Evolution and Genetics - AP Biology

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.

As this is recessive epistasis, only the genotype aa will mask gene B (Aa will not mask B). BB or Bb will give the dominant phenotype as B is dominant to b. Thus, AaBb will yield the dominant phenotype.

Epistasis describes the interaction of genes, where the epistatic locus masks the effects of a gene at another locus. In this example, locus E is epistatic. As this is stated to be recessive masking epistasis, when the E locus is homozygous recessive (ee), this locus will mask the effect of the B locus (color). Thus, any combination of B/b with ee will result in yellow fur. When the E locus is heterozygous (Ee) or homozygous dominant (EE), the effect of the B locus will not be masked. Thus, BbEe will result in black fur (as black B is dominant to brown b), and the E locus does not mask the B locus here, as the E locus is heterozygous in this individual (Ee).

Epistasis describes the interaction of genes, where the epistatic gene masks the effects of another gene, called the hypostatic gene.

Epistasis describes the interaction of genes, where the gene at the epistatic locus masks the effects of another gene at the hypostatic locus.

Epistasis describes the interaction of genes, where the epistatic locus masks the effects of a gene at another locus. In this example, locus E is epistatic. As this is stated to be dominant epistasis, when the E locus is either Ee or EE this locus will mask the effect of the B locus (color). Thus, any combination of B/b with Ee or EE will result in white squash. When the E locus is homozygous recessive (ee), the effect of the B locus will not be masked. Thus, BbEe will result in yellow squash as ee will not mask the color, and B (yellow) is dominant.

Epistasis describes the interaction of genes, where the epistatic locus masks the effects of a gene at another locus. In this example, locus A is epistatic. As this is stated to be recessive epistasis, when the A locus is aa this locus will mask the effect of the B locus (color). Thus, any combination of B/b with aa will result in white snapdragon. Thus, bbaa will result in white snapdragon, as the epistatic locus will mask the effect of the B locus.

Evolution is any change in the proportions of different genotypes in a population from one generation to the next. Mutation, geneflow, nonrandom mating, and natural selection all contribute toward favoring certain alleles over others within a population. This leads to changes in allele frequency, and subsequent evolution.

A population is composed of numerous individuals, each carrying a common set of genes with a unique combination of genetic alleles. The gene pool is the sum of all of these alleles.

The allele frequency for any given gene is the relative proportion of each allele of that gene in a population. This value can be found by dividing the number of a specific allele by the total number of alleles in a population.

The Hardy-Weinberg principle is a mathematical model that states that, under certain conditions, the allele frequencies and genotype frequencies in a sexually reproducing population will remain constant over generations. This consistency means that evolution is not occurring, as evolution (by definition) requires a change in allele frequency.

Requirements for Hardy-Weinberg equilibrium include: large population size, no mutation, no migration, random mating, and no natural selection.

Genetic drift is a change in the allele frequencies of a small population purely by chance.

The bottleneck effect occurs when allele frequencies are affected by a cataclysmic event. Evolution refers to a change in allele frequency, but is not limited to small populations or random chance. Natural selection refers to changes in allele frequency due to specific conditions, as opposed to random chance.

An adaptation is a characteristic of an organism that helps it survive and reproduce in a particular environment. Adaptations are the result of random mutations that have favorable outcomes. The favorability of these traits enables offspring that inherit them to thrive, thus increasing their prevalence in the population.

Chargaff was able to show the diversity of DNA through patterns within base pairs. While studying DNA, he noticed that the concentration of adenine was equal to the concentration of thymine, and the concentration of cytosine was equal to that of guanine. This led him to establish Chargaff’s rules, which state that the base composition of DNA varies between species, and for each species the percentages of adenine and thymine bases are roughly equal and the percentages of cytosine and guanine are also roughly equal.