The Origin of Species
chapter 24
Campbell and Reece
Speciation
process by which one species splits into 2 or
more species
 Speciation explains both the diversity of life
and the unity of living things.

Speciation : forms bridge between:
MICROEVOLUTION MACROEVOLUTION
 Evolutionary
 Evolutionary
change below
change above the
species level
species level
 Example:
 Examples:
◦ change in allele
frequencies in
population over
generations
◦ origin of new
group of
organisms
◦ impact of mass
extinctions
Biological Species Concept

Species: a group of
populations whose
members have the
potential to
interbreed in
nature and
produce viable,
fertile offspring
members of a
species cannot
produce viable,
fertile offspring
with other groups
 emphasizes the
separateness of
species due to
reproductive
barriers

What holds the gene pool of a species
together?
Gene Flow:
transfer of alleles
between
populations of
same species
 exchange of alleles
tends to hold
populations
together
genetically

Reproductive Isolation

existence of biological barriers that
keep members of 2 populations from
interbreeding over long periods of
time
Reproductive Isolation

block gene flow
between the
species & limit
formation of
hybrids
Reproductive Isolation

hybrids: offspring
that result from
the mating of
individuals from 2
different species
or from 2 truebreeding varieties
of same species
Reproductive Isolation
single barrier may not prevent all
gene flow
 combination of several barriers can
effectively isolate a species’ gene
pool
 2 categories:
1. prezygotic barriers
2. postzygotic barriers

Prezygotic Barriers
block fertilization from happening
by:
1. impeding members of different
species from attempting to mate
2. preventing attempted mating from
being completed successfully
3. hindering fertilization if mating
was completed successfully

Postzygotic Barriers
reproductive barrier that prevents
hybrid zygotes produced by 2
different species from developing
into viable, fertile adults
1. lethal developmental errors
2. infertility in viable offspring

Types of Prezygotic Reproductive Barriers
Habitat Isolation
2 species that occupy different habitats
w/in same area may rarely interact
example:
1.
Types of Prezygotic Reproductive Barriers
2. Temporal Isolation:
 species breed during
different times of
day, different
seasons, or different
years
 eastern & western
spotted skunks: 1
breeds in late
summer other in late
winter
Types of Prezygotic Reproductive Barriers
3. Behavioral Isolation:
courtship rituals used to attract mates
are effective barriers
Types of Prezygotic Reproductive Barriers
4. Mechanical Isolation:
 morphological differences prevent
successful completion even if
attempted
Types of Prezygotic Reproductive Barriers
5. Gametic Isolation:
 sperm of 1 species may not be able to
fertilize egg of another:
◦ reproductive tract hostile to sperm
◦ sperm does not have enzymes to
penetrate zona pellicida of another
species
Types of Postzygotic Reproductive Barriers
Reduced Hybrid Viability:
hybrids development or survival is
impaired
1.
Types of Postzygotic Reproductive Barriers
2. Reduced Hybrid Fertility:
hybrids may develop and be healthy
but they are not fertile
Types of Postzygotic Reproductive Barriers
Hybrid Breakdown:
Some 1st generation hybrids are fertile
but those offspring are feeble or
sterile
3.
Species
There is no single, universally
applicable species concept that can
define, explain, and identify all
species.
 There are multiple ways to think
about & define species.

Biological Species Limitations
unable to use these characteristics on
fossils of extinct species
 only applies to organisms that
reproduce sexually
 only applies where there is no gene
flow

Other Definitions of Species
these dfns emphasize the unity w/in
a species.
1. morphological species concept:
 characterizes a species by a
structural feature
 applies to species that reproduce
sexually or asexually
 how scientists distinguish most
species
 disadvantage: subjective

Morphological Species Concept

Problems:
◦ domestic dogs may look very different
but are still same species
◦ mouse lemurs look very similar but
there are 18 species of them
◦ grey mouse lemur
lesser mouse lemur
Ecological Species Concept

views species in terms of its niche
◦ the sum of how members of the species
interact with the nonliving & living
parts of their environment
◦ asexual or sexual species
◦ emphasizes role of disruptive NS as
organisms adapt to different
environmental conditions
Phylogenetic Species Concept
defines species as smallest group of
individuals that share a common
ancestor, forming one branch on the
“tree of life”
 determining degree of differences is
difficult

Species

There are >20 other ways to define
species
Speciation can take place with or w/out
geographic separation
Speciation can occur in 2 main ways:
1. Allopatric Speciation
2. Sympatric Speciation

ALLOPATRIC SPECIATION
“other country”
 gene flow is interrupted when
population is divided into
geographically isolated
subpopulations

Allopatric Speciation Process
once geographic separation has
occurred, the separated gene pools
will each have their own mutations
 NS & genetic drift may alter allele
frequencies in different ways in each
subpopulation group

Allopatric Speciation
Evidence of Allopatric
Speciation
are many studies & examples
supporting this type speciation
 indirect support: regions that are
isolated or highly subdivided have
more species than regions w/out
those features

