Evolution and Genetics - AP Biology

Evolution impacts the allele frequencies within a population through mutation, genetic drift, gene flow, and natural selection. These mechanisms of evolution result in adaptation, cooperation, speciation, extinction, and co-evolution.

Positive selection is a type of natural selection in which the frequency of an allele increases.

Negative selection is a type of natural selection in which the frequency of an allele decreases.

Sexual selection is selection for traits that confer greater reproductive success. This includes selection for traits that better attract mates.

Nodes specifically indicate the most recent common ancestor between the two branches. For example, the ancestor found at the node joining branches from humans to branches from dogs would represent the most recent link between the two branches.

Convergent evolution describes the independent evolution of traits at different points on a phylogeny, and is not related to nodes. Kingdoms and domains are very broad categories. They can be represented by nodes, but only at a very global level.

Genus and species are the two taxonomic classifications that are used to represent the scientific name of an organism. The hierarchy of the binomial classification system is (from the top tier to the bottom): kingdom, phylum, class, order, family, genus, species. Kingdom gives the broadest classification of an organism, while species gives the most specific. The most specific classifications are used to name the organism.

For example, the scientific name for a house cat is felis catus. This indicates that houses cats, like some other species, are in the genus felis and have the unique species tag catus.

The full classification for a house cat would be: anamalia (kingdom), chordata (phylum), mammalia (class), carnivora (order), felidae (family), felis (genus), catus (species).

In the system of biological classification, organisms are classified in a hierarchy, or taxonomy. The highest levels of classification are the most inclusive, while the lower levels become more and more specific until a single species is identified. From most inclusive to least inclusive, organisms are assigned a kingdom, a phylum, a class, an order, a family, a genus, and finally a species.

Organisms sharing the same classification at less inclusive levels will be more closely related. For example, two organisms sharing the same genus will be more closely related than those who only share the same family. Of the given answer choices, family is the most specific level of classification.

A clade is defined by a organisms that possess a shared derived trait. In other words, we need to find a set of organisms in the table that includes all organisms with a certain listed trait, while excluding any that lack that specific trait. The lion, fish, and sea urchin are the only clade listed. They are all triploblastic, segmented deuterostomes, and none of the other species share those characteristics. The trait of being a deuterostome is the shared derived trait that defines this clade.

Sea urchin, jellyfish: The jellyfish is the outgroup of the tree presented in the table, as it does not have any of the listed traits. Its last common ancestor with the sea urchin is also its last common ancestor with all of the other species, so a clade with the jellyfish must include all 5 species in the table.

Fish, sea urchin, scorpion: All share segmentation and three tissue layers, which the jellyfish does not have. However, the lion also has segmentation and three tissue layers, so it must be included to form a clade.

Fish, sea urchin: Both are deuterostomes with segmentation and three tissue layers, but the lion is also a deuterostome, so it must be part of the clade.

Scorpion, sea urchin: They both have segmentation and three tissue layers, but so do the lion and the fish, so they must also be part of the clade.

Cladograms show evolutionary relatedness, usually based on the similarity of the DNA sequences between different species. The closer two branches of the cladogram are to each other, the more closely related the organisms are to each other. The ends of the branches indicate a common ancestor shared by all of the species of that branch. Cladograms do not show geographic relationships. Although primitive cladograms were formulated based on physical characteristics of animals, now, DNA analysis provides a much more accurate comparison between species. Furthermore, cladograms are not limited to animals.

Each branch in the tree represents a break from the common ancestor at the bottom. A and B are both branches off of the same larger branch that C is also a branch of. D, E and F branched off from the common ancestor earlier than A, B, or C. In general, branches that are closer together, and are on the same larger branch, represent organisms which are closely related.

Phylogenetics is the study of relationships between organisms and groups of organisms. This is done through the production of phylogenetic trees, which are used to describe these relationships. To make phylogenetic trees, scientists use molecular sequencing and/or morphological similarities in order to characterize the relationships between organisms.

The answer is directional selection. This environmental change will cause a shift in the mean beak size of the population towards smaller beaks because their food resource was not affected. There is selection against long beak size only.

In this example the extreme beak sizes are favored over the mean; however the mean beak size does not change. There is selection against the mean, without affecting the value of the mean. This type of selection is known as disruptive selection.

Darwin observed that the finches found on the Galapagos had adapted different sized beaks to eat different diets. By doing this, the birds had evolved to eat diets that would be inedible by other finches. This concept of a "niche" means that different species will exploit and use their environment in different ways.

Mitosis does not create genetic variation. The primary roles of mitosis are growth and healing.

Crossing over refers to the exchange of genetic material between chromosomes during metaphase I of meiosis, and is largely responsible for enhancing the genetic variation of eukaryotes. Mutation is necessary to introduce new genetic factors and adaptations into a population, thus furthering evolution and promoting variation.

Artificial selection is a process by which humans have modified species over many generations by selecting and breeding individuals that possess desired traits. The result of such a selection is a narrowing of the traits originally present from its ancestors. The goal of artificial selection is generally to make certain desired traits fixed in the population, with an allele frequency of 100%. These traits are relatively arbitrary, and may be wild type, recessive, advantageous, or disadvantageous. The key factor is a decrease in genetic variability in a small population.

Examples of artificial selection include the derivation of individual breeds of dog and the production of genetically specific lab mice.

Stabilizing selection removes extreme variants, as they do not provide a survival advantage in a given environment, and increases the frequency of the intermediate phenotype. For example, the bone density of a species of bird is likely to experience stabilizing selection. Bones that are too dense will inhibit the bird's ability to fly, and bones that are too light will be brittle and prone to injury. Stabilizing selection moderates the influence of these two factors and selects for the intermediate phenotype that is neither too heavy, nor too weak.

Disruptive selection shows an increase the in the frequency of extreme traits and a decline in the intermediate trait. For example, if two extremes are white and black coloration, disruptive selection will act against grey coloration and favor both white and back. Directional selection favors only one extreme, for example favoring black over grey and grey over white. Artificial selection occurs when humans interfere with breeding habits to promote the inheritance of a specific trait.

The heterozygote advantage is when a heterozygote phenotype increases fitness, as compared to both homozygote phenotypes. An example of this in human populations is sickle cell anemia—a condition where red blood cells cannot properly transport oxygen. Individuals that are heterozygotes are resistant to malaria. The example of sickle cell anemia demonstrates that heterozygosity may confer advantages and disadvantages, but homozygosity only confers a disadvantage.

Disruptive selection occurs when conditions in a habitat favor individuals that are on the extremes of the phenotypic range, instead of individuals with intermediate phenotypic traits. In the example provided in the question, dark-colored moths and light-colored moths each have regions of the habitat where they gain an advantage, but intermediate grey coloring is not favored in any region of the habitat. As a result, both extremes will experience positive selection, while the intermediate will decrease, giving rise to a disruptive selection trend.

Stabilizing occurs when the intermediate phenotype is favored over either extreme. Directional selection occurs when a single extreme is favored over any other phenotype. Artificial selection is the result of human manipulation of breeding, selecting for chosen traits.