Lankester’s Migrational Selection of Blindness

| 26 Comments

Following up PZ’s previous post about blind fish, I want to quote a section of R. A. Fisher’s “The Genetical Theory of Natural Selection”. Fisher is credited as being one of the founders of modern evolutionary biology, which drove him to create modern statistics.

This quote begins on page 128 of the 1999 complete variorum edition. I think it was originally written in 1930.

The power of the means of dispersal alone, without the necessity for selective discrimination in either region, is excellently illustrated by the theory, due to Ray Lankester, which satisfactorily accounts for the diminution or loss of functional eyes by the inhabitants of dark caverns. Ray Lankester pointed out that the possession of the visual apparatus is not merely useless to such animals but, by favoring their migration towards sources of light, will constantly eliminate them from the body of cave inhabitants, equally effectively whether they survive or perish in their new environment. Those which remain therefore to breed in the cavern are liable to selection in each generation for their insensibility to visual stimuli. It should be noted that with such very restricted habitats migrational selection of this sort might attain to very high intensity in consequence produce correspondingly rapid evolutionary effects.

Very interesting perspective isn’t it?

26 Comments

Hunh. That’s a new concept for me- I hadn’t considered the idea of a genuine selection pressure that doesn’t involve individual fitness at all (but simply involves the individuals with said trait wandering out into some other population.)

Has there been significant work on determining whether the conjectured mechanism is actually at play?

Don’t know. It’s definitely not a commonly discussed explanation.

Edwards modeled it in 1963. And he cites (ha a pun!) this reference:

Lankester, R. 1925. The blindness of cave animals. Nature, 116, 745

This is one I’ve often thought about, but more in systems such as the Galapagos finches. Finches that happened to have beaks good at hoking out insects from bark (“hoking” is a Northern Irish term which seems appropriate in this context) tended to meet more like-beaked finches in the places where they tended to hang out, i.e. in the branches of the trees. So they made hoky-beaked finches together, and ignored their nutty-beaked brethren, who cleared off to the bottom of the tree to wait for the nuts to fall. And while they waited, they mated.

So, you get this situation where migration to the habitat that suits you creates a new effective genepool, without any of that nasty “red in tooth and claw” business that we find icky. Of course, while you’re doing this, genetic differences accumulate between the populations, and pretty soon you have speciation going on. But the point here is that your phenotype directs where you go, and hence the local gene distribution, rather than simply finding yourself somewhere where you are outcompeted by birdies with more suitable beaks, and resigning yourself to an offspring-free demise.

The net result (at least in organisms with enough wit to find somewhere comfy) is a rather rapid population stratification, and resultant rapid evolution & speciation. I’m not sure whether this has been adequately modelled - anyone got any refs?

Not buying this hypothesis. I think (in my amature opinion) the previous article that PZ posted is a much better explaination for the loss of sight. It has the added bonus of lab work that shows the loss of sight in that fish is directly connected to the expression of a gene that also increases sensitivity in the lower jaw. The increased jaw sensitivity is a boon in a lightless environment.

As Shane opined above, if a species has populated an environment to the point where the available resources are used up, those members that can exploit a new food, even if poorly, will adapt over time and become more efficient at it.

Not only would a creature with poor sight and better skin sensitiviy have a harder time competing in a well lit habitat, it will initially have little or no competition amongst its peers in the dark area of the cave.

The blind cave dwellers are not evolving the loss of a sense, they are evolving a new (or poorly expressed) useful one at the expense of an existing useless one.

MememicBottleneck said:

Not buying this hypothesis. I think (in my amature opinion) the previous article that PZ posted is a much better explaination for the loss of sight.

well I reckon we should teach both and let the students decide.

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If a population of fish were free to wander in and out of a cave, the ones who most comfortable in the dark could exploit this new food source. If an accident trapped fish in the cave, this would be less of a factor. We need more data.

Frank B said:

If a population of fish were free to wander in and out of a cave, the ones who most comfortable in the dark could exploit this new food source. If an accident trapped fish in the cave, this would be less of a factor. We need more data.

I humbly disagree. If free to wander in and out, the ones most capable of exploiting the new food source would spend more time in the cave, particularly if they are disadvantaged in the light.

If an accident trapped the fish in the cave, still, the ones most “fit” amongst the trapped, are most likely to reproduce. It is a simpler explaination that the propagaters are better adept at catching a meal than saying “don’t go into the light”. :P

I’ll point out that my post isn’t intended to challenge the pleiotropic adaptation explanation given in PZ’s example. I’m just illuminating a under studied explanation for the general loss of sight in cave populations.

MememicBottleneck said: If free to wander in and out, the ones most capable of exploiting the new food source would spend more time in the cave, particularly if they are disadvantaged in the light.

