A review of 40 Years of Evolution: Darwin’s finches on Daphne Major Island by Peter and Rosemary Grant Princeton University Press, Princeton, NJ, 2014. by Jean K. Lightner. Natural selection in the opposite direction, with small birds surviving disproportionately, occurred 8 years later. Three Major Lessons of Peter and Rosemary Grant’s Research 1) Natural selection may be variable. By presenting data before and after an environmental change (i.e., drought), this Data Point does a good job of focusing students’ attention on the process of natural selection leading to adaptation of a population (i.e., change in beak depth in a finch population in response to a change in seed availability after the drought). 19. Figure 1 Darwin’s finches show most extreme differences in beak size and shape. Figure 1 shows two graphs of beak depth measurement for the 50 medium ground finches that died in 1977 and did not survive the drought (nonsurvivors) and the 50 medium ground finches that lived beyond 1977 and survived the drought (survivors). An evolutionary response to directional natural selection followed in the next generation , because beak size variation is highly heritable (Keller et al. a. the mean should be less than 15mm b. the mean should be more than 15mm c. the mean will change randomly d. the mean should not change mass, and beak depth, taken from a sample of 100 medium ground finches (Geospiza fortis) living on the island of ... Evolution by means of natural selection can only occur if heritable traits vary among individuals in a population. Mean beak depth will decrease. 2. assuming the trait is heritable what would happen to the mean beak depth after several generations of stabilizing selection? Survived = 1). Mean beak depth: t obs = 3.27 Mean tarsus length: t obs = 1.82 b. Assuming that beak depth is heritable, what do you predict about beak depth in the next generation? So the next question is how does a particular beak shape become characteristic of an interbreeding population (species) of finches? The mean beak depth of surviving G. fortis before and after the selection event of 1977. Mean beak depth is indicated by the arrowhead. Peter and Rosemary Grant and colleagues (Gibbs & Grant 1987; Grant 1999; Grant & Grant 2002b) have demonstrated that even short-term fluctuations in food availability, driven largely by seasonal variations in weather patterns, lead to evolutionary changes in beak morphology via natural selection. The Warbler finch (a) has the smallest, the Large ground finch (b) the largest beak. You can see from Figure 2 that there is a correlation between the parents' and offsprings' beak size. Unmasking natural selection. 2001). Determine the mean beak depth of the parents (Mother Beak and Father Beak) for each offspring in 1976. How do you know that finches' beak depth is heritable? It is apparent that differences in beak shape can result from shifts in gene expression during the complicated development process.