Exam 1

What additional information is needed to estimate when the experiment shown below began using a molecular clock?

A calibration point such as a fossil or a sample taken at a known date.

Carefully describe two conditions when an asexual species would be at a disadvantage to a sexual species.

Asexual species lack the genetic variation to respond to changing conditions. Thus, any change in the environment would reduce the fitness of an asexual species because the asexual genomes would be less fit in the new environment than in the previous environment. Similarly, any new parasite or pathogen that attacked an asexual species would negatively affect the fitness of an asexual species because it would lack the ability to defend itself from this novel enemy.

Correlation vs causation

Correlation: a measure (usually statistical) that describes the size and direction of a relationship between two or more variables. The variables may or may not have a cause and effect relationship. Causation: one event is the result of the occurrence of the other event; i.e. there is a causal relationship between the two events. Both causation and correlation suggest there is a relationship among the variables, however correlation says nothing about how and why the relationship exists whereas causation means that if there is a change in one variable there will be a corresponding change in the second variable (= cause and effect).

Descent with modification vs natural selection

Descent with modification is the pattern, noted by Darwin, that organisms accumulate differences by small changes over generations. Natural selection is one mechanism of evolution. Individuals vary in how well they match their environment and those that match most closely pass on more of their genes to the next generation. This mechanism results in populations that are increasingly better matched to the environment where they live. Descent with modification is a pattern that can be caused by a number of factors such as genetic drift, natural selection, or catastrophic environmental events. Natural selection is a process that favors individuals with the traits that best match their environment over individuals that are more poorly adapted. It is one of several factors that can explain descent with modification

In a population of 32 diploid individuals, what is the maximum possible gene pool for one gene?

If there were unique alleles in all individuals and each individual was heterozygous it would total to 64 alleles

Carefully describe how the data on Galapagos iguanas (shown below) illustrates a case when natural selection and sexual selection act in opposition (= antagonistic). Closed symbols are females and open symbols are males.

In normal years (Fig.b), at least some of the males are larger-bodied than females, a pattern consistent with the hypothesis that sexual selection favors large males that can maintain territories and achieve more copulations than males that are smaller. In normal years body size of neither males or females changes greatly suggesting there are adequate food resources available to individuals of all sizes. In El Nino years (Fig. a), the largest males lose considerable body mass body mass in contrast to the females and those males that are no larger than the largest females which change little in body mass. Given that El Nino years are periods of low food resources, this suggests that natural selection is acting strongly on the largest males in these year. Thus, in normal years sexual selection shifts the population of males towards larger sizes and in resource-poor (El Nino) years, natural selection acts against large males and shifts the male population downwards

What are two lines of evidence that the mass extinction event at the end of the Cretaceous period was caused by an asteroid strike?

Iridium, an element common in asteroids, is found at sites distributed worldwide in the sedimentary layer deposited at the end of the Cretaceous period. Discovery of the impact crater that extends from the Gulf of Mexico and onto modern-day Yucatan peninsula.

Synonymous substitutions vs missense mutations

Missense mutations are a type of nonsynonymous substitution that results in a change in the nucleotide of a codon that results in a different amino acid. Synonymous mutations are a type of substitution that does not change the codon in a way that a new amino acid is formed. These changes are therefore not phenotypically expressed in the protein. Both are point mutations, but missense mutations may change protein architecture whereas synonymous mutations do not affect the protein.

How are paralogous genes distinguished from xenologous genes?

Paralogous genes arise by gene duplication events. By examination of the sequence of nucleotides, amino acids, etc. paralogous genes can be matched to the orthologous gene they are a copy of in the same genome. Xenologous genes are those that originate in a distantly related species and so are transferred between unrelated species. To determine if a gene is xenologous, one needs to establish that there are intervening species between the species pair without the gene.

Why is reinforcement among closely related species generally stronger where they occur in sympatry than where they live in allopatry? Explain your reasoning fully.

Pre-reproductive isolating barriers evolve quickly when the fitness of hybrid offspring is less than the fitness of offspring from parents in the same species. Hybrid offspring will not occur when species live in different geographic areas (=allopatric) because they live apart so there will be no selection. Species that live in the same geographic area (=sympatric) will encounter the other species and potentially create hybrid offspring. Under these circumstances, individuals that accurately differentiate mates from their own species from individuals in other species will have greater fitness. Over time, the populations will be primarily composed of individuals that pick a mate of their own species. This process will enhance pre-reproductive isolation.

Redraw the image below if human height was a simple Mendelian trait

Ratios such as 1:2:1 are genotypic ratios of Mendelian traits or are phenotypic ratios of a trait with co dominance. Answers with 1:2:1 ratio were not given full credit unless that specified that the ratio was a codominant trait.

Nachmann and Crowell (2000) estimated that there are about 175 new mutations per diploid genome per generation in Homo sapiens. Why are these mutations so rarely expressed in the phenotype?

The approx. 21,000 protein coding genes in humans represent 1.5% of the entire genome. The rest of the genome is not phenotypically expressed and mutations there have no effect on fitness. Mutations in the coding region also do not alter protein function (or fitness) if they are silent substitutions or do not otherwise change protein function.

Why was Lyell's hypothesis of uniformitarianism such an important concept to Darwin as he developed his ideas of evolution by natural selection?

Uniformitarianism is the proposal that the geological processes that alter the Earth are created by slow processes that occur uniformly through time, rather than created by unimaginable catastrophes. This was an important observation to Darwin because he realized natural selection was a very slow process yet was responsible for the incredible diversity of all life on earth. If natural selection had generated this diversity, the earth had to be very old (much older than previously thought), as uniformitarianism suggested was true.

Whole genome duplication events (polyploidy) often result in species formation. Carefully describe why this happens.

When an entire genome does not reduce, meiotic pairing is impossible or greatly impaired because the polyploid individual has, in one generation, a different number of chromosomes than does any other member of the surrounding population.

The following two questions refer to the figure below a) (4 pts) Circle a paraphyletic group. b) (2 pts) What character on the phylogeny is a homoplasy?

a) worm and snake b) Leg loss

Hemophilia occurs in 1 out of 20,000 humans and is a quantitative trait with a simple dominance-recessive relationship. a) (4 points) What is the frequency of carriers in a population that is in Hardy-Weinberg equilibrium? b) (4 points) Why is it not possible for natural selection to eliminate hemophilia completely?

a.) carriers = 2pq, p = sqrt(1/20,000), q= 1-p b.)Because at very low allele frequencies, the fraction of individuals that express the phenotype are very small and most of the recessive alleles are in heterozygotic ("carriers") individuals. Selection cannot act to change allele frequencies unless the phenotype is expressed. When the phenotype is so rarely expressed, selection becomes very weak.