1–10 Animal cells have neither cell walls nor chloroplasts, whereas plant cells have both. Fungal cells are somewhere in between; they have cell walls but lack chloroplasts. Are fungal cells more likely to be animal cells that gained the ability to make cell walls, or plant cells that lost their chloroplasts? This question represented a difficult issue for early investigators who sought to assign evolutionary relationships based solely on cell characteristics and morphology. How do you suppose that this question was eventually decided?
Figure Q1–2 Phylogenetic tree for hemoglobin genes from a variety of species (Problem 1–11). The legumes are highlighted in green. The lengths of lines that connect the present-day species represent the evolutionary distances that separate them. Barley ChlamydomonasParameciumNematode Clam Insect Earthworm Goldfsh Frog Salamander Cobra Chicken Rabbit Whale Cat Human Cow LotusAlfalfaBeanVERTEBRATES INVERTEBRATES PROTOZOA PLANTS 1–11 When plant hemoglobin genes were first discovered in legumes, it was so surprising to find a gene typical of animal blood that it was hypothesized that the plant gene arose by horizontal transfer from an animal. Many more hemoglobin genes have now been sequenced, and a phylogenetic tree based on some of these sequences is shown in Figure Q1–2.
A. Does this tree support or refute the hypothesis that the plant hemoglobins arose by horizontal gene transfer?
b. Supposing that the plant hemoglobin genes were originally derived from a parasitic nematode, for example, what would you expect the phylogenetic tree to look like?
1–12 Rates of evolution appear to vary in different lineages. For example, the rate of evolution in the rat lineage is significantly higher than in the human lineage. These rate differences are apparent whether one looks at changes in nucleotide sequences that encode proteins and are subject to selective pressure or at changes in noncoding nucleotide sequences, which are not under obvious selection pressure. Can you offer one or more possible explanations for the slower rate of evolutionary change in the human lineage versus the rat lineage?
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