How Does Natural Selection Affect The Bivalves?
Introduction
Evolution is the change in genetic composition of organisms between generations. Evolution is the process that results in organisms becoming more varied and better adapted in comparison to their ancestors. The driving force of evolution is natural selection. Natural selection is the process where individuals containing specific traits become more likely to survive compared to individuals without those traits. Because certain individuals have a greater chance to survive, they become more likely to reproduce yielding offspring that contain the same favored characteristics. As this occurs, the number of individuals with preferred traits become more abundant while the population of individuals without these traits begins to decrease, possibly even reaching the point of complete elimination. In lab, natural selection was observed by studying the predated and survival rates of bivalve mollusks. These rates reflected the interaction between the selective pressures and the adaptive traits of the bivalves. In the experiment, the selective pressure that affected the bivalves was the predation they faced from moon snails. The adaptive trait that was observed was the …show more content…
difference in shell size between bivalves. The biological hypothesis used for the experiment stated that the shell size of the bivalve was directly related to the likelihood of being preyed upon. It was predicted that the greater the shell size the less likely it would be for the bivalve to be preyed on. It would be more difficult for a moon snail to prey on a larger bivalve because it would require much more energy compared to a bivalve with a smaller shell.
Methods
The experiment performed consisted of two main parts. The first part of the experiment consisted of each group being given six shells varying in size and type. Each group was tasked with sketching and then identifying the shells using a dichotomous key. The dichotomous key provided pictures and descriptions of different physical traits that allowed each group to differentiate between the bivalve species. For each shell, the groups were given two descriptions on the key and based on which description the shell matched the groups were given one of two outcomes. Either the shell species was identified or the group was then forced to move to a new set of descriptions until the shell was properly classified. In order to obtain the most accurate results the validity of each identification was checked by the instructor.
The second half to the experiment consisted of each group being given eight Anadara brasiliana (incongruous ark) shells. For each shell, groups had to measure the total length of the shell to the nearest millimeter as well as record if the shell displayed a borehole. The presence of a borehole meant that the bivalve had been preyed upon by a moon snail. The data each group recorded was then reported to the instructor and was then compiled to create a class data set as well as three frequency charts. After creating this data set, the class reviewed a larger data set that compared two beaches. The beaches contained either an incongruous ark population or a population composed of Anadara ovalis also known as blood arks. The class was then asked to conclude whether natural selection had occurred based on the data presented. These conclusions are stated within the discussion portion.
No deviations were made to this experiment, the full procedure followed can be found in the “Evolution by Natural Selection” section of the General Biology II Laboratory Manual pages 1-18.
Results
Size (mm) Shells (S) Boreholes (B) Organisms (O= S/2) Frequency (%)
(O/N_O • 100) % Predated
(P= B/O • 100) % Survival (100-P)
1-20 18 10 9 18.75 100 0
21-30 32 19 16 33.3 100 0
31-40 26 6 13 27 46.15 53.85
41-50 15 4 7.5 15.6 53.33 46.67
51-60 5 1 2.5 5.21 40 60
Total N_S= 96 N_B= 40 N_O=48
Table 1. Class Shell Size and Predation Data
Table 1. displays the data collected in class for bivalve shell size as well as the percent predated and percent survival of the shells within each size range. The total number of shells (N_S) was 96, this value was divided in half in order to calculate the total number of organisms being observed (N_O). This calculation was done because one bivalve consists of two shells and so No was found to be 48. The frequency for each size range was found by dividing the number of organisms within each category by the total number of organisms being observed. The percentage of predated bivalves was calculated by dividing the number of boreholes in each size range by the corresponding number of organisms and then multiplying that value by one hundred. Finally, the percent survival was found by subtracting the percent-predated value of each size range from one hundred. Figure 1. Frequency Distribution of All Organisms. Graph one displays the frequency values for all organisms in class. This data was calculated by dividing the amount of organisms in each size range by the total number of organisms (No). The total number of organisms represented by this graph was equal to 48.
Figure 2. Frequency Distribution of Predated Organisms
Figure 2 demonstrates the frequency distribution of the predated organisms seen in class. These values were determined by dividing the number of shells with boreholes in each size range by the total number of organisms within that same size category. The products of those values were then multiplied by one hundred. The total number of predated organisms observed by this graph was equal to 20.
