2007, Volume 5, Number 1
Beak of the Fish: What Cichlid Flocks Reveal About Speciation Processes
IntroductionThe finches of the Galapagos Islands greatly influenced Charles Darwin’s formative ideas on natural selection and its connections with speciation. Today, the adaptive radiation evident in the beaks of those fourteen different species of birds continues to provide the classic textbook example of evolution in action.
However, if the creation of a mere fourteen different species has been instructive, then an explosive creation of well over a thousand should reveal a great deal more about speciation processes. The fish family Cichlidae provides such a case: these fish have evolved thousands of new species within their insular lake homes, and furthermore have done so on extremely rapid time scales. Indeed, their explosive and rapid cladogenesis has made cichlids the greatest extant vertebrate radiation known. The young evolutionary age of cichlids also provides a window into the earliest stages of diversification, such that speciation may be studied as an ongoing process of change. Moreover, the recent multitude of species within the cichlid family provides an opportunity to look for patterns in speciation processes. Such studies have suggested a three-stage pattern underlying speciation that may be generally applicable to the evolutionary origins of many other vertebrate species (Danley & Kocher 2001; Streelman & Danley 2003). Of special interest is the stage of evolution in which different feeding structures arise among these fishes, particularly since parallels exist between the beaks of Darwin’s finches and the jaws of cichlids. Modern genetic techniques are responsible for insight into this three-step pattern, and are increasingly being used to further uncover the detail in the processes responsible for the remarkable diversity of cichlids. Unfortunately, even as cichlids are just beginning to provide the post-Darwinian age with highly valuable insights into the mechanisms of evolutionary change, these fish are being rapidly destroyed by anthropogenic threats, undoubtedly taking their fascinating and irreplaceable stories about evolution with them.
East African Great Lakes and Their Cichlid Flocks
Like Darwin’s finches that were isolated on islands, flocks of cichlid fishes have also undergone adaptive radiations within insular habitats: the Great Lakes of East Africa. These tropical lakes are among the largest lakes in the world, reaching depths of 1500-m and areas of about 69,000-km2. Fortuitously, differences in geologic origins, features, and ages are useful when studying the evolution of endemic cichlid fishes. Of the three lakes, Lake Tanganyika is the oldest (forming 9
-12 million years ago) and deepest. This lake of 34,000-km2 hosts twelve flocks of cichlids that have radiated into about 250 species. The other deep rift lake, Lake Malawi, is about the same size but is younger (formed 1-2 million years ago). This intermediate lake hosts about a thousand species that have all radiated from just one flock ancestor. The youngest (750,000 years old), largest, and most shallow, Lake Victoria hosts about 500 species that have also emanated from one cichlid flock. (Barlow 2000; Salzburger et al. 2005). Since the greater depths of the lakes lack oxygen, cichlids are confined to the surface layers and shallow benthic habitats of the lakes. These habitats are characterized by sandy bottoms punctuated with patches of rocky outcroppings. (Barlow 2000).
Cichlid Species: A Kaleidoscope of Form and Color
The species composing the cichlid flocks of these East African Great Lakes possess a bounty of variation in both form and color. Indeed, these fish are prized for their beauty and interesting diversity, and as such, are popular with aquarium hobbyists. The visual scenes within the lakes are themselves reminiscent of aquaria with swarms of colorful fish in a myriad of hues and shapes. With this abundance of similar fish fauna, species and their phylogenies have been difficult to determine, using only morphology and color characteristics; are similarly shaped but differently colored fish morphs of the same or different species? Behavioral differences in feeding strategies and reproduction often clarified these determinations: fish that looked similar but fed, mated, or reared young differently were considered separate species (Genner & Turner 2005). However, modern molecular techniques that examine genetic characteristics have recently provided an additional means by which cichlid diversification can be described and evolutionary relationships discovered. For example, Meyer et al. (1990) found no overlaps in the mitochondrial DNA types of 14 cichlid species from Lake Victoria; they concluded that these fish were not merely alternative morphs of one species. That is, their classification as separate species was justifiable given their unique genetic characters. Now, cichlid species are regularly defined using genetic characters, in addition to information on morphology, color, and behavioral traits (Turner et al. 2001; Genner & Turner 2005).
