Evolution - MCAT Biology

It is always difficult to rephrase "survival of the fittest" in some new, clever way. The flowers which BY CHANCE have developed a different color, pattern, or odor that better attracts pollinators are indeed more likely to experience reproductive success and pass on these genes to their offspring. Competing plants might do well for a while, but they are already disfavored, and further environmental changes may put them even more at risk (or have no effect, or again favor them over the presently more attractive plants).

The horse choice is an example of intentional breeding—artificial selection.

The storm option does not imply any condition in any of the plants which conferred an advantage against freezing to death, or even any difference between species of plants; it is more akin to a question about mass extinction than to one about evolution.

The giraffe choice relates to the Lamarckian fallacy of being able to pass on acquired characteristics; species that are more successful just plain "luck out" relative to environmental stresses.

The bacterial response discusses a mutation without likely survival implications for the bacterium.

Vertebrates have adapted to terrestial living due to their ability to maintain water inside their bodies, despite no longer being immersed in water. The loop of Henle in the nephrons is designed to concentrate urine, reabsorbing water without unnecessarily excreting it. The longer the loops descend into the medulla, the more concentrated the urine becomes. Shorter loops would not concentrate urine as much, and thus would not contribute to a vertebrate's adaptation to land-based life.

Internal lungs, impermeable skin, and internal fertilization would all protect vital processes from interference by the external environment.

In this scenario, the two extreme phenotypes are selected for, while intermediate phenotypes are selected against. This is disruptive natural selection. Over time, disruptive selection results in a decreased frequency of "middle" phenotypes and an increased frequency of the two groups at the extreme ends. This is a process that can eventually lead to speciation.

The opposite is process stabilizing selection, in which the extreme variations are selected against in favor of more "average" phenotypes. Directional selection occurs when only one end of the spectrum is favored, such as sharp teeth but not dull teeth. Artificial selection involves human intervention in selecting desirable traits. Vestigial selection is not a type of natural selection.

Large size is favored in males and small size is favored in females, but intermediate size is always disfavored. The result is an increase in the two extreme phenotypes (either large or small) and a decrease in the average phenotype. This type of trend is known as disruptive selection.

Stabilizing selection occurs when the extreme phenotypes are disfavored, and the average or intermediate phenotype is preferable. Directional selection occurs when only one extreme phenotype is advantageous, for example if only large felines were able to find mates. Genetic drift is the phenomenon by which the allele frequencies of a population change by chance, due to independent assortment or other random events. The bottleneck effect occurs when an outside event, such as disease or natural disaster, diminishes the original population such that the allele frequencies of the new population differ from those of the original.

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.

Inbreeding increases the expression of recessive traits due to more heterozygous carriers mating with each other. As the same individuals mate, the chance of a homozygous recessive child increases. This is the same as estimating the likelihood of a single healthy child from two carrier parents (0.75) versus eight healthy children from two carrier parents (0.10).

Inbreeding decreases genetic diversity, rather than increasing it. The rate of spontaneous mutation is not impacted by this type of breeding. There is no reason to infer increased levels of aggression.

For the Hardy-Weinberg equation to be true, the population in question must be very large. This ensures that coincidental occurrences do not drastically alter allelic frequencies.

The two equations pertaining to Hardy-Weinberg equilibrium are:

In this second equation, each term refers to the frequency of a given genotype. is the homozygous dominant frequency, is the heterozygous frequency, and is the homozygous recessive frequency.

From the question, we know that:

We now know the dominant allele frequency. Using the other Hardy-Weinberg equation, we can find the recessive allele frequency:

Returning to our genotype frequency terms, we can use this recessive allele frequency to find the homozygous recessive frequency:

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.

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.

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 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.

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).

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.

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.

Allopatric speciation occurs when a physical barrier, or in some cases emigration of subpopulations of a species, prevents interspecies mingling that eventually leads to the inability to interbreed. Sympatric speciation describes a process by which new species form while in the same geographic location. Parapatric and peripatric speciation are subcategories of sympatric speciation.

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

The situation described is a classic example of the evolutionary principle of "bottlenecking," making this the best answer choice. Bottlenecking describes the phenomenon in which the genetic diversity of a population changes suddenly, often due to a natural disaster, which then results in future generations appearing more genetically and phenotypically similar to one another than in the pre-disaster generations. In this instance, pink flmaginos were initially more common than white flamingos, but both were fairly prevalent. They then encounter a hurricane that kills all but a few flamingos, which were almost entirely white-feathered, which led to all ensuing generations being predominantly white-feathered.

This does not represent "selective mating," "spontaneous mutation," or "genetic engineering," as white-feathered flamingos became most prevalent due to a natural disaster, not white flamingos selectively seeking to mate with other white flamingos, not due to a single mutation, and not due to scientists artificially manipulating the genes within the flamingos.

This does not represent "genetic drift," because the changes in phentype prevalence were not due to random chance fluctuations, they were due to an explainable event, a natural disaster that resulted in an initially low number of white flamingos remaining and becoming most predominant.