Classical Species Radiations


Readings: Turelli, M. et al. 2001. Theory and Speciation. TREE 16:330-343.  hotlink to paper


 Most students of biology have become familiar with Charles Darwin and his work with the famous finches of the Galapagos Islands that are named after him.  Indeed, much has been learned from this relatively small group of birds, with important studies on this group of birds continuing to this day by Peter & Rosemary Grant of Princeton University.


We will explore outstanding examples of species radiations here, not only because they give clear hints of how evolution and speciation occur, but because these same taxa are in some cases threatened with extinction as a result of man's activities.  The examples we will consider are all island faunas – Galapagos finches, Hawaiian honeycreepers, Hawaiian Drosophila and cichlid fishes of the 

great lakes of Africa.  These groups of fauna are the best documented examples of species radiations.


Galapagos Finches:


see the Web site: Voyage of the Beagle to find out more about Darwin's travels aboard the Beagle).


see image: Galapagos Islands


The Galapagos islands are a group of 16 major islands plus numerous smaller islets (total = 32) located on the equator about 1000 km west of the Ecuadorian (South American) mainland.  The archipelago was formed following volcanic eruptions of the seafloor ca. 3 - 5 million years ago.  The islands, because of their remoteness from mainland sources of plants and animals, have a relatively impoverished biota.  Twenty-six species of land birds occurred naturally on the Galapagos Islands prior to human introductions of others, and 13 of these are finches.  The islands also support 4 mockingbird, 2 flycatcher, 2 owls, 1 hawk, 1 dove, 1 cuckoo, 1 warbler and 1 martin species (Pianka 1983).

The finches are thought to have evolved from 1 mainland species (or possibly a Cocos Island species that reached the islands long ago.  The island finches belong to a distinct subfamily of finches, endemic to the Galapagos and Cocos Island (Costa Rica).  Cocos Island is several hundred km north of the Galapagos and about the same distance from the mainland.  It supports only 1 finch species, the Cocos Island finch Pinaroloxias inornata.  This finch species, unlike those on the Galapagos, has had no opportunity for geographic isolation (other than from the mainland).  Thus, its gene pool has never been split and additional species have never evolved there. The Cocos finch is a generalist feeder, in contrast to many of those on the Galapagos which exhibit specialist feeding niches.


On the Galapagos Islands, adaptive radiation has resulted from geographic isolation and reduced gene flow among islands. Three distinct genera (Geospiza, Camarhynchus and Certhidea) occur on the islands.  Members of these genera  differ in where they forage, how they forage, and what they eat. The ground-finches (Geospiza) include 6 ground-foraging species with broad beaks that crush different species and/or sizes of seeds, or use flowers of the Opuntia cactus.  The 6 species of Camarhynchus finches forage in trees, have narrower beaks, and eat either vegetation (1 species) or different sizes of insects.  C. pallidus uses a stick or cactus spine to probe wood for insects.  The other genus has one species Certhidea olivacea, is called a 'warbler-finch'. It is widespread on almost all of the islands and islets.


Other than Cocos Island, between 3 and 10 finch species occur on each island.  see image: finches (Ruse 1982).  Beak lengths and depths vary widely from island to island, and in some cases, provide limited evidence for competitive displacement.  For example, on islands (Abington, Bindloe, James, Jervis, Albemarle, Indefatigable, Charles, Chatham) where Geospiza fuliginosa and G. fortis occur sympatrically, they tend to have widely divergent beak depths.  Because beak morphology is a critical feature in diet determination, this separation is viewed as adaptive because it would tend to minimize competition between the taxa during periods of resource scarcity.  On these islands G. fuliginosa has a much smaller beak than co-occurring G. fortis.  However, only one of these closely-related species is supported on the islands of Daphne and Crossman.  The beak morphology of these species are very similar to each other when they species occur in isolation.


see Galapagos Finches

see image: beak shapes)

beak morphology separation

sympatric separation


Many authors have cited this as a classic example of character displacement, though before it can be confirmed, 4 conditions must be met:


1) the change in mean character state (e.g. beak shape) in areas  of overlap should not be predictable from variation within    regions of overlap or areas of isolation;

2) sampling should be conducted at more than one set of locations to eliminate local variation effects;

3) Heritability of the feature must be high if genetic variation is thought to underlie the variation in the feature, and if it is to be passed to subsequent generations;

4) Evidence must be presented to demonstrate that competition occurs and that the measured feature has relevance to competition among groups.


