1. Chapter 24
The Origin of Species

2. Speciation
The origin of a new species is the focal point of evolutionary theory. The appearance of a new species is the main point of diversity.
Microevolution describes the adaptations that arise within a gene pool.
Macroevolution occurs when evolutionary change occurs above the species level. These are big changes.

3. 2 Patterns of Evolutionary Change
1. Anagenesis
2. Cladogenesis

4. 1. Anagenesis
Anagenesis is also called phyletic evolution and it is the accumulation of changes that happens gradually over time and transforms a given species into a species with different characteristics.
An example would be how the color of a bird changes over time.

5. 2. Cladogenesis
Cladogenesis is also called branching evolution and is the splitting of the gene pool into two or more separate pools. These pools give rise to new species. Cladogenesis promotes biodiversity by increasing the number of species.

7. Ernst Mayr and the Biological Species Concept
This concept defines a species as a population or group of populations whose members have the ability to interbreed in nature and produce variable, fertile offspring, and are unable to produce viable, fertile offspring with other populations.

8. Reproductive Isolation
Reproductive isolation is a mechanism by which many species are isolated from one another due to the existence of many different types of biological barriers.
1. Prezygotic barriers impede mating or hinder fertilization.
2. Postzygotic barriers prevent viable, fertile adults from forming.

9. Prezygotic Barriers to Mating
Impede mating/hinder fertilization:
1. Habitat isolation
2. Temporal isolation.
3. Behavioral isolation.
4. Mechanical isolation.
5. Gametic isolation.

10. 1. Habitat Isolation
Occurs as 2 species live in the same area, but encounter each other rarely, if ever.
Example 2 species of garter snakes. One lives in the water, the other lives on land.

11. 2. Temporal Isolation
This occurs when there are differences in breeding times, seasons, or years which prevents gamete mixing.
Example: Eastern and Western spotted skunk. Their geographic ranges overlap, but the Eastern skunk mates in the later winter, and the Western skunk in the late summer.

12. 3. Behavioral Isolation
Courtship rituals which occur between closely related, but different individuals produce effective reproductive barriers.
Example: Blue footed boobies of the Galapagos. The courtship ritual for these organisms males do a dance where he shows off his blue feet to a female in high-step form.

13. 4. Morphological Isolation
Occurs when differences in appearance and/or anatomy cause different species to be unable to mate.
Example: Plants of different color attract different pollinators preventing cross pollination. Also, differences in flower shape and design prevents cross pollination.

14. 5. Gametic Isolation
This happens when the sperm of one species is unable to fertilize eggs of another species.
Reasons for why this does not occur; The sperm may not survive the reproductive tract of a female of a different species; the sperm may not be able to penetrate the egg once it gets there.
Example: Sea urchins are a variety of closely related aquatic animals. They reproduce in a similar way, but they are distinct enough that their gametes do not fuse to form zygotes.

15. Post Zygotic Barriers to Mating:
Prevent production of viable/fertile adults.
1. Reduced hybrid viability.
2. Reduced hybrid fertility.
3. Hybrid breakdown.

16. 1. Reduced Hybrid Viability
Occurs when genes of different parent species interact and impair normal growth and development of the organism.
Example: A specific subspecies of salamander live in areas where they occasionally meet and breed. Often times the offspring do not develop fully and those that do are not very fit.

18. 2. Reduced Hybrid Fertility
Sometimes 2 species can mate and reproduce a viable species that is sterile. The sterility is often a result of the two parents having a different number or structure of chromosomes. Thus, meiosis fails to produce normal gametes.
Example: A donkey and a horse mate and a mule is formed.

19. Donkey Horse = Mule

20. 3. Hybrid Breakdown
Occurs when 2 parents meet and produce offspring. When these offspring mate with each other or the parents, the resulting offspring is very weak and sterile.
Example: different strains of rice that each carry a number of recessive alleles. When the offspring mate, the recessives accumulate in the F1. Thus, the resulting F2 offspring are very weak and sterile.

22. Limitations of the BSC
These barriers to mating are not easily seen because we can’t observe the matings of fossilized remains.
We also can’t evaluate the reproductive isolation of prokaryotes and other organisms.
Additionally, there are a lot of animals we don’t know much about making it difficult to apply this concept.

23. Other Definitions for Species:
1. Morphological species concept.
2. Paleontological species concept.
3. Ecological species concept.
4. Phylogenetic species concept.

24. 2 Main Ways of Speciation
1. Allopatric (other country) speciation.
2. Sympatric (same country) speciation.

