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EVOLUTION AND SYSTEMATICS
SPECIES AND SPECIATION
Ernst Mayr:"the greatest livingevolutionary biologist“
-- S. J. Gould
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 WITHINTHAN AMONG SPECIES: SPECIES AREDISTINGUISHABLE BY THEIR TRAITS
SIBLING (CRYPTIC) SPECIESAND HYBRID ZONES
• Some species are difficult to distinguish
• Hybrid zones: zones of contact where adjacent (sub)species interbreed
cornix
corone
Subspecies of carrion crow(Corvus corone) hybridize
in contact zone
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)
Wintering
Nesting
“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. Eastand 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 populationsB. Peripatric speciation: peripheral isolatesC. Parapatric speciation: speciation “beside”D. Sympatric speciation: speciation in “same
place“
allopatric peripatric parapatric sympatric
Modes of speciation
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 andspeciation 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.
Zambesi R.
Linyanti Swamp
Pollimyrus spp.(Mormyridae)
P. castelnaui
P. marianne Kwando/Linyanti fish P. castelnaui
EOD discharge patterns
voltage
Electric organ discharge (EOD) characteristics ofP. castelnaui and Pollimyrus captured in Kwando/Linyanti system
P. castelnaui
P. marianne
Semi-isolatedintermediate population
intermittent
• 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 sympatric
“Speciation in the same place”: selection must be strong, mate choice must be correlated with cause of divergence
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 fruitDivergent 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 ISOLATIONWITHIN 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)
Proportion of corolla tube + pedicel
Variation in flowers along transect in mountains of southern California(Mimulus longiflorus)
Pleistocene isolates
post-Pleistocenedispersal
reproductive isolationbetween
“Circle of races” ring species
Herring Gull(Larus argentatus)
Ring species (Ensatina salamanders):forms at end of intergrading “ring”
are reproductively isolated
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