Chapter 11 Evolution

molecular mutualism

When different molecules act such that they increase each other's rate of replication.

Which of the following statements regarding the benefit of making the switch from an RNA- to a DNA-based genetic system during the evolution of life is correct? a. DNA is a more stable molecule because deoxyribose is less reactive than is ribose. b. DNA replication systems do not have the proofreading capabilities that are usually present in RNA replication. c. Replication of RNA is too fast to be favored by natural selection. d. The double stranded structure of DNA has a much greater potential for interaction with outside molecules.

a. DNA is a more stable molecule because deoxyribose is less reactive than is ribose. FEEDBACK: In the RNA world, natural selection would have favored any transmission system that was more efficient than that of RNA and ribozymes. DNA is chemically more stable than RNA, primarily because DNA's deoxyribose sugar is less reactive than is RNA's ribose sugar. The double-stranded structure of DNA reduces the potential for outside molecules to interact with and disrupt the nitrogenous bases, while DNA replication systems also have proofreading capabilities that are not present in RNA replication. Read the subsection titled "From RNA to DNA" in Section 11.5 and compare the molecular structure of these molecules in Figure 11.24.

Which of the following statements about LUCA is NOT correct? a. It refers to a single, ancestral organism. b. It represents a base to the tree of life. c. The acronym stands for "last universal common ancestor." d. It represents a phylogenetic event horizon.

a. It refers to a single, ancestral organism. FEEDBACK: The last universal common ancestor (LUCA) itself was not the first life-form, nor was it the only lifeform present at the time. Although LUCA is the base of the phylogeny of life by definition, it was not conceptualized as a single individual, but rather as a population. Finally, LUCA is also a point in the history of life beyond which phylogenetic analysis cannot see, referred to as the "phylogenetic event horizon." Examine the hypothetical tree of life in Figure 11.5.

Why is the higher fidelity of DNA proofreading and repair (compared to RNA) evolutionarily important? Select one: a. A lower mutation rate results in shorter, more efficient genomes. b. A lower mutation rate allows for longer genes and more storage of information in the genome. c. A lower mutation rate allows DNA to specialize in only functioning as a genetic storage system. d. A lower mutation rate allows for longer genes and less storage of information in the genome.

b. A lower mutation rate allows for longer genes and more storage of information in the genome. FEEDBACK: DNA's proofreading and repair system dramatically lowers the mutation rate that occurs during synthesis, transcription, and translation. In turn, this allows DNA genomes to be much larger in size, allowing organisms to store and transmit much more genetic information than an RNAbased system. Scientists also hypothesize that this led to the specialization of DNA in to functioning exclusively as a genetic information storage system. Read Section 11.5.

Which two scientists elaborated on Darwin's idea of the "warm little pond" and formulated the "prebiotic soup hypothesis" as the first hypothesis for the abiotic origin of life on Earth? a. Miller and Urey b. Oparin and Haldane c. Cech and Altman d. Wallace and Lyell

b. Oparin and Haldane FEEDBACK: In the 1920s, both Aleksandr Oparin and J. B. S. Haldane elaborated on Darwin's "warm little pond" idea and proposed the first scientific hypothesis for the origin of life on Earth. They hypothesized that in a reductive atmosphere (in the absence of oxygen), some available sources of energy, such as ultraviolet light and lightning, might have helped to convert atmospheric gases into a range of molecules that served as the basis for early life on Earth.

Stanley Miller and Harold Urey developed and conducted experimental tests for a "prebiotic soup" model of the origin of life, which had been proposed a few decades earlier by Aleksandr Oparin and J.B.S. Haldane. What did Miller and Urey show could be synthesized abiotically? a. single-stranded RNA molecules b. amino acids c. complex proteins d. ribosomes

b. amino acids FEEDBACK: Miller and Urey tested the plausibility of the prebiotic soup hypothesis in the 1950s (Miller 1953). They were able to produce some very important evidence that organic molecules may form in abiotic conditions. They used simulated lightning (an electric current between two electrodes) in a mixture of gases to replicate the ancient atmosphere. Depending on which combination of gases was used, numerous common amino acids, such as glycine, alanine, and valine, were produced in the experimental apparatus. Using a simple experimental protocol, Miller and Urey produced some of the building blocks of life. Examine Figure 11.8.