25. 1. Allopatric Speciation
Occurs when gene flow in a population is interrupted by a geographic barrier.
Example: Lakes may rise and fall separating groups of individuals. Rivers may split land that was formerly as one.
Once the barrier has been set up and populations begin to diverge, mutations and natural selection take over and allele frequencies change as genetic drift alters the gene pool.

28. 1. Allopatric Speciation
Allopatric speciation is likely to occur in these small populations as they are more affected by genetic drift than larger populations.
To confirm allopatric speciation, scientists bring together 2 species in a laboratory and see if they can successfully breed and produce viable, fertile offspring.

29. 1. Allopatric Speciation
The Galapagos ground finch Geospiza dificilis:
Females respond to songs from males from the same island and ignore songs from males of the same species from different islands (allopatric populations) This demonstrates that prezygotic barriers have developed in these allopatric populations and that they are on their way to becoming separate species.

30. 2. Sympatric Speciation
This occurs in geographically overlapping populations. Even though direct contact remains between members of the same species, mechanisms such as chromosomal changes and nonrandom mating alter gene flow.

31. 2. Sympatric Speciation A general example:
Polyploidy occurs when accidents happen during cell division, and cells end up with extra sets of chromosomes.
If a diploid cell becomes a tetraploid (4n) as a result of an error, and the organism is able to self-fertilize, it can become reproductively isolated in just one generation.

32. 2. Sympatric Speciation Example:
The North American apple maggot-fly usually colonizes Hawthorn trees--and eats haws.

33. 2. Sympatric Speciation
Some flies have switched from haws to apples and lay their eggs on the apples while the fruit is still on the tree.
They often compete with each other for territory on apple trees, rather than with other flies on the hawthorn trees.
These flies are becoming more and more entwined with the cycle of the apple trees.

34. 2. Sympatric Speciation
Apples ripen and fall about a month earlier than the haws.
One month is a lifetime for these flies, so the switch from haws to apples by these flies is pushing the two gene pools apart.
One fly is reliant upon the life-cycle of the haw tree, the other is reliant on the life-cycle of the apple tree and they are shifting their breeding cycles to account for their new habitat.

35. Adaptive Radiation
This occurs when a few organisms make their way into a new environment and give rise to diversely adapted populations of new organisms that have descended from a common ancestor.
This is often seen following mass extinctions when numerous niches open up.

36. The Tempo of Speciation
Niles Eldridge and Stephen Jay Gould coined the term punctuated equilibrium to describe the tempo of speciation.
Punctuated equilibrium is marked by periods of apparent stasis in the evolution of an organism and then is followed by points where speciation occurs rapidly.

37. The Tempo of Speciation
Sometimes we see fossils in the strata that never change and then disappear.
This doesn’t mean that the organism didn’t change or evolve. It just means that the events that produced the new species may have occurred too rapidly to have been preserved in the fossil record.

38. The Tempo of Speciation
For example:
Say a species survived for 5 million years, and many of its major morphological changes occurred in the first 50,000 years (1%). Often times this is too quick to be preserved in the fossil record. Then, it would seemingly appear suddenly, linger with no little/no change, and then become extinct.

39. The Tempo of Speciation
Stasis can also be explained this way. Often times, changes go undetected by the fossil record. Consider changes in biochemistry--they can’t be detected by paleontologists.

40. Macroevolution
Macroevolution results as species diverge and speciate again and again resulting in differences that accumulate and become more pronounced. Speciation is the beginning of macroevolutionary change.
These cumulative changes occur as a result of thousands of small speciation events.
Thus, if you accept microevolution, you get macroevolution for free.

41. The Arguments
Many arguments against evolution fail to recognize the fact that many complex structures evolve in small increments from simple ancestral structures that perform the same basic function.
It’s hard to imagine millions of years when you can’t comprehend what a million actually is.

42. The Arguments
For example, consider the human eye. It is a very complex structure that works to form an image and transmit the information to the brain for processing.
How could such a complex structure evolve in gradual increments?
How could a partial eye be of any use to our ancestors?

43. The Arguments
The flaw in the argument is the assumption that partial eyes have no use. Simple light sensors are useful, even though they can’t focus an image.
Many different eyes evolved on different organisms as they diverged from a common ancestor with light-sensing photoreceptors.

45. Remember,
Evolution is not goal oriented, it is the result of the interactions of organisms with their current environments. The most fit organisms survive. As the environments change over time, so to do the organisms.
These small changes (microevolution) accumulate and slowly give rise to big changes (macroevolution).

46. Remember Also,
“Speciation is a process, NOT an event.”