Patterns of Natural Selection Worksheet Key

Name _________________________________ Per____ Patterns of Natural Selection Worksheet – KEY Define the following terms: 1. stabilizing selection__Selection favors the intermediate phenotype _______________________________________________ __________________________________________________________________________________________________________ 2. directional selection _Selection shifts to favor a slightly more or less extreme phenotype ________________________________ __________________________________________________________________________________________________________ 3. disruptive selection __Selection favors both extreme phenotypes___________________________________________________ __________________________________________________________________________________________________________ Write the correct type of selection below (Disruptive, Directional, or Stabilizing). Then draw what you would expect on the graph. _____Stabilizing______________ Starlings produce an average of five eggs in each clutch. If there are more than five, the parents cannot adequately feed the young. If there are fewer than five, predators may destroy the entire clutch. As a result, five eggs becomes the most common clutch size. ______Disruptive____________ Seed cracker birds have either large beaks or small beaks. They do not have medium sized beaks because medium sized beaks do not allow for adequate cracking of seeds. _____Stabilizing_____________ A scientist measures the circumference of acorns in a population of oak trees and discovers that the most common circumference is 2 cm. There are very few acorns with circumferences of 3 cm or circumferences of 1 cm. Only the 2 cm circumference acorns survive. ______Directional__________ A population of Madagascar hissing cockroaches lives in a woodpile. The cockroaches are eaten by lizards. Because the lizards have small heads, the lizards are unable to eat the very largest adult cockroaches, and instead prey upon small and medium sized adults. Over time, only the large headed lizards survive. ______Stabilizing___________ Female birds that lay close to the optimum number of eggs have the most surviving offspring. Those that lay fewer or more eggs have lower relative fitness. ______Directional___________ A population of birds with various size beaks eats seeds. Small seeds can be eaten by birds with small beaks. Larger, thicker seeds can only be eaten by birds with larger, thicker beaks. During a drought, only large thick seeds exist so only the large, thick-beaked birds survive. _______Disruptive____________ Black rabbits (BB) and white rabbits (bb) are both able to survive because they can camouflage into the white and black rocks in their environment. However, the intermediate gray rabbits (Bb) do not survive. This results in only white and black rabbits. _______Stabilizing___________Small lizards have difficulty defending territories so they end up dying out. Large lizards are more likely to be preyed upon by owls. Over time, only medium sized lizards end up surviving. _______Stabilizing___________Women often have complications during labor while giving birth to very large babies, whereas very small babies tend to be underveveloped. As a result, medium sized babies are the ones that end up surviving to adulthood. Label the three types of selection illustrated by the graphs below. Name _________________________________ Per____ For your study, you will be examining a specific population of spiny cacti, which is originally in genetic equilibrium. A road is built quite close to the study site, which keeps away the treacherous and parasitic insects, but with the road comes the tourists. In many desert areas of the United States, passing cactus lovers like to pick up a souvenir cactus to take home with them after a day-trip out into the desert. This is a serious problem in some areas because the tourists always take the better looking cacti, and these happen to be the ones with the middle-spine-numbers. Years of collecting have left their toll on the roadside cacti. Using this background knowledge and the data collected below; create a graph of both the original population of cacti and the current population on a piece of graph paper. Place the number of cacti on the Y axis (count by 1’s) and the number of spines on the X axis (start at 70 and count by 2’s). Remember to label your x and y axis and give your graph a title. 70 74 78 82 86 90 94 98 102 106 110 What type of selection does this graph represent?____Disruptive___________________________ 114 118 122 126 130