1. Chapter 24: The Origin of Species
What is a species?
A population whose members can interbreed in nature and
produce viable, fertile offspring
aka….reproductive isolation
What kinds of barriers keep different species isolated so they cannot mate?
Figure 24.4
Pre–zygotic barriers – before mating &/or zygote is formed
Post–zygotic barriers – after zygote is formed

2. Figure 24.4 Reproductive Barriers
Prezygotic barriers impede mating or hinder fertilization if mating does occur
Individuals of different species
Mating attempt
Habitat isolation
Temporal isolation
Behavioral isolation
Mechanical isolation
HABITAT ISOLATION
TEMPORAL ISOLATION
BEHAVIORAL ISOLATION
MECHANICAL ISOLATION
(b)
(a)
(c)
(d)
(e)
(f)
(g)

3. Reduce hybrid viability Reduce hybrid fertility Hybrid breakdown
Viable fertile offspring
Reduce hybrid viability
Reduce hybrid fertility
Hybrid breakdown
Fertilization
Gametic isolation
GAMETIC ISOLATION
REDUCED HYBRID VIABILITY
REDUCED HYBRID FERTILITY
HYBRID BREAKDOWN
(h)
(i)
(j)
(k)
(l)
(m)

4. Chapter 24: The Origin of Species
What is a species?
What kinds of barriers keep different species isolated so they cannot mate?
How are new species created?
Allopatric speciation
when a geographic barrier isolates a population blocks gene flow
ex. mountain range emerging, new river dividing a field, island
Sympatric speciation
intrinsic factors such as chromosomal changes (plants) or
non-random mating alter gene flow

5. Figure 24.5 Two main modes of speciation
Allopatric speciation. A population forms a new
species while geographically isolated from its parent population.
(b)
Sympatric speciation. A small population becomes a new species without geographic separation.

6. Chapter 24: The Origin of Species
What is a species?
What kinds of barriers keep different species isolated so they cannot mate?
How are new species created?
Allopatric speciation –
when a geographic barrier isolates a population blocks gene flow
ex. mountain range emerging, new river dividing a field, island
Adaptive radiation
evolution of many diversely adapted species from a
common ancestor
Seen on islands
Sympatric speciation
intrinsic factors such as chromosomal changes (plants) or
non-random mating alter gene flow

7. Figure 24.12 Adaptive radiation
Dubautia laxa
Dubautia waialealae
KAUA'I
5.1
million
years
O'AHU
3.7
LANAI
MOLOKA'I
1.3 million years
MAUI
HAWAI'I
0.4
Argyroxiphium sandwicense
Dubautia scabra
Dubautia linearis N

8. Chapter 24: The Origin of Species
What is a species?
What kinds of barriers keep different species isolated so they cannot mate?
How are new species created?
Allopatric speciation –
when a geographic barrier isolates a population blocks gene flow
ex. mountain range emerging, new river dividing a field, island
Adaptive radiation
evolution of many diversely adapted species from a
common ancestor
Seen on islands
Sympatric speciation
intrinsic factors such as chromosomal changes (plants) or
non-random mating alter gene flow
ex. oats, cotton, tobacco, potatoes, wheat
Autopolyploidy
An individual has more than 2 chromosome sets derived from
a single species from an error in meiosis

9. Figure 24.8 Sympatric speciation by autopolyploidy in plants
Failure of cell division in a cell of a growing diploid plant after chromosome duplication gives rise to a tetraploid branch or other tissue.
Gametes produced by flowers on this branch will be diploid.
Offspring with tetraploid karyotypes may be viable and fertile—a new biological species.

10. Chapter 24: The Origin of Species
What is a species?
What kinds of barriers keep different species isolated so they cannot mate?
How are new species created?
Allopatric speciation –
when a geographic barrier isolates a population blocks gene flow
ex. mountain range emerging, new river dividing a field, island
Adaptive radiation
evolution of many diversely adapted species from a
common ancestor
Seen on islands
Sympatric speciation
intrinsic factors such as chromosomal changes (plants) or
non-random mating alter gene flow
Autopolyploidy
An individual has more than 2 chromosome sets derived from
a single species from an error in meiosis
Allopolyploidy
2 different species produce the polyploid hybrid

11. Figure 24.9 One mechanism for allopolyploid speciation in plants
Meiotic error;
chromosome
number not
reduced from
2n to n
Unreduced gamete
with 4 chromosomes
Hybrid with
7 chromosomes
with 7 chromosomes
Viable fertile hybrid
(allopolyploid)
Normal gamete
n = 3
Species A
2n = 4
Species B
2n = 6
2n = 10

12. Chapter 24: The Origin of Species
What is a species?
What kinds of barriers keep different species isolated so they cannot mate?
How are new species created?
What is the difference between gradualism & punctuated equlibrium?

13. Figure 24.13 Two models for the tempo of speciation
Gradualism model. Species
descended from a common
ancestor gradually diverge
more and more in their
morphology as they acquire
unique adaptations.
Time
(a)
Punctuated equilibrium
model. A new species
changes most as it buds
from a parent species and
then changes little for the
rest of its existence.
(b)