Figure 3. Frequency Distribution of Survived Organisms
Figure 3. Displays the frequency for the number of survived bivalves. This data was calculated by subtracting the percent of predated organisms in each size range from one hundred. The total number of surviving organisms represented by this graph was equal to 28.
The data collected in class was used to create the three histograms shown above. Each figure depicts a specific trend. Figure 1 displays which size of bivalve was seen most and least frequently within the observed population. Figure 2 demonstrates the what size class of bivalve was most and least preyed upon by the moon snails. In contrast, figure 3 depicted what size class of bivalve were most and least likely to survive.
Discussion By observing the data collected and the histograms generated, it is clear that both the incongruous ark and blood ark populations are most likely undergoing natural selection.
This conclusion is based on the moon snail’s preference of preying on the small to moderate sized bivalves. Because smaller bivalves are more likely to be preyed upon both the incongruous and blood ark populations will evolve in favor of having a larger shell in order to better survive against their environmental factors. This conclusion is further supported by the type of selection seen within the incongruous and blood ark populations, which was directional. Because one extreme seemed to be favored in terms of survival the frequency of the bivalve population will most likely shift towards the larger sized
individuals.
A plausible explanation for this trend is that moon snails are more easily able to prey upon organisms with smaller shells. Smaller bivalves most likely require less energy and power to prey upon in comparison to larger shelled bivalves. The difference in effort required means that a moon snail will be more able to obtain food successfully while exerting minimal energy. Thus, the moon snail’s predation habits are due to a greater chance of success with less effort. While comparing the data between the beaches the blood ark populated and the beach that the incongruous ark populated it was clear that blood ark population seemed to be seemed to be less predated. This could have been due to the beach that the incongruous ark inhabited having a greater predator population. Another possible explanation could be that the blood ark population may be harder to catch compared to the incongruous ark. In order to see if these
References
Martineau, Aguirre, LaMontagne, and Sparkes. “Evolution by Natural Selection”. Laboratory Manual General Biology II for Science Majors Bio
Evolution is the change in genetic composition of organisms between generations. Evolution is the process that results in organisms becoming more varied and better adapted in comparison to their ancestors. The driving force of evolution is natural selection. Natural selection is the process where individuals containing specific traits become more likely to survive compared to individuals without those traits. Because certain individuals have a greater chance to survive, they become more likely to reproduce yielding offspring that contain the same favored characteristics. As this occurs, the number of individuals with preferred traits become more abundant while the population of individuals without these traits begins to decrease, possibly even reaching the point of complete elimination. In lab, natural selection was observed by studying the predated and survival rates of bivalve mollusks. These rates reflected the interaction between the selective pressures and the adaptive traits of the bivalves. In the experiment, the selective pressure that affected the bivalves was the predation they faced from moon snails. The adaptive trait that was observed was the …show more content…
difference in shell size between bivalves. The biological hypothesis used for the experiment stated that the shell size of the bivalve was directly related to the likelihood of being preyed upon. It was predicted that the greater the shell size the less likely it would be for the bivalve to be preyed on. It would be more difficult for a moon snail to prey on a larger bivalve because it would require much more energy compared to a bivalve with a smaller shell.
Methods
The experiment performed consisted of two main parts. The first part of the experiment consisted of each group being given six shells varying in size and type. Each group was tasked with sketching and then identifying the shells using a dichotomous key. The dichotomous key provided pictures and descriptions of different physical traits that allowed each group to differentiate between the bivalve species. For each shell, the groups were given two descriptions on the key and based on which description the shell matched the groups were given one of two outcomes. Either the shell species was identified or the group was then forced to move to a new set of descriptions until the shell was properly classified. In order to obtain the most accurate results the validity of each identification was checked by the instructor.
The second half to the experiment consisted of each group being given eight Anadara brasiliana (incongruous ark) shells. For each shell, groups had to measure the total length of the shell to the nearest millimeter as well as record if the shell displayed a borehole. The presence of a borehole meant that the bivalve had been preyed upon by a moon snail. The data each group recorded was then reported to the instructor and was then compiled to create a class data set as well as three frequency charts. After creating this data set, the class reviewed a larger data set that compared two beaches. The beaches contained either an incongruous ark population or a population composed of Anadara ovalis also known as blood arks. The class was then asked to conclude whether natural selection had occurred based on the data presented. These conclusions are stated within the discussion portion.
