CHAPTER 15 Species: Concepts & Origins

1. CHAPTER 15 Species: Concepts & Origins

2. Where are We… • Up to this point we have concentrated on evolutionary processes operating within and between populations (ie. population genetics) • Microevolution • Now we are moving towards more coarse-level evolutionary processes operating on larger temporal, geographical and phylogenetic scales • Macroevolution

3. Species • Species form the link between Microevolution and Macroevolution • Processes of Speciation include elements from fine scale Microevolution as well as larger Macroevolutionary processes

4. What is a species? Difficult concept to put boundaries on: We find variation within groups and between groups We find phenotypes that are plasticwe find genetic differences that do not correspond to any visible phenotypic differences

5. What is a species? The difficulty in defining and identifying species points to the gray area that exists between species (ie, intermediates, variants, races, subspecies exist) May be the best evidence that each species was not specially created Check out:www.pbs.org/wgbh/evolution/library/05/index.html

6. What is a species? • Species are a group of interbreeding populations • All groups of individuals that send genetic migrants back and forth belong to a single species • Species have independent evolutionary trajectories • A species evolves without the being influenced by gene flow from other groups of individuals • Species are the fundamental unit of evolutionary change

7. “Typological” or “Essential” Notion of Species • Individuals were members of a given species if they conformed to that “type” or ideal, in certain characters that were “essential” fixed properties • This forms the idea of a “type-specimen” or “type-series” for defining a species…. • Essentially all described species in science are based on a TYPE

8. “Typological” or “Essential” Notion of Species • Basically stems from Linnaeus • Formally discussed by Ernst and Mayr • Relies heavily on fixed (not polymorphic) morphological differences unique to a “species” • Yet, still required common decent or decent from a common ancestor (monophyly!)

9. Species concepts There are something on the order of 32 recognized species conceptsEach seeks to define groups that cannot (or do not) ‘talk to each other’ genetically We’ll limit our discussion to 3 basic ideas of species delineationBiologicalMorphologicalPhylogenetic

10. Morphological (aka Taxonomic) Species Concept Morphological (aka Taxonomic) Species Concept: Species are defined by differences in characters, especially morphologyWorkable for all kinds of organisms, extinct or extant

11. Morphological (aka Taxonomic) Species Concept :Major Difficulties Polymorphism - genetic polymorphisms, sexual dimorphism, social castes

12. Morphological (aka Taxonomic) Species Concept :Major Difficulties Geographic variation - a species may appear different in different parts of its range due to genes or environment Two populations of Thamnophis sirtalis

13. Morphological (aka Taxonomic) Species Concept :Major Difficulties Cryptic species - genetically distinct units might have no outward differences Hyla chrysoscelis (2n) and H. versicolor (4n)

14. Biological Species Concept A group of interbreeding (or potentially interbreeding) populations that is reproductively isolated from other such groups If individuals from two separate populations cannot produce fertile offspring they are said to be two different species A current favorite among biologists, the legal definition of species by the USA, CITES, and the ESA

15. Biological Species Concept:Major Difficulties • Fossils and extinct organisms? • Asexuals? • Geographically separated groups? • How much interbreeding is too much? • What is “potentially interbreeding”?

16. Fem. lion X Male tiger Male lion X Fem. tiger zebroid zonkey

17. Evolutionary Species Concept (Simpson, modified by Wiley) • "A single lineage of ancestor-descendent populations which maintains its identity from other such lineages and which has its own evolutionary tendencies and historical fate" • Four logical corollaries: • All organisms, past and present, belong to some evolutionary species • Species must be reproductively isolated from each other to the extent that this is required to maintain their separate identities, tendencies, and fate. • Evolutionary species may or may not exhibit recognizable phenetic differences (# of species may be over or under-estimated, usually the latter) • No separate, single evolutionary lineage may be subdivided into a series of ancestral and descendent species. • Benefits: clear conceptually; applies to asexual species, through time, and allopatric secies • Problems: application (problem to know the future) asexual species (too many independent lineages)

18. Phylogenetic Species Concept • A species is the smallest, irreducible cluster of organisms that contains all known descendants from a common ancestor  the smallest monophyletic group • Identifiable through phylogenetic reconstruction  any branch is a species; defines species as evolutionarily meaningful units

19. Phylogenetic Species ConceptMajor Difficulty • Unclear what scale differences between species should be separate from variation within a species, ie,how much genetic difference is enough to consider two groups separate species?