 To date conditions 1 and 3 have been demonstrated.  The variation observed in G. fortis in isolation on Daphne greatly exceeds that observed on other islands.  Moreover, heritability of the trait for beak morphology, which can range from 0 (no relation between parental and offspring morphology) to 1 (absolute relationship) has been estimated as 0.82.  Thus, there is good evidence that beak morphology is genetically based. Criterion 2 cannot be met since there is only one Daphne and one Crossman.  Satisfying this requirement would necessitate observing the same pattern on other islands like Daphne and Crossman.  However, Peter and Rosemary Grant recently demonstrated that co-occurring finches diverged in beak morphology after a drought which greatly reduced seed abundance, relative to their respective beak shapes prior to the drought.  This provided good evidence in support of condition 4.  Divergence in beak shape facilitated a divergence in diet and minimization of interspecific competition during this critical period which John Wiens has referred to as a 'resource crunch'.  Indeed, competition may be a very ephemeral or transient process that may occur only sporadically, but its effects may be both long-lasting and very significant.


Other interesting patterns have been reported for the Galapagos Geospiza finches. First, larger islands in the archipelago support more species than smaller islands, likely owing to more habitats and more area of each habitat.  In addition, the number of finch species declines in relation to the extent of isolation  (= mean distance from other islands) of the island relation to others in the group.  This concept will be explained in considering island biogeography.  At the same time, the number of endemic species increases with the degree of isolation.


 Two closely related taxa, G. fortis and G. conirostris, have completely different distribution patterns and never co-occur.  The same pattern holds for G. difficilis and G. fuliginosa, though the patterns are not as clear because they occasionally co-occur.


Grant and Schluter (1984) argued that G. conirostris and G. difficilis are found on very few islands because of interspecific competition. G. conirostris is found on islands at different ends of the archipelago, suggesting that it has had opportunity to colonize islands in between. They attributed its rarity and absences on additional islands to competition from other finch species. 


The pattern for G. difficilis is more compelling.  It is found on 4 widely separated islands and extinct on 4 others. This species co-occurs with G. fuliginosa on large central islands (presumably with large habitat area to support large populations), but do not co-occur on smaller central or small outlying islands.  Grant and Schluter used statistical comparisons to suggest that in both cases the species co-occur much less than one would expect by chance alone.


Dan Simberloff (1984) argued that Grant and Schluter had made incorrect statistical comparisons and that available evidence did not support the competition hypothesis. Specifically, he stated that Grant and Schluter's method of assuming all 6 species were equally likely to co-occur (their null hypothesis) was inappropriate since these two species (G. conirostris and G. difficilis) were so rare.  He argued that these species overlap with few species precisely because they are so rare that they cannot co-occur with other species.  


 It is widely believed that Darwin's finches evolved from a single ancestor, possibly the Cocos finch.  The original colonists to the Galapagos island experienced adaptive radiation, a process whereby the species diversified to exploit a wide variety of available (and in this case, vacant) niches.  As in the following cases of the Hawaiian honeycreepers and cichlid fishes of the African Great Lakes, closely related taxa co-exist by exploiting different habitats (trees vs. ground) or food sources (eg. insects vs. seeds). If individuals with similar niches practiced assortative mating, in which they preferentially mate with individuals sharing similar traits or habits, then sympatric speciation is possible (assuming that the behavior that separates sub-species is heritable).  In each of these cases of species radiations, the diversification of the lineage occurred in an environment of low native species diversity at the time of invasion - in other words, it appears that diversification of the lineage was not constrained or prevented by pre-existing competitors.


see image: honeycreeper - Futuyma 1986 (birds marked with an 'E' are endangered or extinct). 


The honeycreepers of Hawaii are another remarkably speciose group of birds; in fact, these birds experienced much greater adaptive radiation than Darwin's finches, though sadly, many of the taxa have been driven extinct owing to introduction of diseases, other passerine birds and mammals, and destruction of habitat.  For example, on Laysan Island in the northwest region of the Hawaiian chain, introduced rabbits and a large windstorm destroyed vegetation that resulted in the extinction of the Laysan honeycreeper.  As well, bird pox virus and avian malaria, the latter of which was introduced to the Hawaiian islands by mosquitoes on ships in 1826 have been attributed with sharp declines in Drepanidid species diversity. Currently, honeycreepers occur primarily at altitudes above 600 m on the main islands and on several smaller remote islands in the NW part of the archipelago.  Mosquitoes, by contrast, occur primarily below 600 m, and overlap very little with the honeycreepers. Many of the native birds are not found at low latitudes because of habitat conversion (e.g. golf courses, sugar cane).  However, at the lower altitudinal end of the honeycreeper species ranges, between 2 and 7% of individuals have avian malaria.  This frequency is 29% in one species that is highly mobile. Honeycreepers from remote islands in the chain (which were less likely to ever come in contact with  introduced diseases) were highly vulnerable to malarial infection, whereas some infected birds in highly populated regions were apparently very healthy. Thus, it is not clear exactly what role, if any, diseases have played in these extinctions.


