ANT336 Exam Review.docx

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Susan K Pfeiffer

Give an example that illustrates the difference between ultimate causation and proximate causation.  Ultimate causation answers questions of why an animal behaves the way it does. Ultimate causes take into account environmental factors that contribute to why an animal is able to survive and reproduce. They tie directly into the concepts of Darwinism and natural selection. Ultimate causation has an end goal in forwarding genes and ensuring survival. Example: A female songbird is attracted to male songbirds' songs. These songs also warn other males to stay away from the singer’s territory. Male songbirds that sang in the spring were naturally selected for; they attracted more mates, reproduced more, and passed their genes on.  Proximate causation answers questions of how biological traits operate in an animal. Whereas ultimate causation has long-term implications, proximate causes have more immediate effects. These causes explain why certain animals have certain appendages, for example, but are independent of the evolutionary discussion. Example: Increased light during springtime triggered the male songbirds brain to produce a sex hormone. These hormones drove the birds to sing. What is the difference between personal fitness and inclusive fitness?  Inclusive fitness differs from individual fitness by including the ability of an allele in one individual to promote the survival and/or reproduction of other individuals that share that allele, in preference to individuals with a different allele.  Individual fitness - Reproductive success measured by the number of direct descendants an individual has. Different types of Natural Selection  Individual selection – differences in fitness, owing to a genetically variable phenotypic character, among individual organisms within a population  Stabilizing selection – When selective pressures select against the two extremes of a trait, the population experiences stabilizing selection. For example, plant height might be acted on by stabilizing selection. A plant that is too short may not be able to compete with other plants for sunlight. However, extremely tall plants may be more susceptible to wind damage. Combined, these two selection pressures select to maintain plants of medium height. The number of plants of medium height will increase while the numbers of short and tall plants will decrease.  Directional selection - In directional selection, one extreme of the trait distribution experiences selection against it. The result is that the population's trait distribution shifts toward the other extreme. In the case of such selection, the mean of the population graph shifts. Using the familiar example of giraffe necks, there was a selection pressure against short necks, since individuals with short necks could not reach as many leaves on which to feed. As a result, the distribution of neck length shifted to favor individuals with long necks.  Disruptive Selection - In disruptive selection, selection pressures act against individuals in the middle of the trait distribution. The result is a bimodal, or two-peaked, curve in which the two extremes of the curve create their own smaller curves. For example, imagine a plant of extremely variable height that is pollinated by three different pollinators, one that was attracted to short plants, another that preferred plants of medium height and a third that visited only the tallest plants. If the pollinator that preferred plants of medium height disappeared from an area, medium height plants would be selected against and the population would tend toward both short and tall, but not medium height plants. Such a population, in which multiple distinct forms or morphs exist is said to be polymorphic  Frequency-dependent selection – an evolutionary process where the fitness of a phenotype is dependent on its frequency relative to other phenotypes in a given population. - In positive frequency-dependent selection (or purifying frequency-dependent selection) the fitness of a phenotype increases as it becomes more common. - In negative frequency-dependent selection (or diversifying frequency-dependent selection) the fitness of a phenotype increases as it becomes rarer. Negative frequency-dependent selection is an example of balancing selection Sexual Selection  Darwin considered traits selected according to their role in mating to be separate from those acted on by natural selection. He considered this a different type of selection that he called "sexual selection." Sexual selection occurs in two ways: through contests and through choice.  Contest – Contests are competitions between members of the same sex for access to the other sex. This competition may take many different forms. A common example is direct combat between males.  Choice – While contests involve direct physical competition between members of the same sex, choice involves competition for attention from
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