Swordfish and female preference


In one of my earlier posts in this series, I presented two (non mutually exclusive) models of sexual selection. Those were the "good genes" model and the "runaway sexual selection" model. Well, there is actually a third model out there also (which does not exclude the others). I'm not aware of any name for it, I'll just call it the "existing female preference" model. According to this model, females have a built in preference for a certain type of male, even if that type of male does not exist.

The paper I summarize here is in the Nov 9, 1990 issue of Science. In the article, the author claims that, in swordfish, the female preference for males with swords existed before males had swords.

Within the genus Xiphophorus there are swordless platyfish and swordtails. The swordless state is considered to be ancestral. Basolo (the author) experimented with females of the species X. maculatus. Males of this species are swordless. He placed a female in the center of an aquarium that was sectioned into three areas. On one side, he placed a normal male. On the other he placed a male with an artificial sword attached. She noted that the female prefered (stayed on that side of tank and offered mating displays) the male with the artificial sword. The experiment was redone and males switched sides (to control for side bias). The result was the same, the female prefered the male with the sword to the swordless male.

The author further experimented to determine if it was the sword itself the female was cueing on. To do this she repeated the above experiment except in this case both males had artificial swords. One sword was colored, the other was opaque (clear plastic). In this case the female prefered the male with the colored sword. The control (for side preference) was also run. In addition, the author removed the swords and switched them between males and ran the tests (and controls) again. The results were once again, the same. The female prefered the male with the visible sword.

So, the data she collected were. [small aside, yes the word "data" is plural. "Datum" is the singular. Computer types simply misused the term often enough that it has become accepted in computer literature]

  1. Females (from this species that had never seen males with swords) prefered males with swords.
  2. The females were not cueing on some side effect of the sword. (The clear vs. colored sword showed this. One possible side effect the female could have cued on was a unique swimming motion induced by the presence of the sword)
  3. The female (in the colored vs. clear sword experiment) was not cueing on some other trait of the two males. (The switching swords experiment showed this. When the swords were switched, her preference switched.)

The author then concluded that the females in this genus have a pre-existing preference for males with swords. It is not surprising then that many species in the genus have swords, males have exploited this bias. What may be surprising to some is that some species don't have swords. This (IMHO) illustrates a pervasive misunderstanding that most people (and sadly many biologists) have about evolution. Evolution is not goal oriented. In this case there is no "selection pressure" for males to develop swords. They are not being pushed to develop swords. If, by chance, one males fins by chance happen to be longer than the other males in his population, he will enjoy greater reproductive success (because he is more "swordlike" than the others). This could continue until enough mutations have been selected for that males in this species have swords. But (and this is a very important but) there is no mechanism that is directing this to happen. In other words, there is no pressure on the males to develop swords. It's a fairly subtle point that is hard for many in our culture to accept. We live in a culture that likes to view things in terms of progress or heading towards a goal. Evolution is neither progress nor goal oriented.



Basolo, 1990, Female preference predates the evolution of the sword in swordtail fish, Science 250: 808 - 810