Drosophila Experiment
Sympatric Speciation
“same country”
 occurs in populations in same
geographic area
 less common than allopatric
 occurs if gene flow is reduced by
factors like:

◦ polyploidy
◦ habitat differentiation
◦ sexual selection
POLYPLOIDY
means extra sets of chromosomes
 can occur in animals
 gray tree frog (Hyla versicolor)

◦ around Great Lakes
Polyploidy
much more common in plants
 estimate: 80% of today’s plants
species have ancestors that formed
by polyploid speciation
 2 forms

1. Autopolyploid
individual has >2 chromosome sets
all derived from a single species
 plant polyploidy

Plant Polyploidy

tetraploid plant can produce fertile
tetraploid offspring by selfpollinating or mating with other
tetraploids
2. Allopolyploid
2 different species interbreed
making a hybrid
 hybrid reproduces asexually
 over generations sterile hybrid 
fertile polyploid (called an
allopolyploid)

Allopolyploids
can breed with each other but not
with either of their parents so are a
new species
 rare: 5 new plant species since 1850
documented
 Mimulus peregrinus

Allopolyploids
include many agricultural crops
 Triticum aestivum (bread wheat) has
6 sets chromosomes (2 pair from each
of 3 parents), an allohexaploid
 1st polyploidy event probably
occurred ~8,000 yrs ago as
spontaneous hybrid

Top 2 parents
bottom: Triticum aestivum
Allopolyploids
plant geneticists “create” new
polyploids making hybrids with
desired characteristics
 use chemicals that induce meiotic &
mitotic errors

Habitat Differentiation

Sympatric speciation can occur when
genetic factors enable a
subpopulation to exploit a habitat or
resource used by the parent
population
Rhagoletis pomonella
North American
apple maggot fly
 original habitat
was the native
hawthorn tree

Habitat Differentiation


apples mature
faster than
hawthorn fruit, NS
has favored flies
with rapid
development
have an allele that
benefits flies that
feed off only 1 or
the other not both
(post-zygotic barrier
to reproduction)

flies feeding on
apple trees now
show temporal
isolation from flies
still eating
hawthorn fruit
(prezygotic
restriction to gene
flow)
Sexual Selection
can also drive sympatric speciation:
 cichlid fish Pundamilia pundamilia

Cichlid Fish
>600 species found in Lake Victoria
 originated in past 100 000 yrs
 hypothesis : subgroups of original
population adapted to different food
sources  genetic divergence
 female preference for mates may also
be a factor: 1 species breeding males
have blue back another species has
orange back

Breeding Cichlids Colors
Sexual Selection Study
placed the 2 subspecies in same tank
 used monochromatic orange light so
both appeared very similar
 females bred with either
 conclusion: mate choice by females is
based on male coloration so it’s the
main reproductive barrier
(prezygotic behavior)

Allopatric & Sympatric Speciation
are the 2 main modes of speciation
Allopatric
geographic
isolation
NS
genetic drift
sexual selection





Sympatric
requires
emergence of a
reproductive
barrier that
isolates a
subgroup
less common
polyploidy
sexual selection





Hybrid Zones
A region in which members of
different species meet & mate,
producing at least some
offspring of mixed ancestry.
 Hybrids are more likely to die as
embryos & show a variety of
morphological abnormalities

Hybrids of yellow-bellied &
fire-bellied toads
Bombino bambino

Bombino variegata
Hybrids are more likely to die as
embryos & show a variety of
morphological abnormalities
Hybrid Zones
Hybrid Zones over Time


barriers between parent species
may increase (strengthening of
reproductive barriers)
eventually hybrids no longer
formed
Hybrid Zones over Time
Or
 barrier weakens
 2 species fuse
Hybrid Zone over Time
3rd possibility:
 status quo is stabilized and
hybrids continue to be produced
Speciation Time Course

Patterns in the Fossil Record
◦ Punctuated Equilibrium: geologic
periods of apparent stasis then
sudden dramatic changes
demonstrated by some species
◦ other species only show gradual
changes coined Gradualism
Punctuated Equilibrium
Punctuated Equilibrium
&
Gradualism
Speciation Rates
Speciation Rates
Hybrid sunflower
that diverged
reproductively from
parents to form
new species over
short time period
 F1 generation 5%
fertility
 F5 generation >90%
fertile

Genetics of Speciation

fundamental question for
biologists:
How many genes must change
to form a new species?
Single Gene Examples
found in a few cases:
 Euhadra

◦ snail : single gene resulted in
mechanical barrier to reproduction
Small # of Genes

2 monkey flower species have
different pollinators = strong
barrier to cross-pollination

Mimulus lewisii
Mimulus cardinalis
Large #s of Genes

Hybrid of 2 species of fruit fly:
Drosophila pseudoobscura
results from gene interactions
among at least 4 loci
Large #s of Genes

Western sunflower: hybrid
species thrives in dry sand but 2
parents live in nearby moister
environments