1. 1/12/2019
Speciation

2. Today’s OUTLINE: (1) Geographic Mechanisms of Speciation
(What circumstances lead to the formation of new species?)
(2) Species Concepts
(How are Species Defined?)

3. Mechanisms of Speciation
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Mechanisms of Speciation
Last Time: Genetic Models:
The roles of: Mutations
Natural Selection
Genetic Drift
This Time: Geographic Models:
Allopatric Model (difference place)
Sympatric Model (same place)
Parapatric Model (adjoining)

4. (1) Mechanisms of Speciation
Last Time: Genetic Models:
How do Genetic Drift, Natural Selection, Mutations, etc. create new species?
Are there “speciation” genes?
This Time: Geographic Models:
How does speciation occur in Nature?
Is geographic isolation required?

5. Mechanisms of Speciation
Geographic (Ecological) Models:
Allopatric Model (different place)
Disperse to Another Location
Vicariance: a barrier is formed
This geographic split could lead to Dobzhansky-Müller incompatibilities
Sympatric Model (same place)
Polyploid speciation
Mate Choice
Niche Partitioning (e.g. different food source, Host Plant)
Parapatric Model (adjoining)

6. Geographic Models of speciation
Allopatric speciation: geographic isolation
Sympatric speciation: no geographic isolation
Parapatric speciation: geographic separation (or gradient), but not isolation

7. Allopatric Models
Involves Geographic Isolation
Dispersal
Vicariance

8. Allopatric Models Following geographic separation between populations,
Dispersal
Vicariance
This geographic separation provides the setting that allows speciation at the molecular level to occur (last lecture)

9. Allopatric Models Dispersal Vicariance
Random Mutations would arise in the separated populations, and then selection or genetic drift would lead to fixation of those mutations
If different mutations are fixed in the different populations, reproductive isolation could arise through Dobzhansky-Müller incompatibilities (last lecture)

10. Allopatric Speciation
Examples (see book)
Dispersal:
Colonization of islands
Colonization of lakes
Vicariance:
Highway going through a forest
Fragmentation of habitats
Formation of Panama splitting the Caribbean &Pacific Oceans

11. Sympatric models Speciation with no geographic separation
Speciation despite gene flow
Formation of polyploids
(discussed in previous lecture)
(2) Natural Selection due to
Niche Partitioning
Sexual Selection

12. Sympatric Model (1) Formation of Polyploids
(covered in previous lecture)
Important mechanism for plants
Occurs rarely in animals
Autopolyploidy: happening in one spot (in the plant)
Allopolyploidy: the different plant taxa have to be in the same location to hybridize

13. Sympatric Model Niche Partitioning
(2) Selection in the face of gene flow:
Niche Partitioning
Strong assortative mating and sexual selection (disruptive selection)

14. Example of Niche Partitioning:
Soapberry bugs that have adapted to two different host plants

15. Selection drives beak length apart

16. Evolutionary change in beak length on the new small fruit
trend toward smaller beaks on smaller fruit

17. Niche Partitioning Soapberry bugs mate on different host plants
The populations that live and mate on different fruit are unlikely to encounter each other
Reduces gene flow
Isolation
Disruptive Natural Selection
Adaptation to alternative hosts leads to reproductive isolation (through the genetic mechanisms discussed earlier, such as Dobzhansky-Müller model)

18. But, sometimes hybrid zones do form between populations that are in the process of speciating
Sometimes hybridization between different species results in vigorous new species or populations, especially in plants (hybrid vigor, or heterozygote advantage)
The effects vary depending on how distant the two species or populations are… and whether the different alleles at different loci are able to work together (coadapted gene complexes)
Hybrids between different populations within a species do tend to have an advantage (Heterozygote advantage). However, mating between very distant populations (different species) can lead to hybrid breakdown.

19. Increasing genetic distance
1/12/2019
Increasing genetic distance
Mating between different species
(Lions x tiger,
Horse x donkey)
Fitness
Will not mate or
Produce inviable or sterile hybrids
Mating between relatives
Populations within a species
Outbreeding
Depression = Hybrid Breakdown
Inbreeding
Depression
Hybrid Vigor
(due to Heterozygote advantage)

20. (2) How are Species Defined?

