plant evolution of islands - insight cruises · provide evidence of early stages of speciation •...
TRANSCRIPT
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Plant evolution of islands
1. Islands as evolutionary laboratories – Darwin and the Galápagos
2. Colonization and establishment: the reproductive biology and genetics of island plants
3. A glimpse of Caribbean islands and cays
4. Island hopping: Juan Fernández, New Caledonia and Australia
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Island biology
1. Main influences on diversity: island age & size,
distance from mainland, environmental heterogeneity and intensity of human disturbance
2. Geographical isolation & novel environments result in evolutionary diversification (= adaptive radiation) and high levels of endemism
3. Founder effects and genetic bottlenecks a prominent feature of island populations
4. Island novelty includes: evolution of woodiness, high incidence of dioecy, transitions to selfing and wind-pollination
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Charles Darwin & Alfred Russell Wallace gained numerous
insights into evolutionary diversification from studies of island biogeography
Darwin Wallace
Islands as evolutionary laboratories
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Island exploration and the development of Darwin�s ideas on evolution
• Voyage on H.M.S. Beagle around the world (1831-1836) as ship�s naturalist • Made numerous observations and collections of plants, animals & fossils
• His observations on patterns of variation in the Galápagos islands were particularly influential
• Darwin saw many ‘incipient species’ and geographical races and this caused him to doubt the ‘fixity’ of species and their origin by special creation
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H.M.S. Beagle sails to Galápagos Islands
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Galápagos Islands • 15 main islands of volcanic origin; oldest 5-10
million years old; youngest more recent • Flora and fauna colonized by species capable
of long-distance dispersal from South American mainland
• Distinct races and species on different islands provide evidence of early stages of speciation
• Darwin spent only 5 weeks on the islands but his observations formed the foundation for his theory of evolution
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Galápagos Islands now a UNESCO World Heritage site and a major ecotourism destination
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San Cristobal Island (Chatham) site of Darwin’s arrival in the Galápagos Sept 17th 1835 �Nothing could be less inviting than the first appearance�
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Statue of Charles Darwin at arrival site
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Animals groups observed by Darwin
on the Galápagos Islands illustrating
important evolutionary
principles Darwin’s finch
Marine iguana
Frigate bird
Flightless cormorant
Galápagos tortoise
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Adaptive radiation • the evolution of ecological and phenotypic diversity within a rapidly multiplying lineage as a result of speciation
• From a single common ancestor the process results in an array of species that differ in traits allowing exploitation of a range of habitats and resources
• Four features commonly identify an adaptive radiation
1. Recent common ancestry from a single species 2. Phenotype-environment correlation 3. Trait utility 4. Rapid speciation
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Hawaiian honeycreepers have all descended from a single species of finch in the last 10 million years How did this occur?
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What causes adaptive radiations?
1) Ecological opportunity – abundant resources and few competitors often occur on oceanic islands or their aquatic counterparts e.g. African rift lakes 2) High rates of speciation characterize the clade – can be tested by looking at mainland clade, e.g. Darwin�s finches & Hawaiian honeycreepers also radiated on mainland (although not as much), whereas Galápagos mockingbirds have not radiated on islands or continents
3) Origin of a key innovation – e.g. toepad in Anoles; floral nectar spur in Columbines (dealt with in next Lecture)
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Flora of Galápagos
Native species ~ 500 spp.
Endemic* species ~ 450 spp.
Alien species ~ 749 spp.
* A species restricted to a geographical area
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Prickly pear cacti are the first colonizers - how did they get
there?
Volcanic landscapes of recent origin on Isabela
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Cacti have fleshy bird dispersed fruits and are capable of long-distance dispersal
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The islands and cays of the Caribbean
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Red mangrove Rhizophora mangle (Rhizophoraceae) • Outstanding shoreline colonizer
• Adaptations include: prop roots, vivipary and hydrochory
• Unspecialized pollination system
involving wind and self-pollination
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Long Island, Bahamas
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Palms fruits are dispersed to many Caribbean islands by birds
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Cuscuta (Dodder) – a parasitic flowering plant
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Passiflora floral diversity in the Bahamas
Passiflora cupraea Passiflora pectinata
Passiflora sp.