Origins: Out of Lake Tanganyika Comes Explosive, Rapid, and Parallel Evolution
Remarkably, nearly all of the cichlid species in the East African Great Lakes are endemic. Moreover, none of these cichlid species are shared between the lakes despite belonging to the same family (Salzburger et al. 2005). Since they share numerous similarities, an understanding of how these fish are inter-related becomes an important initial step in untangling the mystery of how many differences evolved. Again, modern genetic techniques have proven to be the most useful tool for this detective work: the first examination of mitochondrial DNA of cichlids from the Great Lakes by Meyer et al. (1990) revealed that the cichlids of each lake are monophyletic. The family tree that emerges from this investigation also reveals that Lake Tanganyika cichlids are the ancestors from which the cichlids in the other two Great Lakes are derived (Figure 1). The finding that well over a thousand species of cichlids in Lakes Malawi and Victoria evolved from a single source reveals that speciation processes within these lakes were remarkably explosive.
Furthermore, Meyer et al. (1990) utilized a molecular clock in their analysis of mitochondrial DNA haplotypes, to estimate the ages of different cichlid lake flocks. They concluded that the Victorian cichlids are newcomers originating less than 200,000 years ago, while the ages of the Lake Malawi and Tanganyika cichlids also match with the (relatively young) geologic ages of those lakes. A recent extensive study by Salzburger et al. (2005) that used over twice the number of mitochondrial base pairs and ten times the number of species, confirms that Lake Tanganyika is both the geographic and genetic origin for the cichlids of the other Great Lakes. This finding, that the multitude of species in the Great Lakes occurred over relatively short time scales (0.5-2 million years), reveals that speciation processes within these lake were remarkably rapid.
Not only did cichlids in the Great Lakes explosively and rapidly diversify, they also separately converged into very similar morphologies. Figure 2 illustrates several cichlids from Lakes Tanganyika and Malawi that share similar body forms and engage in parallel lifestyles. Studies prior to Meyer et al. (1990) argued that such cichlids were polyphyletic: these outwardly similar fish from different lakes were derived from different ancestral lines, and in turn were most closely related to each other. Instead, the advent of molecular phylogenetic studies have repeatedly shown that these cichlid radiations have occurred separately, each within their own lake basin (Meyer et al. 1990, Kocher et al. 1993, Meyer 1993, Nagl et al. 2000, Verheyan et al. 2003, Salzburger et al. 2005). For example, the uniquely fleshy lips of the Placidochromis fishes of Lake Malawi arose separately from the fleshy lips of the Lobochilotes cichlids of Lake Tanganyika; Placidochromis are more closely related to their own proximate Lake Malawi flocks than to their albeit morphologically similar cousins of Lake Tanganyika. Even within the same lake, these fish seem to separately evolve similar morphologies. For example, Ruber et al. (1999) found that some cichlids of Lake Tanganyika that share the same dentition patterns are not monophyletic. That is, different species separately evolved the same tooth shape. Similarly, Allender et al. (2003) examined the genetic differences between like-colored fish from the northern and southern portions of Lake Malawi. Using over 2000 genetic loci, they built phylogenies demonstrating that the same color patterns had independently evolved within different parts of the lake. That is, the northern and southern fish are unrelated, but separately evolved the same color types via divergent selection. Thus, both between and within them, the Great Lakes have been reservoirs of frequent parallel evolution (Genner & Turner 2005; Kassam et al. 2006).
