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12 a2 a1

Human a cluster

J Interrupted gene (exons are dark)

▼ Gene of unknown structure

OOOGCO

Figure 2. Representations of the two clusters of genes that code for human globin, the protein portion of hemoglobin. Adapted from <http:// www.irn.pdx.edu/ ~newmanl/GlobinGene Evolution.GIF>.

One controversy about gene families involves whether they have arisen primarily by polyploidy or via tandem gene duplications. Polyploidy means that full genomes in an organism are duplicated either by mitosis or meio-sis without cytokinesis or by matings between organisms with unequal numbers of chromosomes. This is followed by full copying of both parents' full genomes so each haploid set of chromosomes is now diploid. In tandem duplication, one or more copies of a gene lie on the same chromosome adjacent to one another. Polyploidy has been invoked to explain the evolution of complex new functions in taxa. Researchers give five reasons. Polyploids:

(1) have "higher levels of heterozygosity than do their diploid parents";

(2) "exhibit less inbreeding depression than do their diploid parents";

(3) "are polyphyletic . . . [which] . . . incorporates genetic diversity from multiple progenitor populations" and, thus, they have higher genetic diversity "than expected by models of polyploid formation involving a single origin";

(4) have genome rearrangements that are common; and

(5) are like duplicated genes, freed from intense selection pressure, which allows frequent evolution of new functions (Soltis and Soltis 2000, p. 310).

Austin Hughes has been the major critic of the often invoked polyploid hypothesis for origin of major animal groups because of the major substitutional load that would be involved and molecular phylogenetic evidence against it (1999, pp. 205-212). However, the results of most molecular evolutionary studies are more consistent with the gradualist view that new functions are generated primarily by tandem gene duplication and divergence of both sequence and function, spread over a long time. see also Development, Genetic Control of; Evolution of Genes; Polyploidy; Pseudo-genes; Transposable Genetic Elements.

John R. Jungck polyploidy presence of multiple copies of the normal chromosome set il

Bibliography

Henikoff, Steven, et al. "Gene Families: The Taxonomy of Protein Paralogs and Chimeras." Science 278 (1997): 609-614.

Holmes, Roger S., and Hwa A. Lim, eds. Gene Families: Structure, Function, Genetics and Evolution. Singapore: World Scientific Publishers, 1996.

Hughes, Austin L. Adaptive Evolution of Genes and Genomes. New York: Oxford University Press, 1999.

Page, Roderic D. M., and Edward C. Holmes. Molecular Evolution: A Phylogenetic Approach. Malden, MA: Blackwell Science, 1998.

Patthy, Laszlo. Protein Evolution. Malden, MA: Blackwell Science, 1999.

Soltis, Pamela S., and D. E. Soltis. "The Role of Genetic and Genomic Attributes in the Success of Polyploids." In Variation and Evolution in Plants and Microorganisms, Francisco J. Ayala, et al., eds. Washington, DC: National Academy Press, 2000.

Thorston, J. W., and R. DeSalle. "Gene Family Evolution and Homology: Genomics Meets Phylogenetics." Annual Review of Genomics and Human Genetics 1 (2000): 41-73.

Internet Resources

"Phylogenic Relationships between Globin-Type Proteins." Concordia College. <http://www.cord.edu/faculty/landa/courses/b315f99/sessions/phylogeny/globin Phylogeny.jpg>.

"Proposed Evolution of Globin Genes." Portland State University. <www.irn.pdx.edu/ ~newmanl/GlobinGeneEvolution.GIF>.

Gene Flow

WHAT IS AN ALLELE?

Alleles are forms of a gene that may differ between individuals or populations; brown and blue eye colors are due to different alleles for eye color.

gametes reproductive cells, such as sperm or eggs genetic drift evolutionary mechanism, involving random change in gene frequencies

Gene flow is the transfer of genetic material between separate populations. Many organisms are divided into separate populations that have restricted contact with each other, possibly leading to reproductive isolation. Many things can fragment a species into a collection of isolated populations. For example, a treacherous mountain pass may cut off one herd of mountain goats from another. In human beings, cultural differences as well as geographic separation maintain unique populations: It is more likely that a person will marry and have children with someone who lives nearby and speaks the same language.

Over time, reproductive isolation can lead to genetic differences between two populations. Gene flow between populations limits this genetic divergence, serving to inhibit the development of separate species out of the two separated populations.

The essential mechanism of gene flow is movement of individuals (or their gametes) between populations. For example, gene flow can occur in plant species when pollen is carried by bees or blown by the wind from one population of flowering plants to another.

Migration has been a significant feature of human history in both prehistoric and more recent times. No gene flow occurs if an individual migrates into a different population but does not reproduce. The migrant's genes must become part of the genetic makeup of the population into which it has migrated.

In most populations, not all individuals contribute equally to the next generation. Because each individual can have different alleles, when only a subset of individuals reproduce, allele frequencies change from generation to generation, and some alleles may be lost. A change in allele frequency due to random chance is known as genetic drift, whereas a change due to differ-

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