Rats have also played a large role in species extinctions. It is believed that rats were introduced early on during human colonization of the islands (by Polynesians 1000 years ago) and that many (~50%) native birds went extinct at this time; 50% of the remaining species went extinct following European colonization, and 50% of these remaining species are currently endangered.


Nine extinctions have occurred since introduction of malaria to the islands.  Of these, 6 occurred on Lana'i, Moloka'i and O'ahu; however, these small islands have been radically modified by humans, thus habitat destruction appears to have played a large role.  Stuart Pimm has suggested that behaviour may have affected 2 large, nectar-feeding drepanididae species that have been driven extinct and two smaller ones that have been driven to extirpation ('akoekoe and 'i'iwi) on some islands.  The large species were very aggressive  and were capable of defending high quality resources, but were poorly adapted to exploit poor quality resources.  He speculated that human development reduced the high quality resources (e.g. native hibiscus) and that the larger species were more vulnerable to extinction than smaller taxa that were capable of exploiting the poorer quality resources that remained (e.g. 'akoekoe and 'i'iwi).  Another species, the rare 'akiapola'au, a specialized insectivore, has become endangered because it lives primarily in large koa trees, forests of which have been felled as a wood source and for tourist toys. Another endangered species, the palila, a granivore, exploits seeds of one tree, the mamane.  This tree has been adversely affected by introduced goats and sheep, thereby endangering the bird.


Thus, it appears that many of the honeycreepers are endangered because of their extreme specialization (habitat or food), which itself is a result of dramatic adaptive radiation.  This problem may be compounded by introduced diseases, mammals, and exotic birds.


Hawaiian Drosophila

  see image: Drosophila phylogeny of picture-wing Drosophila of Hawai'i, representing only a small fraction of all flies on the islands. (Futuyma 1986)


   The Drosophila (fruit flies) of Hawai'i consist of nearly 700 different species, each typically restricted to one island.  The islands vary tremendously in age, Hawai'i being the youngest (~400,000) years old); even this island has endemic species. Island age increases progressively to the northwest including submerged seamounts in the Pacific off the Asian coast. Speciation of these flies is also speculated to have occurred as a result of adaptive radiation.  The speciation mechanism is not entirely clear, though two main models are:


1) Mayr's founder effect model: a few new colonists to an island represent only a small fraction of the total gene pool.  Genetic drift then results in dramatic 'peak shifts' in the genome of colonizing species relative to the parental stock so long as gene exchange is precluded.  This genetic change in the population would occur very rapidly and would be driven by local selective pressures in a relatively homogeneous environment.  (This idea formed the basis for the idea of punctuated equilibrium model).


see founder effect with flies (Futuyma 1986)



 Hampton Carson modified this model slightly to suggest that certain loci form strong epistatic relationships (synergistic effects caused by multiple loci) and a 'closed variability system'. For example, in fruit flies, genes controlling mate recognition and behaviour may form such a closed variability system' in which females will recognize only specific behaviours or morphologies when preparing to mate (=sexual selection). Carson believes that these blocks of genes become destabilized during founding events as the new population experienced exponential growth in an altered selection environment. Recombinants that previously had low fitness may now thrive, bringing the new population to a new  'adaptive peak'.


Templeton modified this model suggested that change in allelic frequencies for a few key genes (owing to genetic drift) could precipitate changes in modifier loci in the new environment leading to a new co-adapted state of the character.  For example, Ken Kaneshiro has determined that some species differ only by one mutation; this mutation modified the behavioural repertoire of males during mating rituals.  Derived (=new) species females would recognize the ritual and mate with the individual whereas ancestral females (parental species) would not and the species were behaviourally (sexually) isolated from one another.


2) Brian Charlesworth has argued that founding may not be the impetus for speciation, but that it may occur as a result of adaptive divergence from the parental stock under a new regime of selective pressures.  Many studies have illustrated that isolating mechanisms are determined by numerous genes.  It is not clear whether this can be applied to the Hawaiian Drosophila, as this view maintains that speciation occurs as a result of gradual, sequential allele substitutions.