In the country of the blind the one-eyed man is not necessarily king?

To elaborate on Mememic’s point, a fish with poor eyesight would be at a disadvantage in a well lit pond, but would be on an equal fin in the dark. It wouldn’t necessarily have an advantage over others in the dark, but just compete better. But what would it take for even a poor sighted fish not to migrate toward the light, as Ray Lankester was referring to.

So, if the same mutation caused increased tactile sense and reduced visual sense, was the loss of visual relevant to selection, or just along for the ride so to speak?

Ray Lankester pointed out that the possession of the visual apparatus is not merely useless to such animals but, by favoring their migration towards sources of light, will constantly eliminate them from the body of cave inhabitants,

So the more visionary of the inhabitants would up and leave their more touchy feely relatives in the dark? Shame on them.

Henry

The mention of Ray Lankester brings to mind this interesting article from Stephen Jay Gould: http://findarticles.com/p/articles/[…]5698600/pg_1_ in which he points out that Lankester provides a link between the man of evolution (Darwin) and the man of revolution (Marx) and that Lankester was one of the few to attend Marx’s funeral.

Frank B said:

To elaborate on Mememic’s point, a fish with poor eyesight would be at a disadvantage in a well lit pond, but would be on an equal fin in the dark. It wouldn’t necessarily have an advantage over others in the dark, but just compete better. But what would it take for even a poor sighted fish not to migrate toward the light, as Ray Lankester was referring to.

What I was trying to say is that the loss of eyesight is a side effect of increased jaw sensitivity. Therefore, the poor sighted fish is at an advantage in the cave over his well sighted cousins.

Lankester probably had no way of knowing or measuring what other abilities were being developed/improved as a result of loss of sight. He was trying to rationalize the loss of sight as a benefit (thats what I got out of it anyway).

In total darkness, the sighted and blind fish would be on equal footing. The posting implies that there is some source of light present. With any light at all, the sighted fish will still have an advantage (if no other senses evolved). A hungry fish will go toward the food instead of the light.

Does anybody know of any modern data to support Lankester’s hypothesis? I’m sure it was a good guess for the era in which he proposed it, but is probably a bit dated now.

Lankester’s suggestion assumes that fish will migrate towards any light source, which is by no means a given.

Not only is negative phototaxis quite common in nature, but the behaviour itself is equally available for selection to act upon. Fish that tended to move away from available light sources would, in Lankester’s model, equally tend to accumulate in the body of the cave in preference to those who don’t.

That’s not to say that Lankester’s idea is invalid, but IMO it is not enough on its own to explain loss of eye function.

Like all adaptations, eyes have a cost along with a benefit. Eyes are fairly notorious as a source of both injury and infection. In most vertebrates, the surface (i.e. cornea) of the eye is the only genuinely living tissue in constant contact with the external environment (feathers, fur, scales, skin, etc. all being non-living tissues, except for amphibians and some scale-less fish). This makes the surface of the eye a prime location for the entry of pathogens, such as viruses, bacteria, and fungi. Virtually all animals have secondary adaptations that minimize such injuries and infections (eyelids in land vertebrates, and anti-pathogenic enzymes such as lysozyme, etc.) However, these adaptations are costly, and that cost is added to the cost of those injuries and infections that occur despite such protection.

In other words, there is a cost-benefit relationship between having eyes and not having them. When there is sufficient light available for the eyes to function, this relationship is tipped toward having eyes (along with their attendant problems). When there isn’t sufficient light, this tips the relationship back toward eyelessness.

The same explanation has been proposed for the loss of flight in island populations of insects (especially beetles) and birds. Darwin himself pointed out that flightlessness seems to be most common among insects and birds that are isolated on small islands. As in the case for eyes, one possibility is that wings (again, like all adaptations) are both beneficial and costly. If there are predators that can capture insects or birds on the ground (or other surfaces, such as tree limbs), wings can provide a benefit by allowing those who possess them to escape from such predators. However, if there are no such predators, then the costs of having wings (in both the pure physical cost of making and maintaining such a large and elaborate structure), plus the possibility of injury (broken wings are a constant problem among winged animals) outweigh the now-lost benefit of flight, and selection eliminates the large, elaborate wings.

Add to this the liability of having wings that can, on occasion, cause one to be blown far enough away from the island that one cannot return, and the cost of having wings may even outweigh their advantages in the presence of terrestrial predators. This argument is similar to Lankester’s, in that it involves “dispersion” of a sort. In this case, having non-functional wings on isolated islands limits dispersion in the same way that eyelessness does in cave fish, and is beneficial for essentially the same reasons.

