“Evolutionary Ecology of Human Papillomavirus: Trade-Offs, Coexistence, and Origins of High-Risk and Low-Risk Types” Questions

Questions – Cervical Cancer

These questions address the Journal of Infectious Diseases article entitled “Evolutionary Ecology of Human Papillomavirus: Trade-Offs, Coexistence, and Origins of High-Risk and Low-Risk Types” by Orlando et al. (2011).  Be sure to explain your answers.

1.    On page 1, the authors describe two patterns of selection on the quantitative trait virulence.  Name these two patterns, described below:

a.     “Natural selection often favors intermediate phenotypes”

In the above statement, the authors are describing the pattern of selection known as stabilizing selection in which individuals with intermediate values for the trait virulence have the highest fitness. These individuals are favored by selection.

b.    “…some ecological circumstances may promote extremes of persistence or virulence”

In the above statement, the authors are describing disruptive selection in which individuals with the extreme values of virulence, low or high, have the highest fitness. In this type of selection, the extremes are favored while the intermediate phenotypes are selected against.

2.    Apply Darwin’s postulates to HPV populations in human hosts (see page 2 for guidance).

Postulate 1: There is variation within a population. There are 9 different states, allowing for variation, when dealing with this model of HPV. There are 3 celibate states of people, being susceptible (S), infected (I), and resistant (R). There are 6 other states relating those 3 states; SS, SI, SR, II, IR, RR. This relates to people in relationships. There is also variation in different HPV strands. There are high risk (HR) and low risk (LR) types. HR types produce less virions and have a longer duration of infection, immune response is slower. LR types produce more virions and have a shorter duration of infection, so immune response is quicker. HR types strive in long monogamous relationships while LR types strive in many short relationships.

Postulate 2: Variation is passed from parents to offspring. HPV replicates its DNA within human host cells. LR types create more LR types and HR types create more HR types. 

Postulate 3: Some individuals have greater fitness and results in over reproduction. There are many copies of each type of virus in the human body.

Postulate 4: Selection acts on the population. If the human host is in a long monogamous relationship, HR types will thrive and continue to replicate because there will be more sexual encounters allowing for a longer duration of infection. The LR types will die in that particular human. If the human host is in many short relationships, LR types will thrive and continue to replicate because there are less sexual encounters and the virus needs to be transmitted quickly with each encounter and be able to have a shorter duration of infection with more virions. The HR types will die in that particular human. There is also selection acting in resistant humans. They can no longer contract that particular type of virus, so the virus will not be passed on even if they are in a relationship with an infected person. That particular type of virus will eventually die off either if it has nowhere to move on.

3.    What is an adaptive landscape (sometimes called a Wrightian landscape)?  Please include a 3D figure (with citation).

An adaptive landscape represents the mean fitness of a population. It looks like a mountain and involves multiple dimensions in space. As populations evolve adaptively, they move up the mountain towards the peaks. Multiple gene frequencies are represented on the graph. The high peaks represent high fitness and the low peaks represent low fitness, as illustrated in the figure below. Many loci are represented on an adaptive landscape and contribute to the fitness of a population.

 

Figure represents 2 loci: http://evolution.berkeley.edu/evosite/evo101/images/adaptivelandscape.gif

Source: http://evolution.berkeley.edu/evosite/evo101/VIIC1aComplexNovelties2.shtml

4. The authors define Evolutionarily Stable Strategies on page 4. Can you please explain ESSs in understandable terms?

            Basically, ESSs depend on the simple principle of consumer and resources. Since the amount of susceptible individuals depends on the types of viruses present, different strains have different viral fitness, or the “per-capita growth of infected individuals”. A specific virus has to fight against the other phenotypes to have the highest fitness. An ESS is a form of convergent evolution in which the virus’s “strategy” converges upon several different factors to become most advantageous. Once the ESS is fixed in a population, nothing can disrupt it. Since natural selection acts so strongly on the ESS, no other mutations can be large enough to tilt the strategy of the population to have the best fitness.

            A thought experiment given by Cornell University really helps to illustrate this concept. Suppose that one group of people, or population, is using the same strategy in a game, such as poker. The benefit is that everyone is using the same strategy, so all are receiving the same benefits. Now let’s say that one smaller group within the population begins using a different strategy. The minority would win out over the majority if they receive more adaptive benefits. So now, the minority strategy would beat out the majority strategy, which is no longer evolutionarily stable. If the minority has less fitness/survival than the majority, then the majority group has an ESS that is fixed in the population.

Adapted from http://ess.nbb.cornell.edu/ess.html

5. Given the paper’s conclusion (see page 7), what would you predict about the efficacy of HPV vaccines? Why should an OB/GYN know about evolution?

            Since HPV responds to natural selection and other evolutionary forces, getting rid of one strain of the virus would act as an open niche for other strains to fill. This would create even more of a problem due to a larger amount of mutated viruses. HPV strains compete and will evolve at a fairly high rate, so this mutation of virus will occur relatively fast. According to the authors, the vaccine would act as a “strong selective force”. This means that the virus wouldn’t necessarily completely wipe out a selected strain, but it would act as a mechanism of evolution against it. Then, the potentially dangerous viral interactions would be diminished.

            OB/GYNs should definitely have some sort of background in evolution because of the way that the HPV virus can mutate and change with evolutionary forces. HPV is a huge worldwide problem, and OB/GYNs see and treat it almost daily. They need to be able to recognize the strains in order to complete the right testing and treatment if necessary. Also, evolution plays a role in the formation of cervical and breast cancers. People’s individual genetics interact with their environments, and doctors need to be able to understand the risk factors to educate their patients about.