Speciation

1. 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 • 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 • (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 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 Species are dynamic rather than static entities, with boundaries changing constantlyMany groups are in the process of speciation

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

27. 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) Example: all human populations belong to the same biological species

29. Biological Species Concept 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 Strengths

30. 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. Ring Species

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

33. 2. Phylogenetic Species Concept 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 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 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 descendants A A A Group I B B B C C C D D D Group III Group II E E E F F F G G G (b) Paraphyletic group (c) Polyphyletic group (a) Monophyletic group (clade)

38. 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 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 Paraphyly: a group which either does not include all its descendants or the ancestor.

41. 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 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 • 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 Strengths • Most intuitive; the way we recognize species • Easiest. Easier than constructing phylogeny or intermating

45. 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? • 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 Species are dynamic rather than static entities, with boundaries changing constantlyMany groups are in the process of speciation

48. 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 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.