Posted: Jul 27, 2010 12:07 pm
And of course, there's the Double Tail mutation in Betta splendens, which first appeared amongst captive bred specimens back in the 1970s.
In the case of Betta splendens, the Siamese Fighting Fish, there is a trade-off in the wild between the effects of predation and the effects of sexual selection by females. Males have longer dorsal and anal fins than females, and these play a role in the courtship and mating of these fishes. One role that those longer fins play is during the mating embrace itself, where the long and extended anal fin of the male is used to capture the female's eggs as they are released, which are then quickly scooped into the mouth of the male. The male then blows the eggs into the bubble nest that these fishes construct for the purpose of rearing their young. Any decent tropical fish magazine will cover the breeding of Betta splendens in some detail, and indeed, Betta splendens is frequently the first bubble nest breeding labyrinth fish tackled by novice aquarists extending their remit. But I digress.
Longer fins allow the male to put on a more impressive courtship display, and consequently, females will preferentially select mates with large, impressive fins. However, once the fins grow beyond a certain limit, they start to act as an impediment to escape from predation, and therefore there is a dynamic balance struck in nature, between the selection pressure exerted by predators to grow fins within a constrained limit, so that males may escape predation, and the selection pressure exerted by females, who mate preferentially with males possessing large and showy fins.
However, in an aquarium environment, the predation pressure is absent, and thus, the sole selection pressure acting upon the males is sexual selection by females. In such an environment, male Betta splendens will develop larger and larger dorsal and anal fins, and indeed, this is what we have seen happen during generations of aquarium breeding. For comparison, here is a male Betta splendens that conforms to the wild type with respect to fin development (these are known in the aquarium hobby as plakat Bettas):
Compare this with a typical aquarium long-finned Betta:
As can be seen, the effects of sexual selection are pretty substantial once predation pressure is removed! Which once more demonstrates that whether a mutation is deleterious or advantageous frequently depends upon the environment and the niche being occupied - a mutation that would be deleterious in one environment can be advantageous in another, and vice versa. For example, mutations for pigment loss in fur, resulting in an animal being white in appearance, would be deleterious in a temperate rainforest, where such an animal would stand out from the environment like a sore thumb - small animals of this sort would be easy meat for predators, and large predators would be easily evaded by prey. The same mutation taking place in an animal living on a polar ice cap, on the other hand, confers an immediate advantage over those retaining brown fur. In the case of the fishes above, predation acts as a brake upon overexpression of long fins driven by sexual selection, and thus wild male Betta splendens are constrained to a certain maximum fin length, but in an environment where the predation pressures acting as a brake are removed, sexual selection drives fin length development to whatever extremes are physically and metabolically supportable by the males.
Indeed, when experiments are performed, and female Bettas are given a choice of mates of different fin sizes, the females exhibit a significant preference for the males with the large, showy, impressive fins, and the males with shorter, less impressive fins are left on the shelf as it were. It's not difficult to construct the requisite experiment: all one needs to do is construct an aquarium with appropriate dividers, ensuring of course that the male cells are fitted with opaque side walls so that the males cannot see each other (they're not known as Siamese Fighting Fish for nothing!) and have nice transparent windows allowing the females in the undivided part of the aquarium to swim freely and examine all the males in their display cells. Then, pop a range of males in the cells, add the females to the open section, and let the females engage in a little cheesecake window shopping as it were ... it won't be long before you see the females starting to pay attention to the big, showy males with long flowing fins, and ignoring the males with the short fins.
Consequently, we have above an experimental demonstration of the power of sexual selection - picky females shaping male phenotype by their mating choices. If funds could be allocated to the requisite experiments, I suspect it would not be difficult to demonstrate that if wild Betta splendens are placed in a pond where they are free from the action of predators, and excess stock is removed on a purely random basis, eventually, the male fin length will be seen to increase over successive generations.
Now we come to the Double Tail mutation. When a fish inherits this mutation, it develops two tails, arranged in 'over-under' shotgun arrangement. The fish doesn't just have a large, split tail fin: when dissected, these fishes have two sets of caudal peduncle bones, and thus genuinely have two tails. Moreover, the mutation responsible has been demonstrated to be a single-factor Mendelian recessive gene by appropriate mating experiments. Let's compare our single tailed aquarium Betta above with a Double Tail specimen, shall we?
Now the Double Tail mutation also results in an extra wide dorsal fin, and in fine specimens, the fins are truly spectacular to behold. Now, we can perform a second experiment: take a selection of single-tail males with nice, long fins, put them in our "cheesecake shopping" aquarium setup, and let them compete with a Double Tail male for female attention. When the females go window shopping, you'll find that they're most impressed by that big showy Double Tail male.
Now once again, amongst wild fishes, the Double Tail mutation would be deleterious, as it would impact upon male escape from predation, but in an aquarium environment, females select Double Tail males preferentially if given a choice between ordinary single-tailed males and Double Tail males. Likewise, if a suitable pond environment could be set up, and some Double Tail males let loose in there, I suspect that even though the Double Tail mutation is recessive, it would, over time, spread through the population once the females got to work exerting their selection pressure on the males.
