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EVOLUTION AND SYSTEMATICS

EVOLUTION AND SYSTEMATICS. Ernst Mayr: "the greatest living evolutionary biologist“ -- S. J. Gould. SPECIES AND SPECIATION. SPECIATION. Speciation connects micro- and macroevolution “Species“ is basic unit recognized for most life forms ~10 million species exist; ~1.4 million described

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EVOLUTION AND SYSTEMATICS

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  1. EVOLUTION AND SYSTEMATICS Ernst Mayr: "the greatest living evolutionary biologist“ -- S. J. Gould SPECIES AND SPECIATION

  2. SPECIATION • Speciation connects micro- and macroevolution • “Species“ is basic unit recognized for most life forms • ~10 million species exist; ~1.4 million described • Many additional extinct species assumed and known (e.g. fossils)

  3. Living things tend to occur in classes/groups • Individuals in groups resemble one another more than members of different groups • Species are important kind of natural group • Species differ in appearance, ecology, behaviour, genetics, distribution, etc. • Why do natural groups exist? How do they evolve? • Diverse species concepts have been proposed

  4. WHAT IS A SPECIES?EARLY PRACTICES • Early research relied on morphological features, hence (1) Morphological Species Concept • “A species is what a good taxonomist says it is“ • Poulton (1904): interbreeding within species defines them • Some workers also used non-anatomical attributes, e.g. habitat, distribution

  5. TRAITS VARY LESS WITHIN THAN AMONG SPECIES: SPECIES ARE DISTINGUISHABLE BY THEIR TRAITS

  6. SIBLING (CRYPTIC) SPECIESAND HYBRID ZONES • Some species are difficult to distinguish • Hybrid zones: zones of contact where adjacent (sub)species interbreed Subspecies of carrion crow (Corvus corone) hybridize in contact zone corone cornix

  7. Species of fire-bellied toad (Bombina) hybridize in complex contact zone

  8. CHANGING SPECIES CONCEPTS • Genetics and evolution in 1930s, 1940s: Dobzhansky, Mayr, Fisher, Wright, Haldane • Mayr (1942): “species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups” = (2) Biological Species Concept

  9. BSC: SPECIES DISCRETENESS Interbreeding within species and absence of hybridization/interbreeding between them: • relative uniformity within species • absence of intermediate forms Barriers to reproduction essential for differentiation between populations  speciation

  10. SOME PROBLEMS WITH THE BSC • Species may be morphologically similar yet isolated reproductively • Morphologically different species may interbreed American Black Duck (Anas rubripes) Mallard (male) (Anas platyrhynchos)

  11. Nesting Wintering

  12. “REPRODUCTIVE ISOLATION”AND THE BSC • Hybridization common in plants • Many plants reproduce mainly or solely by asexual means such as apomixis • Apomixis in hundreds of species, > 30 families of flowering plants (e.g., dandelion Taraxacum officinale)

  13. Reproductive isolation: • Cannot be tested for fossils • Irrelevant to asexual forms • Hard to apply in groups with much hybridization even between divergent forms

  14. (3) PHYLOGENETIC SPECIES CONCEPT • Monophyletic groups: groups with independent evolutionary histories; smallest group = species • Testable and can be applied to any kind of organism • Problems: hard to apply, would lead to large increase in number of recognized species • Nevertheless, cladistic approaches point to many situations where current taxonomy/classification and recognition of species need to be revised

  15. WHAT IS A SPECIES? • “Smallest independent evolutionary unit” (text) • Lineages with independent evolutionary histories and reproductive integrity • “Independence” follows from mutation, selection gene flow, and drift operating on a population, separately from other populations • In practice, parts of all three concepts often used jointly, with large roles of PSC and genetics

  16. CONCEPTS AND PRACTICE • More disagreement on concepts than practice • Four general species criteria in use: 1) Separation – Species must be separated from one another morphologically, behaviourally, genetically, or in other ways. 2) Cohesion – Populations must be internally cohesive, genetically and ecologically. 3) Monophyly – Individuals and populations within a species must share a single most recent common ancestor.

