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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.