Variability In Biological Samples Winkles Practical Biology Essay

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In biology, there are a number of biological variations which occurs in a variety of organisms from cells to single or a group of organisms of the same species (Cullen. 2009). These may not be noticeable whereas in some cases are noticeable, i.e. colour of skin. Variations are caused by either genetic differences (also known as genotypic variation) or environmental factors which affect the expression of a genetic phenotype (also known as phenotypic variation) or may even be a combination of the two. These variations could manifest as physical appearances, fertility, biochemistry and other measureable characterises such as height and weight of an organism. (Cumming. 2011)

Genotypic variations occur when there is a difference in the number or structure of the chromosomes. (Deshek and Harrison. 2006) Variations could also occur when there is a difference in the genes carried by the chromosomes. An example of genetic variation is human blood groups, eye colour, and body structure and also if an organism is resistant to disease i.e. some individuals who have sickle cell anaemia have the same gene which is shown to have increased resistance malaria (Cumming. 2011).

Environmental caused variations could be caused by either one or several combined factors such as; climate, food supplies, and even competition exhibited by other organisms which live in the same habitats. This type of variation would therefore determine an organism's phenotypic variation (Deshek and Harrison. 2006).

A combination of the two biological variations (genotypic variation and phenotypic variation) could be the height of an individual, where the height of an individual is an inherited characteristics but the availability of nutrients in the environment would determine the individual's actual height (Farris, M. 2008).

In the animal kingdom the biological variation of colours changes in species is often recorded. An example would be the arctic fox where this type of fox could change its fur colour when changes in the environment occur, i.e. from a warm season to a cold season. Environmental factors such as light and temperature changes would enable the artic fox to change from brown (warm season) to white (cold season), this provides a natural advantage of camouflage and also provides adaptation to the arctic cold (Naughton. 2012).

Biological variation is found in a number of different organisms, environmental factors would greatly affect the biological variations of organisms of the same species. Periwinkle shells, Littorina Littorea, show biological variation in height, mass and aperture, where wave exposure is one of the main factors which would affect the periwinkle size distribution (Boulding, Holst, & Pilon, 1999), the periwinkle shells would therefore to adapt to the harsh habitat. The periwinkle size would vary due to the ecological affects such as: the periwinkle, (are herbivorous grazers), display a size dependent grazing rates (Geller. 1991), (which would reduce the interspecific competition due to obtaining smaller food sources than larger organisms (Byers. 2000), and also there would be a differential selection of habitats by age (Saier. 2000) to obtain the most from a habitat.


The materials used to carry out the experiment were, 50 Winkle shells, Vernier callipers were used to measure the periwinkles height and aperture, electronic balance was used to measure the mass of the periwinkle.


There were 50 winkle shells were used where in the experiment, the measurement of each shells height and aperture was measured to the nearest 0.1mm using vernier callipers. Vernier callipers (Fig 3) are an instrument which were used to measure the precise internal and external distances of the peri-winkle shells extremely accurately.

Fig 3: shows a diagram of a manual vernier calliper used to measure objects external distances, and internal distances.

The vernier callipers consists of two jaws which are the external and internal jaws; the external jaw was used to measure the exterior diameter i.e. height, of the peri-winkle shell (from the aperture to the apex) while the interior jaw was used to measure the internal diameter i.e. the aperture, of the shell.

To obtain a reading with the vernier callipers, the measurements must first be understood. The 1cm mark on the vernier calliper is equal to 10mm on a fixed calliper, for example if the scale is adjusted (to the right) by 2mm away from the 10mm mark, this would read as 12mm (10mm + 2mm = 12mm).

Another example for reading the scale was:

The calliper must first be zeroed on the sliding Vernier scale. Read the number from the fixed scale (when a measurement is being taken). If the value is between two values, the lower value must be read.

The line on the Vernier scale must be found which aligns perfectly with another line on the fixed scale. If no lines are perfectly aligned, find the one that would be the closets. Read the number on the Vernier scale for this line, the two readings would then be combined to obtain the measurement. Fig 4 shows the fixed reading is 3 mm and the Vernier reading is 0.58 mm, so when the measurement is combined, 3mm + 0.58, an answer of 3.58 is obtained mm (ehow. 2013).

Fig 4: shows how to read a value which has been obtained in the fix measurement. (ehow. 2013)

To measure the height of the peri-winkle shells (fig 5) using the vernier calliper; the first step that was taken is to set the calliper to 0, the shell was placed flat on the surface and a measurement was taken of the height (fig 5. Labelled as H) of the shell from the axis of the shell to the aperture. The measurement of the aperture was taken by the internal jaws of the callipers by; closing the internal jaws of the calliper (set to 0) then the internal jaws of the callipers was placed into the mouth of the shell, the internal jaws were opened and a measurement of the aperture was taken from lip to lip (fig 5, from a to b) inside the mouth of the aperture. The result (height and aperture) for each shell was then recorded in table (fig 6 in appendix).

Fig 5: shows the basic measurement procedures for the periwinkle shell. Where measurements of the height were taken from the axis to the aperture, labelled as H, and the measurements of the aperture were taken from a to b inside the mouth. (Independent research. 2013)

Each shell was then individually weighed using an electronic top pan balance to measure the mass of the periwinkle shell to the nearest 0.01g. To avoid any systematic errors, the balance was first zeroed before the measurement was taken for each individual shell as to obtain a correct reading. The mass for each of the 50 shells was then recorded in a table (Fig 6 in appendix).


These results are shown for the shell mass against the shell height (fig 1) and also shell mass against the shell aperture (fig 2). Using the correlation coefficient, a linear relationship was identified between the shell mass (g) and the following varibles: shell height (in mm) and shell aperture (in mm).