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Major Evolutionary Transitions in Plant Reproductive Systems
Hermaphroditism animal pollination outbreeding
Wind Pollination
Loss of adaptations for animal pollination
Separate sexes Selfing
Spread of sterility mutations
Decreased flower size
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Evolution of selfing
• Many independent transitions from outcrossing to selfing in angiosperms
• Associated with evolution of selfing syndrome
• Commonly results in reproductive isolation & speciation
• Important genetic, demographic & biogeographical consequences
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Baker’s Law
Herbert G. Baker – California 1973
• Establishment following long-distance dispersal favours self-compatible rather than self-incompatible individuals, because a single individual is sufficient to found a new population
• Predicts that island colonization should favour individuals with the capacity to self-fertilize
• Also predicts that colonizing species including invasives should have the capacity for uniparental reproduction by selfing or clonal propagation
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Cuba
Jamaica
Nicaragua
Mexico
Brazil, North Ecuador
• Trimorphic • Dimorphic • Monomorphic • Unknown
0 Kilometres 1000
Brazil, South
Geographic distribution of Eichhornia paniculata
Predominantly outcrossing and high genetic diversity
Predominantly selfing and low genetic diversity
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Evolution of selfing from outcrossing in annual water hyacinth
Large-flowered bee-pollinated outcrosser
from Brazil
Small-flowered selfer from Jamaica
Brazil
Jamaica
• populations in Brazil are mostly outcrossing and visited by long-tongued bees although selfing forms do occur
• long-distance dispersal favours selfing forms because a single individual can start a colony without mates or pollinators this is known as ‘Baker’s Law’
• Jamaican and Cuban populations are largely selfing
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Evolution of selfing from outcrossing through genetic drift & natural selection
Trimorphic
L M S L M M
Loss of
S-allele
Dimorphic Monomorphic
Increased selfing
Loss of
m-allele
Outbreeding
Evolution (1989)
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0.001
Caribbean
Brazil, north
Brazil, south
C. America
Neighbour network of genetic relationships among range-wide samples indicates multiple origins of selfing
Neighbour network based on sequences of 10 EST-derived nuclear loci. Generated with SplitsTree
New Phytologist (2009) 183: 546-556
Trimorphic Dimorphic Monomorphic
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θCaribbean = 0.0012
θBrazil = 0.0042
θAncestor = 0.0125
Theta (4Neµ)
Time (generations)
T = 125 000
Understanding Colonization History through Coalescent Simulations: Bayesian Estimates of
4Nem for Eichhornia paniculata from Brazil & Caribbean
MIMAR: Becquet & Przeworski (2007)
Colonization of Caribbean estimated at 125,000 yrs BP and therefore not associated with agriculture
RW Ness, SI Wright & SCH Barrett unpubl. data
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Evolution of mating systems in Turnera (Turneraceae)
Mainland South and Central America distylous and outbreeeding
Caribbean Islands homostylous and selfing
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Evolutionary breakdown of distyly and island colonization in
Turnera ulmifolia
L S
Distyly Homostyly
Selfing
Cross over
in distyly supergene
Disassortative mating
Outcrossing via increased herkogamy
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Juan Fernández (Robinson Crusoe) Islands, Chile
• Three volcanic islands ~600km from mainland, largest is 93sq km, highest peak 916m • 209 native plants of which 62% are endemic
• Many transitions to wind-pollination and evolution of woody growth forms
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Fern-rich dwarf woodland
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Remains of Alexander Selkirk’s home 1705-1709
Inspiration for Daniel Defoe’s novel Robinson Crusoe
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Dendroseris litoralis Asteraceae
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Endemic hummingbird Sephanoides fernándensis (Juan Fernandez Firecrown) pollinating
Dendroseris litoralis
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The giant Gunnera peltata (Gunneraceae) which has been described as wind pollinated
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New Caledonia • ~18,000sq km, 1500km east of Australia • 43 endemic gymnosperms & Amborella • ultramafic rocks and metalliferous soils
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Smooth-bark Kauri, Agathis robusta (Araucariaceae) New Caledonia
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Cook Pine, Araucaria columnaris (Araucariaceae) Isle of Pines, New Caledonia
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Chandelier Cypress, Neocallitropsis pancheri (Cupressaceae) La Réserve de la Madeleine, New Caledonia
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Serious loss of endemic biodiversity by nickel mining New Caledonia has ~25% of the world’s reserves
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Amborella trichopoda (Amborellaceae) The most basal extant flowering plant
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Screw Pine, Pandanus sp. (Pandanaceae) New Caledonia
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Deplanchea speciosa (Bignoniaceae) Grand Terre, New Caledonia
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Endemic Pitcher Plant, Nepenthes vieillardii (Nepenthaceae), New Caledonia
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H.M.S. Beagle sails to Australia spends two months there; ours will be a flying visit
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Australia • Distinct flora and fauna with high levels of
endemism* and many unique adaptations • Biological uniqueness due to long history of
isolation from other land masses • Although a continent Australia is also an island
and shows many island characteristics e.g. endemism, radiations & unique adaptations
*Endemic species are restricted to a particular geographical region or habitat
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Dry forests composed of Eucalyptus (Gum Tree)
The dominant tree group in Australia with 700 spp., only 15 of which occur outside the continent
Blue Mtns, New South Wales
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Tropical forest - Queensland, Australia
Epiphytic fern
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Rodent pollinated
Banksia
Although the plant is a shrub the flowers are produced on the ground where rodents forage
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Today’s General Messages • Islands have long held a fascination for evolutionary biologists since Darwin’s time
• Islands are characterized by many unique adaptations and high levels of endemism
• Human disturbance and invasive species are a major threat to island biodiversity
Further Reading
Barrett, S.C.H. (1996). The reproductive biology and genetics of island plants. Philosophical Transactions of the Royal Society Ser. B. 351: 725-733