Background: Fungi not only evolve to avoid the defenses of the plants, but they also attempt to prevent the plant from enacting the mechanisms to improve its defenses. Anything the fungi can do to slow the evolution process of the host plants will improve the fitness of future generations because the plant will not be able to keep up with the evolutionary changes of the parasite. One of the main processes by which plants quickly evolve in response to the environment is sexual reproduction. Without sexual reproduction, advantageous traits could not be spread through the plant population as quickly allowing the fungi to gain a competitive advantage. For this reason, the sexual reproductive organs of plants are targets for attacks by fungi. Studies have shown that many different current types of obligate parasitic plant fungi have developed mechanisms to disable or otherwise affect the sexual reproduction of the plants. If successful, the sexual reproduction process slows for the plant, thus slowing down evolutionary change or in extreme cases, the fungi can render the plant sterile creating an advantage for the pathogens. It is unknown exactly how this adaptive trait developed in fungi, but it is clear that the relationship to the plant forced the development of the process.Some researchers are also studying how a range of factors affect the rate of evolutionary change and the outcomes of change in different environments. For example, as with most evolution, increases in heritability in a population allow for a greater evolutionary response in the presence of selective pressure. For traits specific to the plant-fungi coevolution, researchers have studied how the virulence of the invading pathogen affects the coevolution. Studies involving Mycosphaerella graminicola have consistently showed that virulence of a pathogen does not have a significant impact on the evolutionary track of the host plant.There can be other factors in that can affect the process of coevolution. For example, in small populations, selection is a relatively weaker force on the population due to genetic drift. Genetic drift increases the likelihood of having fixed alleles which decreases the genetic variance in the population. Therefore, if there is only a small population of plants in an area with the ability to reproduce together, genetic drift may counteract the effects of selection putting the plant in a disadvantageous position to fungi which can evolve at a normal rate. The variance in both the host and pathogen population is a major determinant of evolutionary success compared to the other species. The greater the genetic variance, the faster the species can evolve to counteract the other organism’s avoidance or defensive mechanisms.Due to the process of pollination for plants, the effective population size is normally larger than for fungi because pollinators can link isolated populations in a way that the fungus is not able. This means positive traits that evolve in non-adjacent but close areas can be passed to nearby areas. Fungi must individually evolve to evade host defenses in each area. This is obviously a clear competitive advantage for the host plants. Sexual reproduction with a broad, high variance population leads to fast evolutionary change and higher reproductive success of offspring.Environment and climate patterns also play a role in evolutionary outcomes. Studies with oak trees and an obligate fungal parasite at different altitudes clearly show this distinction. For the same species, different altitudinal positions had drastically different rates of evolution and changes in the response to the pathogens due to the organism also in a selective environment due to their surroundings.Coevolution is a process that is related to the red queen hypothesis. Both the host plant and parasitic fungi have to continue to survive to stay in their ecological niche. If one of the two species in the relationship evolves at a significantly faster rate than the other, the slower species will be at a competitive disadvantage and risk the loss of nutrients. Because the two species in the system are so closely linked, they respond to external environment factors together and each species affects the evolutionary outcome of the other. In other words, each species exerts selective pressure on the other. Population size is also a major factor in the outcome because differences in gene flow and genetic drift could cause evolutionary changes that do not match the direction of selection expected by forces due to the other organism. Coevolution is an important phenomenon necessary for understanding the vital relationship between plants and their fungal parasites.

Situation: Ted had two garden plots. The East plot had very healthy plants and good soil. The Western plot was problematic, the plants were attacked by parasitic fungi.

Question: Which plot had more fungi avoiding the defenses of the plants?
Answer: Western

Situation: Ted had two garden plots. The Eastern plot had very healthy plants and good soil. The Western plot was problematic, the plants were attacked by parasitic fungi.

Question: Which plot had fewer fungi avoiding the defenses of the plants?
Answer: Eastern

Situation: Ted had two garden plots. The Eastern plot had very healthy plants and good soil. The Western plot was problematic, the plants were attacked by parasitic fungi.

Question: Which plot had fewer fungi preventing the plants from enacting the mechanisms to improve its defenses?
Answer: Eastern

Situation: Ted had two garden plots. The Eastern plot had very healthy plants and good soil. The Western plot was problematic, the plants were attacked by parasitic fungi.

Question: Which plot had more fungi preventing the plants from enacting the mechanisms to improve its defenses?
Answer: Western

Situation: Ted had two garden plots. The Eastern plot had very healthy plants and good soil. The Western plot was problematic, the plants were attacked by parasitic fungi.

Question: Which plot had more fungi slowing the evolution process of the host plants?
Answer: Western

Situation: Ted had two garden plots. The Eastern plot had very healthy plants and good soil. The Western plot was problematic, the plants were attacked by parasitic fungi.

Question: Which plot had fewer fungi slowing the evolution process of the host plants?
Answer: Eastern

Situation: Ted had two garden plots. The Eastern plot had very healthy plants and good soil. The Western plot was problematic, the plants were attacked by parasitic fungi.

Question: Which plot had fewer fungi disable the sexual reproduction of the plants?
Answer: Eastern

Situation: Ted had two garden plots. The Eastern plot had very healthy plants and good soil. The Western plot was problematic, the plants were attacked by parasitic fungi.

Question: Which plot had more fungi disable the sexual reproduction of the plants?
Answer: Western

Situation: Ted had two garden plots. The Eastern plot had very healthy plants and good soil. The Western plot was problematic, the plants were attacked by parasitic fungi.

Question: Which plot had more fungi affect the sexual reproduction of the plants?
Answer: Western

Situation: Ted had two garden plots. The Eastern plot had very healthy plants and good soil. The Western plot was problematic, the plants were attacked by parasitic fungi.

Question: Which plot had fewer fungi affect the sexual reproduction of the plants?
Answer: Eastern