When genes duplicate and diverge in this way, the individuals of one species become endowed with multiple variants of a primordial gene. This evolutionary process has to be distinguished from the genetic divergence that occurs when one species of organism splits into two separate lines of descent at a branch point in the family tree—when the human line of descent became separate from that of chimpanzees, for example. There, the genes gradually become different in the course of evolution, but they are likely to continue to have corresponding functions in the two sister species. Genes that are related by descent in this way—that is, genes in two separate species that derive from the same ancestral gene in the last common ancestor of those two species—are called orthologs. Related genes that have resulted from a gene duplication event within a single genome—and
Figure 1–19 Four modes of genetic innovation and their effects on the DNA sequence of an organism. A special form of horizontal transfer occurs when two different types of cells enter into a permanent symbiotic association. Genes from one of the cells then may be transferred to the genome of the other, as we shall see below when we discuss mitochondria and chloroplasts.
283 genes in families with 38–77 gene members 764 genes in families with 4–19 gene members 2126 genes with no family relationship273 genes in families with 3 gene members 568 genes in families with 2 gene members are likely to have diverged in their function—are called paralogs. Genes that are related by descent in either way are called homologs, a general term used to cover both types of relationship (Figure 1–21).
Genes can Be Transferred Between organisms, Both in the laboratory and in nature
Prokaryotes provide good examples of the horizontal transfer of genes from one species of cell to another. The most obvious tell-tale signs are sequences recognizable as being derived from viruses, those infecting bacteria being called bacteriophages (Figure 1–22). Viruses are small packets of genetic material that have evolved as parasites on the reproductive and biosynthetic machinery of host cells. Although not themselves living cells, they often serve as vectors for gene transfer. A virus will replicate in one cell, emerge from it with a protective wrapping, and then enter and infect another cell, which may be of the same or a different species. Often, the infected cell will be killed by the massive proliferation of virus particles inside it; but sometimes, the viral DNA, instead of directly generating these particles, may persist in its host for many cell generations as a relatively innocuous passenger, either as a separate intracellular fragment of DNA, known as a plasmid, or as a sequence inserted into the cell’s regular genome. In their travels, viruses can accidentally pick up fragments of DNA from the genome of one host cell and ferry them into another cell. Such transfers of genetic material are very common in prokaryotes.
Horizontal transfers of genes between eukaryotic cells of different species are very rare, and they do not seem to have played a significant part in eukaryote evolution (although massive transfers from bacterial to eukaryotic genomes have occurred in the evolution of mitochondria and chloroplasts, as we discuss below).
genes GA and GB are orthologs genes G1 and G2 are paralogs
Figure 1–20 Families of evolutionarily related genes in the genome of Bacillus subtilis. The largest gene family in this bacterium consists of 77 genes coding for varieties of ABc transporters—a class of membrane transport proteins found in all three domains of the living world. (Adapted from F. kunst et al., Nature 390:249–256, 1997. With permission from macmillan Publishers ltd.)
Figure 1–21 Paralogous genes and orthologous genes: two types of gene homology based on different evolutionary pathways. (A) orthologs.
(B) Paralogs.