21. How are species defined?
So, what criterion should be used? Historically, the most common criteria had been using morphological characters (external phenotype)

22. Speciation is a messy process
Rates of molecular, phenotypic (morphological) evolution and reproductive isolation are not necessarily concordant, but often discordant
Speciation is a jagged messy idiosyncratic process, where species boundaries are often difficult to define

23. Problem: Populations are in the process of speciating from one another, and species boundaries are often difficult to define until the populations are sufficiently divergent by all measures

24. So then, how do you define species???

25. Darwin’s view:
Species are arbitrary constructs of the human mind imposed on a continuum of variation
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Species are dynamic rather than static entities, with boundaries changing constantly Many groups are in the process of speciation

26. Three Main Species Concepts
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Three Main Species Concepts
Biological Species Concept
Phylogenetic Species Concept
Phenetic Species Concept (includes Morphological SC)

27. 1. Biological Species Concept (Ernst Mayr, 1942)
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1. Biological Species Concept (Ernst Mayr, 1942)
A group of interbreeding populations that are evolutionary independent of other populations

28. 1. Biological Species Concept (Ernst Mayr, 1942)
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1. Biological Species Concept (Ernst Mayr, 1942)
Example: all human populations belong to the same biological species

29. Biological Species Concept
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Biological Species Concept
Strengths
An unambiguous empirical criteria which is clearly linked to speciation (if populations can’t intermate they can’t belong to the same species)
Using reproductive isolation as the criterion is meaningful as it confirms the lack of gene flow between groups

30. Biological Species Concept
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Biological Species Concept
PROBLEMS:
Many ‘species’ are asexual and do not intermate (viruses, bacteria, protists)
Many highly divergent species can hybridize (plants)
Only applicable to present (not fossil taxa)
Ability to intermate sometimes drops off gradually (“ring species”)

31. 1/12/2019
Ring Species

32. 2. Phylogenetic Species Concept
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2. Phylogenetic Species Concept
The smallest group that is monophyletic is called a species

33. 2. Phylogenetic Species Concept
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There are several monophyletic groups here
Monophyletic group:
A group with a shared derived (descendant) character
A group that contains a common ancestor and all its descendents

34. Phylogenetic Species Concept
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Phylogenetic Species Concept
Typically, a phylogeny is constructed using DNA or heritable traits (proteins, morphological traits)
The phylogeny reveals hierarchical relationships among groups
The smallest group that has a shared derived character and is monophyletic is called a species

35. Phylogenetic Species Concept
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Phylogenetic Species Concept
There is a derived character that is shared by the 4 populations
Monophyly
The smallest monophyletic group is called a species

36. A monophyletic clade consists of an ancestral taxa and all its descendantsB
Group I B B C C C D D D E E
Group II E
Group III F F F
Figure Monophyletic, paraphyletic, and polyphyletic groups G G G
(a) Monophyletic group (clade)
(b) Paraphyletic group
(c) Polyphyletic group

37. Figure 16.1 Phylogenetic species
The taxa labeled A-G on the tips of this phylogeny represent distinct species. Groups labeled G1, G2 etc. represent populations of the same species.

38. Phylogenetic Species Concept
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Phylogenetic Species Concept
Strengths
Easy to see evolutionary relationships on large and small taxonomic scales
It can be used on any species (sexual, asexual) for which there is phylogenetic information (molecular, morphological, biochemical data) on extant or fossil species

39. Phylogenetic Species Concept
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Phylogenetic Species Concept
Problems:
Need a good phylogeny – time consuming and can be expensive
Not recognize paraphyletic groups (a monophyletic group that does not include all the descendents; reptiles are paraphyletic, as they do not include birds, because birds emerged from within reptiles)
A trivial trait (single mutation or trait) can make a group monophyletic, and may not warrant calling a group a new species

40. Examples of Paraphyletic Groups
Figure 4.13 Monophyletic and paraphyletic groups
The prokaryotes, dicotyledenous plants ("dicots"), and fish are all examples of paraphyletic groups.
Examples of Paraphyletic Groups
Paraphyly: a group which either does not include all its descendants or the ancestor.

41. Phylogenetic Species Concept
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Phylogenetic Species Concept
Problems:
A trivial trait (single mutation or trait) can make a group monophyletic, and may not warrant calling a group a new species
The cut off for a “species” is often arbitrary. For example, 3% sequence divergence is often used for bacteria

42. Phylogenetic Species Concept
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Phylogenetic Species Concept
Monophyly
Sometimes a trivial trait, like a single point mutation could make a group monophyletic, and a “species” according to the phylogenetic species concept
The smallest monophyletic group is a species

43. 3. Phenetic Species Concept
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Traditional Definition: Populations that are phenotypically similar to one another but different from other sets of populations.
Identifies species using overall similarity (often “a key”), but not in a phylogenetic context… no hierarchy – no branching pattern, no ancestral-derived relationships
Encompasses the “Morphological Species Concept”
Most often morphological traits are used, but any phenotype could be used

44. Phenetic (often Morphological) Species Concept
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Phenetic (often Morphological) Species Concept
Strengths
Most intuitive; the way we recognize species
Easiest. Easier than constructing phylogeny or intermating

45. Phenetic Species Concept
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Phenetic Species Concept
Problems:
Different species can look similar due to convergent evolution
Populations that look distinct sometimes belong to the same species
Speciation can occur without changes in morphology or other traits (cryptic species)

46. Which species concept to use?
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Which species concept to use?
When we discuss animals we often use the biological species concept as the gold standard... complemented with the phylogenetic and phenetic species concepts
Plants: it depends, since very distant plants can hybridize… phylogenetic species concept is often used.
Bacteria: poses difficult problems for classification.
Bacteria do not interbreed (≠ Biological Species concept). In some cases massive exchange of genetic material (horizontal gene transfer) leads to phylogenetic confusion.
Often a combination of the Phylogenetic and Phenetic Species Concepts (biochemical and morphological [like cell wall] traits) are used.

