Need help on my BIO 110L Lab

Question

1.     Evidence from the fossil record shows that evolution has resulted in an increase in the number of apertures (areas where the wall is thinner or softer) in pollen grains. Researchers hypothesized that having a higher number of apertures may allow the pollen to germinate faster.

To test this hypothesis, the researchers collected pollen from a population of Viola diversifolia from the alpine region of the Pyrenees Mountains. In this population, approximately 1/3 of the pollen grains have three apertures and 2/3 have four apertures. The pollen grains were germinated on an artificial medium and the percent of pollen that had germinated was counted after one hour and two hours. Six replicates of 100 pollen grains per treatment were completed. The results are presented in the table below. Dajoz I, Till-Bottraud I, Gouyon PH. 1991. Evolution of pollen morphology. Science 253: 66-68. [data simplified for educational activity]

Table 1.

	Percent Germinated			Percent Germinated
3 apertures	1 hour	2 hours		4 apertures	1 hour	2 hours
Rep 1	4	12		Rep 1	17	21
Rep 2	4	9		Rep 2	11	11
Rep 3	0	4		Rep 3	17	39
Rep 4	0	0		Rep 4	27	33
Rep 5	0	0		Rep 5	9	17
Rep 6	3	3		Rep 6	8	14
Average	1.83	4.67		Average	14.83	22.5

a.     Calculate the average for each column. Create a line graph of the averages (vs. time) for each treatment on the same graph in excel (be sure to include 0,0 as the starting point for each). Paste your graph below. [2 pts]

b.     Do the data support or reject the hypothesis? Explain your evidence. [2 pts]

c.      Once pollen lands on the stigma of a flower, the pollen germinates, creating a pollen tube that grows down the ovule. The sperm cell moves through the tube to fertilize the egg. Based on the evidence from the experiment above, explain how natural selection might have led to directional selection toward higher aperture number. [3 pts]

Answer 1

b. The data support the hypothesis that a higher number of apertures may allow the pollen to germinate faster. Over the span of two hours pollen with 4 apertures achieved an average germination of 22.5%. This is more than four times that of the pollen with 3 apertures, which reached an average germination of 4.67%.

c. According to the data from the experiment above, a higher number of apertures allows individuals to germinate faster than those with a lower number. This would mean that those individuals could out compete others, successfully and quickly germinating and therefore producing more offspring. This directional selection would change the composition of each subsequent generation to include more individuals with an inherited higher aperture number.

Answer 2

Hi Juanita R.,

I can help you with some of the above. If you don't at all know how to use Excel and cut and paste (not that it can be done on Wyzant here, but they're working on it!), you should get someone to walk you through that, it isn't hard.

For the graph, you need a data table: time = 0, 1, 4 hours and the respective averages (0, 1.83, 4.67) and (0, 14.83, 22.5) . That's 3 columns, 3 rows, with time as column A (or whatever else is convenient). The graph will be a Chart, x-y scatterplot, data in columns; time row will be the "x" and the other 2 columns (of data) will be y1 and y2 respectively. Choose distinguishing markers for each data set, and choose whatever option for connecting the dots is. Don't forget your axis labels, and a title!

(b) The data certainly support your hypothesis; technically you should have all the trials as data so that your averages have sample deviations associated with them. Excel also has statistical functions for that (AVE, STDEV) if I recall --. Now you need slightly higher powered math: a t-test for significance of the difference of 2 means (look it up, it isn't so bad. You may encounter some new math terms, look them up also). You assess the 1-hour data and the 4-hour data separately, then combine your conclusions verbally. It really looks like these data sets have a significant difference, just eyeballing. What's more amazing, is the size of the effect of that 4th aperture!

(c) Assuming that the 4th aperture development is genetically caused, by natural variation, such faster-germinating pollen grains will grow and fertilize eggs with higher speed, all other things being equal. Since an egg may be only fertilized once (for genetic purposes, I'm aware of polar bodies,etc.) the offspring mix will be weighted towards possessing a 4th aperture, compared with their parental mix.

Now, the crucial question for follow-up -- given such an overweight towards a 4th aperture in potential pollenizing, why is the natural population at the proportions of apertures that it is? A hint towards one possibility: what if the conditions for germination of the pollen grain aren't optimal? (You may be aware that for humans, at any rate, "Y" sperm swim faster to any currently viable egg, but "X" sperm have longer longevity in the body, and can therefore fertilize an egg after all the hot-shot "Y"'s have become immotile.)

-- Cheers, -- Mr. d.