Download Evolution and Systematics Biology 2900

EVOLUTION AND SYSTEMATICS
Ernst Mayr:
"the greatest living
evolutionary biologist“
-- S. J. Gould
SPECIES AND SPECIATION
SPECIATION
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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
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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
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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
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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
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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
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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
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Peripheral populations differ in environment
Peripheral populations differ demographically
allopatric
peripatric
parapatric
sympatric
PARAPATRIC SPECIATION
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*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
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Okavango vs. Zambesi samples differ in:
-- EOD discharge patterns
-- mtDNA
-- allozymes
-- morphometrics
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Kwando/Linyanti vs. Zambesi samples differ in
EOD discharge patterns and morphometrics
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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