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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
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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) 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
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 – • 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)
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)
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: • Allopatric speciation: Due to geographic separation of populations • Peripatric speciation: peripheral isolates • Parapatric speciation: speciation “beside” • 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
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
voltage P. marianne Kwando/Linyanti fish P. castelnaui EOD discharge patterns
Electric organ discharge (EOD) characteristics of P.castelnaui and Pollimyrus captured in Kwando/Linyanti system
P. marianne intermittent P. castelnaui Semi-isolated intermediate population