A biologist studied a population of squirrels for fifteen years. During that time, the population was never fewer than thirty squirrels and never more than forty-five. Her data showed that over half of the squirrels born did not survive to reproduce, because of both competition for food and predation. In a single generation, 90% of the squirrels that were born lived to reproduce, and the population increased to eighty. Which inference(s) about this most recent surge in the population size might be true?

In the given case, the upsurge in the size of the squirrel population would have taken place due to genetic variation and more availability of food. Genetic variation plays the most important role in the process of natural selection and is the basic mechanism of evolutionary change. Genetic variation refers to the variation in DNA between the individuals.  

Thus, it can be said that in the given case, the amount of food, which was less in the past may have got increased, and the squirrels of the coming generations would have demonstrated higher levels of genetic variation than shown by the previous generations. Thus, the number of squirrels got increased in the population.