A wolf kills a deer that is infected with of Echinococcus granulosis in its liver and eats it. After consuming the deer, the wolf's intestine becomes filled with a number of produced from scoleces. These eventually release eggs into the wolf's intestines, which are released in its feces.

Explanation: Echinococcus granulosis biological cycle.

The adult form of the parasite Echinococcus granulosis inhabits the intestines of canids, such as the dog or in the wolves, as in the exposed example. They might be 2 to 7 millimeters long, and in their anterior extreme, there is a segment known as scolex or head. The invertebrate attaches to the intestine wall by using its scolex. In the posterior extreme, there are the fertile segments of the animal, formed by three or four proglottids. The proglottides, carry the eggs and release them in the canid´s intestine. These eggs can get out of the animal´s body when it defecates. The parasite´s eggs are now in the feces of the canid.

Once on the ground or in the water, these eggs might be ingested by an intermediate host. In general, these hosts are animals such as sheep, goats, horses, pigs, and even humans. In the exposed example, the intermediate host is the deer.

The eclosion of the parasite´s eggs occurs in the thin intestine of their host, during digestion. The invertebrate penetrates the intestinal wall, from where it migrates to other organs, especially the liver and the lungs. At this point, the organism known as the oncosphere, turn into hydatid cysts. In its interior, it starts the protoscolex formation, making the size of the cyst to get bigger. When the protoscolex is completely formed, the fertile cyst is now infectious to the final host.

The final host (wolves) becomes infected when it feeds on the intermediate host. After ingestion, protoscolex gets released in the interior of the duodene. Their free state is known as scolex. At a certain point, scolex gets attached to the intestine wall where they stay until they become adults. And the cycle starts again.    

•there must be variation for the particular trait within a population.

•the variation must be inheritable (that is, it must be capable of being passed on from the parents to their offspring).

•individuals with one version of the trait must produce more offspring than those with a different version of the trait.good

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