What keeps a species from subdividing into other species?
What causes a species to branch into two new species?
QUESTIONS TO ANSWER
HW
Finish Chapter 22
 We will lecture Today and Tomorrow
 Friday we will begin the H-W Modeling Lab

 2-3

days
Quiz on CH 22 and 21 Next week
THE ORIGIN OF
SPECIES
Speciation is the
process by which one
species splits into two
or more species
MICRO VS MACROEVOLUTION
 Microevolution
consists of changes in allele
frequency in a population over time
 Macroevolution refers to broad patterns of
evolutionary change above the species level
WHAT IS A SPECIES?

Biological Species Concept
OTHER SPECIES CONCEPTS
Morphological – Species based on shape, size,
color, etc.
 Ecological – based on niche
 Phylogenetic – based on evolutionary
relationships (smallest group of individuals that
come from a common ancestor)

WEAKNESSES
Biological – asexual
 Ecological – gene flow
 Phyolgenetic – genetically
subjective
 Morphological – subjective bridge

WHAT HOLDS A SPECIES TOGETHER?
WHAT CAUSES SPECIATION?

Allopatric speciation

Sympatric speciation
 Polyploidy
 Habitat
Differentiation
 Sexual Selection
FIGURE 22.3
Prezygotic barriers
Habitat
isolation
Temporal
isolation
Behavioral
isolation
Postzygotic barriers
Mechanical
isolation
Gametic
isolation
MATING
ATTEMPT
(a)
(c)
(d)
(e)
(f)
Reduced
hybrid
viability
Reduced
hybrid
fertility
Hybrid
breakdown
VIABLE,
FERTILE
OFFSPRING
FERTILIZATION
(g)
(h)
(i)
(j)
(b)
(k)
(l)
1. GEOGRAPHIC ISOLATION
A physical barrier separates (splits) the original
population
 As time passes, the environment may selects for
different traits and the 2 populations become
genetically different AND can’t reproduce
 Barriers: Rivers, mountains, bodies of water

KAIBAB AND ABERT SQUIRRELS
Kaibab
Abert
- The Kaibab squirrel
(Sciurus aberti
kaibabensis) became
geographically isolated
from the common
ancestor about 10,000
years ago.
- Its closest relative is
the Abert squirrel
(Sciurus aberti aberti)
- They live in opposite
sides of the Grand
Canyon
Example: Emus, ostriches, rheas, Cassowaries,
 Common ancestor lived in the continent of
Gondwana (all southern continents together)
 When Gondwana broke up, the populations
separated and changed over time. All these birds
are closely related but live far apart.
WRASSES: ORIGINAL POPULATION SPLIT UP WHEN THE
ISTHMUS OF PANAMA WAS FORMED
- 2 SPECIES NOW, ONE ON EACH SIDE OF CENTRAL AMERICA
2. TEMPORAL ISOLATION
(TIME OF REPRODUCTION)
Groups of individuals reproduce at different times.
The genes of the 2 groups do not mix.
 It is happening now with some of the migratory
birds that decide not to migrate.
 The birds that do not migrate start mating before
the migratory ones come back.
 Eventually they might become different enough
that they may not recognize each other.

ROBINS USUALLY MIGRATE SOUTH TO WARMER CLIMATES. SOME
ARE NOT MIGRATING. THEY REPRODUCE BEFORE THE MIGRATING
INDIVIDUALS RETURN.
TEMPORAL ISOLATION
Experiment
Initial population
of fruit flies
(Drosophila
pseudoobscura)
Some flies raised
on starch medium
Some flies raised
on maltose medium
Mating experiments
after 40 generations
Results
9
8
20
Number of matings
in experimental group
Starch
population 1
22
18
15
Starch
population 2
Maltose
Starch
Starch
population 1 population 2
Male
Starch
Starch
Female
Maltose
Female
Male
FIGURE 22.8
12
15
Number of matings
in control group
FIGURE 22.8A
Experiment
Initial population
of fruit flies
(Drosophila
pseudoobscura)
Some flies raised
on starch medium
Some flies raised
on maltose medium
Mating experiments
after 40 generations
FIGURE 22.8B
Results
22
9
8
20
Number of matings
in experimental group
Maltose
Starch
population 2
Starch
population 1
Maltose
Male
Starch
Starch
population 1
18
15
Starch
population 2
Female
Male
Starch
Female
12
15
Number of matings
in control group
POLYPLOIDY
 Polyploidy
is the presence of extra sets of
chromosomes due to accidents during cell division
 Polyploidy is much more common in plants than in
animals
 An autopolyploid is an individual with more than two
chromosome sets, derived from one species
 The offspring of matings between autopolyploids
and diploids have reduced fertility
FIGURE 22.UN01
Cell
division
error
2n  6
2n
Gametes produced
by tetraploids
Tetraploid cell
4n  12
New species
(4n)
 An
allopolyploid is a species with multiple sets
of chromosomes derived from different species
 Allopolyploids cannot interbreed with either
parent species
FIGURE 22.9-1
Species A
2n  6
Species B
2n  4
Meiotic error; chromosome
number not reduced from 2n to n
Normal
gamete
n3
Unreduced gamete
with 4 chromosomes
FIGURE 22.9-2
Species A
2n  6
Species B
2n  4
Meiotic error; chromosome
number not reduced from 2n to n
Normal
gamete
n3
Unreduced gamete
with 4 chromosomes
Hybrid with
7 chromosomes
FIGURE 22.9-3
Species A
2n  6
Species B
2n  4
Meiotic error; chromosome
number not reduced from 2n to n
Normal
gamete
n3
Unreduced gamete
with 4 chromosomes
Hybrid with
7 chromosomes
Normal
gamete
n3
Unreduced gamete
with 7 chromosomes
FIGURE 22.9-4
Species A
2n  6
Species B
2n  4
Meiotic error; chromosome
number not reduced from 2n to n
Normal
gamete
n3
Unreduced gamete
with 4 chromosomes
Hybrid with
7 chromosomes
Normal
gamete
n3
Unreduced gamete
with 7 chromosomes
New species:
viable fertile hybrid
(allopolyploid)
2n  10
 Many
important crops (oats, cotton, potatoes,
tobacco, and wheat) are polyploids
HABITAT DIFFERENTIATION