Multistage Evolutionary Patterns
The frequent occurrences of parallel evolution within cichlid flocks, added to their numerous and recent cladogenesis events, provide a highly useful case study of adaptive radiation. What can cichlids reveal about speciation processes; are there common factors or underlying patterns? Once again, modern phylogenetic techniques have been useful for these evolutionary investigations. The 1990 mitochondrial DNA sequencing study by Meyer et al. first found that the cichlids in Lake Malawi could be divided into two monophyletic groups based on habitat usage: the rock-dwellers and the sand-dwellers. But, given that they had sampled only 24 of the 200 species of this lake, they could not fully generalize the result. Moreover, they found that the amount of genetic variation in molecular markers was extremely low (less than that in the human species). This paucity of DNA sequence variation has posed challenges for subsequent investigations attempting to build cichlid phylogenies at finer scales.
However, increases in the types and extent of molecular techniques have recently begun to address this problem. Notably, Albertson et al. (1999) utilized over two thousand amplified fragment length polymorphism (AFLP) loci, in a genetic survey of representatives from many of the rock-dwelling cichlids in Lake Malawi. With this data they were able to confirm the findings of Meyer et al. (1990): rock-dwellers are monophyletic. In addition, the large extent of their investigation, in which they examined 2,247 characters spread across the entire genome, allowed researchers to examine the inter-relatedness of these fish at a much finer scale. The resulting more detailed phylogeny shows that fishes having morphological similarities related to feeding, are themselves monophyletic. That is, each rock-dweller has a unique jaw morphology that best serves its feeding habits and the family tree groups of the species by such physical differences. For example Pseudotropheus species have a mouth that is somewhat subterminal, at the bottom of steeply sloping face; and this flock is all very closely related. However, Labeotropheus species are grouped together based on their completely subterminal mouths and protruding fleshy snouts. Consequently, the genetic similarities between species in these flocks parallel their morphological groupings.
Albertson et al. (1999) initially hypothesized that if early speciation processes were not driven by adaptations to different feeding habits, then closely related species would differ in jaw morphology. The genetic similarities that they found between morphologically similar species showed the opposite to be true; closely
related species may differ in color or other traits but not in their general jaw features. So, their findings suggest that diversification between these species first occurred as a result of trophic influences.
Danley and Kocher (2001) have incorporated these results regarding the phylogenetic histories of the cichlids of Lake Malawi, into a generalized description of cichlid speciation processes that involves three evolutionary stages. At each stage, a different primary force acts to drive selection along a different adaptive axis. In the first stage, a single cichlid progenitor colonizing the lake divides up the available habitat into rock versus sand substrates.
This division is then followed by diversification in regards to the exploitation of food resources. Lastly, differences in color and other communication-related mechanisms result in even greater diversification. Simplification of the explosive radiations of cichlids into the three-stage model will ultimately lead to a
better understanding of the remarkable speciation that has occurred in this vertebrate group.
Speciation Process Stage 1: Division of Habitat
One key to the early success of cichlids may be that they were simply in the right place at the right time; they were among the first to colonize the East African Great Lakes (Barlow 2000). With a minimum of competitive interspecific influences, early cichlids were able to seek food resources throughout all the available
shallow lake habitat. Thus, one cichlid group adapted to the benthic sand habitat, while others adapted to the other available benthic substrate that consisted of rock. Such a split very early in their evolutionary history is well supported by cichlid phylogenies from all three Great Lakes; the cichlid family trees for Lake Tanganyika (Sturmbauer 1998), Malawi (Meyer 1993), and Victoria (Nagl et al. 2000) all feature an early split between rock and sand-dwellers.
Speciation Process Stage 2: Division of Food Resources
As illustrated in the phylogenetic tree of the cichlids of Lake Malawi built by Albertson et al. (1999), the fish next began to divide food resources. They accomplished divisions by exploiting food resources using a multitude of strategies. Their differing strategies are now reflected in their adaptive feeding morphologies. For example, some cichlids (e.g. Labeotropheus) specialize in feeding on rock-clinging algae by swimming parallel to the rock surface. Their fleshy snouts and ventral-facing mouths are adaptations that increase the efficiency of this feeding mode. In contrast, Metriaclima cichlids utilize an approach to grazing on algae that involves a perpendicular orientation to the rock surface. Their sloped head and forward-facing mouth are ideal adaptations for their alternative algae feeding mode.
