Natural Selection

This is an essay I wrote for a science course which describes a plausible natural selection scenario for how a parasitic lancet fluke evolves to be capable of a neurochemical 'mind control' of an ant host. I am not a professional scientist, so I claim no expertise on these subjects. The post to which I am responding was made by a fellow student, and was a response to an earlier post I made in a discussion of environment and responsibility. Here, I am countering a statement that my references to the lancet fluke (more info here: http://www.ted.com/talks/dan_dennett_on_dangerous_memes.html ) and the fish-tongue-eating-crustacean known as Cymothoa exigua (more info: http://www.youtube.com/watch?v=vAlEfrwr8aU ), which he refers to as "two horrifying examples", demonstrate that "nature [is] out of balance, or is a battle of Good-vs.-Evil? Perhaps even an unsettled balance of natural selection, due to our negligence of humans disrupting nature's preservation?." My rebuttal follows, edited for typos, and with off-topic text removed.

As to the 'two horrifying examples', I would argue that neither example is one of an out-of-balance nature nor a battle of good-vs-evil. Neither appears to be affected at all by human activity. The lancet fluke, for instance, is not alone in its ability to take over another creature in some useful way, there are literally countless examples like this one. Nature simply favors any ability which gives a member of the species an advantage over others of its species, and by extension, sees advantages in species which are generally better adapted than other species competing for the same resources -- which should not be confused with group selection, a concept which is controversial and outside the mainstream in biology, though there are absolutely well-respected biologists holding positions in favor of group selection, but I digress.

Let me create a hypothetical progression of lancet fluke behavior. This is purely a mind exercise with few details and backed only by my readings in biology and behavior, but I think it serves us well and presents a plausible pathway, even if incorrect and flawed in real details. Also note that when I use singular terms, such as 'fluke', I'm referring to a statistically signficant set of flukes acting in some way or possessing some characteristic, and am not suggesting that a single specific fluke was responsible for each stage.

The lancet fluke, like all parasites, requires a host, and we know that the first cycle in the fluke's existence takes place in the body of a snail ("Dicrocoelium dendriticum"). Originally, it is easy to imagine a life cycle that saw the fluke exist completely within the snail, and there are countless parasites for which this model is quite typical.

However, natural selection will favor any genetic change which gives an advantage over other members of the species, even if that edge is very small. Imagine, then that the snail spits up a slime ball which contains living lancet flukes. (Snails do in fact do this and theory holds that this is a likely vector taking the lancet fluke into the ant's body ("Dicrocoelium dendriticum")). The ant, which either physically contacts or otherwise ingests the slime ball, becomes a secondary host for any members of the fluke species which can survive and reproduce during this second stage. Initially, this was probably only a bare few, but enough flukes possessed the ability to live in the snail, survive in the slime ball, and then live in the ant's body that this ability was strongly favored by natural selection to the exclusion of the flukes which could not do so. ***

So, now we have a fluke which has evolved a survival trick which extends its life and offers it an advantage which allows it to reproduce and pass down genes which also tend to favor this trick. Again, this isn't anything inherently good or bad, and certainly didn't require human intervention, this is really the way natural selection works. It is cold, without emotion, and without forethought. It is simply a process which rewards (without intent) any advantage to survival possessed in the genes of a replicating entity. The fluke which survived the slime ball stage and took advantage of the ant-body stage was more likely to reproduce than one which died in the ant's body, and given that this survival had a large reliance on genetics, was also more likely to produce offspring which could likewise take advantage of the ant-body stage.

From there, you can imagine that ants are regularly eaten by ruminants such as cows and sheep. Ants crawl all over grass and other delicacies enjoyed by grazers, so it is inevitable that large numbers are swallowed by such beasts. Natural selection again gives pressure: if any fluke was capable of surviving in the gut of a cow, that fluke was again more likely to reproduce offspring capable of doing the same.

So at this point, we have a fluke which has gone from living solely in the belly of snail to a naturally-selected subset that was capable of surviving the snail coughing it up in a slime ball, being swallowed by an ant, and then being swallowed by a cow. We haven't yet gotten to the 'mind control' ability, but that comes next.

