
 Four races of mole rats
show different numbers
of chromosomes
 Races live in areas with
differing moisture content
and soil type
 Hybrids rarely form
Adaptive Radiation
• One species gives rise to many new
species in a short period of time
– populations invade a variety of habitats and
evolve in response to differing
environmental pressures in those habitats
– results from
• availability of unoccupied habitats
• lack of competition
Sympatric Speciation
Single species
(white mice);
homogeneous habitat
(a)
Climate change;
two habitats;
isolated because don’t mix
(b)
(c)
(d)
Environmental pressure to adapt;
genetic divergence;
tan vs. white mice
Sufficient divergence;
now different species
Speciation by Polyploidy
Diploid with
chromosome set A and
chromosome set B.
Chromosomes duplicate
but do not separate
Tetraploid with two sets of
A and B.
Cross between diploid and
tetraploid species 
Triploid with one each of
chromosome sets A, B and D.
Modern Wheat
Chromosomes duplicate
but do not separate 
Hexaploid with three sets
of A, B and D.
Fertility of Polyploids
Tetraploid
Plant
Tetraploid
Plant
Meiosis
Diploid
Gametes
Fertilization
Viable Tetraploid
Zygote
Meiosis
Diploid
Gametes
Diploid
Plant
Meiosis
Diploid
Gamete
Haploid
Gamete
Fertilization
Viable Triploid
Zygote
Meiosis
(faulty)
Meiosis yields
unbalanced
products
no viable zygotes.
Applying Your Knowledge
1. Sympatric Speciation
2. Speciation by Polyploidy
3. Allopatric Speciation
A. Which process involves a sudden, large
change in chromosome number?
B. Which process requires geographical
separation?
C. Which process can occur as a result of small
differences within the same local environment?
Maintaining Reproductive Isolation
• Premating Isolating Mechanisms
– Prevent mating between individuals of
different species
• Postmating Isolating Mechanisms
– Interfere with development, viability or
fertility of offspring from matings between
species
Premating Isolating Mechanism:
Geographical Isolation
 The Grand Canyon is a geographical barrier
that can promote speciation.
 The Kaibab squirrel (left) lives on the
North Rim while the Avert squirrel (right)
lives on the South Rim.
Premating Isolating Mechanism:
Ecological Isolation
White-throated Sparrow
feeds in dense thickets
White-crowned Sparrow
inhabits fields and meadows
Premating Isolating Mechanism:
Ecological Isolation
Each of the 750 species of
fig wasps pollinates a
different species of fig plant.
Premating Isolating Mechanism:
Temporal Isolation
Premating Isolating
Mechanism:
Temporal Isolation
Monterey Pine releases
pollen in early spring
Bishop’s Pine releases
pollen in summer
Premating Isolating Mechanism:
Temporal Isolation
Periodic Cicada: Males emerge
on a 13 or 17-year cycle.
Premating Isolating Mechanism:
Behavioral Isolation
Firefly signaling patterns
differ between species
Fruit fly species have
different wing beat
frequencies for the
mating ritual.
Premating Isolating Mechanism:
Behavioral Isolation
White-crowned sparrows from different areas
have distinct song patterns for territorial displays.
Premating Isolating Mechanism
Mechanism
Examples
-Male sex organs of separate
insect species vary greatly
differences in structure of sexual -Pollen from one plant species
female
organs
does not germinate on female
part of another species
Mechanical Isolation:
clasper
Postmating Isolating Mechanism
Mechanism
Example
-Sperm may not have the proper
Gametic
Incompatibility enzymes to remove the egg’s
protective coat
- Pollen may not be able to grow
toward the egg in plants of different
species
Postmating Isolating Mechanism:
Hybrid Inviability
Hybrids between Sheep and Goats
do not survive
Postmating Isolating Mechanism:
Hybrid Infertility
horse
+ donkey
Mule
Cannot Reproduce
Postmating Isolating Mechanism:
Hybrid Breakdown
Liger
Tiglion
Hybrids of Lions and Tigers have been obtained
through zoo breeding programs. While the first
generation may be healthy, hybrids become weak
or sterile in future generations.
Applying Your Knowledge
1. Premating Isolating Mechanism
2. Postmating Isolating Mechanism
A. Hybrids of dogs and wolves can occur. Some
have more dog traits and others favor wolves.
After two or more generations, the hybrids are
feeble and cannot reproduce.
B. The male fruit fly flaps his wing to attract a
nearby female. Wing beat frequencies differ
between species so a female will only accept
the male that has the correct wing beat.
Applying Your Knowledge
1.
2.
3.
4.
5.
Temporal Isolation
Ecological Isolation
Behavioral Isolation
Geographical Isolation
Mechanical Isolation
A. English Oak is found in Europe and Scrub Oak is
found in California.
B. Lions live in groups called prides on open grassland.
Tigers are solitary and live in forests. These species
do not form hybrids even though their ranges
overlap.
C. The male Reggiana bird of paradise displays his
feathers to a potential mate. The female trumpet
manucode, a related species, is not attracted by this
display.
Extinction
Death of all members of a species
Factors that contribute to extinction
localized distribution of a species
overspecialization
 competition
habitat change and destruction
Localized Species Distribution
The Devil’s Hole Pupfish
lives in a single pool at
the mouth of a water-filled
cave system in Nevada.
Overspecialization
The Everglades Kite depends on a single
species of snail as a food source.