Speciation most important problem in evolutionary biology - change over time

1. Speciation most important problem in evolutionary biology - change over time - diversification Species concepts Mechanisms of speciation

2. Species Concepts Mayden (1997) 22 different species concepts species as categories -- taxa species as evolutionary groups -- “smallest independent evolutionary unit” importance -- conservation of biological diversity understanding the process of diversification

3. Biological Species Concept -- Ernst Mayr (1947) “Species are actual or potentially interbreeding natural populations which are reproductively isolated from other populations” emphasizes: genetic exchange within species reproductive isolation between species can be difficult to apply and, doesn’t apply to many species: asexual species -- bacteria, fungi, many invertebrates and plants hybridizing species -- wolves and coyotes; cottonwood and balsam poplar fossil species

4. Phylogenetic Species Concept -- Joel Cracraft (1989) “An irreducible (basal) cluster of organisms diagnosably different from other such clusters, and within which there is a parental pattern of ancestry and descent” species are monphyletic groups can be applied to any type of organism can be difficult to implement difficulties with matching gene trees to speciation events

5. Morphospecies Concept fossil species problems of cryptic species because of limited characters

6. Geographical classification (passJ) allopatric vicariance founder event parapatric sympatric Classical model of allopatric speciation isolation of populations allopatry genetic divergence of populations allopatry development of reproductive isolation when populations come back into contact sympatry (secondary contact and reinforcement)

7. geographic isolation due to range splitting -- vicariance populations may diverge due to: natural selection sexual selection (genetic drift) type of divergence may be pre-mating or post-mating or both

8. Vicariant speciation in snapping shrimp Isthmus of Panama closes ~3 mya Caribbean, Pacific populations both pre-mating and post-mating divergence

9. geographic isolation due to a founder event founder population is genetically different from source population; genetic drift is important at the start of the process divergence via: natural selection in a novel environment sexual selection type of divergence may be pre-mating or post-mating or both

10. Patterns of speciation in Hawaiian Drosophila are consistent with founder events

11. outcomes of divergence in allopatry insufficent divergence ---> fusion reproductive isolation due to assortative mating within populations ---> speciation mating between populations leading to a hybrid zone hybrid fitness hybrid zone eventual outcome lower than narrow, short- reinforcement parental forms lived equal to wide, parental populations parental forms persistent coalesce greater than ecotone stable hybrid zone, parental forms new habitat new species

12. Classical model of allopatric speciation isolation of populations allopatry genetic divergence of populations allopatry development of reproductive isolation when populations come back into contact sympatry (secondary contact and reinforcement) Not unique to allopatric speciation *first two stages frequently happen in ‘sympatry’ differentiation despite gene flow due to selection reproductive isolation is a consequence of selection on ecological attributes

13. Ecological speciation in three-spined sticklebacks Rundle et al. 2000 Science 287:306 pairs of species in freshwater lakes; independently derived from a marine stickleback species benthic form -- large, heavy-bodied; feeds on invertebrates in the near-shore habitats limnetic form – slender-bodied; feeds on plankton in open water Predict: limnetic forms from different lakes should not be reproductively isolated from each other limnetic forms from one lake should be isolated from benthic forms from a different lake species pairs from three different lakes

14. different lakes same lake

16. Natural selection, sexual selection selection and speciation in the cichlids of Lake Victoria Verheyen et al. 2003 Science 300:325 Lake Victoria -- more than 500 species of haplochromine cichlids morphological, ecological, and behavioral diversity

18. Lake Victoria cichlids are a monophyletic group most closely related to taxa from Lake Kivu

19. algal grazer insect eater piscivore algal grazer

21. Sexual selection drives speciation in African great lakes cichlids Couldridge and Alexander 2002 Behavioral Ecology 13:59 Pseudotropheus zebra complex of at least four species (Lake Malawi) differ in color pattern; similar in other morphology, ecology, behavior Test for assortative mating female preference with choice (conspecifics and heterospecifics) female preference without choice (heterospecifics only)

