Populations geographically isolated from one another evolve differences. As populations diverge from one another, individuals from one population become less likely to reproduce successfully with individuals of the other populations. Eventually these differences between the populations become so great that they are considered to be different species. Thus species that share a fairly recent evolutionary history are generally more similar to each other than are species that share an ancestor in the more distant past. By identifying, analyzing, and quantifying similarities and differences among species, biologists can construct phylogenetic trees that portray the evolutionary histories of the different groups of organisms. As an example, we can show the evolutionary relationships among humans and our closest living relatives in a branching diagram that shows how they diverged from a common ancestor. In this tree, the living species are shown at the tips of the branches, and the branches show when these groups diverged from one another along a time scale. Our convention in this book will be to place phylogenetic trees on their sides, with the oldest lineages on the left and the most recent to the right.

Tens of millions of species exist on Earth today; many times that number lived in the past but are now extinct. Biologists give each of these species a distinctive scientific name formed from two Latinized names—a binomial. The first part of the name identifies the species’ genus (plural genera)—a group of species that share a recent common ancestor. The second part of the scientific name identifies a particular species within the genus. For example, the scientific name for modern-day humans is Homo sapiens: Homo is our genus, sapiens refers to our species. Homo is Latin for “man,” and sapiens is from the Latin word for “wise” or “rational.” The binomial for our close but extinct relatives, the Neanderthals, is Homo neanderthalensis. Note that it is conventional to write the binomial in italics with the genus capitalized but the second part not capitalized.

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Much of biology is based on comparisons among species. Meaningful interpretations of those comparisons require an understanding of evolutionary relationships of the respective species. Our ability to reconstruct evolutionary relationships has been greatly enhanced in recent decades by gene sequencing. Genomic sequence analysis and other molecular techniques have enabled biologists to augment evolutionary knowledge based on the fossil record with a vast array of molecular evidence. The result is the ongoing compilation of phylogenetic trees that document and diagram evolutionary relationships. The broadest categories of the tree of life are shown in Figure 1.9, and they will be surveyed in more detail in Part Seven. (The tree is expanded in Appendix A, and you can also explore the tree interactively online.)

Question

This tree shows that the last common ancestor for fungi and animals was much more recent than the last common ancestor for fungi and plants. Fungi are therefore more closely related to animals than to plants.

Although many details remain to be clarified, the broad outlines of the tree of life have been determined. Its branching patterns are based on a rich array of evidence from fossils, structures, metabolic processes, behavior, and molecular analyses of genomes. Recall that the primary division of life—into Archaea and Bacteria—occurred among the early single-celled prokaryotes. Members of these two groups differ so fundamentally that they are believed to have separated into distinct evolutionary lineages early in the history of life. One of the lineages of Archaea incorporated a nucleus and internal, membrane-bound organelles and gave rise to the major group Eukarya. Plants, fungi, and animals are examples of familiar multicellular eukaryotes that evolved independently, from different groups of the unicellular eukaryotes known as protists. We know that plants, fungi, and animals had independent origins of multicellularity because each of these three groups is most closely related to different groups of protists, as can be seen from the branching pattern of Figure 1.9.