1. Ch. 24 – The Origin of Species

2. What is a Species? Speciation – the origin of new species
The biological species concept defines a species as a population or group of populations whose members have the potential to interbreed with one another in nature to produce viable, fertile offspring, but who cannot produce viable, fertile offspring with members of other species
Eastern and Western Meadowlarks

3. How does speciation occur?

4. Isolation of a Species
Reproductive Isolation – biological species concept states that each species is isolated by factors (barriers) that prevent interbreeding
Prezygotic Barriers – impede mating between species or hinder fertilization of ova
Habitat Isolation
Behavioral Isolation
Temporal Isolation
Mechanical Isolation
Gametic Isolation

5. Prezygotic Barriers
Habitat Isolation – two species living in different habitats within the same area will not encounter one another
Example: garter snakes in the same genus that live in the same area, but one lives mainly in water and one on land; they will most likely NOT mate.

6. Prezygotic Barriers
Behavioral Isolation – Special signals used by a species to attract mates
Examples: fireflies recognize signals of their own species, frogs have calls that only attract their own species, and birds have songs that only attract their own species
Blue-Footed Boobies

7. Prezygotic Barriers
Temporal Isolation – Species that breed during different times of the day, season, or year
Example: Similar frog species that do not interbreed

8. Prezygotic Barriers
Mechanical Isolation – Structural differences in genitalia or flowers
Example: flowers whose pollen is carried by insects to flowers of other species; insects whose copulatory organs may not fit together, so no sperm would be transferred

9. Prezygotic Barriers
Gametic Isolation – male and female gametes fail to attract each other or are unviable
Example: aquatic species who release sperm and egg into the water where the eggs are fertilized. Gamete recognition does not occur and the sperm are unable to fertilize the egg. Similar to the mechanism used by flowers to recognize sperm (pollen) of their own species.

10. Postzygotic Barriers
If a sperm cell from one species does fertilize an ovum of another species, then postzygotic barriers usually prevent the hybrid zygote from developing into a viable, fertile adult
Reduced Hybrid Viability
Reduced Hybrid Fertility
Reduced Breakdown

11. Postzygotic Barriers
Reduced Hybrid Viability – hybrid zygotes fail to develop or fail to reach sexual maturity
Example: Some species of frogs in the genus Rana may interbreed, however the offspring usually do not complete development and those that do are frail.

12. Postzygotic Barriers
Reduced Hybrid Fertility – hybrid fails to produce functional gametes
Example: mules – a hybrid of a donkey and a horse. Mules cannot backbreed with either species

13. Postzygotic Barriers
Hybrid Breakdown – offspring of hybrids have reduced viability or fertility
Example: different cotton species can produce fertile hybrids, but breakdown occurs in the next generation when offspring of the hybrids die as seeds or grow into weak and defective plants

14. Reproductive Barrier Summary

15. Three Modes of Speciation
Allopatric Speciation
Adaptive Radiation
Sympatric Speciation
Polyploidy

16. Allopatric Speciation
Geographic Barrier
Interbreeding between two resulting populations is prevented
Barriers include rivers to mountain ranges
Gene flow is initially interrupted because they are separated in space
Gene flow then diverges

17. Allopatric Speciation
Likelihood of allopatric speciation increases with small, isolated populations; Ex. Galapagos Islands
The barrier’s effect depends on the ability of the organisms to move about
The gene pool can be changed dramatically by genetic drift and natural selection

18. Allopatric Speciation

19. Allopatric Speciation
Antelope Squirrels
Interbreeding cannot
occur

20. Does Speciation Always Occur with Allopatric Conditions?
Allopatric conditions could lead to reproductive isolation
Evolutionary divergence of the ring species
Adaptive radiation of the island species
Origin of prezygotic barriers (ex. Fruit flies)
Origin of postzygotic barriers between distant populations of the monkey flower
The isolated population’s gene pool evolves by genetic drift and natural selection; reproductive isolation evolves as a byproduct

21. An Example of Adaptive Radiation on Island Chains - Species diverge as populations specialize for each set of geographical and ecological conditions

22. “Lobster Claw Syndrome”

23. Sympatric Speciation
Formation of a new species without the presence of a geographic barrier
Gene flow is reduced due to chromosomal changes and nonrandom mating

24. Sympatric Speciation In plants
Autopolyploids have more than 2 chromosome sets
Allopolyploids are hybrids of two different species with more than 2 chromosome sets

25. Sympatric Speciation
Autopolyploidy in plants
Allopolyploidy in plants

26. Sympatric Speciation In animals
Genetic factors could cause “daughter” populations to become dependent on resources other than the parent population
Ex. Cichlids in Lake Victoria show sexual dimorphism and females tend to mate only with males who have their same coloration although they are able to mate with both morphs. Over time the two cichlid populations may diverge into two species

27. Divergent Evolution
Two or more species originate from a common ancestor.
Results from Allopatric speciation, Adaptive Radiation and/or Sympatric speciation

28. From Speciation to Macroevolution
Microevolution – change over generations in a population's allele frequencies (by genetic drift and natural selection)
Speciation – population’s genetic divergence from its ancestral population that results in reproductive isolation.
Macroevolution – Level of change evident over the time scale of the fossil record

29. Morphological Transformations
The evolution of complex structures from simpler versions (ex. eyes)
Exaptations – structures that evolve in one context, but become co-opted for another function (ex. birds hollow bones must have had some adaptation on the ground)

30. Punctuated Equilibrium Model
Paleontologists rarely find gradual transitions of fossil forms.
They often observe species appearing as new forms rather suddenly in a layer of rock, persisting essentially unchanged, and then disappearing from the fossil record as suddenly as they appeared.

31. Punctuated Equilibrium Model
Punctuated Equilibrium – species diverge in spurts of relatively rapid change Long periods of stasis punctuated by episodes of speciation.

32. Species Selection
Species that endure the longest and reproduce the most offspring, determine the direction of major evolutionary trends
Evolution is never-ending, it is still going on today
Evolution is a response to interactions between organisms and their current environments. If conditions change, an evolutionary trend may cease or even reverse itself.