Itay Budin and Jack Szostak sought to understand how cell membranes composed of single-chain lipid molecules could possibly evolve to the more complex phospholipids seen in modern cell membranes (Budin and Szostak 2009). Which statement best summarizes their results? a. Adding a small fraction of phospholipid molecules resulted in vesicles with lower phospholipid content; these cells tended to be reduced in size, conferring a selective advantage. b. Adding a large fraction of phospholipid molecules resulted in vesicles with lower phospholipid content; these cells tended to be reduced in size, conferring a selective advantage. c. Adding a small fraction of phospholipid molecules resulted in vesicles with higher phospholipid content; these cells tended to grow in size conferring a selective advantage. d. Adding a large fraction of phospholipid molecules resulted in vesicles with higher phospholipid content; these cells tended to grow in size conferring a selective advantage.

c. Adding a small fraction of phospholipid molecules resulted in vesicles with higher phospholipid content; these cells tended to grow in size conferring a selective advantage. FEEDBACK: Adding just a few phospholipid molecules slowed down the rate of movement of fatty acid molecules out of the membrane. This resulted in vesicles with the higher phospholipid content tending to grow in size, rather than shrink. Cells with a greater ability to synthesize phospholipids outcompeted other cells, leading to the evolution of membranes dominated by phospholipids. Read the subsection titled "Lipid Membranes and Reproduction in Early Cells" in Section 11.3, and examine Figure 11.16.

Which of the following statements about the Lincoln and Joyce experiment on selfreplicating ribozymes is most likely correct? a. Ribozymes produced true protein-based enzymes. b. In the self-replicating ribozymes, variation in the rate of replication was not heritable. c. The self-replicating ribozymes that had more efficient catalytic activities would soon began to dominate their populations. d. Truly self-replicating enzymes had not evolved in Earth's early environment

c. The self-replicating ribozymes that had more efficient catalytic activities would soon began to dominate their populations. FEEDBACK: After establishing that the template RNAs in this system could self-replicate, Lincoln and Joyce conducted an experiment in which several variant template molecules were supplied with variant substrate molecules (Lincoln and Joyce 2009). This variation was heritable, and different templates replicated at different rates. They also found evidence for selection, because the self-replicating ribozymes that had more efficient catalytic activities soon began to dominate their populations of selfreplicating ribozymes. Read Section 11.4 and examine Figure 11.23.

All of the following are true regarding Sol Spiegelman's experiment on the origins of life, except that __________. a. the most common size of the RNA strand in the final test tubes was a little over 200 nucleotides long b. variation in the length of the RNA strands occurred at each round of replication c. there was no selection for any of the variations in length d. shorter RNA sequences were favored by selection because they took less time to replicate

c. there was no selection for any of the variations in length FEEDBACK: At the end of Spiegelman's serial transfer experiment (Figure 11.22), he did not find strands of 4000 nucleotides, as in generation 1. This is because the shorter RNA sequences were favored by selection because of their fast reproduction. Since it was also evident that the variation in size was heritable, it was interesting to see that a typical final strand was a little over 200 nucleotides long. Apparently, mutations were damaging to the smaller sequences (those consisting of 50-100 nucleotides). Read the subsection titled "Experimental Evidence on the Origins of Natural Selection" in Section 11.4 and examine Figure 11.22.

In 1977, Sidney Fox tried a different approach to testing the prebiotic synthesis of biological molecules. He mixed a number of different amino acids together at a high temperature (120°C) in an environment lacking water. When he subsequently placed the mixture into water to investigate what the amino acids would form, what happened next? a. He found ribose in very low supply. b. He found that a mechanism for joining the amino acids together to make proteins was completely absent. c. He added CO and had success with creating polypeptides. d. He found some peptide-like structures, but the bonds between the amino acids were weak and unstable.

d. He found some peptide-like structures, but the bonds between the amino acids were weak and unstable. FEEDBACK: After the experiments of the 1950s and 1960s, many controversies remained regarding the prebiotic soup model. For instance, the sugar ribose was found in very low supply, and a mechanism for joining the amino acids together to make proteins was completely absent. Sidney Fox mixed a number of different amino acids together at a high temperature (120°C) in an environment lacking water (Fox and Dose 1977). When he mixed large amounts of aspartic acid and glutamic acid, and subsequently placed the mixture into water, the amino acids present were strung together in a peptide-like structure. The bonds between the amino acids, however, were weak and unstable. Subsequent work by Claudia Huber and her colleagues found that amino acids do link together via stable peptide bonds in the presence of a compound such as carbon monoxide (CO), which is thought to have been present in early Earth's atmosphere. Read Section 11.2 titled "The Origin and Evolution of the Building Blocks of Life".