20. Restricted Gene Exchange • Narrow Hybrid Zones: genetically distinct pops. meet & interbreed to a limited extent (semispecies, superspecies…We don’t like these terms!) • Sympatric Hybridization: some hybridization occurs w/in sympatry but only occasionally • Geographic Variation: Genetically distinct pops. appear conspecific in certain areas…

21. Ring Species • A chain of interbreeding pops. that loop around an uninhabited area so that terminal pops. are so different when they meet, they cannot interbreed! • eg. Ensatina salamanders

22. Ring Species: Ensatina

24. Allopatric Populations • This is one of the main limitations of the Biological Species Concept! • The BSC. uses “potentially interbreeding”…. • What if populations NEVER come into contact but may be able to interbreed in captivity… (eg. Mountain top species, species that live in patchy habitats… )

25. Example 1: Montane Pitvipers • Live at high elevation (above 2500M) in Middle America • Most populations are isolated on these mountain tops • Expect restricted gene flow between populations

27. Phylogeny of Cerrophidion (Montane Pitvipers) Cerrophidion godmani Costa Rica C. petlacalensis Veracruz, Mexico C. tzotzlilorum Chiapas, Mexico C. godmani Oaxaca, Mexico & Guatemala C. godmani Huehuetenango, Guatemala C. godmani N.W. and Interior Guatemala

28. C. petlacalensis C. tzotzliroum Montane Pitvipers: Cerrophidion C. godmani C. godmani C. godmani C. godmani HOW MANY SPECIES????

29. EXAMPLE 2:Western Diamondback Rattlesnakes Lives at lower elevations in hot-dry desert and grassland areas across the US and northern Mexico No obvious drastic boundaries across its range that isolate populations High degree of morphological polymorphism across its range

30. South-Central Mexico (San Luis Potosi)

31. S-Eastern Arizona

32. Central Texas

33. East-Central Mexico (Veracruz)

34. PROPOSED BIOGEOGRAPHIC REGIONS Colorado Desert Sonoran Desert Chihuahuan Desert/ Trans Pecos Region Southern Plains/ Ozarks Coastal Plains

35. Is Crotalus atrox monophyletic...?? Crotalus tortugensis Crotalus atrox Crotalus atrox Crotalus ruber

36. ML (and Bayesian) Phylogeny for all 50 C. atrox specimens plus outgoups mt ND4 gene (~850bp)

37. NESTEDCLADEDESIGN

39. C. tortugensis GENE FLOW Between Desert Regions!!!

40. Side Note: Gene VS. Species Trees • The previous examples were based on one or two gene sequences • We used these to infer the “species tree or phylogeny • We made the assumption that the Species Tree = Gene Tree • This assumption is based on the assumption that the genes we examined evolved with the species over time • Also depends on Coalescent Theory, as we discussed previously

41. Allopatric Populations • The BSC requires gene flow between populations of different species to be prevented • What are the mechanisms which may prevent gene flow and promote speciation… • Well…. Allopatry is one, what are the rest?

42. Reproductive Barriers: 2 Major Categories • Prezygotic barriers – prevents the formation of hybrid zygotes • Postzygotic barriers – reduced fitness of hybrid zygotes

43. Prezygotic Isolation - I • Temporal Isolation – different mating times • Habitat or Resource Isolation – isolation by using different habitats, different host plants, etc.. • Ethological (=Behavioral) Isolation – very important, courting behavior, seasonal timing of mating, species specific recognition, chemical recognition of species..

44. Prezygotic Isolation - II • Mechanical (or physical) Isolation – mainly in plants, yet lizards and snakes also show this (Hemipenes  lock and key…) • Gametic Incompatability – External fertilization – Marine Inverts cast sperm & eggs into the water column (Lysin protein dissolves a hole only in egg of same species) Atractus ventrimaculatus hemipenis

45. Hemipenes of snakes (Atractus)

46. Postzygotic Isolation • Hybrid Inviability – hybrids fitness = ZERO! • Hybrid Sterility – again… No fitness conferred to hybrids!

47. Where from here… • Next, we will jump into how species are formed…. • How do processes that we have discussed for populations play out over the long haul in creating species