Biol 121- 2010.03.22- Evolution- Sexual Selection, Definition of Species (Ch. 25-26).docx

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22 Apr 2012
Biol 121 225
Freeman 520-523, 526-531 Mar 22, 10
Evolution: Sexual Selection, What is a species?
What does natural selection favour?
-traits with a fitness advantage
What does sexual selection favour?
Two important consequences of the
fundamental asymmetry of sex
-traits with a mating advantage, leading to differential mating success
-individuals have heritable traits that enhance their ability to obtain mates
-traits that attract members of the opposite sex are much more highly
elaborated in males (sexual selection acts more strongly on males)
-therefore, females invest much more in their offspring than do males
-two important consequences:
1) Eggs are large and energetically expensive and thus females produce
relatively few young during lifetime
-female’s fitness limited primarily by her ability to gain resources to produce
more eggs and healthier young not by ability to find mate
2) Sperm are so simply to produce that male can father an almost limitless
number of offspring
-for males, fitness is limited not by the ability to acquire the resources needed
to produce sperm, but by the number of females they can mate with
Mating success
-the number of mates that produce offspring over a period of time
Why is sexual reproduction so
important? (3)
-ensure that genes are passed on to offspring
-genetic variation
-prevent inbreeding resurgence of homozygous recessive alleles
-sexual dimorphism
-refers to any trait that differs between males and females (size, weapons
males use to fight over females, elaborate ornamentation/behaviour, etc)
-different morphologies b/t males and females can easily tell different b/t a
male lion and lioness
Intersexual selection
-choosiness on the part of one sex for mates (typically female)
Intrasexual selection
-competition within a sex (male-male competition)
Male-male competition
-is an example of intrasexual selection (competition w/n a sex)
-since males invest little in each offspring (sperm = simple to produce), then
males should be willing to mate with almost any female and if there are =
numbers of m and f in a pop, then males have to compete with each other for
-therefore, sexual selection should act more strongly on males
-weaponry structures are examples of traits useful in competition
-e.g. elephants have tusks and they can fight with each other using them and
tusks indicate defence against predators, strength, and also of health and
therefore sexual vigour
-e.g. seals fight each other using head blows therefore the larger seal is
generally a better fighter (male seals can be 4x female seals in size)
-e.g. in bird species, colourful feathers or beak indicates health because the
most colourful males have a lot of energy invested into their appearance and
therefore since they have lots of energy ‘to spare’ they are in good health
Female choice
-choosy females get direct benefits
-they can have an increased chance that the mate will provide parental care
-increased change that a fertile male is chosen
-increased change that the male has a better genotype
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