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There, the genes gradually become differentin the course of evolution, but they are likely to continue to have correspondingfunctions in the two sister species. Genes that are related by descent in thisway—that is, genes in two separate species that derive from the same ancestralgene in the last common ancestor of those two species—are called orthologs.Related genes that have resulted from a gene duplication event within a singlegenome—and are likely to have diverged in their function—are called paralogs.Genes that are related by descent in either way are called homologs, a generalterm used to cover both types of relationship (Figure 1–25).The family relationships between genes can become quite complex (Figure1–26). For example, an organism that possesses a family of paralogous genes (forexample, the seven hemoglobin genes a, b, g, d, e, z, and q) may evolve into twoseparate species (such as humans and chimpanzees) each possessing the entireset of paralogs.
All 14 genes are homologs, with the human hemoglobin a orthologous to the chimpanzee hemoglobin a, but paralogous to the human or chimpanzee hemoglobin b, and so on. Moreover, the vertebrate hemoglobins (theoxygen-binding proteins of blood) are homologous to the vertebrate myoglobins (the oxygen-binding proteins of muscle), as well as to more distantancestral organismancestral organismearly ancestral organismgene Ggene GSPECIATION TO GIVE TWOSEPARATE SPECIESgene GGENE DUPLICATIONAND DIVERGENCEGENE DUPLICATIONAND DIVERGENCEgene G1species Aspecies Bgene GAgene GBlater ancestral organismgene G2gene G1SPECIATIONgene G2genes GA and GB are orthologs(A)genes G1 and G2 are paralogs(B)species Aspecies Bgene G1Agene G1Bgene G2Agene G2Ball G genes are homologsFigure 1–25 Paralogous genes and orthologous genes: two types of genehomology based on different evolutionary pathways.
(A) and (B) The mostbasic possibilities. (C) A more complex pattern of events that can occur.G1A is a paralog of G2A and G2Bbut an ortholog of G1B(C)THE DIVERSITY OF GENOMES AND THE TREE OF LIFE21Drosophila globinshark myoglobinancestralglobinhuman myoglobinchick myoglobinshark Hb bchick Hb bchick Hb echick Hb rhuman Hb bhuman Hb dhuman Hb ehuman Hb Aghuman Hb Ggshark Hb ahuman Hb q-1chick Hb a-Ahuman Hb a1human Hb a2chick Hb a-Dchick Hb phuman Hb zgenes that code for oxygen-binding proteins in invertebrates, plants, fungi, andbacteria. From the DNA sequences, it is usually easy to recognize that two genesin different species are homologous; it is much more difficult to decide, withoutother information, whether they stand in the precise evolutionary relationshipof orthologs.Genes Can Be Transferred Between Organisms, Both in theLaboratory and in NatureProcaryotes also provide examples of the horizontal transfer of genes from onespecies of cell to another.
The most obvious tell-tale signs are sequences recognizable as being derived from bacterial viruses, also called bacteriophages (Figure1–27). Viruses are not themselves living cells but can act as vectors for genetransfer: they are small packets of genetic material that have evolved as parasiteson the reproductive and biosynthetic machinery of host cells. They replicate inone cell, emerge from it with a protective wrapping, and then enter and infectanother cell, which may be of the same or a different species.
Often, the infectedcell will be killed by the massive proliferation of virus particles inside it; butsometimes, 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 asequence 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 ferrythem into another cell. Such transfers of genetic material frequently occur in procaryotes, and they can also occur between eucaryotic cells of the same species.Horizontal transfers of genes between eucaryotic cells of different speciesare very rare, and they do not seem to have played a significant part in eucaryote evolution (although massive transfers from bacterial to eucaryotic genomeshave occurred in the evolution of mitochondria and chloroplasts, as we discussbelow). In contrast, horizontal gene transfers occur much more frequentlybetween different species of procaryotes. Many procaryotes have a remarkablecapacity to take up even nonviral DNA molecules from their surroundings andthereby capture the genetic information these molecules carry.
By this route, orby virus-mediated transfer, bacteria and archaea in the wild can acquire genesfrom neighboring cells relatively easily. Genes that confer resistance to anFigure 1–26 A complex family ofhomologous genes. This diagram showsthe pedigree of the hemoglobin (Hb),myoglobin, and globin genes of human,chick, shark, and Drosophila. The lengthsof the horizontal lines represent theamount of divergence in amino acidsequence.22Chapter 1: Cells and Genomesantibiotic or an ability to produce a toxin, for example, can be transferred fromspecies to species and provide the recipient bacterium with a selective advantage.
In this way, new and sometimes dangerous strains of bacteria have beenobserved to evolve in the bacterial ecosystems that inhabit hospitals or the various niches in the human body. For example, horizontal gene transfer is responsible for the spread, over the past 40 years, of penicillin-resistant strains of Neisseria gonorrheae, the bacterium that causes gonorrhea.
On a longer time scale,the results can be even more profound; it has been estimated that at least 18%of all of the genes in the present-day genome of E. coli have been acquired byhorizontal transfer from another species within the past 100 million years.Sex Results in Horizontal Exchanges of Genetic InformationWithin a SpeciesHorizontal exchanges of genetic information are important in bacterial andarchaeal evolution in today’s world, and they may have occurred even more frequently and promiscuously in the early days of life on Earth. Such early horizontal exchanges could explain the otherwise puzzling observation that theeucaryotes seem more similar to archaea in their genes for the basic information-handling processes of DNA replication, transcription, and translation, butmore similar to bacteria in their genes for metabolic processes.
In any case,whether horizontal gene transfer occurred most freely in the early days of life onEarth, or has continued at a steady low rate throughout evolutionary history, ithas the effect of complicating the whole concept of cell ancestry, by making eachcell’s genome a composite of parts derived from separate sources.Horizontal gene transfer among procaryotes may seem a surprising process,but it has a parallel in a phenomenon familiar to us all: sex. In addition to theusual vertical transfer of genetic material from parent to offspring, sexual reproduction causes a large-scale horizontal transfer of genetic information betweentwo initially separate cell lineages—those of the father and the mother. A keyfeature of sex, of course, is that the genetic exchange normally occurs onlybetween individuals of the same species.
But no matter whether they occurwithin a species or between species, horizontal gene transfers leave a characteristic imprint: they result in individuals who are related more closely to one set ofrelatives with respect to some genes, and more closely to another set of relativeswith respect to others. By comparing the DNA sequences of individual humangenomes, an intelligent visitor from outer space could deduce that humansreproduce sexually, even if it knew nothing about human behavior.Sexual reproduction is widespread (although not universal), especiallyamong eucaryotes. Even bacteria indulge from time to time in controlled sexual exchanges of DNA with other members of their own species.
Natural selection has clearly favored organisms that can reproduce sexually, although evolutionary theorists dispute precisely what the selective advantage of sex is.The Function of a Gene Can Often Be Deduced from Its SequenceFamily relationships among genes are important not just for their historicalinterest, but because they simplify the task of deciphering gene functions.
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