Dark Banquet - Bill Schutt [80]
In reality, most of what you do or experience in your lifetime has little or no effect on the genetic makeup of your offspring. Whether we’re talking about longer snouts and legs for Miocene protohorses, or sharper teeth for ancient vampire bats, any inheritable modifications inevitably came about as a result of changes that occurred at the genetic level (i.e., changes in part of a genetic blueprint or in the timing of genetically programmed events).*125
It’s this change in the timing of genetically programmed events that explains how ticks may have evolved from chiggers. In this case, blood feeding may not have been a novel characteristic (as in ancient vampire bats), but possibly the timing of its appearance was. In a process known as heterochrony, the timing of developmental events is altered. Heterochrony, then, could explain the origin of the first tick from an ancestral mite—a mite that somehow maintained its larval feeding behavior into adulthood.
How could something like that come about?
There are numerous examples of this process in nature, but the most well known is neoteny—in which an organism reaches sexual maturity while retaining juvenile characteristics. The classic example concerns the giant salamander Necturus (the mudpuppy), which retains its gills throughout adulthood. In the vast majority of amphibians (like most salamanders, as well as mudpuppy cousins, like frogs and toads), these respiratory structures are lost as the larvae metamorphose into semiterrestrial adults.
It’s been hypothesized that in this famous case of neoteny, a mutation allowed some salamanders to retain their gills as they reached sexual maturity. The obvious question is, Why would that particular characteristic become an adaptation? The best hypothesis thus far is that the selection pressure to retain gills as an adult might have been a change in the terrestrial environment (e.g., a new predator or drier conditions), making it safer to extend the time salamanders spent in the ponds where they swam as larvae.
Similarly, with regard to the evolution of ticks, perhaps increases in local vertebrate populations or species diversity (both are also forms of environmental change) led to an evolutionary advantage for some mites that accidentally retained the parasitic dietary habits they had as larvae. Basically, more vertebrates meant more exploitable sources of food. As in Necturus, this adaptation had evolved from a mutation that hadn’t produced a new character but instead had changed the developmental timing of a previously existing character. Following this hypothetical scenario to its conclusion, true ticks would have evolved as prototicks transitioned from feeding on liquefied cell contents (like their mite ancestors) to feeding on blood.
However ticks came about, most researchers think that the first ticks appeared sometime during the early Cretaceous period (around one hundred million years ago) and, not coincidentally, during a period of tremendous vertebrate diversity.
Within the arachnids, chiggers, ticks, and mites belong to the order Acari (or Acarina), which contains between 850 and 900 species of ticks and approximately 50,000 species of mites.
According to Gwilym O. Evans, author of Principles of Acarology, acarines are unlike other arachnids because of the intimate associations they’ve developed with other animals. In mites, these associations range from symbiosis to commensalism to parasitism.
Briefly, symbiotic relationships are those between two different organisms in which both derive some benefit. Among the acarids, perhaps the strangest example of symbiosis is the relationship between the eastern subterranean termite Reticulitermes flavipes and the slime mite Histiostoma. Researchers have found that termite colonies often become infected with a pathogenic fungus (Metarhizium anisopliae). The fungus invades the termite’s body, secretes a fatal