Speciation of fish in East African lakes
perhaps the greatest challenge for allopatric speciation comes from adaptive radiation of fish (and other organisms) in various lakes around the world
most famous cases are the lakes of East Africa in the Rift Valley . . . Lake Victoria is geologically less than 1 million years old -- it has over 238 species of fish, all but 38 of which belong to one family (Cichlidae) -- and nearly all of which are endemic to this lake (occur nowhere else) cichlids in Lake Tanganyika have evolved into highly diverse and specialized forms, those in Lake Malawi less so, those in Lake Victoria the least -- note the correspondance with the ages of the lakes (see table, below) -- the older the lake, the more diverse and specialized the cichlids in each lake, the numerous species of cichlids are each other's closest phylogenetic relatives -- both morphological and DNA sequences support this conclusion -- just like Darwin's finches on the Galapagos archipelago, the cichlids in each lake evolved from one ancestral species into many present-day species -- and they did so within the lake (not by repeated colonizations from outside) these cichlids are thus an extraordinary case of adaptive radiation to exploit virtually every source of food for fish -- detritus, surface insects, benthic (bottom-dwelling) insects and larvae, burrowing benthic insects, molluscs, zooplankton, other fish, algae on other plants, algae on rocks, floating plants, rooted plants, and (even) broods of other cichlids this last specialization needs explanation -- in all Lake Victoria cichlids, after the eggs are fertilized externally, females brood their eggs and young in their mouths until they are 10-15 mm in length -- at least 10 species of cichlids feed mainly on the eggs and young of other species -- either by grabbing the snout of a brooding female and sucking the eggs out or by chasing her until she disgorges her brood all of these feeding specializations are achieved by relatively minor alterations in anatomy of head, mouth, teeth and body shape -- yet distinct genetic differences indicate reproductive isolation between populations that differ only slightly in morphology (like Darwin's finches again) -- almost all species in Lake Victoria are similar enough to be classified in one genus (Haplochromis) [examples of body shapes in cichlids of Lake Victoria (all in the genus Haplochromis)] in Lake Tanganyika, cichlids have evolved more extreme feeding specializations and morphology -- consequently, ichthyologists recognize more endemic genera
allopatric speciation might explain speciation within these lakes . . . possibilities exist for isolation on separate reefs or in temporarily isolated satellite lakes -- mouth-brooding (or in L. Tanganyika, hole-nesting) results in limited natal dispersal of young in comparison to planktonic larvae -- limited natal dispersal perhaps explains why cichlids have radiated in these lakes but other families of fish with planktonic larvae have not -- a lake looks uniform to us, but to a shallow-water fish with prolonged parental care it might in fact be an "archipelago" of opportunities -- but these fish clearly stretch the case for allopatric speciation (see reading list for more discussion -- especially Goldschmidt's Darwin's Dream Pond) although the cichlid fishes of Lake Victoria evolved in less than 1 million years (at a time when humans were also evolving in Africa), most have become extinct even faster -- in a few decades since humans removed the forests around the lake (so the water became turbid) and introduced a large predacious fish (the Nile perch)
NC has a modest example of a lake with endemic species -- Lake Waccamaw has 2 endemic species of fish and possibly 3 more -- but all are in different families -- so no indication of speciation within the lake
diversity of fish within a lake does not necessarily require speciation . . . Iceland is an oceanic island, never connected to continents -- in Icelandic lakes there are only three species of fish (as in other arctic areas, the numbers of species are much lower than in the tropics) -- Three-spined Stickleback, Brown Trout, and Charr (a species of salmon related to trout) -- in some lakes, Charr develop into four distinct forms -- benthic-feeding dwarf Charr (8-15 cm long as an adult), plankton-eating intermediate Charr (18-20 cm), fish-eating large Charr (25-50 cm) and mollusc-eating large Charr (25-50 cm) -- the first and last have blunt snouts and mouths underneath their heads, intermediate ones have streamlined snouts and mouth in front yet all have the same frequencies of alleles at two genetic loci -- apparently all are the same species with four distinct developmental pathways! -- evidently there is strong disruptive selection for ecological differences -- but not strong enough for sympatric speciation
why have Charr not speciated in Icelandic lakes -- but some ancestral cichlid in Lake Victoria evolved into over 200 species in only 1 MY?
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||