NA maggot fly can live on native hawthorn trees
as well as more recently introduced apple trees
SEXUAL SELECTION
Experiment
Normal light
P. pundamilia
P. nyererei
Monochromatic
orange light
FIGURE 22.6
SYMPATRIC SPECIATION
SEXUAL SELECTION
(a) Under high predation:
body shape that enables
rapid bursts of speed
(b) Under low predation:
body shape that favors
long, steady swimming
WHAT HYBRIDS TELL US ABOUT SPECIATION

Hyrbid Zone - region in which members of
different species mate and produce hybrids
FIGURE 22.11
Fire-bellied
toad range
Hybrid zone
Fire-bellied toad, Bombina
bombina
Yellow-bellied
toad range
Yellow-bellied toad, Bombina
variegata
Frequency of
B. variegata-specific allele
0.99
Hybrid
zone
0.9
Yellow-bellied
toad range
0.5
Fire-bellied
toad range
0.1
0.01
40
30
20
10
0
10
20
Distance from hybrid zone center (km)
Frequency of
B. variegata-specific allele
FIGURE 22.11B
0.99
Hybrid
zone
0.9
0.5
Yellow-bellied
toad range
Fire-bellied
toad range
0.1
0.01
40
30
20
10
0
10
20
Distance from hybrid zone center (km)
FIGURE 22.12-3
Isolated
population
diverges.
Gene flow
Population
Barrier to
gene flow
Hybrid
zone
Hybrid
individual
FIGURE 22.12-4
Isolated
population
diverges.
Hybrid
zone
Possible
outcomes:
Reinforcement
Gene flow
Population
Barrier to
gene flow
Fusion
Hybrid
individual
Stability
 Reinforcement
– hybrids less fit than the parent
species

Natural selection reinforces reproductive barriers, and,
over time, the rate of hybridization decreases
 Where
reinforcement occurs, reproductive
barriers should be stronger for sympatric than for
allopatric species
 Fusion
of the parent species into a single
species may occur if hybrids are as fit as
parents, allowing substantial gene flow between
species
 For example, researchers think that pollution in
Lake Victoria has reduced the ability of female
cichlids to distinguish males of different species
 This might be causing the fusion of many
species
FIGURE 22.13
Pundamilia nyererei
Pundamilia pundamilia
Fusion
Pundamilia “turbid water,”
hybrid offspring from a
location with turbid water
Rates of Speciation
HOW FAST DO SPECIES FORM?
HOW FAST IS EVOLUTION?
HOW FAST DO ORGANISMS CHANGE?
Gradualism
Punctuated Equilibrium
GRADUALISM

Organisms go through gradual and continuous
change
PUNCTUATED EQUILIBRIUM

Organisms go through fast periods of change,
followed by long periods of no change (according
to fossil record)
PUNCTUATED EQUILIBRIUM
PROPOSED BY STEPHEN JAY GOULD
(1941-2002)
http://www.youtube.com/watch?v=8XggWLp
upp4
GRADUALISM
PUNCTUATED EQUILIBRIUM

Repeated Speciation and Macroevolution
THINGS WE’VE MISSED

Maintaining Variety
 Diploidy,
heterozygote advantage, Frequency
dependant Selection
Sympatric Speciation - Polyploidy
 Vestigial Structures
