adaptive radiation - tcd · adaptive radiation • the formation of many new species from a single...
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Adaptive Radiation
Adaptive Radiation
• The formation of many new species from a single progenitor
• Usually associated with opening up of new ecological niche
• Species vary adaptively to exploit the niche
• Founder effects have been proposed to assist the rapid formation of new species
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Adaptive radiations in Vertebrates
• Honeycreepers on Hawaiian islands • Cichlid fishes in great African lakes • Finches on the Galapagos Islands
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Background
• Finches collected by Darwin on visit of Beagle to Galapagos Islands
• Not initially prominent in Darwin’s work but later a paradigm of species origin
• 13 species endemic to Galapagos (1000km from Ecuador)
• Differ particularly in habitat, feeding and morphology (beak)
Background cont.
• Geological evidence indicates that the current Galapagos Islands are up to 4MY old but that islands were present for >20MY
• Darwin’s Finches (DF) may be a monophyletic group that underwent adaptive radiation on colonization of the Galapagos
• DF probably originated on South American mainland among the subfamily Emberizinae.
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3 Lineages of Darwin’s finches
• Ground finches (6 species) – Geopsiza; Seed eaters
• Tree finches (6 species) – Camarhynchus; Insect eaters – Platyspiza Vegetarian finch
• Warbler-like finch (1 species) – Certhidea; insect eater
Cactus finches
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Above: Darwin's own sketches of Galápagos finches. From Darwin's Journal of Researches: 1.Large ground finch Geospiza magnirostris; seed eater 2.Medium ground finch Geospiza fortis ; seed eater 3.Small tree finch Camarhynchus parvulus ; insect eater 4.Warbler finch Certhidea olivacea ; insect eater
Galapagos islands: Many Arid regions, cactus and scrub vegetation
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Small ground finch: Geospiza fuliginosa
Small tree finch: Camarhynchus parvulus
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Medium ground finch: Geospiza fortis
1976 – 77 severe drought.
Decline in the production of the seeds that are the dietary mainstay of Geospiza fortis, the medium ground finch.
Boag & Grant Science 214:82, 1981
Population decline from 1400 to 200 on the tiny Galapagos island of Daphne Major.
Drought
Species has become completely specialised (adapted) to one food source
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Sharp- beaked ground finch: Geospiza difficilis a.k.a. Vampire finch
Cactus ground finch: Geospiza scandens
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Large ground finch: Geospiza magnirostris
MHC based study of DF
• MHC= Major Histocompatibility Complex • Large amount of ancient MHC polymorphism
present in DF • Much of the polymorphism predates arrival on
Galapagos (16-18 alleles) • Colonization must have begun with a sizable
(>30 individuals) flock.
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From an initial founder population from South America, these finches have radiated out into 13 species. Similar in body size and colour, but highly differentiated in size and shape of beak. Each has adapted to a different food source
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From an initial founder population from South America, these finches have radiated out into 13 species. Many are similar in body size and colour, but highly differentiated in size and shape of beak. Each has adapted to a different food source
Darwin’s Finches in the Galápagos Islands
Hawai’i – 4000km from nearest continent
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Endemic species in Hawai’i
Group % endemic
Mosses 46
Ferns 70
Angiosperms 91
Gymnosperms 91
Arthropods 99
Birds 81
Endemic birds Hawai’ian Honeycreepers
Insect eating
Nectar eating
Finch like
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Hawai’ian Honeycreepers
• Probably descended from finch-like seed-eating ancestor
• Radiated adapting to new food sources and habitats
• Genetic divergence indicates that founders arrived 3.5 – 8 mya
• 29-33 species and 14 more fossil species
Cichlid fish species of the Great African Lakes
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Cichilds are distributed throughout the World
Africa is the centre of cichlid biodiversity
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Victoria
Tanganyika
Malawi
East African Lakes
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Cichlid Species in the East African Lakes
• Lake Victoria – Formed 250,000-750,000 years ago – Saucer-shaped (shallow) & size of Ireland – 500+ species of cichlids
• Tanganyika & Malawi – Deep - fill rift between tectonic plates
• Malawi – 4 million years old – 500-700 species
• Tanganyika – 9-12 million years old – 200-250 species
Cichlids possess two sets of jaws
Mouth jaws
Throat jaws
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Cichlids have adapted to different ecological niches within the lakes
• Species co-exist without being in direct competition • The sets of jaws are adapted (“fine-tuned”) to different
food sources – algae scrapers –eat algae off rocks – insect pickers –pluck larvae from between rocks – scale eaters –scrape scales off other cichlids (who
eventually become wary: c.f. frequency dependent selection)
• Cichlid species have also evolved different reproductive strategies – isolates the groups and reinforces speciation – mouth brooding
• Many colour morphs exist
Mouth Brooding
Protects the young
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Tanganyika Malawi
Very similar-looking species in different lakes occupy similar ecological niches.
BUT are only distantly related
Morphological similarity has little correlation with evolutionary relatedness.
Evolutionary Relationships • Lake Tanganyika has
the smallest number of species, but the greatest evolutionary diversity (longest time)
• In Lake Malawi and in Lake Victoria, any of the species within the lake is more closely related to other species of the same lake than to species from another lake
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Cichlids – summary • There has been a huge radiation of cichlid species
over the last few million years • Different lakes in Africa contain similar-looking
species that have adapted to similar ecological niches, but that are not close evolutionary relatives
• Speciation has followed similar trends in each of the lakes independently
• Lake Victoria’s cichlid population is under threat from the introduced perch species.
• The destruction of the cichlid populations is in turn threatening the entire ecosystem of the lake.