Given that this fluke has now a genetic advantage, however small, over other flukes and over other competing species, it may appear well off. But natural selection does not offer species many opportunities to rest. Likely, the snail, ant, and cow bodies have experienced pressure to be able to survive this invasion. Any fluke infestation which killed its host too soon probably didn't live long enough to reproduce (and likewise, neither did the host). This fatality-condition is shared by both fluke and host genes. Any combination which resulted in death for the pre-reproductive fluke or host whittled down the combination of fluke and host genes which were selected for, and only those flukes with genes which allowed the host to survive and reproduce while also allowing itself to suvive and reproduce enjoyed significant advantage over those which did not, in terms of leaving offspring.

Imagine, then, that the ant develops a set of defenses against the fluke. Perhaps natural selection favors ants which are less likely to position themselves somewhere they might be grazed upon. Perhaps behaviors which result in staying low to the ground or holding along the edge of fields (rather than in the middle of them) would find these ants less likely to be eaten. Naturally, this is bad news for the fluke which, by now, rather expects to get on with its third stage of life in the belly of the cow.

So here comes one of the amazing parts of natural selection: its ability to derive complex and apparently abstract behavior out of a few basic rules. Differential survival means that any advantage, no matter how small, is enough for the process of natural selection to reward these small advantages with the ability to leave more offspring. Imagine that a line of flukes possess a mutation which allows it to secrete a chemical trigger to the ant which prevents the (newly developed, in evolutionary time) behavior which has protected the ant from being eaten as often by cows. We don't need to posit that there was intent here, perhaps various mutations in the fluke had occurred which generated different chemical excretions. We only care hear about the one that actually affected the behavior of the ant in a way advantageous to the fluke.

Suddenly, the fluke has pressured the ant to change its behavior, and the ant is now more likely to return to its original behavior, or demonstrate a new behavior, which left it more likely to be eaten by a cow. This sort of chemical triggering is well-documented and I won't go into details here, except to mention that bacterial and viral infections use all manor of chemical triggers to affect a myriad of things. Again, there doesn't need to believe this is done with intent. For every thousand mutations which change the way chemicals are produced by a parasite, perhaps only one will ever have anything other than a neutral (or negative) effect on its host. This variety, between parasites which possess this one-in-a-thousand mutation and those which do not, is all natural selection needs to differentiate between them. If the chemical allowed its possessors to survive and reproduce more frequently, it is straightforward to see that this would be an advantage that future generations were more likely to possess. The fluke never needed to plan this devious trick, it was just along for the ride built by its own (marginally, but significantly) superior genes and subjected to pressure from natural selection.

The last step here is just an extension of the previous. Once a chemical trigger is in place which advantaged flukes being able to produce it, it is just a short distance to chemical triggers which directed the ant to repeatedly climb the blade of grass. The process is the same as above, so I won't repeat it here.

And so, we've looked at an entirely plausible explanation for how natural selection would deliver a lancet fluke capable of 'mind control' over an ant, living originally only in the belly of a snail, but evolving a complicated adaptation which saw it survive several stages of life in hosts from snail to ant to cow. I'll not go into the tongue-eating-crustacean situation since it too would have gone through a process mirroring this one.

Again, there is no need to posit intent or 'good vs bad', this is just the way natural selection works. Harsh, cold, or heartless, perhaps, but those really are human concepts which we anthropically apply to the world around us.

I'll not get into the 'reason for human existence' since that is a teleological subject probably best left alone here (and to avoid any heated emotions from all of us who have our beliefs or lack-there-of). I am, however, completely in agreement with you about reducing the toxins we dump, and I hope that rational and reasonable people, maybe even ourselves, can study and implement strategies which do just that.

References:

Wikipedia. (n.d.) Wikipedia - Dicrocoelium dendriticum. Retrieved on October 2, 2009 from http://en.wikipedia.org/wiki/Dicrocoelium_dendriticum

*** as a digression, natural selection doesn't necessarily produce an all or nothing differential survival, as Darwin himself explained in On the Origin of Species. The hypothetical situation described above might find that one species of lancet fluke, which evolved from the original species found only in snails, evolved adaptations which required life cycles in both snail and ant, while a second species evolved, also from the original species, which did not. This process works again at the point in which the fluke moved into a third stage in a ruminant body. Differential selection is a manner in which species are not only selected for, but the fuels the divergence of species from their common ancestry.