Predictions of the Bateman-Trivers Theory:
1) If females invest a great deal in each offspring, then they should protect the investment by being choosy about their mates.
Conversely if males invest a little in each offspring, then they should be willing to mate with any female.
2) 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
Basically there should be 2 types of sexual selection: a female choice and male-male competition
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.
o Sexual selection should act more strongly on males than females.
Female Choice for “Good Alleles”
• Experiment with bird species found that females prefer to mate with males that are well fed and in good health.
• key observations:
• 1) in many bird species, the existence of colourful feathers or a colourful beak is due to the presence of the red and yellow
pigments called carotenoids
• 2) Carotenoids protect tissue and stimulate the immune system to fight disease more effectively
• 3) animals cannot synthesize their own carotenoids, but plants can. To obtain carotenoids, animals have to eat carotenoid
rich plant tissues.
• Observation suggest healthiest and best nourished birds in population have the most colourful beaks and feathers.
• Sick birds have dull colouration because they are using all of their carotenoids to stimulate their immune system
Female Choice for Parental Care:
• In many species, females prefer to mate with males that care for young or that provide the resources requires to produce
• Females may choose mates on the basis of:
o Physical characteristics that signal male genetic quality OR
o Resources or parental care provided by males OR
Study done on elephant seals, found that:
Males that own beaches with large congregations of females father large numbers of offspring. Males that lose fights father
few or no offspring
The alleles of territory-owning males rapidly increase in frequency in the population
If the ability of win fights and produce offspring is determined primarily by body size, then alleles for large body size have
a fitness advantage, leading to the evolution of large male size. Fitness Advantage is due to sexual selection.
Consequences of Sexual Selection:
Since sexual selection is more intense in males than females, males tend to have more traits that function only in courtship
or male-male competition
o Females survive to adulthood to get a mate, males do not.
Sexual Dimorphism refers to any trait that differs between males and females
o E.g. male lions have a mane, female lions do not.
Sexual selection also violates the Hardy Weinberg Principle causes certain alleles to increase or decrease in frequency
and results in evolution (different from inbreeding and Assortative mating).
ANIMALS USUALLY MAKE DECISIONS IN A WAY THAT MAXIMIZES THEIR FITNESS
Phylogenies and the History of Life
• Phylogeny evolutionary history of a group of organisms
• Phylogenetic Tree shows the ancestor-descendant relationships among populations or species, and clarifies who is
related to whom.
o A Branch represents a population through time
o Node point at which two branches diverge. Represents the point in time when ancestral species split into two or
o Tip (terminal node) the endpoint of a branch, represents a group (a species or larger taxon) that is living today or
ended in extinction.
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