Evolutionary Factors - GRE Subject Test: Biology

Genetic drift occurs when a gene's frequency is changed in a population due to pure chance. Consider a rock rolling down a hill and crushing all flowers that have white petals. The population will now have only red petals because the white ones were destroyed. Were the red petal flowers more suited to their environment? No, it just so happened that all of the white were removed from the gene pool. This shows that evolution can occur due to random chance as well as natural selection, and both forces can have an impact on a population.

Using the Hardy-Weinberg equilibrium equations, you can determine the answer.

The value of gives us the frequency of the dominant allele, while the value of gives us the frequency of the recessive allele. The second equation corresponds to genotypes. is the homozygous dominant frequency, is the heterozygous frequency, and is the homozygous recessive frequency.

16% of the population shows the recessive phenotype, and therefore must carry the homozygous recessive genotype. We can use this information to solve for the recessive allele frequency.

We can use the value of and the first Hardy-Weinberg equation to solve for .

Knowing both and , you can use the second equation to find the percent of heterozygous organisms in the population.

The variables and are specifically referring to the allele frequencies of the dominant and the recessive allele in a population, respectively.

Expected genotype frequencies can be seen in the equation:

In this equation, represents the expected genotype frequency of homozygous dominant organisms, represents the expected frequency of heterozygous organisms, and represents the frequency of homozygous recessive organisms. These values are the expected frequencies in the population, based on the Hardy-Weinberg conditions and allele frequencies; they may not be the values actually observed. To get observed genotype and phenotype frequencies, more information about the size and makeup of the population would be needed.

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:

Hardy Weinberg equilibrium has requirements that must be met by a population in order to confirm that evolution is not taking place:

1. The population must be large in number.

2. There can be no new mutations entering the population.

3. Immigration and emigration cannot change the allelic frequencies of the population.

4. Mating must be random.

5. Natural selection cannot be taking place.

Since it was said that females are selectively choosing which males they mate with, Hardy Weinberg equilibrium is being violated.

Since we know that the population is in Hardy-Weinberg equilibrium and that there are only two alleles, we can use the Hardy-Weinberg equation to solve this problem:

Lets say that represents the black allele, and represents the red allele. Since we know that red is recessive to black, only animals with two red alleles will be red. Fortunately, the portion of the equation is the only portion that deals with red animals (the other two variables are black: both homozygous dominant as well as heterozygous). This means that is equal to the frequency of red animals in the population:

Since we now know the frequency of the red allele in the population, we simply subtract it from one in order to find the frequency of the black allele, which turns out to be 0.6.

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.