BIO220H1 Lecture Notes - Lecture 15: Gamete, Limiting Factor, Mate Choice
Lecture 15 reading
sexual selection
Peacock’s tail
decreases male survivorship but only exists because it confers an advantage to its bearer
in the form of increased reproductive success
History of sexual selection
•Darwin primarily knew only about NS: organisms better adapted to their environment
would benefit from higher rates of survival than those less well equipped to do so.
•then he saw organisms that develop elaborate and apparently non-adaptive, sexual
traits that would clearly not aid in the survival of their bearers. He suggested that such
traits might evolve if they are sexually selected.
•Sexual selection: depends on the struggle between males to access females. two
mechanisms of sexual selection:
1) intrasexual selection: competition between members of the same sex (usually
males) for access to mates,
2) intersexual selection, where members of one sex (usually females) choose members
of the opposite sex.
Which sex is under stronger selection?
•Sex roles are defined by differences in gametes. asymmetry in gamete production
leads to Bateman’s principle: female reproduction is primarily limited by their access
to resources to nourish and produce these large gametes, whereas male reproduction
is mainly limited by access to females. So females are usually the choosy ones.
•The higher the reproductive variance, the stronger the effects of sexual
selection (the average reproductive success is equal for both males and
females, but variance is different).
•Strong sexual selection typically results in sexually dimorphic traits that
are exaggerated, or more elaborate, in the sex with highest reproductive
variance
find more resources at oneclass.com
find more resources at oneclass.com
The role of parental care
Most species provide little or no care to their offspring, but in species where parental
care is required, variance in reproductive success will be impacted not only by
fertilization success, but also by the contribution of each sex to the care of the offspring
•If females provide more parental care than males, the variance in male reproductive
success can be expected to be large, since females providing offspring care will not be
immediately available for further reproduction and competition for available females
will increase among males.
• in species where biparental care is required to successfully raise o spring, variance in
male reproductive success is generally lower, since males that are engaged in
providing parental care will not be able to invest as much energy in pursuing
additional mating opportunities. This situation often results in the emergence of
sexually monomorphic species, in which the male and female look and behave in
similar ways
•in rare situations where only males provide parental care, males can become the
limiting resource for females. So, the variance in reproductive success may be high for
females who then tend to monopolize access to one or more males to care for their
offspring. Males may then become choosy about which females they mate with. This
often results in reversed sexual dimorphism, such that females evolve more elaborate
secondary sexual characters than males.
How does sexual selection operate
2 mechanisms:
1) intrasexual selection: members of the same sex attempt to outcompete, often
during direct encounters. Intrasexual selection is typically responsible for the
evolution of male armaments such as deer antlers, beetle horns, and large body size,
that provide individuals with an advantage when fighting off potential competitors.
Individuals who are better able to exclude competitors, have a greater chance to
acquire mates and father offspring.
2) intersexual selection: interactions between the sexes, typically involving mate
choice. The evolution of elaborate behavioral displays and morphological traits can
often be explained as the result of intersexual selection. Usually, females tend to be
more choosy to increase their fitness. Males tend to compete with one another to
gain the female's attention.
find more resources at oneclass.com
find more resources at oneclass.com
38
BIO220H1 Full Course Notes
Verified Note
38 documents
Document Summary
Peacock"s tail decreases male survivorship but only exists because it confers an advantage to its bearer in the form of increased reproductive success. He suggested that such traits might evolve if they are sexually selected: sexual selection: depends on the struggle between males to access females. two mechanisms of sexual selection: 2) intrasexual selection: competition between members of the same sex (usually males) for access to mates, intersexual selection, where members of one sex (usually females) choose members of the opposite sex. So, the variance in reproductive success may be high for females who then tend to monopolize access to one or more males to care for their offspring. Males may then become choosy about which females they mate with. This often results in reversed sexual dimorphism, such that females evolve more elaborate secondary sexual characters than males. 2) intrasexual selection: members of the same sex attempt to outcompete, often during direct encounters.