Indeed, cichlids have exploited a plethora of all possible food sources, which is reflected in both their behavior and myriad trophic adaptations. There are vegetarian cichlids that are highly specialized phytoplankton collectors, algae grazers, algae rippers, or algae scrapers. Each has physically evolved to best pursue the specific vegetative food of choice. Carnivorous cichlids specialize in stalking, ambushing, or out-running their prey. Those needing speed are streamlined powerful swimmers; while those utilizing the element of surprise possess speed at suddenly sucking in prey. Other cichlids methodically sift the bottom for detritus, while others aggressively nip the scales of their neighbors. Fascinatingly, some scale nippers are so specialized at attacks to one particular side (e.g. the left) that their jaws are appropriately inclined in that direction. A particularly mischievous cichlid carnivore from Lake Malawi feigns death by lying flat on its side so that it can spring up quickly to attack those that are lured into feeding on the "corpse;" it wears a blotchy color pattern suggestive of decomposition to complete the trickery of its feeding strategy. Carnivorous cichlids do not limit their predation to conspecifics, as many specialize in consuming molluscs, insects, or insect larvae; those needing shell crushing power are equipped with powerful jaws. (Barlow 2000).
Speciation Process Stage 3: Division via Communication
Many closely related species of cichlids share physical attributes that match their feeding lifestyles, but they often differ in color or other traits. Often, the differences are most notable in male secondary characteristics that involve rainbows of color. Since females are usually doing the choosing during mating, male coloration may serve as a kind of advertising. In addition, females may utilize colors to select not only the best male, but also to select a male of her same species. So, while adaptive forces have physically shaped and diversified cichlids, the forces of sexual selection have played a leading role in subsequent cladogenesis (Danley & Kocher 2001). Certainly, since all female cichlids invest heavily in their offspring by faithfully brooding their eggs, the choice of mate is an important one.
Some female cichlids brood their eggs in their mouths where the eggs can be kept especially safe until hatching. For these mouth-brooding cichlid species, the actual fertilization process is rather unique and is also usually driven by visual cues. For example, after laying eggs, the female will quickly gather them into her mouth. Her persistence at collecting all of her eggs draws her to her mate’s anal fin spots. These spots mimic the shape, size, and color of the eggs. With the female thus appropriately positioned, the male then fertilizes the eggs she holds in her mouth. Fertilization rates are thought to increase when females can see male egg-spots (Wickler 1962), potentially further driving the forces of diversifying markings and color via sexual selection.
Males of some cichlid species have expanded their advertising activities to go well beyond just colorful good looks; they also show off with sandcastle building activities. Like bowers constructed by many birds, males will construct and then defend towers of sand that can be meticulously created. Females are free to select any sandcastle territory, and its owner, with whom to mate. So, male reproductive success also increases as a function of coloration and/or bower building finesse.
Thus, the last stage in cichlid speciation involves numerous divisions, all based on colors and behaviors that communicate reproductively significant information.
Similar Speciation Processes in Other Vertebrates
The generalized three-stage model for speciation also successfully describes patterns in the phylogenies, and thus evolution of other vertebrate taxa. This is especially true for many other fish species. For example, stickleback fish also diversified first into bottom- versus water column-dwelling types, and did so independently in a number of different lakes. Like cichlids, sticklebacks subsequently radiated based on body morphology. Tropical parrotfish are another analogous example. These fish first ecologically split available habitat between reef and sea grass. The reef-occupying parrotfish further diversified morphologically into either algae scrapers or algae excavators. Later these groups radiated into many different color forms related to reproduction (Streelman et al. 2002). So like cichlids, parrotfish species may have similar feeding morphologies, but differ in male mating colors. Terrestrially, an example of this behavior is found in the Anolis lizards of the Caribbean who have undergone diversification first by dividing available habitat types, and later by changing physical form to best suit their lifestyle. Evidence for the operation of these stages in the speciation of Anolis comes from a number of different islands where they evolved separately. Finally, even Darwin’s finches have, from their genetically-based phylogenies, first speciated based on habitat (tree- versus ground-dwellers) and then by their remarkable beak-shape adaptations to different feeding strategies. (Streelman & Danley 2003).
