Lecture #18— Intraspecific Competition: Sexual Selection
Individuals of the same species are in direct competition for all of the same resources (Intraspecific Competition) and some are more successful than others in competing for resources (food, space, water, shelter, etc.) This is a fundamental premise of evolution—natural selection.
But there is another force at work in some species where individuals compete for mates. Darwin called it Sexual Selection = competition between individuals of the same sex for reproductive success. There are two types: Male competition and female choice. ♀
Sexual Dimorphism = Males and females of a species often look different, (e.g. male and female peacocks). When this occurs you know that different selective forces are acting on the different sexes—they are doing different things in their lives.
Male Competition
1. Male fighting for access to females (e.g. ♂ Lions fighting)
2. Male threat displays—fighting is to be avoided if possible. Involves physical displays of color, aggressive action, sound, etc. all showing off menacing behavior.
3. Sperm competition—when a species is promiscuous with females mating with many males—then males with the greatest sperm production and ejaculation will have an advantage. Consequently, testes size among species varies greatly depending on the number of male competitors in pursuit of a female.
4. Sperm removal—some species remove sperm previously deposited in female reproductive tracts by competitive males. (e.g. damsel fly).
5. Male guarding female until she lays eggs (e.g. dragon fly).
Female Choice (especially common among birds)
1. Males attempt to attract females on the basis of their territory with rich resources. (e.g. red- winged black birds).
2. Males attempt to attract females with their physical (attractive) attributes
(e.g. female barn swallows choose males with the longest tails).
Parental investment: It has been argued that females need to be more discrimating in selecting a mate than males as they are more limited in the number of offspring they can parent in a lifetime. Males are said to have a low cost of parental investment while females have a much higher cost. This may be true in some species where the male merely inseminates the female and moves on (e.g. deer) but not other species where both parents raise the offspring.
Why do females choose particular traits in males? Two opposing views:
a. The selective advantage hypothesis: They choose them because the traits are a direct signal of the health or “wealth” of the bearer. A good territory with adequate food signals that the offspring will be well cared for. Good plumage and a vigorous courtship indicate a healthy strong male which will serve as a good partner.
Some of the traits seem to be genetic and some appear to be learned.
However, some traits that a female selects for appear disadvantageous for survival: The male peacock tail display attracts predator attention; the barn swallow tail is not the most favorable aerodynamic shape for flying. How can we explain this? The handicap hypothesis has been suggested. This is the idea that any male who has survived in spite of the disadvantageous handicap must be a superb specimen as a potential mate.
b. The beautiful hypothesis: They choose them because the female simply finds them attractive or beautiful. Charles Darwin was in favor of this idea.
If females select for a genetic trait in males for whatever the reason, then this sets the stage for Runaway Selection. That is, because she has chosen the most elaborate male, her male offspring will have more beautiful plumage and they in turn will be selected and their offspring in turn will be chosen—with each generation with more and more glorious plumage. And her daughters will inherit the same preferences for fancy plumage. Runaway Selection is an example of a positive feedback loop. The only thing stopping this would be if the plumage is so elaborate that it interferes with general survival and then there will be a countervailing selection.
Kin Selection:
Kin Selection is the type of selection that occurs when individuals of a species favor closely related individuals. This is observed when individuals live in a group and know who their close relations are. In some colonies like bees and ants all of the hive members are apt to be sisters. So, when we see members of the colony risking their lives for other members, they are saving individuals with very similar genetic constitutions. They are saving their own genes. It has been argued that we should evaluate not personal fitness but Inclusive Fitness. Thus, evolutionary success ultimately depends on leaving behind the maximum number of copies of itself in thepopulation regardless of who sends them along—all the kin have similar genetic constitutions and they all contribute not just the immediate parents.
Terms/Concepts to Define
Intraspecific Competition
Sexual Selection
Sexual Dimorphism
Sperm Competition
Cost of Reproduction
Parental Investment
Kin Selection
Altruism
Inclusive Fitness
Individual Fitness
Can you answer these questions?
1. List and explain three types of male competition strategies in sexual selection.
2. Is there sexual selection among humans? If so, what kind of sexual selection strategies are involved?
3. How does sexual dimorphism play a role in sexual selection?
4. What is the advantage that occurs in male competition when threat displays are involved?
5. In primate species which are promiscuous, what predictions can you make about their testes size and sperm production in the ejaculate? What is your logic involved in making these predictions?
6. If female birds develop a preference for a male plumage display that has been learned, is this trait likely to be passed along to the next generation?
7. Runaway selection will progressively lead to more and more exaggerated traits in birds such as the peacock display, what is to stop this from happening?
8. Many species are social, living in large groups of cooperating individuals, what are three possible advantages that accrue in this group behavior?
9. Using evolutionary arguments, how can we explain situations where we find individuals putting themselves at risk to themselves to save the lives of other members?