1. The How and Why of Species
Speciation
The How and Why of Species

2. What is a Species?
A species is one or more populations of organisms with the potential to interbreed with one another but NOT with members of other such groups naturally.
“But… What about the liger?”
We’ll get there. I promise!

3. Determining Separate Species

4. Biological Species Concept
Proposed by Ernst Mayr who said:
“Species are groups of interbreeding natural populations that are reproductively isolated from other such groups."
Explains why the members of a species resemble one another and differ from other species.
Breeding organisms pass genes to offspring
By contrast, genes are not transferred to other species, and different species therefore look different
Has some fallacies: asexual organisms, hybrids, ring species, chronospecies

5. What Leads
to New Species?

6. What leads to New Species?
Diversifying or Directional Selection can lead to new species.
More on this later!
When differences between subpopulations become large enough that gene flow between them may stop.

7. Reproductive Isolating Mechanisms
Prezygotic Barriers

8. Reproductive Isolating Mechanisms
Prezygotic Barriers
Geographic Isolation
Ecological Isolation.
Temporal(Time) Isolation
Behavioral Isolation
Mechanical Isolation
Gametic Isolation
Prezygotic Barriers
Geographic Isolation: A physical barrier does not allow the species to mate.
Ecological Isolation: Species occur in the same area but occupy different habitats.
Temporal(Time) Isolation: Species breed during either different times of the day, different seasons, or different years.
Behavioral Isolation: Unique behavior patterns isolate species.
Mechanical Isolation: Morphological differences can prevent successful mating.
Gametic Isolation: The sperm of one species may not be able to fertilize the egg of another species.

9. Reproductive Isolating Mechanisms
Postzygotic Barriers
Reduced Hybrid Viability
Reduced Hybrid Fertility
Hybrid Breakdown
Reduced Hybrid Viability- The hybrid would be very fragile. The genes of the different parents will interact and impair the hybrids development.
Reduced Hybrid Fertility- The hybrids will survive and live a full life but they are unable to produce offspring.
Hybrid Breakdown- hybrids could be fertile and viable in the first generation but when they mate offspring are feeble and sterile.

10. Modes of Speciation

11. Allopatric speciation of squirrels in the Grand Canyon

12. Allopatric Speciation
New species arise as a result of geographic isolation
“Rivers change course, mountains rise, continents drift, organisms migrate, and what was once a continuous population is divided into two or more smaller populations”
Allopatric means “different homelands”
More likely to occur in small populations because the smaller gene pool can be changed easier

13. Sympatric Speciation
Cichlids from Lake Victoria

14. Sympatric Speciation

15. Sympatric Speciation
Two subpopulations become reproductively isolated within the same geographic area.
First proposed by Darwin in the 1850s.
A population of insects feed on a single type of plant. Some begin to feed on another plant – they might not interbreed. The two groups could then evolve independently.

16. Parapatric Speciation
Images from Evolution Berkeley

17. Parapatric Speciation
No specific extrinsic barrier to gene flow
Continuous population exists but the population does not mate randomly
Individuals are more likely to mate with their geographic neighbors than with individuals in a different part of the population’s range
Divergence may happen because of reduced gene flow within the population and varying selection pressures across the population’s range

18. Competitive Exclusion

19. Competitive Exclusion
Also known as Gause’s Law
Two species that compete for the exact same resources cannot stably coexist.
As a result, competing related species often evolve distinguishing characteristics in areas where they both coexist

20. Rate of Speciation

21. Rate of Speciation
Often can take millions of years, but can occasionally occur faster.
Banana trees  moth species
Gradualism
Punctuated equilibrium
Gradualism – occurs at a regular, gradual rate
Punctuated equilibrium – occurs in “bursts” followed by periods of little change

22. And Now, Those Hybrids…

23. And Now, Those Hybrids… Ligers – hybrid between lions and tigers
Zebroids – hybrid between horses and zebras
Cama – hybrid between a camel and a llama (artificial insemination)
Wolphin - bottlenose dolphin and a false killer whale
Remember, most of these hybrids are sterile and cannot reproduce with each other.