Co-evolution is defined as a change of two or more species in close association with one another. Co-evolution is most common with mutualistic relationships, in which both species gain benefit. Such evolution can result in symbiotic relationships, in which the organisms depend on one another for survival. For example, bees make honey from flowers and flowers are pollinated by bees. Neither species can survive without the other.

Parasitism, commensalism, and predator-prey relationships can also demonstrate co-evolution, as one species develops defenses and the other develops offensive traits.

The endosymbotic theory describes the evolution of eukaryotic cells from prokaryotes. Small independent organisms were engulfed by larger ones; both were prokaryotic and unicellular. This engulfment benefited both organisms, and the genes that promoted this engulfment were selected for. Over a long time, eukaryotic cells evolved with many membrane-bound organelles, each with their own specific structure.

Evolution must occur over many generations and a long time scale. Changes within the life of an individual do not constitute evolution. Thus, a tadpole becoming a frog is not evolution.

After the Industrial Revolution's smog polluted the trees and resulted in them darkening in color, it was much easier for the black moths to survive and reproduce, because the darkened trees provided them with better camouflage than the light-colored trees had. The white-colored moths, on the other hand, were now exposed to predation at a greater rate, because they were not camouflaged. Due to this change in the environment, the moth population became predominantly black-colored as it adapted. This is an example of natural selection in action.

The mechanism of evolution was described by Darwin as the "survival of the fittest." We need an answer that addresses the fact that the individuals best suited to survival pass on their genes more often than individuals that aren't as well-suited, and that over long periods of time, these changes an accumulate until new species arise. This means that the correct answer is "There are always differences in the genes of individuals of a species. Some differences give certain individuals a better chance of survival. In each generation, those individuals best-suited to survive the current conditions will have the highest success in passing on their genes. Over many generations, the accumulation of these changes leads to the creation of new species from existing species."

A selective pressure is a factor that promotes evolution by selecting for a trait that will allow better survival for the organism that is at risk for compromised survival. In the case of anoles, the presence of the brown anole has promoted an evolutionary change for the green anole. There is no evolutionary change that can be observed in the other instances. 

Evolution occurs over a given time and may be punctuated or stagnant.  As individuals are produced with different characteristics more fit for their environments, the population gradually changes.  Evolution may even cause speciation or the merging  of two species.  Evolution is the response of a changing environment and different species interactions- individuals do not evolve; rather, natural selection works in favor of those individuals who are more fit for the current environment.  Those more fit individuals will go on to produce more offspring with their same qualities, leading to a gradual change, or evolution, of a species, population, and community. 

Coevolution describes how two species that interact closely with each other for their survival can evolve in response to one another. Another example is lichens, which are fungi and bacterial cells that grow together to act as a single living organism. 

Sexual dimorphism is a phenotypic differentiation between males and females of the same species. These altered phenotypes occur in organisms that reproduce through sexual reproduction. Commonly referenced possible examples are body size, physical strength and morphology, ornamentation, behavior and other bodily traits. An example would be comparing bright green male peacocks to the brown and grey female peacocks. Genetic drift is a different, unrelated evolutionary concept. Evolutionary monomorphism and the sperm-egg model do not refer to real concepts.

The disease is recessive and found on the X-chromosome. Remember that males only have one X-chromosome, while females have two. As a result, males only require one mutant allele to express a recessive phenotype, while females need to have two mutant alleles in order to have hemophilia.

In this scenario, both parents are healthy, but the female is a carrier for the disease. This means that one of her alleles codes for hemophilia.

Parents: XX^h x XY

Since daughters will need two copies of the mutant allele, they will all be healthy; by default they will receive the healthy X-chromosome from the father.

Daughters: XX or XX^h; all healthy phenotype

Sons, however, have a 50% chance of receiving the mutant allele from the mother, because the father will always contribute the Y-chromosome to sons.

Sons: XY or X^hY; half healthy, half hemophilic

As a result, 50% of the sons will have hemophilia.