Question

7. A violent storm sweeps over northern Belize flooding the Rio Bravo River. As the flood waters subside the bank of the river is undercut and a "raft" of vegetation drifts away and is carried to a deserted island 100 miles off the coast. The raft carries 1122 seeds of the Bombux tree, a plant with white or blue (recessive) flowers. Eighteen percent of the seeds that germinate grow to maturity. Of those, 12% have blue flowers. a. Assuming that the frequency of the blue-flowered allele does not change as the population grows, what will be the instance of blue-flowered Bombux on the island? b. Blue-flowered Bombux on the island is how many times greater than the original population on the mainland? The frequency of the blue phenotype on the mainland is 11.2%. Is this an instance of evolution? How could one demonstrate so?

Answer 1

a) The instance in terms of frequency are actually just 12% of tthe population, but in terms of current number of individuals it has to be calculated using the first number of seeds that came into the island, then calculate the germinating seeds from those and calculate the blue flowered plants from those:

(1122)(0.18) = 201.96, or 202 seeds germinated, now

(202)(0.12) = 24.24, or 24 plants are blue. In that point 24 plants are blue, they will increase with population growth, but its stated the allele frequencies will stay the same.

b) We have a blue phenotype frequency of 0.12 in the island, and 0.112 in mainland:

0.12/0.112 = 1.07, that means the island is 1.07 more times greater

Is it an instance of evolution? Well yes, it occured because of a bottleneck, since the whole island population has an origin in only 1122 seeds in a recent event, allele frequencies in the island depended on the frequencies from the relocated seeds, they don't respond to the frequencies in mainland, only to the ones selected by the bottleneck.

How could one demonstrate so? Well, bottlenecks are special evolutionary events related to genetic drift, because genetic drift effect depends on the population size. Bottlenecks greatly decrease population size, leading to a strong effect by genetic drift. We could demonstrate it by periodically analysing the population and comparing it with Hardy Weinberg equilibrium calculation. Remember such equilibirum works under no effect from any evolutionary force, so if there is a strong effect from genetic drift then we will be able to observe a deviation from HW equilibrium.

Small populations tend to get close or completely fix an allele, that's why in the island we can see a higher frequency for the blue instance, over time it might get fixed for the whole population, but we can only observe that across many generations.