An interesting problem developed when immunological tests with Drosophila suggested that the flies were far older than the islands.  If this were so, how could all of these endemic species occur on the islands? Where did they come from?  Dating of rock from each of the islands and from the seamounts west of the islands have demonstrated that island age increases east to west and that the oldest seamounts are older than the flies.  Thus, it appears likely that the ancestral flies once occupied the seamount (when it was above sea level) and that they island-hopped to the newly formed Hawaiian islands (which are located along a zone of crustal weakness; it is believe the islands were formed and moved west on their plate), undergoing adaptive radiation as they exploited new islands and habitats.


see image: leapfrogging Hawai'i (Ruse 1992)


Cichlid Fishes of the Great Lakes of Africa


Picture: cichlid fish


see lakes (Meyer 1993)


The great lakes of east Africa - Lakes Victoria, Malawi and Tanganyika are far older than the North American Great Lakes

(~15,000).  These lakes are famous for their rich cichlid fishes - each has hundreds of endemic species. 


(see image: lake statistics - Lowe-McConnell 1994).  

Many of these species are highly specialized to exploit a specific habitat and food sources.


(see image: fish morphology - Meyer 1993).


For example, in Lake Malawi, different cichlids feed on molluscs, plankton, insects, benthic algae, and other fishes.  The mouth parts of these closely related species are often completely different, though highly adapted for their particular form of feeding.  Two genera feed exclusively on scales of other fish, while one species feeds on other fish's eyes.  Indeed, if it was not known how closely related these taxa are, they would almost certainly be assigned to different families.  The species have undergone dramatic adaptive radiation, facilitated by the paucity of other fishes exploiting particular habitats.  As with the Hawaiian Drosophila, these species have complex mating and behavioural rituals, suggesting that sexual selection is an important factor isolating some species.


The lakes:


Lake Victoria: (see image: lake morphometry - Kaufman 1992) the largest tropical lake in the world (69,000 km2)with a maximum depth of 84 m.  The lake formed between 250,000 and 750,000 years ago.  There is some evidence that the lake level dropped drastically about 14,000 years ago, precipitating many extinctions but also permitting allopatric speciation to occur.  Indeed, Johnson et al. (1996) have used seismic profiles to suggest that the lake was completely dried up before 12,400 years ago; this would indicate very, very rapid evolution of the cichlid assemblages in the lake. Coastal population growth with attendant inflows of nutrients has caused exceptional cultural eutrophication problems.  While the lake was once seasonally oxygenated throughout the water column, it is now permanently anoxic below 20-30 m. 


  (see image: lake oxygen levels)(Kaufman 1992)


This dramatic increase in anoxia has drastically reduced suitable fish habitat.  As well, the Nile perch (Lates niloticus) was introduced to the lake, ironically to improve fisheries. 


(see image: big fish - little fish)(Kaufman 1992).


 The fish has caused the extinction of native cichlid fishes and has reduced the fishery of these and other native fishes (Kaufman 1992)


(see image: percent fish catch).


  The lake had as many as 300 cichlid species and 338 total species earlier this century (Worthington and Lowe-McConnell 1994).  These cichlids had radiated to occupy the many feeding and habitat niches available in the lake.  Sadly, as many as 200 of these cichlid species are now extinct owing to the water quality changes and effects of Nile perch. Historically, the lake supported a very large fishery (5x larger than any other African lake) based on these cichlids (=haplochromines).  This fishery was based on many different species including Bagrus catfish, lungfish, mormyrids and many others; today this fishery has been virtually destroyed, and in its place is a fishery dominated by Nile perch, Oreochromis niloticus and the endemic pelagic cyprinid Rastrineobola argentea. 


(see image: total fish catch)(see Kaufman 1992)


 The fish community in Lake Victoria was also affected by intensive harvesting using increasingly small-mesh gill nets during the 1960's and 1970's.  Declining catches prompted fish managers to introduce many tilapia species from adjacent areas; these species replaced the native tilapia species in lake (Worthington and Lowe-McConnell 1994). Nile perch were most likely introduced clandestinely as early as 1954, but were not observed until 1960.  They were stocked into the lake in 1962 to improve the fishery by preying on the unused cichlid fishes (or so thought the managers); these fish could be harvested and, at the same time, provide hours of enjoyment for fishermen.  As with many other invader species, the population grew very slowly but gradually until about 1980 when populations exploded.  These fish have, ironically, proved a problem for 2 other reasons:


  1) they blow right through the weak nets used by fishermen;

  2) they are a lower quality fish than the natives they replaced.


