An organism's evolutionary "fitness" depends on its ability to reproduce and create viable offspring, or contribute its genes to future generations.

Even if an organism is in perfect health, it is considered to have very low fitness if it cannot produce viable offspring. In an evolutionary sense, the perseverence of certain genes in a population defines the favorability of those genes. An increased prevalence of certain genes can be interpreted as evolution. The activities of a single individual (aside from reproductive viability) are relatively ineffective in determining its ability to pass on its genes to future generations.

Mitochondrial DNA (mtDNA) is only inherited directly from a mother to her offspring and can be used to directly track lineage of a population or species. Nuclear DNA (nDNA) is inherited from both the father and mother of the offspring; it can be used to track lineage as well, but mtDNA similarity is enough to conclude a close relationship between the two populations described in the question.

Color, diet, and location are all distinguishing features of the populations and help characterize their niche in the ecosystem. Diet and location (territory) are not heritable traits, and do not signify ancestry. Color is genetic, but could result from convergent or divergent evolution. mtDNA similarity is the strongest available evidence for a close ancestral link between populations A and B.

The higher the taxonomic group, the less similar the members are. This is true for appearance, behavior, and genetics. The order of taxonomic groupings, from most general to most specific is: kingdom, phylum, class, order, family, genus, species.

Of the given answers, phyla are the highest taxonomic rank. Species of different phyla would show the greatest genetic difference. In contrast, genera are the lowest taxonomic rank of the given answers; species of the same genus would show the least genetic difference.

Chordates must have, at some point in their embryogenesis, all features except vertebrae. Although the vast majority of chordates are vertebrates, Amphioxus has only a notochord. This primitive organism never develops vertebrae. Man has gill arches, and remnants of these are seen in certain congenital malformations. The notochordal remnants can give rise to the tumor known as a chordoma. The brain has ventricles (it is tubular) and it is certainly dorsal. Finally, human tail persists in about one per million live births.  

The phylum chordata has a few key characteristics. They are deuterostomes, meaning that the anus arises from the blastopore. They have a coelom that arises from the mesoderm during development, and at some point they have a tail, pharyngeal slits, and a notochord.

Vertebrata is a subphylum of chordata, so not all chordates are vertebrates (though all vertebrates are chordates).

All chordates are deuterostomes, not protostomes. During early development, the formation of the blastopore leads to the growth of the digestive tract. In protostomes, the blastopore develops into the mouth, while in deuterostomes it becomes the anus.

All chordates have a notochord during development, which eventually gives rise to the nervous system. Chordates also have a coelom, or fluid filled cavity, somewhere within the body and they all exhibit bilateral symmetry.

The only absolutely correct answer is the one that states that an animal can successfully mate with another animal and produce viable offspring that are capable of successfully reproducing. The key to defining a species is that the offspring are both viable and fertile. The correct answer encompasses both of those tenets.

"One animal mates with another animal and produces viable offspring that are not capable of reproducing successfully." This choice is incorrect because it states that the offspring are not fertile.

"One animal lives in a closed environment with 250 to 500 other animals that look similar to one another and support each others' basic needs (food, shelter, protection)." This example may describe a species, but there is not enough information to definitively conclude that. It does not explicitly state that the animals successfully mate with one another, or that their offspring (if they do mate with each other) are fertile. 

"One animal lives in a closed environment with greater than 500 other animals that look similar to one another and support each others' basic needs (food, shelter, protection)." This example may describe a species, but there is not enough information to definitively conclude that. It does not explicitly state that the animals successfully mate with one another, or that their offspring (if they do mate with each other) are fertile.

The Urey-Miller experiment was used to determine if the early atmospheric conditions were favorable for the creation of organic materials. Their experiments determined that basic organic molecules, such as urea and amino acids, were able to form in early atmospheric conditions.

The Mehselson-Stahl experiment revealed the semi-conservative nature of DNA replication.

A physiological condition making two speciated animals unable to mate is a mechanical difference.

Directional selection is when a population undergoes a change biased in a certain direction away from the original average of the population. Since the fish are getting bigger in once sense and smaller in another, this is directional selection.