If the fish are free to wander in and out of the dark environment, there might be some advantage to maintaining some ability to detect light (ie: they can detect when they wander out of their favored environment). Are these fish completely blind, or are the eyes just severely atrophied? (Is it possible to know the difference?)

Nentuaby said:

Hunh. That’s a new concept for me- I hadn’t considered the idea of a genuine selection pressure that doesn’t involve individual fitness at all (but simply involves the individuals with said trait wandering out into some other population.)

Has there been significant work on determining whether the conjectured mechanism is actually at play?

The major concern is what really is fitness. There is a lizard that runs on two legs. Research has found that the lizard runs no faster upright than on all fours. I would suspect that it startles its predators, but the research concluded that there is no evolutionary advantage.

Conceptually it can be hard for laypeople, especially like me, to determine what features would give an animal an advantage. However, in an environment without light could make having fully functional eyes a disadvantage.

Reed A. Cartwright said:

I’ll point out that my post isn’t intended to challenge the pleiotropic adaptation explanation given in PZ’s example. I’m just illuminating a under studied explanation for the general loss of sight in cave populations.

Why must it be either/or? Couldn’t both mechanisms be in operation in some ratio between 100/0 and 0/100

The same explanation has been proposed for the loss of flight in island populations of insects (especially beetles) and birds. Darwin himself pointed out that flightlessness seems to be most common among insects and birds that are isolated on small islands. As in the case for eyes, one possibility is that wings (again, like all adaptations) are both beneficial and costly. If there are predators that can capture insects or birds on the ground (or other surfaces, such as tree limbs), wings can provide a benefit by allowing those who possess them to escape from such predators. However, if there are no such predators, then the costs of having wings (in both the pure physical cost of making and maintaining such a large and elaborate structure), plus the possibility of injury (broken wings are a constant problem among winged animals) outweigh the now-lost benefit of flight, and selection eliminates the large, elaborate wings.

Perhaps. But you don’t really need a liability of wings per se. Being large, complex structures, it is likely that many types of mutations would impair the ability to fly. For example, an increase in body weight would impair flight, absent compensatory changes in the wings. So it is likely that some mutations that were beneficial for other reasons impaired flight as a side effect, but were favored in the absence of strong selection for flight capability. In this scenario, loss of flight becomes more like Gould’s spandrels. Of course, even without this, the ability to fly would eventually be lost by drift in the absence of selection to preserve it.

There are two parts to the development of blind fish. First, there needs to be a separation of the two populations of fish, one staying in the light and another in the dark. Secondly, with reproductive isolation there will be evolutionary change.

The question in my mind is, what traits would a fish have that would lead it to select the dark? That is what some here have written about. I think there may be a variety of traits or environmental circumstances that would lead to selecting the dark. But maybe I am wrong. Maybe one trait is most important, like unresponsiveness to light.

Shubin in his ‘Inner Fish’ mentions that the higher ability to distinguish colours that we have comes at the expense of reduced olfactory capabilities (or something to that effect). Will the loss of sight in fishes (and other animals) in complete darkness enhance any other of their senses, like smell? Similar with the blind and near-blind moles.

Also, do animals choose darkness, or do they (when enough of them to breed) adapt because they are trapped there? Guess the availability of food, and a stable climate could be deciding factors.

The question in my mind is, what traits would a fish have that would lead it to select the dark?

Maybe lack of food in lighted areas, or maybe too many predators in lighted areas, or maybe too much competition for food, mates, or nesting sites in lighted areas?

Henry

Maybe lack of food in lighted areas, or maybe too many predators in lighted areas, or maybe too much competition for food, mates, or nesting sites in lighted areas?

Well, all organisms are subject to environmental forces. One point is that all organisms are different, some are more bold or adaptable than others, and those are traits that some have and some don’t.

Another point is that all species have access to other niches, a few may be easy to move into, most are impossible to move into. How easy can a sighted fish, adapted to the light, move into a totally dark niche? I bet that most fish can’t do it, but a few can, or a few did it gradually over a number of generations.

Henry J said:

The question in my mind is, what traits would a fish have that would lead it to select the dark?

Maybe lack of food in lighted areas, or maybe too many predators in lighted areas, or maybe too much competition for food, mates, or nesting sites in lighted areas?

Henry

Or, it could be that some may have simply swam too far upstream, and forgot to, or were unable to swim back downstream.

And once stuck there, they lost the use of their eyes, which then atrophied with each successive generation.

At the risk of sounding like an idiot (not really all that worried), can somebody tell me what the term is for astyanax and other cave dwelling creatures loss of sight through the evolutional process. It sounds kind of like apathy, but i cant seem to come by it through any other article, and the original place i heard it was on a doco i cant find either.….pleeeeasee its doing my head in. My email is [Enable javascript to see this email address.] thanks

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