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