7.7 Common Ancestry

1. AStructure W
   A student who thinks only eukaryotes have cell walls picks this. Most bacteria also have cell walls, so a cell wall does not show that the organism is a eukaryote.
2. BStructure X Correct
   A nucleus enclosed by a membrane is a membrane-bound organelle, a feature that eukaryotes share and prokaryotes lack, so it is evidence of common ancestry with other eukaryotes.
3. CStructure Y
   A student who thinks ribosomes are membrane-bound organelles picks this. Ribosomes have no membrane and are found in all cells, including bacteria.
4. DStructure Z
   A student who thinks prokaryotes have no membranes picks this. Every cell has a plasma membrane; prokaryotes lack only internal membrane-bound organelles.

CED 7.7.A.1.i · Read this in Fix

1. AThe moss and the mouse are more highly evolved than the yeast and the green alga.
   A student who thinks multicellular organisms are more evolved picks this. The table shows that all four eukaryotes have the same features; living single-celled and multicellular lineages have evolved for equally long.
2. BThe four eukaryotes evolved from this bacterium by gaining each of the four features.
   A student who thinks one living group descended from another picks this. A living bacterium is not the ancestor of living eukaryotes; both lineages descend from ancestors that lived long ago.
3. CThe four eukaryotes inherited these features from a common ancestor that had all four of them. Correct
   Organisms as different as a yeast, an alga, a moss and a mouse share all four features, which the bacterium lacks. Inheritance from a common eukaryotic ancestor explains the shared set far better than four separate origins in each lineage.
4. DEach eukaryote evolved these four features on its own because it needed them.
   A student who thinks needs produce traits picks this. The same set of complex features in four different lineages is best explained by inheritance, not by four independent origins.

CED 7.7.A.1 · Read this in Fix

1. AIt is the ancestor from which fungi, plants and animals all evolved.
   A student who thinks one living organism can be the ancestor of other living groups picks this. A living organism shares ancestors with other eukaryotes but is not their ancestor.
2. BIt will in time become a multicellular organism, since it is still primitive.
   A student who sees evolution as progress toward complexity picks this. Single-celled eukaryotes are not unfinished multicellular organisms, and evolution has no set direction.
3. CIt is a bacterium, since bacteria are the organisms that are made of a single cell.
   A student who thinks every single-celled organism is a bacterium picks this. Bacteria lack a nucleus and other membrane-bound organelles; this organism is a single-celled eukaryote.
4. DIt shares an ancestor with fungi, plants and animals that had these organelles. Correct
   Membrane-bound organelles are shared by eukaryotes and indicate their common ancestry, so the organism is a eukaryote that shares a common ancestor with fungi, plants and animals.

CED 7.7.A.1.i · Read this in Fix

1. AIt would block intron removal in yeast, which inherited related machinery, but not in bacteria. Correct
   Genes with introns, and the machinery that removes them, are shared by eukaryotes through common ancestry, so the yeast machinery is likely to be related to the human target. Most bacterial genes have no introns to remove.
2. BIt would affect neither yeast nor bacteria, as both are too distantly related to humans.
   A student who thinks humans and yeasts are too different to share molecular machinery picks this. Yeasts and humans are both eukaryotes and inherited their intron-removal machinery from a common ancestor.
3. CIt would block intron removal in both yeast and bacteria, as all organisms remove introns.
   A student who thinks all organisms have introns picks this. Most bacterial genes lack introns, and bacteria do not have the eukaryotic intron-removal machinery.
4. DIt would stop DNA replication in yeast, as introns must be cut out of DNA before it copies.
   A student who thinks introns are removed from DNA picks this. Introns stay in the DNA; they are removed from pre-mRNA, so the drug affects mRNA processing, not DNA replication.

CED 7.7.A.1.iii · Read this in Fix

1. AThe introns code for extra parts of the protein that the bacterial version of the gene lacks.
   A student who thinks introns code for protein picks this. Introns are removed from the RNA before translation; the exons, which match the bacterial coding sequence in total length, are translated.
2. BThe eukaryotic versions of the gene evolved from the gene of the present-day bacterium.
   A student who thinks a living organism can be the ancestor of other living groups picks this. The bacterium and the eukaryotes share distant ancestors; the living bacterium is not their ancestor.
3. CEach eukaryote added introns to the gene at the positions where it needed them most.
   A student who thinks features appear where they are needed picks this. Introns at the same positions in three different lineages are best explained by inheritance, not by three separate insertions.
4. DThe eukaryotes inherited this gene, with introns at these positions, from a common ancestor. Correct
   In all three eukaryotes the introns interrupt the gene at the same two positions, between the same exons, although the introns differ in length. Inheritance from a common ancestor whose gene had introns at these positions best explains this.