The origin of cell structures might be hypothesized to have involved a hypercycle, based on mutualism at the molecular level. Which of the following is correct regarding the hypercycle model? a. Molecular mutualisms may have been important among the cells in the DNA world, but not for the replicators in the RNA world. b. Natural selection will favor a hypercycle that is not enclosed in a membrane. c. Molecular mutualisms always favor replicators that increase their own replication rates but not those of others. d. Natural selection will favor a hypercycle that is enclosed in a membrane.

d. Natural selection will favor a hypercycle that is enclosed in a membrane. FEEDBACK: Molecular mutualisms may have been important among replicators in the RNA world. The hypercycle model proposed that if the replicators are enclosed in a membrane and the replication rate of the membrane-bound ensemble depends on the replication rate of the RNA species within, then natural selection will favor the hypercycles in which one RNA species increases the rate of replication of the others. On the contrary, in a hypercycle without a membrane, natural selection favors variants that increase their own rate of replication, but not those that increase the rates of others. Read the subsection titled "Hypercycles" in Section 11.3 and examine Figure 11.16

In order to understand the origin of life on Earth, evolutionary biologists collaborate with__________. a. chemists b. geologists c. atmospheric scientists d. all of the answer options are correct

d. all of the answer options are correct FEEDBACK: Even with recent advances in evolutionary biology, the question of the origin of life on Earth remains a truly interdisciplinary endeavor. While the theory of evolution explains how life diversified subsequent to its origin, understanding the origin of life itself requires that evolutionary biologists collaborate with chemists, geologists, atmospheric scientists, and researchers from other disciplines. For instance, chemists can help us understand what the initial chemical building blocks of life might have been, while geologists and atmospheric scientists can shed light on the possible physical characteristics of the environment in which life originated. See Section 11.1 titled "What is Life?" and examine the timeline in Figure 11.2, "Early Events in the History of Life on Earth."

abiogenesis

The emergence of life from a nonliving precursor.

minimal gene set

The hypothetical minimal number of genes thought necessary to allow for cellular-based life.

Which gas was most likely the LEAST abundant in early Earth's atmosphere? a. O2 b. CH c. NH d. H

a. O2 FEEDBACK: Based on evidence from chemistry and physics, methane (CH ), hydrogen (H ), ammonia (NH ), and water (H O) might best represent the components of early Earth's atmosphere (about 4 billion years ago). Oxygen (O ) would not be expected to have been in this atmosphere. See Section 11.2.

RNA world

A hypothetical early stage in the history of life in which RNA was the fundamental unit upon which life was based, fulfilling both an informational role (much as DNA does today) and a catalytic role (much as protein-based enzymes do today).

hypercycle model

A model for the evolution of early life involving multiple types of replicators, each of which facilitates the replication of another in cyclical fashion.

mutualism

An ecological interaction in which different individuals, often of separate species, act so as to increase each other's fitness.

prebiotic soup

An ensemble of simple organic molecules suspended in liquid water prior to the emergence of life on Earth.

The first genetic material on Earth was most likely __________. a. DNA b. RNA c. a small protein d. a large lipid.

RNA

ribozymes

RNA molecules with enzymatic function.

prebiotic soup hypothesis

The idea that the earliest life emerged in a "souplike" liquid environment, drawing upon energy from cosmic rays, volcanic eruptions, and Earth's own internal heat.

protocell

A simple cell-like entity that predated cellular life-forms in the history of life

vesicle

A small, fluid-filled compartment surrounded by a lipid membrane.

phylogenetic event horizon

The point in the history of life beyond which phylogenetic analysis is uninformative because there are no surviving descendants from ancestors before this point. See also last universal common ancestor (LUCA).

last universal common ancestor (LUCA)

The population of organisms at the base of the tree of life. All living things today are descended from this one lineage.