Dark Banquet (26 page)

Are ticks (like bed bugs) a pest on the rise, and if so, why? Are tick-transmitted pathogens also becoming immune to our treatments, or is there another answer? And what about Lyme disease? Why are the symptoms so variable (ranging from minor annoyance to catastrophic and life altering)? Some people believe that there's a chronic version of the disease—one that the experts refuse to talk about. And while we're in conspiracy mode, whatever happened to Lymerix, the Lyme disease vaccine?

All right, before we deal with the grassy knoll (and the chiggers lying in wait there), let's cover some basics. First of all, chiggers, mites, and ticks are
not
insects, but like insects, they belong to the enormous invertebrate phylum Arthropoda. In fact, they're members of the only arthropod group that rivals the insects in diversity—Arachnida (a subphylum that also includes the spiders and scorpions).

While chiggers aren't
exactly
blood feeders, they merit interest because their blood-sucking cousins, ticks, share much of their biology as well as their penchant for strange behavior. Additionally, hundreds of mite species are vampires as adults. It's just that many of them feed on nonvertebrate blood—like hemolymph, the blood found in arthropods like insects. Some of these mite/insect interactions have also been in the news lately, especially as they relate to agriculture and the honey bee industry.

Evolutionarily, mites and ticks are
extremely
close. In all likelihood, tick ancestors (prototicks) were actually mites that somehow evolved to become obligate blood feeders (similar to vampire bat ancestors, which were non–blood feeders).

Why did some mites retain their larval blood-feeding lifestyles into adulthood while other body characteristics (like sexual organs) developed normally? The answer is that maintaining larval (or juvenile) characteristics as an otherwise mature adult is yet another example of how new species can evolve. The basic premise, proposed by the evolutionary embryologist Gavin de Beer (1930) and reinvigorated by Stephen Jay Gould in his book
Ontogeny and Phylogeny
(1977), is that “evolution occurs when ontogeny
^*120
is altered in one of two ways: when new characters are introduced at any stage of development with varying effects on the subsequent stages, or when characters already present undergo changes in developmental timing.”

In the first scenario, Gould's “introduced characters” result from genetic alterations like mutations—changes in an individual's genetic blueprint that occur during DNA replication.
^*121
It is at this time that errors (variations to some) in the copying mechanism result in mutated strands of DNA. In this classic explanation of how evolution operates, in some instances this mutant DNA results in new characteristics for the individual.
^†122

We've already seen the hypothetical results of such mutations (horses and vampire bats), but let's look at the second of these examples a bit closer. Let's say that a protovampire bat had a genetic mutation that resulted in a change in tooth structure. If this mutation happened to produce sharper teeth (giving the protovampire a better chance of biting animals without being detected and thus increasing its chances of surviving and reproducing), then this novel characteristic would be considered an “adaptation.” Subsequent generations of protovampires, which would include the progeny of the bat with the mutation, would exhibit greater incidences of sharper teeth since those protovampires without this trait would be less likely to survive and reproduce in their local environments. In time, protovampire populations might accumulate more and more new characteristics (like salivary anticoagulants and amped-up excretory systems), adaptations similarly “selected” by the existing environmental conditions. Eventually, these bats would become different enough from their ancestors to be considered a new species—in this case, true vampire bats. Alternatively (and this is the part that many people overlook), the mutation that altered tooth sharpness could have just as easily been one that produced duller teeth, and this “maladaptive” character would have lessened the chances of that individual surviving to reproductive age, where it could pass that mutation (and the trait) on to the next generation.

Considering this last possibility makes it easier to understand how evolution is, in some ways, a genetic crapshoot. It also allows us to recognize the problem with such assumptions as “The environment changed and ancient vampire bats
needed
sharper teeth to make painless bites—so they evolved sharper teeth.” This reasoning, which seems to make sense, betrays a common misconception that people have about the mechanism of evolution.

For Jean-Baptiste Lamarck (1744–1829), the first naturalist to propose a mechanism for evolutionary change, the concept of need-based evolution (and other related ideas) got the Frenchman buried in a rented grave and his life's work all but forgotten. In reality, Lamarck was a scientific heavyweight, and his résumé included prescient insights into botany, taxonomy, and organic evolution. He may have been, in fact, the first scientist to propose that species actually changed gradually over time and that they did so because of natural processes (as opposed to supernatural ones). In his spare time, Lamarck was the first naturalist to separate crustaceans, arachnids, and annelids from insects (although housecleaners had been coping with that very problem for many years), and he also coined the term
invertebrate.
All in all, a fairly hefty set of accomplishments—most of which go completely unmentioned, unappreciated, and most important (for high school students, at least), unmemorized. Instead, Lamarck has been hammered in nearly every introductory biology text ever written. Indeed Lamarck's folly, referred to as “the inheritance of acquired characteristics” has hung around the poor man's neck like an albatross (or more accurately, a giraffe).

In Lamarck's giraffe story, used to explain how evolution might proceed, there was once a population of short-necked animals (let's call them protogiraffes) feeding happily on low-lying leaves. For whatever the reason, the environment changed, these plants died out, and the short-necked animals were left with a dwindling food supply.
^*123
According to Lamarck, the protogiraffes that had previously fed on the vertically challenged (and now extinct) foliage,
needed
longer necks in order to feed from the higher branches of trees that hadn't been wiped out. This need somehow produced elongated necks, resulting in the evolution of the modern giraffe.

Although it took a bit of time to discredit Lamarck (Charles Darwin actually fell back on Lamarckian concepts in his later editions of
Origin of Species
), eventually folks came up with questions like “If Lamarck was right, then why are boys with circumcised fathers born with a foreskin?”
^†124

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.