Once again, no magic needed in order to understand all of this.
In the case of Betta splendens, the Siamese Fighting Fish, there is a trade-off in the wild between the effects of predation and the effects of sexual selection by females. Males have longer dorsal and anal fins than females, and these play a role in the courtship and mating of these fishes. One role that those longer fins play is during the mating embrace itself, where the long and extended anal fin of the male is used to capture the female's eggs as they are released, which are then quickly scooped into the mouth of the male. The male then blows the eggs into the bubble nest that these fishes construct for the purpose of rearing their young. Any decent tropical fish magazine will cover the breeding of Betta splendens in some detail, and indeed, Betta splendens is frequently the first bubble nest breeding labyrinth fish tackled by novice aquarists extending their remit. But I digress.
Longer fins allow the male to put on a more impressive courtship display, and consequently, females will preferentially select mates with large, impressive fins. However, once the fins grow beyond a certain limit, they start to act as an impediment to escape from predation, and therefore there is a dynamic balance struck in nature, between the selection pressure exerted by predators to grow fins within a constrained limit, so that males may escape predation, and the selection pressure exerted by females, who mate preferentially with males possessing large and showy fins.
However, in an aquarium environment, the predation pressure is absent, and thus, the sole selection pressure acting upon the males is sexual selection by females. In such an environment, male Betta splendens will develop larger and larger dorsal and anal fins, and indeed, this is what we have seen happen during generations of aquarium breeding. For comparison, here is a male Betta splendens that conforms to the wild type with respect to fin development (these are known in the aquarium hobby as plakat Bettas):
Compare this with a typical aquarium long-finned Betta:
As can be seen, the effects of sexual selection are pretty substantial once predation pressure is removed! Which once more demonstrates that whether a mutation is deleterious or advantageous frequently depends upon the environment and the niche being occupied - a mutation that would be deleterious in one environment can be advantageous in another, and vice versa. For example, mutations for pigment loss in fur, resulting in an animal being white in appearance, would be deleterious in a temperate rainforest, where such an animal would stand out from the environment like a sore thumb - small animals of this sort would be easy meat for predators, and large predators would be easily evaded by prey. The same mutation taking place in an animal living on a polar ice cap, on the other hand, confers an immediate advantage over those retaining brown fur. In the case of the fishes above, predation acts as a brake upon overexpression of long fins driven by sexual selection, and thus wild male Betta splendens are constrained to a certain maximum fin length, but in an environment where the predation pressures acting as a brake are removed, sexual selection drives fin length development to whatever extremes are physically and metabolically supportable by the males.
Indeed, when experiments are performed, and female Bettas are given a choice of mates of different fin sizes, the females exhibit a significant preference for the males with the large, showy, impressive fins, and the males with shorter, less impressive fins are left on the shelf as it were. It's not difficult to construct the requisite experiment: all one needs to do is construct an aquarium with appropriate dividers, ensuring of course that the male cells are fitted with opaque side walls so that the males cannot see each other (they're not known as Siamese Fighting Fish for nothing!) and have nice transparent windows allowing the females in the undivided part of the aquarium to swim freely and examine all the males in their display cells. Then, pop a range of males in the cells, add the females to the open section, and let the females engage in a little cheesecake window shopping as it were ... it won't be long before you see the females starting to pay attention to the big, showy males with long flowing fins, and ignoring the males with the short fins.
Consequently, we have above an experimental demonstration of the power of sexual selection - picky females shaping male phenotype by their mating choices. If funds could be allocated to the requisite experiments, I suspect it would not be difficult to demonstrate that if wild Betta splendens are placed in a pond where they are free from the action of predators, and excess stock is removed on a purely random basis, eventually, the male fin length will be seen to increase over successive generations.
Now we come to the Double Tail mutation. When a fish inherits this mutation, it develops two tails, arranged in 'over-under' shotgun arrangement. The fish doesn't just have a large, split tail fin: when dissected, these fishes have two sets of caudal peduncle bones, and thus genuinely have two tails. Moreover, the mutation responsible has been demonstrated to be a single-factor Mendelian recessive gene by appropriate mating experiments. Let's compare our single tailed aquarium Betta above with a Double Tail specimen, shall we?
Now the Double Tail mutation also results in an extra wide dorsal fin, and in fine specimens, the fins are truly spectacular to behold. Now, we can perform a second experiment: take a selection of single-tail males with nice, long fins, put them in our "cheesecake shopping" aquarium setup, and let them compete with a Double Tail male for female attention. When the females go window shopping, you'll find that they're most impressed by that big showy Double Tail male.
Now once again, amongst wild fishes, the Double Tail mutation would be deleterious, as it would impact upon male escape from predation, but in an aquarium environment, females select Double Tail males preferentially if given a choice between ordinary single-tailed males and Double Tail males. Likewise, if a suitable pond environment could be set up, and some Double Tail males let loose in there, I suspect that even though the Double Tail mutation is recessive, it would, over time, spread through the population once the females got to work exerting their selection pressure on the males.
Once again, no magic needed in order to understand all of this.