radiation within biology is one of the single most important processes within
Evolution. It describes the process of
diversification among one species, and thus eventually sdeveloping into two or
more species, all from a single ancestral line. Schluter
(2006), described adaptive radiation as the ‘hallmark of adaptive
evolution’ and goes on to state how most, if not all, taxa arose from this
process. The most famous example of this process tends to present itself as ‘Darwin’s
Finches’, the extremely varied group of birds described in Darwin’s venture to
the Galápagos Islands. What he observed was radically different characteristics
of beak shape within closely related species on the islands. These features
arose from a need to overcome to varying problems obtaining food from different
sources around the islands (See Figure 1).
Figure 1: Diagram
showing beak shape compared with food source. (www.mun.ca, 2018)
Small islands, such as the Galápagos, tend to
have very easily observable examples of
adaptive radiation. This
is usually a result of such islands having a vast array of differing ecosystems
combined with a relatively small land mass. The islands of Hawaii have such
observable adaptations in the form of many species. Plants, such as the infamous
silversword (Witter and Carr, 1988), show extensive differences
depending on geographical location. These adaptations can almost certainly be
attributed to the climate, elevation and nutrient levels found in these areas.
Another group of animals with easily observable
adaptations comes in the form of the crickets native to Hawaii. Around 240
species of crickets reside in Hawaii and most are thought to have come from 3
common ancestors. These ancestors being a species of tree cricket, a sword-tail
cricket and a ground cricket (Meyer, 1995), all coming from mainland North
America. The large amount of separate species evolving from these few genera is
an example of adaptive radiation, as the expansion of the cricket’s territory places
them in different habitats and new survival problems. As the islands of Hawaii are
relatively new (and still developing), it is easy to see the chronological spread
of species from island to island. Indeed, even with cricket species, the older
the island the higher the taxonomic diversity tends to be (Gillespie and
Teleogryllus oceanicus (the oceanic field cricket), is endemic to many of the Pacific
islands, as well as Australia. It has been theorised that this species arrived
in Hawaii at the beginning of the last century. Usually the male of this
species possesses a very conspicuous
call, although in recent years, the population of T. oceanicus on Hawaii has become silent. In less than 30
generations, the presence of a parasitic fly (Ormia ochracea) has caused this recently introduced species of
cricket to fall silent. It has been shown that the call of the males attracts
this parasite and in turn, the cricket has evolved a visible lack of the wing
structure that enables the males to create this sound (Zuk, Rotenberry
and Tinghitella, 2006). It has been observed
that 95% of the male T. oceanicus on
the island of Kauai are now
completely silent, differing from even the populations T. oceanicus on nearby islands.
is found only within the Hawaiian island of Nihoa (Howarth, 1987).
4 Thaumatogryllus species have been described
so far, all with different adaptations. One species from this genus, (T. cavicola) has evolved in such a way that
it is able to spend its entire life cycle residing on the ceilings of lava
tubes underground. While its immediate anatomy is almost identical to Thaumatogryllus conanti (Conant’s
giant Nihoa tree cricket), the most obvious difference is the incredibly
pale, almost translucent exoskeleton of T.
cavicola. This adaptation has arisen due to not needing to develop camouflage
or any kind of pigmentation to hide or deter predators respectively, as this
uses up precious resources and nutrients. Whilst pale pigmentation is of benefit
to T. cavicola, T. conanti, as it is
a nocturnal insect, would be easily
spotted by predator animals during periods of lower light, and thus has
retained its original dark brown pigmentation to aid in camouflage.
All 4 species of Thaumatogryllus present a trait known as ‘Island gigantism’. This is a phenomenon in
which island dwelling animals evolve to be significantly larger than their
counterparts from the mainland in which they originate (Lomolino, 2005).
This is most likely due to constraints, such as competition and threat from predators
being removed, along with being able to fill and exploit new niches their
ancestors simply did not have access to. Island gigantism can also be seen in relatives
to crickets, weta, endemic to the islands of New Zealand. Around 70 species and
subspecies have been described, most of which are again substantially larger
compared to their mainland counterparts.
Island gigantism can often be attributed
to species which have spent a relatively short time within island habitats and
have not evolved alongside natural predators. This can then also be described
as adaptive radiation, as a common ancestor has evolved varying traits (i.e.
size) to align, thrive, and take advantage of the niche the organism finds
itself within. As newer islands form, like those in the area of Hawaii,
established species from older land masses will travel and become more specialised
to deal with their ever changing surroundings.