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Section 2 of 3: Widowbird Sex: A Case Study of Sexual Selection and Evolution Adapted from J. Philip Gibson Widowbirds are small, finch-like birds in the genus Euplectes. They are striking members of the bird community in grasslands and shrubby savannas of southern and eastern Africa. This group is noted for the pronounced sexual dimorphism between males and females. During the non- breeding season, both male and female widow birds have a brownish or buff coloration that blends with the grass and other vegetation. During the breeding season, however, males molt and produce black feathers on most of their body, they produce characteristic bright red and/or yellow epaulets and chevrons on their wings, and the males of several widowbird species grow elaborately long tail feathers that can be up to half a meter in length (see Figure 1). During the breeding season, males secure and defend a territory from other males where they then multiple nest frames. A nest frame is a nest made from sticks but it lacks the inner lining of build necessary for laying and incubating eggs. Males then perform a flight display that has a cy rowing appearance with loops and exaggerated wing beats to attract females to their fine grass, territory. Females choose a male for breeding, line a nest frame in his territory with and then incubate the eggs and feed the nestlings in that frame. After the breeding season, males molt to return to their pre-breeding coloration and appearance. Beyond initially building the nest frame, males do not participate further in raising their offspring. The elaborate behaviors and the striking seasonal sexual dimorphism in male widowbirds have provided researchers with an intriguing system in which to study how natural selection can shape traits of a species. Figure 1. Left Long-tailed widowbirds showing breeding and non-breeding plumage. (Long-Tailed Whydah, Frederick William Frohawk, printed by Brumby &Clark Ltd. in Hull and published Frohawks Foreign Finches in Captivity. Image courtesy of ancestryimages.com.). Right. Male widowbird in flight Sexual Selection The bright breeding coloration and long tails produced by male widowbirds are thought to be adaptations to attract the attention of female widowbirds. Mating systems in which individuals differentiate among potential mates and choose one based upon some trait or set of traits is known as sexual selection. The fundamental idea of sexual selection is that choosing the best mate increases the probability of producing the highest quality offspring. The more offspring an individual produces, the more its genes make it into the next generation, the greater its fitness. One idea that has been developed to explain the evolution of elaborate characteristics in males can be thought of as Truth in Advertising or Good Genes. The Truth in Advertising hypothesis proposes that females can assess the quality of a male by his ability to survive despite having an elaborate trait that could potentially be detrimental to his health. For
example, while bright coloration indicates Rood health, it also increases visibility to predators. the Truth in significant cost to tail and also the increased risk of predation. A bird with a long tail is signaling to females: w look at me, I have a huge tail that is very costly in the Darwinian world of the struggle existence, yet I am spry and vigorous. Squawk ticularly those that produce long tails, present excellent systems for evaluating individuals, both in terms of the energy required to grow and maintain a long Advertising hypothesis in sexual selection because the long tail must impose a for If you have not already, watch V3 on widowbird behavior. This is a different species (Jacksons Euplectes jackson) but the behavior is very similar Making predictions: Researchers conducted several studies to investigate the influence of tail length in widowbird mate choice and reproduction. In an initial field study, researchers examined the number of active nests for a sample of males with different tail lengths. Draw the graph you predict you would see based on your understanding of sexual selection. There is no correct answer. Note: There is not a correct answer. You will receive credit based on the completeness of your graph (see checklist) and the quality of your explanation of why you drew the graph the way you did. Graph checklist O O Include at least 10 points (representing 10 males) that have different tail lengths Include a best fit line Tail Length Write a brief justification for your graph, including why you distributed the points in the pattern you did. *Important: Upload a picture of your graph above in your Weekly Quiz by Sunday at 9am. You can upload your concept sheet if you did a fancy computer graph, or just snap a picture of this page with your phone. The quiz question should allow you to Insert stuff or Insert Image by hitting the icon above the answer box.
section 3 of 3: Practice Questions: NOTE- The questions in this section are for your mental practice and are good examples of potential test questions :Predict how allele frequencies differ from Hardy-Weinberg equilibrium under directional selection. Describe how gene flow affects genetic variation in neighboring populations, and why inbreeding may have detrimental consequences. . Explain why beak size in Darwins finches provides a classic example of evolution by natural selection. What type of selection likely occurred? Defend your answer. A population of birds contains 16 animals with red tail feathers and 34 animals with blue tail feathers. Blue tail feathers are the dominant trait. What is the frequency of the red allele? . What is the frequency of the blue allele? What is the frequency of heterozygotes? What is the frequency of birds homozygous for the blue allele? .
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