Evolution Lab 1: Genetic Drift and the Pinto-astic Four Beans!

Alondra Sotelo, Jinmei Wang, Radhika Dalal, Sharon Huynh

In this lab, we explored the Hardy-Weinberg conditions using a model with beans. Using four bean species to represent four unique alleles, we randomly paired the beans, and doubled the unique frequencies of the bean species to create a simulation of genetic drift.

Hypothesis: The bean populations in our model are not following the Hardy-Weinberg principles and their allele frequencies are changing after each subsequent generation.

Prediction:  If there are significant differences between the bean allele frequencies in generation 1 and generation 10, then the bean population is not in Hardy-Weinberg Equilibrium.

Figure 1: The four species of beans used in the experiment.

Clockwise: Pinto, White, Soy, Black beans

Figure 2: The graph above illustrates the number of soy, pinto, black, and white beans in a population over ten generations with a total of fifty beans per generation, and varying totals of each individual bean species. Pinto beans made up the largest portion of population A and had the largest increase in total number of beans from generation one to generation ten. However, the other species of beans showed a steady decrease in their total number of beans during the course of ten generations.

Population A

Generations
Allele	1	2	3	4	5	6	7	8	9	10
Soy	8	4	4	4	5	1	1	1	1	1
Pinto	16	23	32	32	37	42	42	44	44	46
Black	12	9	7	7	5	2	3	2	3	2
White	14	14	7	7	5	5	4	3	2	1

Table 1: Numbers of soy, pinto, black and white beans we found from 1 to 10 trails in population A.

	Expected	Observed	O-E	(O-E)^2	(O-E)^2/E
Soy	8	1	-7	49	6.125
Pinto	16	46	30	900	56.25
Black	12	2	-10	100	8.333
White	14	1	-13	169	12.07
Total	50	50			82.78=Chi-square

Table 2: Chi-squared statistic calculation for population A.

Figure 3: The graph above illustrates the number of soy, pinto, black, and white beans in a population over ten generations. Each generation contains a total of fifty beans with varying totals of each individual bean species. From generation one to generation ten the total number of pinto beans made up the largest portion of the population, while the other species of beans continued to decrease in numbers significantly even leading to the extinction of both soy and white beans during generation seven.

Generations
Allele	1	2	3	4	5	6	7	8	9	10
Soy	17	14	12	8	6	1	0	0	0	0
Pinto	9	15	18	25	29	34	39	43	47	47
Black	13	12	14	12	10	11	11	7	3	3
White	11	9	6	5	5	4	0	0	0	0

Table 3: Numbers of soy, pinto, black and white beans we found from 1 to 10 trails in population B.

	Expected	Observed	O-E	(O-E)^2	(O-E)^2/E
Soy	17	0	-17	289	17
Speckled	9	47	38	1444	160.4
Black	13	3	-10	100	7.69
White	11	0	-11	121	11
Total	50	50			196.09=Chi-square

Table 4: Chi-squared statistic calculation for population B.

Conclusion:

We hypothesized at the beginning of the experiment that the bean populations are not in Hardy-Weinberg Equilibrium. Our model was supposed to reflect Hardy-Weinberg’s conditions, which are as follows:

• A large breeding population.
• Random mating.
• No change in allelic frequency due to mutation.
• No immigration or emigration.
• No natural selection

The random allele pairing of the beans was supposed to be reflective of random mating, mutations, and natural selection were not tested in this model (they were kept unchanged), and with an initial population of 50, there was a large “breeding” population. Though by theory our experiment should have followed Hardy-Weinberg Equilibrium, our results showed otherwise. Table 4 details our Chi-Square calculations, which indicated an extremely high critical value, which means our null hypothesis can be rejected, and it becomes evident that our experiment did not follow Hardy-Weinberg conditions.

Our Chi Square values were 82.78 for population A and 196.09  for population B. These values are significantly higher than 7.82 -- the critical value of Chi Square; this shows that the allele frequencies changed radically throughout the 10 generations. Looking at Figure 2, the pinto bean alleles increased from 9 in the population to 47 alleles, at the same time, the other three bean alleles decreased. This visual presentation also agree with our hypothesis that the allele frequencies changed and that the population is not in Hardy-Weinberg Equilibrium. Therefore, our hypothesis was correct, and the allele frequencies did change with each generation, evolving the bean population.

It seems odd that though all the variables were controlled in the bean model, it still did not follow Hardy-Weinberg Equilibrium. A possible factor that affected our results may have ‘bean’ the size of the alleles used. Looking at Figure 1, it is evident that the pinto beans were much larger than the other beans. The setup of our experiment involved randomly picking two beans, and ideally the chance of picking any bean should have been equal. However, the disparity in legumes’ size made it easier to select a larger bean in comparison to a smaller one. In fact, looking at Figure 2 and 3, in both the pinto bean allele steadily increased while the other bean populations sharply decreased for the two independently conducted trials. The selection of alleles by each partner to create the allele pairs for each generation (as detailed in the procedure of the experiment) was supposed to represent the Hardy-Weinberg condition of “random mating,” but because the chance of picking an allele was affected by its size, this principle of the equilibrium was broken and evolutionary processes proceeded as a result.