What is
Adaptive radiation? Firstly, when discussing evolutionary biology, Schutler
defined adaptive radiation as “the differentiation of a single ancestor into
and array of species that inhabit a variety of environments and that differ in
traits used to exploit those environments”. An example of adaptive radiation
would be the Cichlid fish. Cichlids are a family of fish found in the lakes of
the East African Rift., mainly Lakes Malawi, Tanganyika and Victoria with
radiations producing 250 and 500 species per lake (Brawand et al, 2014) and
other smaller African lakes contain small numbers of endemic cichlids (Table 1).
They all differ in their body shapes and sizes, pigmentation patterns and
social behaviors. Even though the lakes haven’t been thoroughly sampled the
main species are haplochromines, molecular phylogenic studies have shown that
cichlids have evolved more lately than the origin of the lake.Cichlid
fish have a large species richness. When investigating specific lakes e.g. Lake
Malawi which encompasses the largest radiation. Its Comprised of different species
such as several species that feed on eggs and larvae carried by mouth brooding
female cichlids, species that clear parasites from skins of fish another that
feeds on pieces of their skin. Additionally, there are various species that are
scale eaters, fin biters, rock scrapers, sediment shifters and zooplankton
feeders. There are numerous streamline silver offshore shoaling fish and their sharp
toothed predators. There are crab and snail eaters and one particular species
that feeds mainly on flies that rest on rocks just above the surface of the
water, another flips over sediment to look for hidden larvae from insects.

These are
all behavioral and ecological adaptation that the cichlid fish have taken which
associate with morphological changes to the body shape and size, head shape,
jaw size, shape, orientation and shape and number of cusps of the teeth. For example,
body shape evolution is strongly affected by feeding habits. Piscivorous fish have
a much larger head and benthivorous fish tend to have a slender body. Thus,
body shape is not independent from tropic morphology. The body shapes are
generally associated with swimming modes in fish, suggesting that the divergent
body shapes of the Cichlids also relate to other ecological factors, such as
the efficiency of escaping from predators. Some species

Have evolved
lateral line canals to detect movements of prey hidden in the mud and huge eyes
to enable them to see in the dim light at depths of over 100 meters.

Molecular
phylogenic studies have shown that cichlid adaptive radiation have the tendency
for similar adaptations to appear in different lakes this is a demonstration of
parallel evolution.

However,
cichlids have failed to develop some forms. The restrictions on cichlid
adaptive evolution allows insight on the reasons of variation and sheds light
on the considerable diversification among cichlids ancestries in their affinity
to undergo adaptive radiation. E.g. the genera Pseudocrenilabrus and Tilapia
are widely distributed in Africa, yet have shown no adaptive radiation in
Malwai, Tanganyika or Victoria, the main 3 African lakes. Even though in the 3
largest African lakes, cichlids have filled the niches of small fast-moving
plankton, eel shaped species and truly nocturnal forms, there are no predatory cichlids.
Even the larges cichlid predators only weigh 3-3.5 kg, compared to other fish
e.g. 60-200kg for the largest catfish that lives in the same waters

Cichlid
fishes exhibit complex physiological behavior, this makes them a suitable vertebrate
model for the study of reproductive strategies. They demonstrate high levels of
parental care, even after their offspring has been hatched. This is uncommon
among fishes as cichlids continue to guard the larvae and then the independent offspring.

Females have
to be small enough to fit inside empty snail shells and rear their young.
Dominant males are large enough to pick up the shells and their reproductive success
is related to the size of their shell collection. Smaller males show
alternative strategies, either hunting for food and mates in packs or even mimicking
females to sneak into the snail shells.

Mouthbrooding
is not uncommon in fishes, but maternal care is, yet cichlids seems to have evolved
maternal mouth brooding on different occasions. In Lake Victoria and Malwai,
adaptive radiation in ecomorphology have been accompanied by radiations in
social behavior- haplochromiens are maternal mouth brooders.

Furthermore,
a change in the evolution of cichlid fish could be down to the consequences of the
aggressive behavior of male’s cichlids driving the differentiation of species.
Individuals of other species then try to avoid this behavior or strive to
compete for critical resources that both of these species use. Thus, the
increase in frequency in the population helps to drive sympatric speciation.

Suggestions
of a role of allopatric speciation is geography. Over time in the larger lakes,
water levels have fluctuated which led to isolation and reconnection of different
species around the main lake, sub-basins and patches of habitat within a continuous
water body. Even though molecular studies have shown that speciation takes
place within surrounding lakes, there have been theories of the creation of
hybrids from multiple colonisations of a lake. The hybrids contain greater
adaptive genetic variation that any other individual original species, allowing
larger genetic combinations., evidence for this is found in molecular phylogenic
studies and the potential of fertile offspring from hybrids are formed.