1. THE ORIGIN OF SPECIES
Chapter 24

2. Speciation Speciation The origin of a new species
Speciation is a focal point of evolutionary theory
Must explain how new species originate and how populations evolve
Microevolution: Changes in allele frequency in a population over time
Macroevolution: Broad patterns of evolutionary change above the species level

3. Biological Species Concept
Biological Species Concept (used by Campbell’s Biology) defines a species as:
A group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring
Gene flow between populations keeps the populations as one species
Drawbacks:
Can not use biological species concept to analyze fossil species
Can not apply to species that reproduce asexually (all prokaryotes)

4. Other Definitions of Species
Morphological Species Concept:
Defines species by structural and anatomical differences and similarities
Ecological Species Concept:
Defines species by their ecological niche
Phylogenetic Species Concept:
Defines species as the smallest group of individuals on a phylogenetic tree

5. Getting Back to Biological Species Concept
By defining a species as a group of individuals that can successfully reproduce, let’s focus on what keeps organisms from NOT reproducing
Reproductive isolation: Existence of biological barriers that prevent two species from producing viable, fertile offspring
Hybrids
Offspring of crosses between species
Can classify reproductive isolation as before or after fertilization

6. Prezygotic Barriers: Before Fertilization
Block fertilization from ever happening by:
Impeding different species from attempting to mate
Preventing the successful completion of mating
Hindering fertilization if mating is successful

7. Prezygotic barriers (a) (c) (e) (f) (g) (d) (b) Habitat Isolation
Temporal
Isolation
Behavioral
Isolation
Mechanical
Isolation
Gametic
Isolation
Individuals of
different
species
MATING
ATTEMPT
FERTILIZATION
(a)
(c)
(e)
(f)
(g)
Figure 24.3 Exploring: Reproductive Barriers
(d)
(b)

8. Postzygotic Barriers: After Fertilization
Prevent hybrid zygote from developing into a viable, fertile adult by:
Reduced hybrid viability
Reduced hybrid fertility
Hybrid breakdown

9. Postzygotic barriers Reduced Hybrid Viability Reduced Hybrid Fertility
Breakdown
VIABLE,
FERTILE
OFFSPRING
FERTILIZATION
(h)
(i)
(l)
Figure 24.3 Exploring: Reproductive Barriers
(j)
(k)

10. How Does Speciation Occur?
Speciation can occur with or without geographic isolation
Allopatric speciation (with isolation)
Sympatric speciation (without isolation)

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

12. Allopatric Speciation
Allopatric (“other country”)
Gene flow is interrupted when population divided into geographically isolated subpopulations
Geographic barrier prevents members of subpopulation from interacting
Ability of population to disperse defines how much of a barrier is needed
Separate populations may evolve independently through mutation, natural selection and genetic drift

13. Sympatric Speciation Sympatric (“same country”)
Speciation takes place in geographically overlapping populations
Although less common than allopatric, speciation can occur if gene flow is reduced by factors such as:
Polyploidy
Habitat differentiation
Sexual selection

14. Polyploidy Polyploidy
Mistakes in cell division result in extra sets of chromosomes
Rare in animals but common in plants
Many important crops such as oat and wheat are polyploids

15. Polyploidy Cell division error 2n = 6 Tetraploid cell 4n = 12 2n 2n
Figure 24.UN01 In-text figure, p. 495 2n 2n
New species
(4n)
Gametes produced
by tetraploids

16. Habitat Differentiation
Ecological niche
Sum of an organism’s use of biotic and abiotic resources in an environment
If some organisms can exploit a new niche from the parental population, they may eventually evolve into a new species

17. Hybrid Zones Hybrid zone
Region in which members of different species mate and produce hybrids (incomplete reproductive barriers)
Hybrids often have reduced fitness compared to either parental species

18. Hybrid Zones Over Time
When closely related species meet in a hybrid zone, 3 possible outcomes:
Reinforcement
Hybrids less fit than parent species, fewer and fewer hybrids
Fusion
Hybrids as fit as parents, forms single species
Stability
Continued production of hybrids

19. Outcomes in Hybrid Zones
Possible
outcomes:
Isolated
population
diverges
Hybrid
zone
Reinforcement
Figure Formation of a hybrid zone and possible outcomes for hybrids over time. OR
Fusion OR
Gene flow
Population
Hybrid
individual
Barrier to
gene flow
Stability

20. How Long Does Speciation Take?
Hard to say but patterns in fossil records, morphological data or molecular data can give some clues
Fossil record shows some species appearing suddenly, remaining unchanged and then disappear
Punctuated equilibrium
Contrasts with model of gradual change
Speciation by punctuated equilibrium model appears fast in geological time (still a long time!)

21. Speciation Patterns (a) Punctuated pattern Time (b) Gradual pattern
Figure Two models for the tempo of speciation.
(b)
Gradual
pattern

22. How Long Does Speciation Take?
Lots of variation in speciation time
Interval can range from 4,000 years in some species (like cichlids) to 40 million years (some insects)
Average is 6.5 million years