Chapter 24:
The Origin of
Species
Essential Knowledge
1.c.1 – Speciation and extinction have
occurred throughout the Earth’s history
(24.3 & 24.4).
 1.c.2 – Speciation may occur when two
populations become reproductively isolated
from each other (24.1).
 1.c.3 – Populations of organisms continue
to evolve (24.2).
 2.e.2 – Timing and coordination of
physiological events are regulated by
multiple mechanisms (24.1).

Two Concepts of Species
1. Morphospecies
2. Biological Species
Morphospecies
 Organisms
with very similar
morphology
Two Schools
1. Splitters - Break apart species
into new ones on the basis of
small phenotype changes
2. Lumpers - Group many
phenotype variants into one
species
Biological Species
 A group
of organisms that could
interbreed in nature and produce
fertile offspring
 Don’t identify species based upon
similar likeness ALONE!
Key Points
 Could
interbreed
 Fertile offspring
 May or may not
be similar in
phenotype
Morphospecies & Biological
Species
 Often
overlap
 Serve different purposes
Other Concepts of Species
1. Recognition Species Concept
2. Cohesion Species Concept
3. Ecological Species Concept
4. Evolutionary Species Concept
Ecological Species Concept
a specie’s role or
function in the environment.
 Emphasizes
 Called an organism’s niche
(Added Slide ) Lions and tigers are ecologically isolated
Live in
open
grassland
Live in
forest
Tiglon

Hybridization has been successful in captivity
 But it does not occur in the wild
Evolutionary Species Concept
 Emphasizes
evolutionary lineages
and ecological roles
 Used extensively in comparative
biology
 Popular among paleontologists
Speciation
 Speciation
= formation of a NEW
species
 Caused by disruptive selection (see
Ch 23)
 3 Requirements for speciation:
1. Variation in the population
2. Selection
3. Isolation
Reproductive Barriers
 Serve
to isolate a populations from
other gene pools
 Create and maintain “species”
Main Types of Reproductive
Barriers
 Prezygotic –
 Prevent mating or fertilization
 Postzygotic –
 Prevent viable, fertile offspring
Prezygotic - Types
1. Habitat Isolation
2. Behavioral Isolation
3. Temporal Isolation
4. Mechanical Isolation
5. Gametic Isolation
Habitat Isolation
 Populations
live in different habitats or
ecological niches
 Ex – mountains vs. lowlands
Behavioral Isolation
 Mating
or courtship
behaviors different
 Different sexual
attractions operating
 Ex – songs and
dances in birds
Temporal Isolation
 Breeding
seasons or time
of day different
 Ex – flowers
open in morning
or evening
Mechanical Isolation
 Structural
differences that prevent
gamete transfer
 Ex – anthers of flower not positioned to
put pollen on a bee, but will put pollen
on a bird
Gametic Isolation
 Gametes
fail to attract each other and
fuse
 Ex – chemical markers on egg and
sperm fail to match
Postzygotic - Types
1. Reduced Hybrid Viability
2. Reduced Hybrid Fertility
3. Hybrid Breakdown
Reduced Hybrid Viability
 Zygote
fails to mature or develop
 Ex:
 When two different species of frogs
hybridize
○ Egg is fertilized (zygote IS formed)
○ Zygote never develops properly (if at
all)
Reduced Hybrid Fertility
 Hybrids/offspring
are living, viable
 Offspring cannot reproduce sexually
(nonfertile)
 Chromosome count often “odd”
 Meiosis doesn’t work out
 Ex:
mules, ligers
Hybrid Breakdown
 Hybrid/offspring
are living, viable AND
fertile
 Offspring cannot compete
successfully with the “pure breeds”
 Ex:
 Most plant hybrids
 Plants are “born” and can produce seeds
 Blooms/plants may lack certain color or
size of petal
•Hybrids between them produced defective embryos in the lab
•Their mating calls also differ substantially
Modes of Speciation
1. Allopatric Speciation
2. Sympatric Speciation
 Both work through a block of
gene flow between two
populations
Allopatric Speciation
 Allopatric
= other homeland
 Ancestral population split by a
geographical feature.
 Comment – the size of the
geographical feature may be very
large or small
Example
 Pupfish
populations in Death Valley
 Generally happens when a species
range shrinks for some reason
Another Example
Conditions Favoring
Allopatric Speciation
1. Founder's Effect - with the peripheral
isolate
2. Genetic Drift – gives the isolate
population variation as compared to
the original population
3. Selection pressure on the isolate
differs from the parent population
Result
 Gene
pool of isolate changes
from the parent population
 New species can form
Comment
 Populations
separated by
geographical barriers may not evolve
much
 Ex - Pacific and Atlantic Ocean
populations separated by the Panama
Isthmus
 Fish - 72 identical kinds
 Crabs - 25 identical kinds
 Echinoderms - 25 identical kinds
Adaptive Radiation
 Rapid
emergence of several
species from a common ancestor
 Common in island and mountain
top populations or other “empty”
environments
 Ex – Galapagos Finches
Sympatric Speciation
 Sympatric
= same homeland
 New species arise within the range of
parent populations
 Can occur In a single generation
Plants
 Polyploids
may cause new
species because the change in
chromosome number creates
postzygotic barriers
 Animals: Don't form polyploids
and will use other mechanisms
Polyploid Types
1. Autopolyploid - when a species
doubles its chromosome number from
2N to 4N
2. Allopolyploid - formed as a polyploid
hybrid between two species
 Ex: wheat
Autopolyploid
Allopolyploid
Rate of
Evolution??
Gradualism Evolution
 Darwinian
style of evolution
 Small gradual changes over long
periods of time
Gradualism Predicts:
 Long
periods of time are needed
for evolution
 Fossils should show continuous
links
 Problem: Gaps in fossil record
Punctuated Evolution
 New
theory on rate/pace of
evolution
 Elridge
and Gould – 1972
Punctuated Equilibrium
 Evolution
has two speeds of
change:
1. Gradualism or slow change
2. Rapid bursts of change/speciation
Predictions
 Speciation
can occur over a very
short period of time
 Fossil record will have gaps or
missing links
 New species will appear in the
fossil record
 Established species will show
Comment
 Punctuated
Equilibrium is the
newest, most widely accepted
view
 Best explanation of timing
Summary
 Identify
several concepts of "species".
 Recognize reproductive barriers that keep
two species from reproducing with each
other.
 Recognize various mechanisms of
speciation and hybrid zones.
 Identify the Gradual and Punctuated
interpretations of speciation.
 Identify genetic mechanisms of speciation.