Gene flow

Prior to the nineteenth century, religious dogmatism impeded the activities of scientists investigating the origins of life. This was primarily done by an insistence on the unchanging nature of species created by God. Despite mounting fossil evidence that many species of flora and fauna that once inhabited the Earth had disappeared and that many extant species could not be found in the fossil record, pre-nineteenth-century naturalists could find no viable explanation (other than divine intervention) for the disappearance of life-forms and their replacement. This changed in 1859 when Charles Darwin published his ground-breaking On the Origin of Species, which proposed the theory that all contemporary life forms evolved from simpler forms through a process he called “natural selection.”

Many individuals before Darwin had proposed theories of evolution, but Darwin’s became the first to be widely accepted by the scientific community. His success resulted from the careful and objective presentation of an overwhelming amount of evidence showing that species can and do change, and his advancement of a convincing explanation of the mechanism that produces that change—natural selection. Since Darwin, scientists have modified and added new concepts to his theory, especially concerning the ways in which species change, or evolve, over time. One of those new concepts, which was only dimly understood in Darwin’s lifetime, is the importance of genetics in evolution, especially the concepts of migration and gene flow, also called allele flow.

Genes and Gene Exchange

Genes are elements within the germ plasm of a living organism that control the transmission of a hereditary characteristic by specifying the structure of a particular protein or by controlling the function of other genetic material. Within any breeding population of a species, the exchange of genes is constant among its members, ensuring genetic homogeneity. If a new gene or combination of genes appears in the population, it is rapidly dispersed among all members of the population through inbreeding. New alleles may be introduced into the gene pool of a breeding population (thus contributing to the evolution of that species) in two ways: mutation and migration. Gene flow is integral to both processes.

Gene flow may occur by gamete movement, like when pollen moves between plants; by vertical gene transfer, which is the transfer of genes from parent to offspring; and by horizontal transfer, which occurs when genetic material from one species is transferred to another species. These functions help prevent population fragmentation, reduce genetic drift, improve species fitness, and vary genetic material in animals, which enhances their adaptability.

A mutation is the appearance of a new gene or the almost total alteration of an old one. The exact causes of mutations are not completely understood, but scientists have demonstrated that they can be caused by exposure to mutagens like radiation or chemicals, DNA replication errors, and some viral infections and environmental toxins. Mutations occur constantly in every generation of every species. Most of them, however, are either minor or detrimental to the survival of the individual and thus are of little consequence. Very few mutations may prove valuable to the survival of a species and are spread to all of its members by migration and gene flow.

When immigrants from one population interbreed with members of another, an exchange of genes between the populations ensues. If the exchange is recurrent, biologists call it “gene flow.” In nature, gene flow occurs on a more or less regular basis between demes, geographically isolated populations, and even closely related species. Gene flow is more common among the adjacent demes of one species. The amount of migration between such demes is high, thus ensuring that their gene pools will be similar. This sort of gene flow contributes little to the evolutionary process, since it does little to alter gene frequencies or to contribute to variation within the species. Much more significant for the evolutionary process is gene flow between two populations of a species that have not interbred for a prolonged period.

Populations of a species separated by geographical barriers, called allopatric speciation, often develop very dissimilar gene combinations through the process of natural selection. In isolated populations, dissimilar alleles become fixed or are present in much different frequencies. When circumstances do permit gene flow to occur between two such populations, it results in the breakdown of gene complexes and the alteration of allele frequencies, thereby reducing genetic differences in both. The degree of this homogenization process depends on the continuation of interbreeding between members of the two populations over extended periods of time.

