Module 8
1. Module 8
1.23. Page 4
Module 8—Populations, Individuals, and Gene Pools
Lesson Summary
This lesson focused on the following questions:
- What relationships exist between species and ecosystems?
- What effects do these interactions have on population changes?
Relationships between organisms occur when an organism’s struggle for survival is affected by another organism. In some cases, alliances form that benefit both species; in some, the success of one species is, by definition, the death of the other. The relationships of interest in this lesson are the three kinds of symbiosis: parasitism, commensalism, and mutualism.
A symbiotic relationship ties two specific species together for life, usually in a feeding relationship. In mutualism, the relationship is positive for both species. In commensalism, one species benefits and the other species is unaffected. In parasitism, one member of the pair benefits while the other is harmed.
Another important relationship is that of predator and prey. Predator and prey population cycles keep each other at the carrying capacity of the environment: high prey numbers produce more predators that eat more prey, reducing prey densities back to normal. Predators improve the prey gene pool by hunting down the sick, the weak, and the poorly adapted, sieving out the poor alleles and leaving a stronger, more genetically fit prey population. Most human hunters weaken prey gene pools by killing the superior members of the population, eliminating the best alleles from the population. In predator-prey population cycles, there is always more prey than predators, and the numbers of predators follow the trend of the prey species.
Lesson Glossary
Consult the glossary in the textbook for other definitions that you may need to complete your work.
commensalism: a type of symbiosis in which one species benefits and the other is not affected one way or another
host: the organism in a type of symbiotic relationship that provides food or a means to complete reproduction for the parasitic organism of another type of species
mutualism: a type of symbiosis involving two organisms of different species in which both benefit or depend on the relationship to survive
parasite: the organism in a symbiotic relationship that benefits by living on or in another organism (host) as a source of food or means of reproduction
The host is harmed in this relationship.
predator-prey: a relationship in which one organism (predator) hunts and kills another organism (prey) for food
symbiotic relationship: any close relationship in which individuals of different species live together in a feeding or protective relationship