Evolution Update

Evolution Update

Evolving During Disaster: How Diversification Affects Species Survival

Maria Iannucci March 26, 2015

Using sophisticated simulations, scientists assess the benefit of an adaptive technique known as diversification.

Every organism experiences risk during its lifetime that can threaten its survival and ability to pass on its genes to the next generation. This risk can come from abnormal weather events such as flooding, droughts, or increased temperatures, or from a lack of food due to an increase in competition. Sometimes the risk occurs because of human actions such as habitat destruction or chemical spills. For the microorganism world, that risk could be exposure to another organism’s immune system or antibiotics. All of these environmental changes threaten the continuation of a species.

The term “fitness” in a biological context refers to an organism’s ability to survive and reproduce. Reproduction passes on genetic information to the next generation. The higher an organism’s fitness is the more likely it is for that individual to pass on its genes.

Because risk is a constant part of our world, organisms have adapted “bet-hedging” strategies that rely on phenotypic variation to increase their fitness in the face of disaster. When an organism reproduces, most of the offspring will share a phenotype, a set of physical traits, with their parent. However, a small number of the offspring with have a different phenotype. The process of producing a population with different phenotypes is known as “diversification.”

These strategies can help the species survive, even if a disaster were to hit their population. For example, some plants will produce two different seed types: one that will sprout soon after production and one that will stay dormant until a large rainfall event. This plant is using this strategy because it ensures that no matter what the current conditions are some offspring will be produced in the next generation. If the seeds are produced during a drought, the seeds that sprout right away are not likely to survive, however the dormant seeds will only sprout if there is enough rainfall to guarantee their survival and reproduction.

Researchers from Georgia Tech have used a computer simulation to investigate how different types of risks can influence the variation in phenotypes and potentially increase species fitness in the face of risk. By understanding how species use various “bet-hedging” strategies, we will be better able to recognize how evolutionary pressure is working in our world today.

The researchers set up a number of simulations in which populations containing both Phenotype A and Phenotype B were exposed to a disaster. In a selected number of groups, all members of one phenotype would be wiped out by the disaster, then the population would be given time to grow and recover. If Phenotype A was killed, the groups that experienced disaster would the population would be made up almost completely of individuals with Phenotype B after recovery. The only Phenotype A individuals present would have been born after the disaster.

This simulation was run numerous times, each time manipulating different variables, such as what percentage of the population would be impacted by the disaster, how frequently disasters occurred, if migration occurred, and which phenotype was killed. After analyzing their results, the researchers found that both fast and slow diversification can be effective bet-hedging strategies depending on what type of risk a population faces.

Fast diversification is a good strategy when the risk is not linked with time, so the same risk is unlikely to occur again in the near future. Fast diversification is also best when the risk impacts a large area, like in the case of drought. By quickly developing diverse populations, the organisms are more likely to survive the disaster and reproduce. In these cases, diversity helps to increase the fitness of the groups.

Slow diversification is a good strategy when the risk is linked to time, so the same risk is likely to occur in the near future. For example, if a bacterial community in your body is exposed to an antibiotic that kills Phenotype A and leaves Phenotype B behind, it is likely to experience that same disaster again with the next dose of antibiotic. For the bacteria it is better to diversify slowly so that when the next dose of antibiotic arrives most of your population was Phenotype B, which will survive.

While many factors can impact the type of diversification that is most beneficial to a population, this knowledge can help us to understand how both micro- and macro- organisms are adapting to their ever-changing environments.

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