CED 7.7.A.1.iii · Read this in Fix

1. AHumans evolved from yeasts, so human genes are modified versions of genes found in yeasts.
   A student who thinks one living group descended from another picks this. Humans and yeasts share a common ancestor; neither evolved from the other.
2. BThe yeast cells altered the inserted human gene to suit their own needs during the experiment.
   A student who thinks cells change genes to meet their needs picks this. Cells do not rewrite inserted genes to suit themselves; the human protein worked because it is similar to the yeast protein.
3. CYeasts and humans inherited this gene from a shared ancestor; the proteins work alike. Correct
   The human gene can take the place of the yeast gene because both descend from a gene in the common ancestor of eukaryotes, and their proteins still carry out the same function: functional evidence of common ancestry.
4. DAll eukaryotes carry identical copies of this gene, so either version of it can be used.
   A student who thinks common ancestry means identical genes picks this. The human and yeast genes differ in sequence after a long time apart; they are similar enough for the protein to work, not identical.

CED 7.7.A.1 · Read this in Fix

1. AThey are linear, unlike the single circular chromosome of most bacteria. Correct
   The nuclear chromosomes of eukaryotes are linear DNA molecules, a feature shared across eukaryotes. Most bacteria have one circular chromosome.
2. BThey are a feature that bacteria lack, since bacteria have no DNA.
   A student who thinks DNA can be found only in a nucleus picks this. Bacteria have DNA, usually in a circular chromosome in the cytoplasm.
3. CThey are found only in the reproductive cells of the organism.
   A student who links chromosomes only with inheritance picks this. Every cell with a nucleus contains chromosomes, not just eggs and sperm.
4. DTheir number tends to rise with complexity, so humans have more than gorillas.
   A student who thinks chromosome number reflects complexity picks this. Chromosome number does not track complexity: humans have 46 chromosomes, fewer than the 48 of gorillas and chimpanzees, and a dog has 78.

CED 7.7.A.1.ii · Read this in Fix

1. AMulticellular eukaryotes will have more linear chromosomes than single-celled eukaryotes.
   A student who thinks chromosome number rises with complexity picks this. Common ancestry predicts shared features, not more chromosomes in multicellular organisms; chromosome number does not track complexity.
2. BFungi and plants will have end-maintaining proteins related in sequence to those of animals. Correct
   If linear chromosomes, and the means of maintaining their ends, were inherited from a common ancestor, the proteins involved in fungi and plants should be homologous to those in animals, with related sequences.
3. CEukaryotes with the same number of chromosomes will be each other's closest relatives.
   A student who takes a similar value of one trait as a sign of close relatedness picks this. The hypothesis is about inheriting linear chromosomes, and species with equal chromosome numbers need not be close relatives.
4. DEach eukaryote group will use unrelated proteins that it evolved for its own chromosome ends.
   A student who thinks each lineage evolves the features it needs separately picks this. Unrelated proteins would support separate origins, the opposite of the hypothesis.

CED 7.7.A.1.ii · Read this in Fix

1. AThe multicellular species have higher percentages than any of the single-celled ones.
   A student who expects multicellular organisms to be more complex genetically picks this. Single-celled species A (70%) has a higher percentage than the animal D (60%).
2. BSpecies with similar percentages, such as A and D, are each other's closest relatives.
   A student who takes a similar value of one trait as a sign of close relatedness picks this. A single-celled eukaryote and an animal can have similar percentages without being close relatives.
3. CAll four eukaryotes have genes with introns, though the percentages vary widely. Correct
   All four eukaryotes (A, B, C and D) have genes with introns, from 5% in B to 80% in C, while the bacterium E has none: introns are a shared eukaryotic feature, present at very different levels.
4. DThe bacterium lacks introns because its simple genes did not need to have any.
   A student who explains traits by need picks this. The data show only that E has no introns; features do not appear or disappear because organisms need them.

CED 7.7.A.1 · Read this in Fix