These correlations were then presented in fig 1 and fig 2.

Fig 1: shows a graph for Shell mass (g) against hell height (mm)

Fig 2: shows a graph for shell aperture (mm) and shell mass (g).


The R2 value shows if the line of best fit for the data points, is strong or weak, the closer the value of R2 is to 1, the stronger the line of best fit for the results presented whereas the R value shows weather the line of best fit has a positive or negative correlation, this value is known as the correlation coefficient, depending on weather the R value is a positive number (positive correlation) or negation number (negative correlation).

Fig 1 shows that there is a linear relationship between shell mass and the shell height. Fig 1 shows that the R2 value is 0.774 which shows that the line of best fit is relatively strong for the data points, the graph also shows a R value of 0.88 which indicates that there is a strong positive correlation between the shell mass and shell height, the data points also show a degree of precision, therefore figure 1 indicates that, as the mass of the shells increase there is also an increase in height of the shells, this would therefore indicate that as the height increases of the periwinkle littorina Littorea due to environmental factors such as, wave shock (Raffaelli. 1982), the closer the periwinkles are to the shore, the height would increase as there is more wave exposure whereas the smaller periwinkle shells which show a decrease size in height and aperture would be found further away from the shore.

Fig 2 also shows a linear relationship between the shell mass and the shell aperture. Fig 2 shows a R2 value of 0.056 which shows that the line of best fit is relatively weak for the data points presented. The graph also shows an R value of 0.24 which indicates line of best fit presents a relativity weak positive correlation since the data points are very much scattered this would therefore indicate that as the mass of the shells increase there is also a (relatively low) increase in the shell aperture however the graph does show some variation with the results because some of the data points show that as the mass increases there is not a high increase in shell aperture, for example: shell no. 3 (fig 6) has a mass of 3.287g but has a shell aperture of 6.9mm while the majority of shells with a mass of 3g or higher show a aperture value of 7.7mm or higher. There are also other variations in fig 2, five of the data points show aperture values which do not exceed 8mm as the shell mass increases, this may therefore indicate that the periwinkle shells which show a aperture value of 8mm or less would therefore be found further away from the shore where there is less wave exposure so the periwinkle would have in decrease foot size as extra adhesion is not needed comparing to the periwinkle found closer to the shore.

The size of the periwinkle, Littorina Littorea, shell size is genetically determined however environmental factors may have an effect on the phenotypic expression of the shell. Environmental factors such as wave exposure, interspecific competition, predation and also food availability would have an effect on the shell size (Lawrence. 2001). Shells with an increase height combined with the increased aperture would therefore indicate that the periwinkle would have been found much closer to the shore due to environmental factors, such as wave exposure, where the extra height of the shells would withstand extra wave exposure whereas the increase in aperture would enable an increase in foot size for greater adhesion (Raffaelli. 1982) to the soft sediment found closer to the shore, furthermore the shells would have adapted to suit the environment factors found in the area.

However there may some variations in the results obtained (i.e. fig 2), where this relatively low sample size of the shells may not have been enough to determine the (overall) actual size of the periwinkle shells aperture or height, where the experiment could have been improved by: a larger sample could have been taken to get a much clearer overview of the periwinkles in the environment.


There does appear to be a very strong link between the shell mass and the shell height, and there seems to be a relatively weak link between the shell mass and shell aperture. Examples of size variations for the periwinkle shells shows how the shells became adapted to their environment where this would conclude with Charles Darwin's theory of evolution, where these variations would occur for the survival and reproduction of the species so the adapted genes, therefore would pass onto the next generation of periwinkle Littorina Littorea.

Total word count: 1,991


Boulding, E. G., Holst, M., & Pilon, V. (1999). Changes in selection on gastropod shell size and thickness. Journal of Experimental Marine Biology and Ecology, 232: 217-239.

Buffler, A. (2003). Using the vernier callipers and micrometre screw gauge. [Online]. Available at: (Accessed on 17th march 2013)

Cullen, K. E. (2009). Encyclopaedia of Life Sciences. New York: Info base Publishing. Pp. 70

Cumming, M. R. (2011). Human Heredity: Principles and Issues. USA: Cengage Learning. Pp. 426

Cumming, M. R. (2011). Human Heredity: Principles and Issues. USA: Cengage Learning. Pp. 98

Deshek, V. W. and Harrison, M. (2006) Plant Cell Biology. USA: Science Publishers. Pp. 261

Lawrence, J. M. (2001). Edible Sea Urchins: Biology and Ecology: Biology and Ecology. Amsterdam: Elsevier Science. Pp. 169

Farris, M. (2008). The Altitude Experience: Successful Trekking and Climbing above 8000 feet. USA: Globe Pequot. Pp. 15, 16

Geller, J. B. (1991). Gastropod grazers and algal colonization on a rocky shore in northern California: the importance of the body size of grazers. Journal of Experimental Marine Biology and Ecology, 150: 1-17.

Independent research. (2013). NAHANT Those Curiously Interesting Snails. [Online]. Available at: (accessed on 17th march 2013)

McKenzie, G. (2013). How to Read Callipers On Micrometres. [Online]. Available at: (Accessed on 17th march 2013)

Naughton, D. (2012). The Natural History of Canadian Mammals. Canada: University of Toronto Press. Pp. 391-392

Saier, B. (2000). Age-dependent zonation of the periwinkle Littorina Littorea (L.) in the Wadden Sea . Helgol Mar Res, 54: 224-229.

Raffaelli, D. (1982). Recent ecological research on some european species of littorina. Journal of Molluscan Studies, 48: 342-354.