But Why So Many More Cichlids?
While evolutionary scientists have made significant progress in describing speciation patterns with the three-stage model, understanding the basal evolutionary mechanisms responsible for the uniquely explosive and rapid speciation in cichlids is more elusive. Why and how have cichlids radiated into so many more species than other vertebrates? If there are thousands of cichlid species, why aren’t there thousands of Darwin’s finches? A complete answer to this question continues to evade theorists, but numerous clues to the puzzle have been uncovered by a veritable army of investigators. One set of clues revolves around the details of the cichlid feeding apparatus.
The overall cichlid body plan tends to be highly conserved, but their heads and jaws have changed into numerous different forms. From the perspective of the fish, a body plan lacking limbs leaves jaws as the only mechanism by which food can be captured and manipulated. Cichlids have responded evolutionarily by renovating their outer oral jaws into ones that are remarkably flexible and multifunctional (Liem 1980). Such jaws are able to fantastically protrude, and each jaw side can move independently of the other. These abilities greatly increase the numbers and types of food items that can be manipulated (Hulsey & De Leon 2006). In fact, of all fish, the outer jaws of cichlids are the most prehensile in their abilities (Figure 3).
However if one highly flexible jaw is good, then two might be better. In fact, fish do possess a second set of jaw-like structures within their throats called pharyngeal jaws (Figure 3). In most fish, this structure mostly aids directional movement of food from the mouth into the body. Cichlids have expanded its usefulness by physically modifying their pharyngeal jaws so that they can perform even more food manipulation feats (Liem 1974; Hulsey 2006; Hulsey et al. 2006).
Their pharyngeal jaws can squeeze, chew, and grind material much like regular jaws. For example, cichlids that have specialized their diet to molluscs, suction up snails with their outer jaws and then crush the shell using their powerful pharyngeal jaws. Often the snail shell must be exactly positioned so that its weakest element can be exploited for crushing. The outer jaws serve to repeatedly and adroitly perform this positioning to enhance the squeezing power of the pharyngeal jaws (Hulsey 2006). Other cichlids utilize their pharyngeal jaws for different specialized consumption services such as stacking fish scales or masticating filamentous algae (Liem, personal communication).
These jaw adaptations may have been instrumental in allowing cichlids to take advantage of every possible food resource, and so to radiate into many different feeding specialists during the second stage of their speciation process (Liem 1974; Hulsey & De Leon 2006; Hulsey et al. 2006). That is, cichlid jaw adaptations increased the diversity of trophic resources, and fine-tuning of these structures enabled increased ecological speciation. Thus, it has been proposed that these jaw adaptations, unique to cichlids, were the ‘key-innovation’ that fueled their explosive radiations (Liem 1974). This hypothesis is supported by recent bio-mechanical investigations (Hulsey et al. 2006).
Additional clues to the mechanisms of evolution in cichlids have recently been provided by studies that are now seeking to uncover the connections between genotype and phenotype. Since trophic-related morphology changes may have played a crucial role in cichlid speciation, discovery of the genes responsible for these modifications should be illuminating. In 2003, Albertson et al. took the first step in this direction of study, by performing genetic experiments with two cichlids from Lake Malawi. They chose two monophyletic rock-dwelling species that both feed on algae but employ radically different strategies and so possess quite different head and jaw features. Labeotropheus fuelleborni (L) swims parallel to rocks and so its ventral-facing mouth allows it to most efficiently collect algae in this position. Metriaclima zebra (M) approaches rocks at a ninety degree angle and so its forward-facing mouth is also specially well positioned. These two species also differ in their dentition characteristics, with L having shearing scissor-like teeth and M having comb-like teeth (Figure 4).