Biologists often explain, at least in part, the poorly understood phenomenon of speciation through migration and gene flow—or rather, by a lack thereof. If some members of a species become geographically isolated from the rest of the species, migration and gene flow cease. The isolated population will not share in any mutations, favorable or unfavorable, nor will any mutations that occur among its own members be transmitted to the general population of the species. Over long periods of time, this genetic isolation will result in the isolated population becoming so genetically different from the parent species that its members can no longer produce fertile progeny should one of them breed with a member of the parent population. The isolated members will have become a new species, and the differences between them and the parent species will continue to grow as more ages pass. Scientists, beginning with Darwin himself, have demonstrated that this sort of speciation has occurred on the various islands of the world’s oceans and seas.

Mendel and Darwin

Gregor Johann Mendel (1822-1884) was an important contributor to the understanding of genes and hybridization, and is known as the father of modern genetics. In the early nineteenth century, Mendel was a monk who lived in a monastery in the modern-day Czech Republic. Assigned to work in the monastery garden, Mendel was fascinated with the notion of heredity and how characteristics are passed down through generations. He began to catalog and study variations in plants. Over eight years (1856-1863), Mendel would conduct a meticulous series of experiments involving over 10,000 pea plants that he grew. Mendel studied traits in pea plants that included color, shape, height, and the position of the flower. Mendel would later publish his findings, establishing that genes are passed to offspring in pairs. Traits inherited between generations are contained in these genes, where the offspring receives one gene from each parent. Some genes contain dominant traits, which will appear in the offspring. In doing so, the dominant trait supersedes the same characteristic, or recessive trait, that exists in the other parent.

Mendel was fully aware of Darwin’s work. Darwin, however, apparently never became familiar with Mendel’s findings. Unlike Darwin, who lived in London and with widespread notoriety, Mendel worked in obscurity. His work never achieved its deserved recognition or acceptance until approximately three decades after his death. Since this time, a divergence of opinion has developed on whose work better explains the process of continuous change in species. In his 2016 book, The Kingdom of Speech, Pulitzer Prize-nominated author Tom Wolfe sought to undermine Darwin and his theory of the evolution of species. Wolf asserted the principles of Mendelian inheritance better explained this phenomenon than Darwin. While many in the scientific community came to the defense of Darwin and castigated Wolfe for his depiction of Darwinian theories, these criticisms did not dispel the validity of Mendel’s work. Mendel had, in fact, disagreed with Darwin’s assertions in several cases.

Hybridization

The migration of a few individuals from one breeding population to another may, in some instances, also be a significant source of genetic variation in the host population. Such migration becomes more important in the evolutionary process in direct proportion to the differences in gene frequencies—for example, the differences between distinct species. Biologists call interbreeding between members of separate species “hybridization.” Hybridization usually does not lead to gene exchange or gene flow, because hybrids are not often well adapted for survival and because most are sterile. Nevertheless, hybrids are occasionally able to breed (and produce fertile offspring) with members of one or sometimes both the parent species, resulting in the exchange of a few genes or blocks of genes between two distinct species. Biologists refer to this process as “introgressive hybridization.” Usually, few genes are exchanged between species in this process, and it might be more properly referred to as “gene trickle” rather than gene flow.

Introgressive hybridization may, however, add new genes and new gene combinations, or even whole chromosomes, to the genetic architecture of some species. It may thus play a role in the evolutionary process. Introgression requires the production of hybrids, a rare occurrence among highly differentiated animal species. Areas where hybridization takes place are known as contact zones or hybrid zones. These zones exist where populations overlap; in some cases of hybridization, the line between what constitutes different species and what constitutes different populations of the same species becomes difficult to draw. The significance of introgression and hybrid zones in the evolutionary process remains an area of some contention among life scientists.

Biologists often explain, at least in part, the poorly understood phenomenon of speciation through migration and gene flow—or rather, by a lack thereof. If some members of a species become geographically isolated from the rest of the species, migration and gene flow cease. The isolated population will not share in any mutations, favorable or unfavorable, nor will any mutations that occur among its own members be transmitted to the general population of the species. Over long periods of time, this genetic isolation will result in the isolated population becoming so genetically different from the parent species that its members can no longer produce fertile progeny should one of them breed with a member of the parent population. The isolated members will have become a new species, and the differences between them and the parent species will continue to grow as more ages pass. Scientists, beginning with Darwin himself, have demonstrated that this sort of speciation has occurred on the various islands of the world’s oceans and seas.

