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Section 24_3: Sexual Selection

Page history last edited by Derek Weber 10 years, 5 months ago

A. Learning Objective

Explain why intrasexual selection and intersexual selection causes the evolution of certain traits to occur differently in males and females of the same species.


B. Section Summary


This section of the book examines Sexual Selection: a form of natural selection which explains certain traits of sexually reproducing species. Sexual Selection results in traits which increases the chance an individual has to engage in successful mating. Charles Darwin originally explained this phenomenon as "the advantage that certain individuals have over others of the same sex and species solely with respect to reproduction." Females are usually uniform in their reproductive success while males have varied abilities. The traits which evolve through sexual selection are called secondary sex characteristics. The evolution of secondary sex characteristics results in sexual dimorphism. Sexual Dimorphism is the difference in appearance between the two sexes in the same species. There are two major distinctions in sexual selection: Intrasexual Selection and Intersexual Selection.


 Intrasexual Selection occurs between members of the same sex. For example, some male sheep have horns while others do not and some male moose have antlers while others do not. In these cases, the males are directly competing for mating opportunities of territories. Male Fiddler Crabs display this phenomenon through claw size. These crabs enter the burrows of females who are ready to mate. Males who have larger claws can cover the entrance inhibiting the entrance of other males. Hence, having a larger claw provides a male fiddler crab with a sexual advantage.


Intersexual Selection occurs between members of the opposite sex; it usually results in showy uncharacteristic for males which impact a female's choice. To clarify: while males compete against other males in Intrasexual Selection, Intersexual Selection has to do with males trying to attract females. For example, male Indian Peafowls display long and brightly colored feathers; these characteristics are assessed by females when selections are made for mating. Another example of Intersexual Selection, is cryptic female choice. This is when the female reproductive system can influence the relative success of sperm. This phenomenon usually occurs in species in which females mate with more than one male. For example, the female genital tract of certain females selects sperm that is genetically unrelated. 


Sexual Selection can also be a combination of both Intersexual and Intrasexual Selection. This is seen during the breeding season of elks. Intrasexual selection results in some male elk being stronger then others. These strong males tend to push the weaker males; this shows dominance to the female elk which is attractive. Intersexual selection is seen when the females choose the stronger males.  Besides for impacting sexual results, Sexual Selection also affects survival. This phenomenon is known as Sexual Selection balanced by Predation. Animals who have showy characteristics are at a disadvantage if there predators can easily see and kill them. Hence, in places with few predators these showy characteristics such as brightly colored tail feathers (in male Indian Peafowl) can be seen more. 


Sexual selection is responsible for the evolution of secondary sex characteristics. But why does sexual selection occur? Males who have more reproductive success can pass their genes to the next generations. Females select males that will provide them a survival advantage such as males who will best defend territory.


C. Virtual Lectures



D. Useful Materials

THIS IS A MUST WATCH VIDEO. First, because some girl walks in late and the professor is not amused. Besides for demonstrating the hard knock Yale life, the professor covers the topic of sexual selection in depth. Let's be honest, he explains it better than me.  I would recommend watching at least the first ten minutes.


In the first ten minutes, he starts to explain the basics of natural selection and sexual selection.He explains "how sexual sexual works" by providing analogies and other useful

materials. He also goes on to explain why it occurs. He uses general terms to explain complex situations. For example, he examines sexual selection's causes with words such as "direct benefits, good genes, and sexy sons."


Beyond the first ten minutes, the professor starts to diverge into more information than needed for my section. He goes into explaining three hypotheses about the cause of sexcual selection. He explains scenarios in which his students can make experiments in the future to isolate one hypothesis over the others. He also highlights exceptions to sexual

selection. For example. he gives an example of a bird which has females with bright feathers. He contends that in this case, the sperm is more expensive than the egg.

This is a great and short video which elementarily explains sexual selection. It is only about a minute long as the it really starts at 15 seconds and finishes at 1:05. I recommend this video toanyone who is not so ready to jump into my lectures but, rather is just looking to figure out what  my section is about.


The speaker does a great job of explaining sexual selection on a basic level. She goes over the simple concepts in brief detail. She gives a few examples and BAM! You know very little about sexual selection.


But hey, its still a good video to watch. 


University at California Berkley: Evolution Website

This link is to University of California at Berkeley's evolution website. It is not interactive or filled with games, but it gets the job done. I suggest using it because the examples and explanations are

really really good! I mean cali was always awesome but, this just is to another level. The first page relates sexual selection to natural selection in a simple manner using pictures to show extreme



The second page on the link does a great job of explaining the multiple aspects of sexual selection. For example. it demonstrates the difference between female choice and male competition in just a few sentences. Moreover, it also shows the effects of sexual selection.  It explains how sexual selection is related to evolution. A diagram shows that as certain traits are favored they become more abundant through natural selection, which leads to evolution! 

University of Missouri: Article on Sexual Dimorphism

This last link is to an article provided by the University of Missouri. I recommend this for anyone who is having trouble understanding why sexual dimorphism occurs. This is an essential piece of knowledge especially when you continue on to my primary literature. The article highlights the work of a University of Missouri researcher who expanded on Darwin's book, Male,Female: The Evolution of Human Sex Differences.


The article is filled with quotes from David Geary, the researcher. He explains why his book provides the best explanation for gender differences. He conducted research to add to Darwin's

conclusion and further emphasize a point. Geary is sure that Sexual Selection Is the Best Explanation for Gender Differences.


E. Primary Literature

The Origins of Sex Differences in Human Behavior


So far in this lesson, we have discussed some of the differences in males and females of a species. We have learned that differences in secondary sex characteristics results in a phenomenon called sexual dimorphism. In my virtual lecture, I provided examples of physical differences. The male Indian peafowl, for example, has brightly colored tail feathers to attract female mates. However, I did not discuss any psychological differences. David Buss of the University of Michigan theorizes that psychological difference between human males and females have also stemmed from sexual selection. This paper also provides a brief explanation of Sexual Selection so, I suggest reading it as further reinforcement.


There are many topics reviewed in the initiation of his discussion. First, recognizes that males and females of our species, Homo sapiens, have faced many similar problems which needed adaption. These adaptations were caused by natural selection. However, he makes sure to mention the difference between the male and female challenges. For example, he notes that women have faced childbirth, which leads to various adaptations. For example, female cervixes expand to 10 cm right before childbirth and the hormone oxytocin is released into the blood during childbirth. These events don't occur in men and show that females face different stimuli to lead to their adaptations. He adds that along with these difference, the human sexes have encountered psychological differences caused by sexual selection. Buss highlights certain hypothesis which add to his conclusion. First, he talks about how a mother is certain a child is her's but, a father may not know exactly if the child is his. Another example. includes his idea that men have to "gain sexual access" to women. Females have more "consequences" as they become pregnant and go through 9 months of gestation. In the paper he highlights more support such as "the investment of men" on a willing and able basis and the "reproductive value of women."


He also provides empirical data. For example, he notes that tests of general cognitive ability show differences among genders. In addition, he cites studies which refer to the difference in views on sex. These studies show that males have contrasting views on casual sex to females. these attitudes are nothing short of support for his theory that male and female psychological differences stem from sexual selection. Based on much evidence, David Buss concludes that differences in psychology between sexes could have been stimulated by Sexual Selection.


F. Lecture Slides


 G. Quiz


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