Albertson et al. (2003) crossed L and M, and then examined both the F1 and F2 progeny. Their results indicated that many teeth, jaw, and head features are inherited together, and that remarkably few (1-11) genes control these morphologies. For example, the dentition features in the F1 generation were intermediate to that present in the parents, and this pattern persisted into the F2 generation. Such a segregation pattern is consistent with control of this trait by a single gene. The lateral lower jaw also becomes intermediate in the F1 generation, but the spread in frequency becomes wider in the F2 generation. Their QTL analysis found control of the trait was best explained by about 11 genes. Similarly the lower jaw was found to be controlled by 9 genes, the maxilla by another 9 genes, and the premaxilla by 8 genes. This discovery, that so few genes control such great differences in the teeth and jaws of cichlids, may have unveiled one means by which cichlids diversified so numerously and rapidly in response to selection on these trophic features.
Terai et al. (2002) have endeavored to specifically identify these jaw-shaping genes, and their findings are also illuminating in regards to exploring the mechanisms underlying cichlid speciation processes. They hypothesized that if genes controlling jaw morphology have changed along with morphological diversification in cichlids, then the amino acid substitutions should have occurred more frequently in Great Lake cichlids than in other fishes. Conversely, if these genes are not responsible for producing the cichlid diversification in jaws, then the rate of amino acid substitutions should be similar in these cichlids and other fishes. To test this, they amplified parts of the cichlid genome known to be homologous with genes that control jaw and dentition features in mice. They also amplified the genome of an outgroup fish. From the DNA sequences, they found that the highest evolutionary rate of change in this part of the genome corresponded to a single gene previously known from mice called Bmp4. Furthermore, they found that the rate of amino acid substitutions in Bmp4 was five times higher among the Great Lake cichlids than among the outgroup fish. Thus, results suggest that Bmp4 changed at an accelerated rate, as did jaw morphology changes, during the speciation of cichlids. Albertson et al. (2005) have confirmed the role of Bmp4 in regulating jaw features.
Terai at al. (2002) then analyzed the specific protein domain areas affected by these numerous amino acid substitutions in the Bmp4 gene. They found that all substitutions affected the portion of the protein responsible for post-translational control of the protein. The amino acid substitutions in cichlids do not change the function of the protein made by the Bmp4 gene, but instead affect how that protein is used downstream in more complex molecular signaling pathways. This discovery, that a small change in the Bmp4 gene can generate bigger changes via a cascading pathway, may have unveiled another way in which cichlids generated so many different adaptive jaw forms so quickly.
It is further interesting to note that it has recently been found that the Bmp4 gene also plays an important role in the cranio-facial development of Darwin’s finches (Abzhanov et al. 2004; Grant et al. 2006). In fact, the investigators concluded from their study that, “…variation in Bmp4 regulation is one of the principal molecular variables that provided the quantitative morphological variation acted on by natural selection in the evolution of the beaks of the Darwin’sfinch species.” Thus, more detailed comparisons of Bmp4 sequences, expression, and regulation between cichlid flocks and finches should provide future clues to the puzzle of explosive speciation in cichlids.
Undoubtedly such examinations of the genetic factors involved in the control and modification of phenotype will continue to shed new light on the puzzle of speciation processes. However, studies and reflection on a more macroscopic-scale are also beneficial; one reason that there are so many more cichlids than finches, despite so many similarities in the mechanisms involved in their speciation, may simply be due to time. Darwin’s finches have been evolving for about 3 million years, whereas the cichlids of Lake Malawi have only been evolving for about half that time (or less). Thus, the case of cichlids provides a kind of time-travel backward, showing the very earliest events in speciation. As time and evolution proceed toward higher levels of ecological stability, the number of species may decline. Such a pattern is suggested by comparisons between the species of cichlids themselves. As the oldest lake, Tanganyika’s cichlid species number is much less
than the younger Lake Malawi. Also, Tanganyika’s cichlid species are more diverse. Such differences in both species number and diversity suggest that after speciation rates level off, some species out-compete others to extinction; then interspecific competition continues to drive a wedge between remaining successful species. This temporal pattern in speciation is also evident from comparisons of cichlids to their cousins, parrotfishes (Scaridae). Cichlids and parrotfish share components of the same ‘key-innovation’ (i.e. pharyngeal jaws), yet there are about ten times more species of cichlids than parrotfish. One significant difference between them, however, is that parrotfish originated about 40 million years ago, and have had a great deal more time to stabilize into a lesser number of highly diverse species. If we could time-travel back to the early days of parrotfish speciation, we might be met with the kind of profusion in species that we see today in cichlids. (Barlow 2000; Streelman & Danley 2003).