Studying Gene Flow

Scientists from many disciplines study migration and gene flow in various ways. For decades, ornithologists and marine biologists have placed identifying tags or markers on members of different species of birds, fishes, and marine mammals to determine the range of their migratory habits to understand the role of migration and subsequent gene flow in the biology of their subjects. These studies have led and will continue to lead, to important discoveries. However, most migration and gene flow studies relate to humans.

Many of the important discoveries concerning the role of gene flow in the evolution of life come from the continuing study of the nature of genes. A gene, in cooperation with such molecules as transfer ribonucleic acid (tRNA) and related enzymes, controls the nature of an organism by specifying amino acid sequences in specific functional proteins. In recent decades, scientists have discovered that what they previously believed to be single pure enzymes are actually groups of closely related enzymes, which they have named “isoenzymes” or “isozymes.” Current theory holds that isozymes can serve the needs of a cell or of an entire organism more efficiently and over a wider range of environmental extremes than can a single enzyme. Biologists theorize that isozymes developed through gene flow between populations from climatic extremes and enhance the possibility of adaptation among members of the species when the occasion arises. The combination and recombination of isozymes passed from parent to offspring are apparently determined by deoxyribonucleic acid (DNA). Investigation into the role of DNA in evolution is one of the most promising avenues to an understanding of the nature of life.

A classic example of the importance of understanding migration and gene flow in the animal kingdom is the spread of the so-called "Killer Bees." In the 1950s, a species of aggressive African bee was accidentally released in South America. The African bees mated with the more docile wild bees in the area; through migration and gene flow, they transmitted their violent propensity to attack anything approaching their nests. As the African genes slowly migrated northward, they proved to be dominant.

Research in the twenty-first century provided further insight into the nuances of gene flow and its applications. For example, scientists used assisted gene flow to successfully fertilize coral eggs from genetically distinct colonies from Florida and Puerto Rico. Using sperm that was flash-frozen, or cryopreserved, and eggs from Puertorican coral, hundreds of Elkhorn corals were bred in captivity to help repopulate the oceans’ critically endangered coral reefs. Because these corals have genes from such a diverse pool, the coral produced has adaptability characteristics that are not observed in natural coral. This success offers promise for the preservation and conservation of the world’s oceans and many threatened species.

In another study of the adaptive evolution of giraffes, scientists discovered that these animals are highly genetically structured, making population isolation easy. In an analysis of the gene flow of lineages between giraffes, scientists found significant hybridization. Additionally, in analyzing the evolutionary history of giraffes, there were so many gene flow events in the historical gene flow of giraffes that scientists referred to it as a “reticulated network” rather than a genealogy tree. This information has applications for giraffe conservation efforts and offers an interesting evolutionary perspective of the animal.

Further research into migration and gene flow promises to provide information indispensable to the attempt to unravel the mysteries of life. Coupled with the concept of mutation, gene flow is a crucial component of evolution.

Principal Terms

Allele: one of a group of genes that occurs alternately at a given locus

Deme: a local population of closely related living organisms

Fossil: a remnant, impression, or trace of an animal or plant of a past geologic age that has been preserved in the Earth’s crust

Gene pool: the whole body of genes in an interbreeding population that includes each gene at a certain frequency in relation to other genes

Mutation: a relatively permanent change in hereditary material involving either a physical change in chromosome relations or a biochemical change in the codons that make up genes

Population: a grouping of interacting individuals of the same species

Speciation: the process whereby some members of a species become incapable of breeding with the majority and thus form a new species

Species: a category of biological classification ranking immediately below the genus or subgenus, comprising related organisms or populations capable of interbreeding

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