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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 Many additional extinct species assumed and known (e.g. fossils) • 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 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 TRAITS VARY LESS WITHIN THAN AMONG SPECIES: SPECIES ARE DISTINGUISHABLE BY THEIR TRAITS SIBLING (CRYPTIC) SPECIES AND 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 Species of fire-bellied toad (Bombina) hybridize in complex contact zone 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 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 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) Nesting Wintering “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) Reproductive isolation: • Cannot be tested for fossils • Irrelevant to asexual forms • Hard to apply in groups with much hybridization even between divergent forms (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 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 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. 4) Distinguishability – (a) Species diagnosable on morphological or genetic grounds: species possesses unique traits. (b) 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) Japan Caspian & Black Seas Copepoda: Temoridae: Eurytemora affinis (Poppe 1880) -- morphospecies -- geographically widespread -- ecologically important in estuaries: abundant grazer, prey species Eurytemora affinis: Morphometrically-based phylogeny Japan; Caspian & Black Seas Remainder: California; Pacific Northwest; St. Lawrence R., U.S. East and Gulf Coasts Eurytemora affinis: Molecularly-based phylogeny 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) Pine marten [From: Kyle & Strobeck (2003) Can. J. Zool. 81:57-66] 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) MODES OF SPECIATION Despite complications, many forms speciate by: A. Allopatric speciation: Due to geographic separation of populations B. Peripatric speciation: peripheral isolates C. Parapatric speciation: speciation “beside” D. Sympatric speciation: speciation in “same place“ Modes of speciation allopatric peripatric parapatric sympatric 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 Vicariance = splitting of area by appearance of barrier (e.g., mountain range, strait) Kakapo (Strigops): nocturnal flightless ground parrot of New Zealand Kiwi (Apteryx): primitive nocturnal flightless ground bird of New Zealand 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 Endemic single-species family (Degeneriaceae), Fiji Hundreds of endemic plant taxa inhabit Socotra HAWAIIAN SILVERSWORDS (ARGYROXIPHIUM) • 5 endemic species on slopes of highest volcanoes • At 10-15 yr of age, grow high stalks with flowers, die HAWAIIAN DROSOPHILIDAE • • • Adaptive radiation: allopatric speciation by geographic isolation ~900 species (described and undescribed) Diverse in morphology, ecology, behaviour Hawaiian Drosophila: Successive founder events and speciation as new land masses arose 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 allopatric peripatric parapatric sympatric • • Peripheral populations differ in environment Peripheral populations differ demographically allopatric peripatric parapatric sympatric PARAPATRIC SPECIATION * *selection must be strong mate choice must be correlated with cause of divergence Kwando R. Pollimyrus spp. (Mormyridae) Zambesi R. Linyanti Swamp P. castelnaui voltage P. marianne Kwando/Linyanti fish EOD discharge patterns P. castelnaui Electric organ discharge (EOD) characteristics of P. castelnaui and Pollimyrus captured in Kwando/Linyanti system P. marianne P. castelnaui intermittent Semi-isolated intermediate population • Okavango vs. Zambesi samples differ in: -- EOD discharge patterns -- mtDNA -- allozymes -- morphometrics • Kwando/Linyanti vs. Zambesi samples differ in EOD discharge patterns and morphometrics • Okavango (P. castelnaui) genetically distant from P. marianne; in latter, K/L vs. Z likely different biological species allopatric peripatric parapatric “Speciation in the same place”: selection must be strong, mate choice must be correlated with cause of divergence sympatric APPLE AND HAWTHORN FLIES Apple and hawthorn flies (Rhagoletis) classic example: • Courtship, mating on or near fruits; eggs laid on and larvae develop within fruits; drop to pupate in soil • Apple trees introduced < 300 yr ago (hawthorn native) • Apple fruits ~3 weeks earlier than hawthorn fruit Divergent selection: • Hawthorn flies should develop rapidly so pupation takes place before ground freezes • Apple fly larvae should develop slowly so they do not emerge as adults too early REPRODUCTIVE ISOLATION WITHIN SPECIES: RASSENKREIS • Geographic variation and gene exchange may occur throughout geographic range • Populations at range extremes may be reproductively isolated GEOGRAPHIC VARIATION WITHIN SPECIES • Much geographic and other spatial variation described within species, especially early in systematics • Many forms, races, varieties, subspecies described • Lines often drawn to depict boundaries between subspecies Subspecies of Song Sparrow (Melospiza melodia) Variation in flowers along transect in mountains of southern California (Mimulus longiflorus) Proportion of corolla tube + pedicel Herring Gull (Larus argentatus) post-Pleistocene dispersal reproductive isolation between Pleistocene isolates “Circle of races” ring species Ring species (Ensatina salamanders): forms at end of intergrading “ring” are reproductively isolated