47. Darwin’s view:
Species are arbitrary constructs of the human mind imposed on a continuum of variation
1/12/2019
Species are dynamic rather than static entities, with boundaries changing constantly Many groups are in the process of speciation

48. However, concept of species is still useful:
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However, concept of species is still useful:
Species are considered the largest group with a common evolutionary fate

49. Concepts Geographic Models Allopatric Sympatric Reinforcement
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Concepts
Geographic Models
Allopatric
Sympatric
Reinforcement
Problems with the concept of “Species”
Species
Biological
Phylogenetic
Phenetic (Morphological)
Monophyly

50. 1. Which of the following is a species according to the biological species concept?
(A) All hominin species (most are fossil species).
(B) A population of bacteria for which 80% of their DNA sequences are identical.
(C) All allopolyploid plants.
(D) A set of populations of beetles that can intermate and produce offspring for multiple generations, but cannot intermate with other populations.

51. 2. Which of the following is NOT a reason that Species are difficult to define?
(A) Many plants that are genetically divergent are able to mate
(B) Many organisms that are morphologically similar are genetically distinct
(C) Many organisms are asexual
(D) Sometimes groups split off from within a monophyletic group (such as birds splitting off from the reptiles)
(E) Sometimes sexual populations that are unable to interbreed could still be the same biological species

52. 3. Which of the following is most likely to be a "species" according to the Phylogenetic Species Concept? (a) A population of bacteria that has a gene that allows glucose metabolism (b) Bird populations, which share a unique heritable feather structure (c) Spider populations that can interbreed and produce fertile offspring (d) Crustacean populations that form a clade (genetically-related group), except for one population within the clade that colonized land and became insects (e) Populations of deer that share similar antler shape

53. answers 1D 2E 3B

54. Optional Slides

55. Table 16.3 Outcomes of secondary contact and hybridization

56. Reinforcement
So, when hybrids are formed between different species, they are often costly and maladaptive because of hybrid breakdown (the hybrids are maladaptive)

57. Reinforcement
So, when hybrids are formed between different species, they are often costly and maladaptive because of hybrid breakdown
In such cases, you would predict that mechanisms to avoid mating would evolve to avoid the production of maladaptive hybrids (= Reinforcement)
Predict the formation of prezygotic isolation to prevent the creation of maladaptive hybrids

58. Reduced Hybrid Viability Reduced Hybrid Fertility
Reproductive isolation could occur at different stages of reproduction
Prezygotic Reproductive Isolation (before egg is fertilized)
Failure to Mate
Genetic drift and divergence in bird songwon’t mate
Selection on coat colordon’t recognize each other
Sperm-egg incompatibility
Postzygotic Reproductive Isolation (after egg is fertilized)
DM incompatibilities cause embryo to not develop (Example: enzymes don’t work together)
Figure 24.4 Reproductive barriers
Prezygotic barriers
Postzygotic barriers
Gametic Isolation
Reduced Hybrid Viability
Reduced Hybrid Fertility
Hybrid Breakdown
Viable,
fertile
offspring
Fertilization

59. Reinforcement
So, the prediction is that in sympatry (when two different species are in the same place), mechanisms to avoid mating (prezyotic isolation) would be strong
Whereas in allopatry, prezygotic isolation would not be needed because the different species would not come into contact

60. Figure 16.12 Prezygotic isolation in allopatric versus sympatric species pairs of Drosophila
These graphs (Coyne and Orr 1997) plot degree of prezygotic isolation versus genetic distance in a variety of sister-species pairs from the genus Drosophila. Prezygotic isolation is estimated from mate-choice tests performed in the laboratory. A value of 0 indicates that different populations freely interbreed (0% prezygotic isolation) and 1 indicates no interbreeding (100% prezygotic isolation). Genetic distance is estimated from differences in allele frequencies found in allozyme surveys. Sibling species with the same degree of overall genetic divergence show much more prezygotic isolation if they live in sympatry.

61. 4. Under which of the following scenarios is reinforcement most likely to evolve? (a) Different fish species, with each living in a separate pond (b) Two snail species, where each lives on opposite sides of a freeway (c) Different species of crickets living together in a park, where hybrids between them have low survival rates (d) Different insect species, each living on a different species of fruit in a forest (e) Different species of allopolyploid plants living in a field

62. answers 4C