22. C P. callainos pale blue, no bars R P. zebra “red dorsal” pale blue; dark blue bars; orange dorsal fin G P. zebra “gold” pale orange, faint brown bars L P. lombardoi yellow, 5 vertical black bars Predict: 1) in choice tests, females will prefer conspecific males 2) in no choice tests, females will prefer the heterospecific male whose color pattern is most similar

23. C P. callainos pale blue, no bars R P. zebra “red dorsal” pale blue; dark blue bars; orange dorsal fin G P. zebra “gold” pale orange, faint brown bars L P. lombardoi yellow, 5 vertical black bars * * * * * * * *

24. Disruptive selection and speciation in red crossbills, Loxia curvirostra highly specialized -- seeds of coniferous trees (seeds retained in cones) use the crossed bill to extract seeds from within the cone extraction efficiency depends on bill shape and palate structure bill structure and cone structure affect feeding performance ---> no generalist crossbill nine different call types; seven are specialized on different species of conifers

25. Benkman 2003 Evolution 57:1176 detailed study of one population in southern Idaho feeds on Rocky Mountain lodgepole pine (Pinus contorta latifolia) estimated form of selection on bill shape (bill depth) relationship between bill depth and fitness (survival) fitness surface based on feeding performance (bill depth and palate structure) for five species

26. Selection on bill depth is stabilizing

29. Selection and speciation Natural Selection for different feeding methods or habitat preferences Sexual Selection for mate preferences Hybrids do poorly because they do not work well in the environment (physical or social). They may function properly as organisms i.e. viable and fertile

30. What are the requirements for this to work? Strong selection Little genetic exchange/assortative mating Predictions Prerequisite of close linkage/pleiotropy between the traits of ‘mating’ and ‘performance’ or, Recombination will erode weaker associations leading to no speciation

31. Genetics of speciation -- performance and habitat preference in pea aphids Via 2002 two host races/incipient species -- alfalfa and clover known trade-off between performance on clover and alfalfa

32. reaction norms mean fitness of clones tested on both hosts

33. Genetics of speciation -- performance and habitat preference in pea aphids Via 2002 two host races/incipient species -- alfalfa and clover known trade-off between performance on clover and alfalfa what is the genetic architecture of this trade-off? are genes for performance independent of genes for habitat choice/assortative mating? negative genetic correlation: pleiotropy or LDE of closely linked loci vs. LDE of unlinked loci QTL analysis

34. Performance

35. Acceptance and Performance on alfalfa

36. Acceptance and per- formance on clover

37. Intrinsic postmating incompatibility Drosophila frogs Lepidoptera birds

38. Post Lepidoptera Pre- plus post

39. Hybrid sterility and inviability evolve gradually; positively correlated with the time since divergence -- gradual accumulation of deleterious epistatic interactions between species Hybrid sterility evolves faster than inviability Haldane’s Rule: if hybrid sterility or inviability is present in only one sex of F1 offspring, it will be the heterogametic sex For Drosophila it is males--for Lepidoptera it is females

40. Haldane’s Rule: if hybrid sterility or inviability is present in only one sex of F1 offspring, it will be the heterogametic sex inviability due to lethal incompatibilities between autosomal and X-linked loci dominance theory: hybrid incompatibilities are partially recessive XX hybrids are heterozygous and protected X Y hybrids express all X-linked recessives faster-male evolution: - sexual selection drives rapid divergence of male fertility factors ---> hybrid male sterility - spermatogenesis is inherently more sensitive to perturbation

41. Species are evolutionarily independent groups. A species is often defined as a group of populations that are capable of interbreeding, but this definition applies best to diploid, sexually reproducing taxa. Populations which are phenotypically or ecologically distinct, yet regularly interbreed, or species that reproduce largely asexually may still be valid species. The process of adaptive divergence that leads to speciation may occur either in allopatry or sympatry. Speciation is a result of divergent natural (ecological) or sexual selection. Genetic drift may accelerate the process of divergence during founder events. Reproductive isolation between species may result from assortative mating (prezygotic) or from hybrid sterility/inviability (postzygotic). Haldane’s rule is a general description of the early stages of speciation. Genetic correlations for performance in different environments and for habitat choice may facilitate the rate at which divergence occurs.