  17. 4) Distinguishability – • Species diagnosable on morphological or genetic grounds: species possesses unique traits. • Presence of phenetic or genetic clusters: single traits not unique, but overlap with other species. Multiple traits  determination of clusters. NOTE: Species may be morphologically indistinguishable but genetically different (cryptic species)

  18. Japan Caspian & Black Seas Copepoda: Temoridae: Eurytemora affinis (Poppe 1880) -- morphospecies -- geographically widespread -- ecologically important in estuaries: abundant grazer, prey species

  19. Eurytemora affinis: Morphometrically-based phylogeny Japan; Caspian & Black Seas Remainder: California; Pacific Northwest; St. Lawrence R., U.S. East and Gulf Coasts

  20. Eurytemora affinis: Molecularly-based phylogeny

  21. SPECIATION IN AFRICAN ELEPHANTS • Genetics  58% difference between Asiatic (Elephas) and African (Loxodonta) genera • Loxodonta from savannah/bush and tropical forests differ in ecology, habitat, morphology, behaviour • Very limited gene flow detected • Divergence ~2.6 Ma • Recognize: -- L. africana (Blumenbach 1797) -- L. cyclotis (Matschie 1900)

  22. Pine marten

  23. [From: Kyle & Strobeck (2003) Can. J. Zool. 81:57-66]

  24. PRACTICE vs. THEORY • Life originated by descent with modification • Characters change over time • So speciation usually not in sharp steps • To expect only clearly defined species not realistic • Groups in varied states of divergence (Table 15.1)

  25. MODES OF SPECIATION Despite complications, many forms speciate by: • Allopatric speciation: Due to geographic separation of populations • Peripatric speciation: peripheral isolates • Parapatric speciation: speciation “beside” • Sympatric speciation: speciation in “same place“

  26. Modes of speciation allopatric peripatric parapatric sympatric

  27. ALLOPATRIC SPECIATION • Speciation between geographically isolated populations due to barrier (e.g., mountains) • Many kinds of barriers, scales of barriers • Small populations not assumed but may occur • Lack of gene flow permits adaptation to different environments • Evolutionary divergence and reproductive isolation follow • Can distinguish allopatric speciation due to vicariance, founder events

  28. Vicariance = splitting of area by appearance of barrier (e.g., mountain range, strait)

  29. Kakapo (Strigops): nocturnal flightless ground parrot of New Zealand

  30. Kiwi (Apteryx): primitive nocturnal flightless ground bird of New Zealand

  31. OCEANIC ISLANDS:MUCH SPECIATION, HIGH ENDEMISM • Polynesia & Micronesia: ~50% of ~7,000 species of vascular plants are endemic • Degeneriaceae with single tree species, (Degeneria vitiensis) endemic to Fiji • Much endangerment: >60 species of endemic Hawaiian plants have <10 remaining wild individuals

  32. Endemic single-species family (Degeneriaceae), Fiji

  33. Hundreds of endemic plant taxa inhabit Socotra

  34. HAWAIIAN SILVERSWORDS (ARGYROXIPHIUM) • 5 endemic species on slopes of highest volcanoes • At 10-15 yr of age, grow high stalks with flowers, die

  35. HAWAIIAN DROSOPHILIDAE • Adaptive radiation: allopatric speciation by geographic isolation • ~900 species (described and undescribed) • Diverse in morphology, ecology, behaviour

  36. Hawaiian Drosophila: Successive founder events and speciation as new land masses arose

  37. PERIPATRIC SPECIATION • Speciation between geographically isolated main and peripheral populations • Small peripheral population assumed (founder effect) • Lack of gene flow permits adaptation to different environment; main divergence in peripheral population • Small founding and existing population • Peripheral populations with low population densities, extreme and unpredictable environments • Peripheral populations also at range extremes

  38. allopatric peripatric parapatric sympatric

  39. Peripheral populations differ in environment • Peripheral populations differ demographically

  40. allopatric peripatric parapatric sympatric

  41. PARAPATRIC SPECIATION * *selection must be strong mate choice must be correlated with cause of divergence

  42. Kwando R. Pollimyrus spp. (Mormyridae) Zambesi R. Linyanti Swamp

  43. P. castelnaui

  44. voltage P. marianne Kwando/Linyanti fish P. castelnaui EOD discharge patterns

  45. Electric organ discharge (EOD) characteristics of P.castelnaui and Pollimyrus captured in Kwando/Linyanti system

  46. P. marianne intermittent P. castelnaui Semi-isolated intermediate population

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