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

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BIOL 121
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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? -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: Two important consequences of the 1) Eggs are large and energetically expensive and thus females produce fundamental asymmetry of sex 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 -ensure that genes are passed on to offspring important? (3) -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 mates -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 1 Biol 121 225 Freeman 520-523, 526-531 Mar 22, 10 -since females invest a great deal in each offspring (care, eggs) they should protect that investment by being choosy about their mates Song and dance -song and dance indicate healthy male because of his energy -elaborate song and dance is associated with ‘good’ alleles Summary of female choice -basically, females will choose males with “good alleles” as shown by their health which is shown by energy invested in stuff like coloured feathers or size or song and dance -however, females also may choose (depending on species) males that care for young or that provide resources required to produce eggs Irish elk -the Irish elk (now extinct) had very large antlers -the antlers are believed to intimidate other males and also to impress females -they were not for fighting though, upon examining the angle and placement of the antlers on the elk’s head Long-tailed widow birds experiment -these are birds with very long tails in the males -four treatments: 1) Shortened tail (cut) 2) Elongated tail 3) Normal tail (control) 4) Cut tail and glue back on -initially: all groups before trea
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