evolution and systematics

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