No deviations were made to this experiment, the full procedure followed can be found in the “Evolution by Natural Selection” section of the General Biology II Laboratory Manual pages 1-18.
Results
Size (mm) Shells (S) Boreholes (B) Organisms (O= S/2) Frequency (%)
(O/N_O • 100) % Predated
(P= B/O • 100) % Survival (100-P)
1-20 18 10 9 18.75 100 0
21-30 32 19 16 33.3 100 0
31-40 26 6 13 27 46.15 53.85
41-50 15 4 7.5 15.6 53.33 46.67
51-60 5 1 2.5 5.21 40 60
Total N_S= 96 N_B= 40 N_O=48
Table 1. Class Shell Size and Predation Data
Table 1. displays the data collected in class for bivalve shell size as well as the percent predated and percent survival of the shells within each size range. The total number of shells (N_S) was 96, this value was divided in half in order to calculate the total number of organisms being observed (N_O). This calculation was done because one bivalve consists of two shells and so No was found to be 48. The frequency for each size range was found by dividing the number of organisms within each category by the total number of organisms being observed. The percentage of predated bivalves was calculated by dividing the number of boreholes in each size range by the corresponding number of organisms and then multiplying that value by one hundred. Finally, the percent survival was found by subtracting the percent-predated value of each size range from one hundred. Figure 1. Frequency Distribution of All Organisms. Graph one displays the frequency values for all organisms in class. This data was calculated by dividing the amount of organisms in each size range by the total number of organisms (No). The total number of organisms represented by this graph was equal to 48.
Figure 2. Frequency Distribution of Predated Organisms
Figure 2 demonstrates the frequency distribution of the predated organisms seen in class. These values were determined by dividing the number of shells with boreholes in each size range by the total number of organisms within that same size category. The products of those values were then multiplied by one hundred. The total number of predated organisms observed by this graph was equal to 20.
Figure 3. Frequency Distribution of Survived Organisms
Figure 3. Displays the frequency for the number of survived bivalves. This data was calculated by subtracting the percent of predated organisms in each size range from one hundred. The total number of surviving organisms represented by this graph was equal to 28.
The data collected in class was used to create the three histograms shown above. Each figure depicts a specific trend. Figure 1 displays which size of bivalve was seen most and least frequently within the observed population. Figure 2 demonstrates the what size class of bivalve was most and least preyed upon by the moon snails. In contrast, figure 3 depicted what size class of bivalve were most and least likely to survive.
Discussion By observing the data collected and the histograms generated, it is clear that both the incongruous ark and blood ark populations are most likely undergoing natural selection.
This conclusion is based on the moon snail’s preference of preying on the small to moderate sized bivalves. Because smaller bivalves are more likely to be preyed upon both the incongruous and blood ark populations will evolve in favor of having a larger shell in order to better survive against their environmental factors. This conclusion is further supported by the type of selection seen within the incongruous and blood ark populations, which was directional. Because one extreme seemed to be favored in terms of survival the frequency of the bivalve population will most likely shift towards the larger sized
individuals.
A plausible explanation for this trend is that moon snails are more easily able to prey upon organisms with smaller shells. Smaller bivalves most likely require less energy and power to prey upon in comparison to larger shelled bivalves. The difference in effort required means that a moon snail will be more able to obtain food successfully while exerting minimal energy. Thus, the moon snail’s predation habits are due to a greater chance of success with less effort. While comparing the data between the beaches the blood ark populated and the beach that the incongruous ark populated it was clear that blood ark population seemed to be seemed to be less predated. This could have been due to the beach that the incongruous ark inhabited having a greater predator population. Another possible explanation could be that the blood ark population may be harder to catch compared to the incongruous ark. In order to see if these
References
Martineau, Aguirre, LaMontagne, and Sparkes. “Evolution by Natural Selection”. Laboratory Manual General Biology II for Science Majors Bio