1. chapter 24 Campbell and Reece
The Origin of Species
chapter 24
Campbell and Reece

2. Speciation process by which one species splits into 2 or more species
Speciation explains both the diversity of life and the unity of living things.

3. Speciation : forms bridge between:
MICROEVOLUTION
Evolutionary change below species level
Example:
change in allele frequencies in population over generations
MACROEVOLUTION
Evolutionary change above the species level
Examples:
origin of new group of organisms
impact of mass extinctions

4. Biological Species Concept
Species: a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring
members of a species cannot produce viable, fertile offspring with other groups
emphasizes the separateness of species due to reproductive barriers

5. What holds the gene pool of a species together?
Gene Flow: transfer of alleles between populations of same species
exchange of alleles tends to hold populations together genetically

6. Reproductive Isolation
existence of biological barriers that keep members of 2 populations from interbreeding over long periods of time

7. Reproductive Isolation
hybrids: offspring that result from the mating of individuals from 2 different species or from 2 true- breeding varieties of same species

8. Prezygotic Barriers block fertilization from happening by:
impeding members of different species from attempting to mate
preventing attempted mating from being completed successfully
hindering fertilization if mating was completed successfully

9. Postzygotic Barriers
reproductive barrier that prevents hybrid zygotes produced by 2 different species from developing into viable, fertile adults
lethal developmental errors
infertility in viable offspring

10. Types of Prezygotic Reproductive Barriers
Habitat Isolation
2 species that occupy different habitats w/in same area may rarely interact
example:

11. Types of Prezygotic Reproductive Barriers
2. Temporal Isolation: species breed during different times of day, different seasons, or different years

12. Types of Prezygotic Reproductive Barriers
3. Behavioral Isolation: Courtship rituals used to attract mates are effective barriers

13. Types of Prezygotic Reproductive Barriers
4. Mechanical Isolation:
morphological differences prevent successful completion even if attempted

14. Types of Prezygotic Reproductive Barriers
5. Gametic Isolation:
Sperm of 1 species may not be able to fertilize egg of another:
Reproductive tract hostile to sperm
Sperm does not have enzymes to penetrate zona pellicida of another species

15. Types of Postzygotic Reproductive Barriers
Reduced Hybrid Viability:
hybrids development or survival is impaired

16. Types of Postzygotic Reproductive Barriers
2. Reduced Hybrid Fertility: hybrids may develop and be healthy but they are not fertile

17. Types of Postzygotic Reproductive Barriers
Hybrid Breakdown:
Some 1st generation hybrids are fertile but those offspring are feeble or sterile

18. Species
There is no single, universally applicable species concept that can define, explain, and identify all species.
There are multiple ways to think about & define species.

19. Biological Species Limitations
unable to use these characterisitics on fossils of extinct species
only applies to organisms that reproduce sexually
only applies where there is no gene flow

20. Other Definitions of Species
These dfns emphasize the unity w/in a species.
morphological species concept:
characterizes a species by a structural feature
applies to species that reproduce sexually or asexually
how scientists distinguish most species
disadvantage: subjective

21. Morphological Species Concept
Problems:
domestic dogs may look very different but are still same species
mouse lemurs look very similar but there are 18 species of them
grey mouse lemur lesser mouse lemur

22. Ecological Species Concept
views species in terms of its niche
the sum of how members of the species interact with the nonliving & living parts of their environment
asexual or sexual species
emphasizes role of disruptive NS as organisms adapt to different environmental conditions

23. Phylogenetic Species Concept
defines species as smallest group of individuals that share a common ancestor, forming one branch on the “tree of life”
determining degree of differences is difficult

24. Species
There are >20 other ways to define species

25. Speciation can take place with or w/out geographic separation
Speciation can occur in 2 main ways:
Allopatric Speciation
Sympatric Speciation

26. ALLOPATRIC SPECIATION
“other country”
Gene flow is interrupted when population is divided into geographically isolated subpopulations

27. Allopatric Speciation Process
Once geographic separation has occurred, the separated gene pools will each have their own mutations
NS & genetic drift may alter allele frequencies in different ways in each subpopulation group

29. Allopatric Speciation

30. Evidence of Allopatric Speciation
There are many studies & examples supporting this type speciation
Indirect support: regions that are isolated or highly subdivided have more species than regions w/out those features

31. Drosophila Experiment

32. Sympatric Speciation “same country”
occurs in populations in same geographic area
less common than allopatric
occurs if gene flow is reduced by factors like:
polyploidy
habitat differentiation
sexual selection

33. POLYPLOIDY means extra sets of chromosomes can occur in animals
gray tree frog (Hyla versicolor)
around Great Lakes

34. Polyploidy much more common in plants
estimate: 80% of today’s plants species have ancestors that formed by polyploid speciation
2 forms

35. 1. Autopolyploid
Individual has >2 chromosome sets all derived from a single species
Plant polyploidy

36. Plant Polyploidy
tetraploid plant can produce fertile tetraploid offspring by self- pollinating or mating with other tetraploids

37. 2. Allopolyploid 2 different species interbreed making a hybrid
hybrid reproduces asexually
over generations sterile hybrid  fertile polyploid (called an allopolyploid)

38. Allopolyploids
can breed with each other but not with either of their parents so are a new species
rare: 5 new plant species since documented
Mimulus peregrinus

39. Allopolyploids include many agricultural crops
Triticum aestivum (bread wheat) has 6 sets chromosomes (2 pair from each of 3 parents), an allohexaploid
1st polyploidy event probably occurred ~8,000 yrs ago as spontaneous hybrid

40. Top 2 parents bottom: Triticum aestivum

41. Allopolyploids
plant geneticists “create” new polyploids making hybrids with desired characteristics
use chemicals that induce meiotic & mitotic errors

42. Habitat Differentiation
Sympatric speciation can occur when genetic factors enable a subpopulation to exploit a habitat or resource used by the parent population

43. Rhagoletis pomonella North American apple maggot fly
Original habitat was the native hawthorn tree

44. Habitat Differentiation
As apples mature faster than hawthorn fruit, NS has favored flies with rapid development
have an allele that benefits flies that feed off only 1 or the other not both (post-zygotic barrier to reproduction)
The flies feeding on apple trees now show temporal isolation from flies still eating hawthorn fruit (prezygotic restriction to gene flow)

45. Sexual Selection can also drive sympatric speciation:
cichlid fish Pundamilia pundamilia

46. Cichlid Fish >600 species found in Lake Victoria
originated in past yrs
hypothesis : subgroups of original population adapted to different food sources  genetic divergence
female preference for mates may also be a factor: 1 species breeding males have blue back another species has orange back

47. Breeding Cichlids Colors

48. Sexual Selection Study
placed the 2 subspecies in same tank
used monochromatic orange light so both appeared very similar
females bred with either
Conclusion: mate choice by females is based on male coloration so it’s the main reproductive barrier (prezygotic behavior)

49. Allopatric & Sympatric Speciation are the 2 main modes of speciation
geographic isolation NS
genetic drift
sexual selection
Sympatric
requires emergence of a reproductive barrier that isolates a subgroup
less common
polyploidy
sexual selection

50. Hybrid Zones