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Gene Genealogy

In any species, there will usually be several variants, or alleles, of each gene. The alleles of a specific gene are related to each other -- new alleles arise from older ones by mutation or transcription errors (mistakes in copying). Molecular biologists can work out these relationships and draw up family trees that show which alleles are most closely related. When new species evolve from this ancestral population, some of the alleles may be lost through genetic drift or natural selection. As still more species branch off from these descendant populations, the most closely related alleles will not necessarily end up in the most closely related species. In this case, relying on DNA analysis of a single gene may lead to a mistaken conclusion about the relationships between species. Techniques that compensate for this potential for error have been developed.

Credits: Figure 14.5, "Gene trees versus species trees." From Evolutionary Analysis, by Scott Freeman and Jon C. Herron, 1998, Prentice-Hall, Inc. Reproduced by permission of the publisher.

Gene Genealogy

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Evolution of Diversity

Backgrounder

Gene Genealogy:

The biotech revolution of the past few decades has made possible hundreds of new drugs, DNA fingerprinting in crime cases, and the Human Genome Project that is reading out the entire genetic code of humankind.

For biologists, the breakthroughs that have made it possible to isolate and decipher genes -- the units of heredity -- have provided new methods for measuring how closely or distantly related different species are on the evolutionary tree. These molecular tools have greatly aided scientists in tracing ancestor-descendant relationships (which show how later organisms developed from earlier ones) among various organisms on the tree of life.

When scientists study fossils and classify animals or plants only on the basis of visible features, they can be misled by incomplete evidence. The new molecular techniques detect differences in genes between organisms, which provides another kind of evidence to help determine how closely related they are.

In this molecular approach, researchers examine a gene (a DNA blueprint for making a protein) in one organism and compare it to the corresponding gene in one or more other organisms. This enables the researchers to represent evolutionary relationships on a phylogenetic tree by the extent to which the organisms' gene sequences differ from each other. In fact, the genome, or entire set of genes, of an organism contains a record of the organism's evolutionary history. This is true of humans as well.

While molecular analysis has become a standard method of tracing evolutionary relationships, it too has limitations, and in certain situations it can yield wrong answers. When genetic measurements and evidence from fossils conflict, scientists recognize that no one technique is always correct.

One particular limitation of DNA analysis lies not in the technique, but in the nature of the DNA itself: The most closely related genes aren't necessarily in the most closely related organisms. This can happen because a single gene may have several variations at a particular letter of its genetic code. These variations are called "alleles".

When new species evolve from an ancestral population, some of the alleles may be lost through genetic drift or natural selection. As still more species branch off from these descendant populations, the most closely related alleles will not necessarily end up in the most closely related species. In this case, the gene tree and the species tree will have different branching patterns, which could lead to a mistaken conclusion about the species' actual relatedness.

Scientists have recognized this potential for error and have discovered ways to correct for it. One technique is to analyze the sequence data for several different genes, and generate "consensus trees," which illustrate the best fit for all the data combined. In this way, they combine several independent sources of data to increase the accuracy of their work as they trace the evolutionary relationships that link all of life.

The most robust results often come from combining analyses of the comparative morphology of different species with molecular techniques. Drawing insights from each of the various methods enables scientists to construct a more accurate picture of life's history.

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