BIOL 180 Chapter Notes - Chapter 23.3, 50.6: Sexual Selection, Carotenoid, Natural Selection 2
Pgs. 466-469, 1064-1069
Behavioral Ecology; Sexual Selection
I. Sexual Selection
A. Introduction
1. Selection based on success in courtship
2. Intersexual selection-the selection of an individual of one sex for mating
by an individual of the other sex
3. Intrasexual selection-form of selection within a sex--competing with one
another to obtain mates
B. The Fundamental Asymmetry of Sex
1. Pattern
: Traits that attract members of the opposite sex are much more
highly elaborated in males
2. Process:
The energetic cost of creating a large egg is enormous whereas
sperm contain few energetic resources. That is, “eggs are expensive but
sperm are cheap”
3. Consequences of the Fundamental Asymmetry of Sex-characteristic of
many sexual species
a) Because eggs are large and energetically expensive, females
produce relatively few young over the course of a lifetime.
Females fitness is limited not by her ability to find a mate but
primarily by her ability to gain the resources needed to produce
more eggs and healthier young
b) Because sperm are so energetically inexpensive to produce, a
male can father an almost limitless amount of offspring. Thus, a
male's fitness is limited not by the ability to acquire the resources
needed to produce sperm but by the number of females he can
mate with
4. Bateman-Trivers Hypothesis of Sexual Selection
a) If females invest a great deal in each egg, then they should protect
that investment by being choosy about their mates. Conversely, if
males invest little in each sperm, then they should be willing to
mate with almost any female
b) If there are an equal number of males and females in the
population, and if males are trying to mate with any female
possible, then males will compete with each other for mates
c) If male fitness is limited by access to mates, then any allele that
increases a male's attractiveness to females or success in
male-male competition should increase rapidly in the population,
violating the HW. Thus, sexual selection should act more strongly
on males than on females.
C. Female Choice for Good Alleles
1. In several bird species females prefer to mate with males that are well fed
and in good health. Motivated by three key observations:
a) In many bird species the existence of colorful feathers or a colorful
beak is due to the presence of the red and yellow pigments called
carotenoids
b) Carotenoids protect tissues and stimulate the immune system to
fight disease more effectively
c) Animals usually cannot synthesize their own carotenoids, but
plants can. To obtain carotenoids, animals have to eat
carotenoid-rich plant tissues
2. These observations suggest
a) Healthiest and best nourished birds in a population have the most
colorful beaks and feathers
b) Conversely, dull coloration can occur because birds are sick and
using their carotenoids to stimulate their immune system or
because they're poorly fed and have few carotenoids available
c) By choosing a colorful male, more likely to have offspring with
alleles that will help offspring fight disease effectively
3. Testing the Hypothesis that females prefer colorful males
a) Experiment with zebra finches
b) Identified pairs of brothers and randomly assigned one brother to
treatment group and one brother to control group
c) Treatment group=diet heavily supplemented with carotenoids
d) Control group=diet similar except for additional carotenoids
e) As predicted, males eating carotenoid supplemented diet
developed more colorful beaks than males fed with carotenoid
poor diet
f) When given a choice of who to perch with, 9/10 females preferred
more colorful male
g) Results support the hypothesis that they are choosy and prefer to
mate with healthy, well fed males
4. General Conclusion: Colorful markings, along with songs and dances and
other types of courtship displays, carry the messages “I'm healthy and
well fed because I have good alleles, mate with me”
a) Honest signals of male genetic quality
b) Indicate not only good health but ability of males to provide
resources to female or provide care to offspring
5. Summary: Females may choose mates on basis of
a) Physical characteristics that signal male genetic quality
b) Behavioral characteristics of the males that indicate their ability to
provide parental care
c) Or Both
D. Male-Male Competition
1. Some species, females do not have luxury of choosing mates
2. Elephant Seals Example
a) Male elephant seals establish territories on breeding beaches by
fighting
b) A territory is an area that is actively defended and that provides
exclusive or semi-exclusive use by the owner
c) Usually won by larger male
d) Why male northern elephant seals larger on average than females
(1) Males that dominate beaches with large congregations of
females father large number of offspring. Males that lose
fights father few or no offspring
(2) The alleles of successful males rapidly increase in
frequency in the population
(3) If ability to win fights and produce offspring is determined
primarily by body size, than alleles for large body size have
a significant fitness advantage, leading to the evolution of
large male size. Fitness advantage is due to sexual
selection and the consequence is directional selection on
large body size
E. Sexual Dimorphism Results from Sexual Selection
1. Because sexual selection tends to be much more intense in males than
females, males tend to have many more traits that function only in
courtship or male-male competition
2. Stated another way, sexually selected traits often differ sharply between
the sexes
3. Sexual dimorphism (two forms) refers to any trait that differs between
males and females
a) Humans sexually dimorphic in size, muscle mass, distribution of
body hair, etc.
F. Take-Home Messages
1. Sexual selection is just one type of natural selection
2. All the other types, referred to collectively as ecological (or environmental)
selection, favor traits that enable organisms to do things other than obtain
mates--such as survive in physical and biological environments
3. Female choice and male-male competition illustrate how selection can
favor certain phenotypes in a population
a) The adaptive alleles responsible for these phenotypes increase in
frequency over time
b) Thus, evolution occurs and the assumptions of the HW principle
are violated
4. Different agents of natural selection can act on organisms simultaneously,
sometimes favoring the same traits, other times resulting in fitness
trade-offs
a) E.g. sexual selection may favor long train in peacocks while
ecological selection may favor shorter trains that make peacock's
more able to maneuver and less vulnerable to predators
b) Relative importance of different agents of selection can change
over time and over the geographic range of the species
II. Cooperating with Others
A. Introduction
1. Altruism is behavior that has a fitness cost to the individual exhibiting the
behavior and a fitness benefit to the recipient of the behavior
a) Decrease an individual's ability to produce offspring but helps
others produce more offspring
2. In terms of ultimate causation, is paradoxical
a) If certain alleles make an individual more altruistic, they should be
selected against
b) Over time, selfish alleles should increase in frequency
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Document Summary
Introduction: selection based on success in courtship. Intersexual selection-the selection of an individual of one sex for mating by an individual of the other sex. Intrasexual selection-form of selection within a sex--competing with one another to obtain mates. That is, eggs are expensive but sperm are cheap : consequences of the fundamental asymmetry of sex-characteristic of many sexual species, because eggs are large and energetically expensive, females produce relatively few young over the course of a lifetime. Thus, a male"s fitness is limited not by the ability to acquire the resources needed to produce sperm but by the number of females he can mate with: bateman-trivers hypothesis of sexual selection a) b) c) If females invest a great deal in each egg, then they should protect that investment by being choosy about their mates. Conversely, if males invest little in each sperm, then they should be willing to mate with almost any female.
