260 CHAPTER 10Sexual selection in flowering plants
It may sound odd to suggest that sexual selection occurs in plants: males have no
weapons, females no nervous system. But individuals of the same sex compete for
reproductive access to the other sex, just as in animals.
The vast majority of flowering plants are hermaphrodites. As discussed earlier,
the operational sex ratio is a key ingredient that determines which sex experiences
sexual selection. The operational sex ratio is often male-biased in plants because
even after all of an individual’s ovules have been fertilized, it can continue to send
out pollen. That is, more individuals are available to act as males (pollen donors)
than as females (pollen acceptors). This situation sets the stage for male-male com-
petition. Individuals with more attractive floral displays attract more pollinators.
This gives them increased fitness because they export more pollen than do indi-
viduals with less attractive displays [8]. You can admire the evolutionary outcome
the next time you see a field of wildflowers or smell a rose.
A second opportunity for intense male-male competition in plants happens
once a pollen grain arrives on a flower [49]. It is now in a race against the other
pollen grains on the same stigma to fertilize an ovule. This situation parallels
sperm competition in animals: the evolutionary prize is reproductive success at the
expense of other individuals of the same sex.Sex Ratios
In most species with separate sexes, the sex ratio—the relative numbers of males
and females—is about equal at birth. In animals such as humans and in plants
such as papayas, the sex of an individual is determined by its chromosomes. A
human embryo that inherits an X chromosome from the father is female, while an
embryo that inherits a Y chromosome is male. Meiosis in males typically trans-
mits the X and the Y chromosome with equal probability, which suggests that the
sex ratio might be fixed at 50 percent male. But in fact there are species with sex
chromosomes that have unequal sex ratios, and even species that can adjust the
sex ratio in their offspring. Females of the Seychelles warbler (Acrocephalus sechel-
lensis) produce up to 87 percent daughters when conditions on their territories
are good, but only 23 percent daughters when conditions are bad [28]. This shift
in the sex ratio is thought to be adaptive because daughters sometimes help their
parents raise more offspring when the territory has enough food to support them
all. Although the mechanism behind the shift is not known, this warbler does
show that sex chromosomes do not always lock in the sex ratio to equal numbers
of females and males.
Earlier in the chapter, you saw that in some species sex is determined not by
chromosomes but by the physical or social environment [4]. This is called envi-
ronmental sex determination. In turtles and lizards in which sex is determined by
the temperature during egg development, the sex ratio can evolve simply through
changes in where females lay their eggs.
The Hymenoptera (ants, bees, wasps, and their relatives) use yet another way to
determine sex. When a female lays an egg, she can fertilize it with sperm that she
has stored from an earlier mating. All eggs that are fertilized develop into females.
But the female can also lay an unfertilized egg, which will then develop into a
male. Males are haploid, and so this system is called haplodiploid sex determina-
tion. Females can therefore adjust the sex ratio of their offspring behaviorally, by
altering the number of eggs they fertilize. While this system for sex determination
may seem bizarre, it is used by 12 percent of animal species (see Figure 10.3). You
will see shortly that this enables certain wasps to do something remarkable.10_EVOL4E_CH10.indd 260 3/22/17 2:25 PM