A4 B5

Molecular Cladistics

A biologist can count the shared, derived amino acids at each position in a protein and, from the analysis, construct a tree that hypothesizes relationships between various species. On a molecular cladogram, branch lengths are proportional to the number of amino acid changes. Such molecular data are independent of physical similarities or differences. The analysis shown in Figure 17-5 is of the amino acid sequence of a protein involved in flower development.

Biologists have used evolutionary changes in the sequence of macromolecules, such as DNA, RNA, and proteins, as a form of molecular clock, a tool for estimating the sequence of past evolutionary events. The molecular clock hypothesis suggests that the greater the differences between a pair of sequences, the longer ago those two sequences diverged from a common ancestor. A researcher who matches a molecular clock carefully with the fossil record can use it to hypothesize when various characteristics arose and when organisms diverged from ancestral groups.

Chromosomes

Analyzing karyotypes can provide still more information on evolutionary relationships. As Figure 17-6 shows, chromosomes can be stained to reveal a pattern of bands. If two species have the same banding pattern in regions of similar chromosomes, the regions are likely to have been inherited from a single chromosome in the last common ancestor of the two species. Karyotypic data are totally independent of both physical similarities and molecular data.

For example, the chromosomes of two species are shown in Figure 17-6. Several of the chromosomes have similar banding patterns, suggesting that the chromosomes are homologous. In addition, two of species B's chromosomes are similar to parts of one of species A's chromosomes. In such cases, biologists may still hypothesize that all of the chromosomes were inherited from the same ancestor. It is possible that in one of the descendants, one chromosome became two or two chromosomes became one.

Xenarthra

(anteaters, Tubulidentata armadillos,

Pholidota

Pholidota

Proboscidea (aardvarks) and sloths) Primates Rodentia (pangolins) Carnivora Perissodactyla Artiodactyla

Xenarthra

(anteaters, Tubulidentata armadillos,

Proboscidea (aardvarks) and sloths) Primates Rodentia (pangolins) Carnivora Perissodactyla Artiodactyla

Placental mammals figure 17-7

Placental mammals

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