At present, the lake is in grave danger; eutrophication occurs unabated owing to large nutrient inflows from the large human population living on the lake banks. As well, increasing land-use in the basin around the lake has resulted in loss of riparian vegetation.  Deforestation has exacerbated direct nutrient increases to the lake since this plant material previously acted as a biological filter removing nutrients before the water entered the lake. Finally, and perhaps most importantly, the lake has been invaded by aquatic plants that forms a nearly continuous surface mat in shallow regions of the lake. These plants strongly change light penetration, water quality and habitat for native fishes and other organisms.  


 Lake Malawi


Like its larger cousin Lake Tanganyika, Lake Malawi was formed millions of years ago following faulting of the earth's crust. This faulting activity resulted in deep fissures - the present day lakes.  Lake Malawi is the 3rd largest African lake but the second deepest.  This lake has defined seasonal changes, but is permanently stratified and anoxic below 250m (Lowe-McConnell 1993). The SE and SW sections of the lake have a sandy bottom and support commercial fisheries of tilapia and cichlids.  The other regions of the lake are steep and rocky, and support vastly different types of fish than sandy bottom areas.  The rocky areas support ~200 species of 'mbuna' cichlids. These fishes are highly specialized and restricted to small patches of rocky shore.  They are separated from other taxa by stretches of sandy shore or open water.



The lake receded on 2 recent occasions - 25,000 and 10,740 years before present; lake levels dropped dramatically ~1150 AD. and again between 1500 and 1850 AD. Malawi is thought to have supported c.a. 545 fish species, of which 500 were cichlids.  Many of the cichlids apparently radiated because of their localized  distributions and specialized diet - water level fluctuations are thought to have isolated different groups, resulting in micro-allopatric speciation.  Many non-cichlid fishes occupy the sand and mud areas (200 species).  Little is known about this group of fish.  Because of the basin's steep morphometry, its fishery production has never compared to that of Lake Victoria, (a situation similar to a comparison of Lake Erie vs. Lake Superior). As with Lake Victoria, it is believed that the cichlid fishes evolved very rapidly through adaptive radiation.

Intensive fishing pressure in the lake into which Malawi flows (Malombe) resulted in a collapse of the fishery there. In Lake Malawi, some fisheries have suffered from deforestation and its effects on breeding streams, though fortunately, exotic species play a very minor role in this system.  Suggestions to stock the lake with fishes from Lake Tanganyika were rebuffed and Nile perch has never been stocked.  The lake and its biodiversity is recognized as a World Heritage site. Despite this, the rapidly increasing population of humans around the lake is imposing tremendous fishing pressure, and is apparently responsible for the 'Utaka' (a schooling planktivore) and predatory catfish decline (Lowe-McConnell 1996). 



Lake Tanganyika (see image: the big deep one)(Sturmburger & Meyer 1992).


  Lake Tanganyika is the largest and deepest lake in Africa and the second deepest in the world. It is also one of the oldest lakes in the world, aged between 9 and 12 million years old. The lake is permanently stratified and anoxic below 200 m. The lake supports 14 fish families and 12 tribes (flocks) of cichlid fishes.  Mouth-brooding cichlids occur only in this lake.  More than 220 species, mostly cichlids, inhabit rocky coastal water, though open water fishes are also found.  Most of the deep-water fishes are poorly understood and little studied.  In rocky areas, cichlids occupy many feeding niches (algal-grazers, zoobenthic feeders, omnivores, piscivores, plankton feeders). The cichlids belong to 49 endemic genera. Tribes are morphologically and electrophoretically distinct and old, and can be traced back to 7 ancestral lines.  These fishes are also much older than those in Malawi or Victoria (and thus are distinct from them). 


(see image: phylogenetic tree of African cichlids)(Meyer 1993).


  Studies by Sturmbauer and Meyer (1992) indicate that the lake probably existed as three separate basins during dry periods.  As with Lake Malawi, it is thought that these drought events tended to augment behavioural separation of shoreline species, resulting in their divergence and speciation.  Many of the cichlids in Tanganyika (as in Malawi) are brightly coloured, raising the possibility that sexual or social selection is an important component of their behaviour.  The lake is threatened by inflows of pollution, mainly silt-laden waters from rivers owing to forest destruction in the watershed.  Silt covers the algae, the base of much of the foodweb and alters fish spawning sites. Nutrients and industrial contaminants are a growing problem.  Because flushing time of the lake is exceptionally long (thousands of years), any chemicals that enter the lake will cycle indefinetly (Lowe-McConnell 1996). Petro-chemicals are one of the chemical groups most threatening to the lake. Fishing pressure is also intense.



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