Disappearing Cichlids
Unfortunately, anthropogenic causes are destroying cichlids, and with them the clues they hold regarding speciation processes. Due to the remote location of the Great Lakes, rigorous scientific examination of these endemic cichlids has only been undertaken within the last forty years or so. In the meantime, the nature of threats facing cichlids has rapidly multiplied and intensified.
Fish from Lake Malawi provide communities there with seventy-five percent of their animal protein; cichlids are the primary catch. The entire fishing industry supports about two million people, but is doing so in a non-sustainable way. Cichlids captured are decreasing in size and quantity. Similarly, the introduction of commercial methods of fishing to Lake Tanganyika in the 1960s precipitated a decline in cichlid catch after only twenty years of exploitation. Besides being a food resource, cichlids are also valued by the aquarium trade and such harvests place additional pressure on them. In addition, certain particularly valuable cichlids are being moved to locations more convenient for harvesting. Since phylogeographic patterns of speciation are evident in cichlids at small scales, this practice will undoubtedly disrupt cichlid population structures and also confound scientific studies. (Barlow 2000).
However, perhaps a greater threat to cichlids are human changes to their environment. For example, in Lake Victoria pollution and increased run-off has caused rapid eutrophication and increased turbidity. Seehausen et al. (1997) report that the transparency of the water decreased from eight to three meters between 1920 and 1990. And in the 1990s, transparency decreased by another 50%. Noting these changes, they investigated what consequences this may have for Victorian cichlids.
Unfortunately, they found that females were no longer able to recognize conspecific males. Their experiments demonstrated that cichlids choose their mates and distinguish their own species based primarily upon color (i.e. color is the reproductively isolating mechanism). When the ability to properly perceive color was interfered with, females mated indiscriminately with males of many different species. They inferred that a deterioration of water quality in Lake Victoria was responsible for observed decreases in cichlid species diversity resulting from hybridization.
Humans have further changed cichlids’ environment by introducing the Nile Perch to the Great Lakes. Present in Lake Victoria probably since the 1950s, these fish erupted in the 1980s and a precipitous decline in cichlids was noted. Fishing catch of cichlids plummeted by 80%. Nile perch are huge (up to 2 meters and 135 pounds) predators that directly wiped out much of Lake Victoria’s cichlid fauna. As an invasive species, Nile Perch have also indirectly contributed to declines in cichlids. Increased destruction of local forests for firewood used in the processing of the Nile perch catch has further contributed to the increasing turbidity in the lake. (Barlow 2000).
Conclusion
It is perhaps ironic to note that many of the cichlid traits involved in their explosive and rapid adaptive radiations into a spectacular menagerie of species, are the same traits that are now subjecting them to greater threats, putting them at risk for extinction. They are highly specialized and so require habitat that is narrowly defined (e.g. very clear water). While albeit numerous, they exist in small populations (e.g. subpopulations within lakes) that can easily be wiped out by either slow, deteriorating influences or sudden stochastic catastrophes. (Barlow 2000). Having ecologically dominated their own lake basins throughout their evolution, they are perhaps poorly equipped to meet the challenges and predation threats of an invasive species (e.g. Nile Perch). While a mere fourteen dull brown-black finches inspired Darwin in his greatest contribution to biology, perhaps thousands of spectacularly colored fish will inspire future conservation biologists toward their preservation.
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This paper was written for CBES 694: Special Topics - Tropical Forests. The assignment was for a literature review of contemporary scientific investigations on the origins of biologic diversity. The professor was Dr. Elizabeth Stacy.