BIOLOGY Volume 3 - A Guide to General Biology - 2004

27. MECHANISMS OF SPECIATION

27.2. Factors Causing Changes in Populations

The Hardy-Weinberg equilibrium principle states that, under certain conditions, allele frequencies remain constant from generation to generation. Under these conditions, a population will be in a state of genetic equilibrium, and no evolutionary changes will occur. However, the Hardy-Weinberg principle is purely theoretical. Very few populations exist under conditions that maintain this equilibrium (Section 27.1.3).

Section 24.8.4 detailed four Main sources of genetic variation: 1) Crossing Over during Meiosis; 2) independent assortment of Chromosomes during meiosis; 3) random Fertilization; and 4) Mutations. The first three sources are often collectively referred to as sexual recombination; they result in the shuffling of genes. Although these processes lead to The formation of new genotypes and alter genotype frequencies, they do not cause any change in existing alleles, so allele frequencies in the population remain constant. However, many evolutionary changes typically follow the appearance of new alleles, with mutations serving as the primary source of the latter.

The conditions necessary for the Hardy-Weinberg equilibrium are also violated in A number of other cases:

1) when mating is non-random;

2) when the population is small, leading to Genetic Drift;

3) when different genotypes result in differing fertility among the individuals carrying them, creating a genetic load;

4) when Gene exchange occurs between populations.

Each of these situations is examined below.

Class="center">27.2.1. Non-random Mating

In most natural populations, mating occurs non-randomly. In all cases where the presence of one or more inherited traits increases the probability of successful gamete fertilization, sexual Selection takes place. Plants and animals possess many structural and behavioral mechanisms that preclude purely random mating. For example, flowers with larger petals and more nectar than usual are likely to attract more insects, increasing the likelihood of Pollination and Fertilization. The coloration patterns of insects, fish, and birds, as well as their behavioral traits related to nest building, territorial defense, and courtship rituals, enhance selectivity in mating.

The Effect of non-random mating on allele frequencies is demonstrated, for instance, by experiments conducted on Drosophila. In a fruit fly culture initially containing an equal number of red-eyed and white-eyed males and females, all white-eyed individuals disappeared after 25 generations. Observations showed that both red-eyed and white-eyed females preferred to mate with red-eyed males. Thus, sexual selection, as a mechanism of non-random mating, provides certain individuals with a higher reproductive potential, thereby increasing the probability that their genes will be passed on to the next generation. The reproductive potential of individuals with less favorable traits is reduced, and the transmission of their alleles to subsequent generations occurs less frequently.

27.2.2. Genetic Drift

Genetic drift refers to cases where changes in gene frequencies within populations are random and independent of natural selection. Random genetic drift, or the Sewall Wright effect (named after the American geneticist who understood its evolutionary significance), can serve as an important mechanism of evolutionary change in small or isolated populations. A small population may not contain all the genes typical of a given species. Random events, such as the premature death of an individual that was the sole possessor of a particular allele, will lead to the loss of that allele from the population. For example, if a given allele occurs in a population of 1,000,000 individuals with a frequency of, say, 1% (i.e., q = 0.01), it will be possessed by 10,000 individuals, whereas in a population of 100 individuals, this allele will be present in only a single individual, making the probability of its loss by a small population much higher.

Just as an allele can disappear from a population, its frequency can also increase purely by chance. As the name implies, random genetic drift is unpredictable. It may lead to the extinction of a small population, or it may even render it better adapted to its environment or enhance its divergence from the parent population. Over time, under the action of natural selection, this may lead to the formation of a new species. Genetic drift is considered a significant factor in speciation within island and other reproductively isolated populations.

Associated with genetic drift is a phenomenon known as the founder effect. This occurs when a small fraction of a population separates from the parent population and may, by chance, not be entirely representative of the parent population in its allele composition. Some alleles may be absent, while others appear with a disproportionately high frequency. Continued interbreeding within such a pioneer population will create a gene pool that differs in allele frequencies from that of the original parent population. Genetic drift generally reduces genetic variation within a population, primarily through the loss of rare alleles. Prolonged interbreeding among individuals within a small population decreases the proportion of heterozygotes and increases the proportion of homozygotes. Examples of the founder effect have been identified through The Study of small populations established in the USA by religious sects that emigrated from Germany in the 18th century. In some of these sects, marriages took place almost exclusively among their members. In such cases, the frequency of a number of alleles differs greatly from their frequency in populations of both Germany and the USA. For example, the studied community of Dunkers—a religious sect that settled in Pennsylvania—consisted of approximately 100 families; genetic drift must have occurred in such a small population. Blood typing yielded the following results:

These data presumably reflect the results of genetic drift occurring in small populations.

Genetic drift can lead to a decrease in variation within a given population, but it can also increase variation within the species as a whole. Small isolated populations may develop traits uncharacteristic of the main population, which, in the event of environmental change, could confer a selective advantage. Thus, genetic drift plays a role in The process of speciation (The Emergence of new species).

27.2.3. Genetic Load

The presence of unfavorable alleles within heterozygous genotypes in a population is called the genetic load. As noted in Section 27.1.5, certain recessive alleles, which are disadvantageous in the homozygous state, can be maintained in heterozygous genotypes and, under specific environmental conditions, confer a selective advantage to the phenotype; the sickle-Cell allele in malaria-endemic regions is a prime example. Any increase in the frequency of recessive alleles in a population resulting from harmful mutations increases its genetic load.

27.2.4. Gene Flow

In the gene pool of a given interbreeding population, a continuous exchange of alleles takes place among individuals. If allele frequencies do not change As a result of mutations, the gene shuffling resulting from this exchange leads to genetic stability or equilibrium within the gene pool. If a mutant allele arises, it will spread throughout the gene pool as a result of random fertilization.

Such movement of alleles within a population is referred to as "gene flow," though strictly speaking, this term applies to The transfer of alleles from one population to another as a result of interbreeding between members of the two populations. The random Introduction of new alleles into a recipient population and their removal from a donor population alters allele frequencies in both populations and leads to increased genetic variation. Although gene flow introduces genetic variation into populations, its evolutionary effect is ultimately conservative. By spreading mutant alleles across all populations, gene flow ensures that populations of a given species share a common gene pool, thereby reducing differences between them. Consequently, the interruption of gene flow between populations serves as a prerequisite for speciation.

The intensity of gene flow between populations depends on their geographic proximity and the ease with which organisms or Gametes can move from one population to another. For example, two populations may lie so close to each other that interbreeding occurs continuously; in genetic terms, they can then be considered a single population because they share a common gene pool—such as two populations of snails living in adjacent gardens separated by a hedge.

For flying animals and pollen grains, it is relatively easy to disperse actively or passively into new locations. Once there, they may interbreed among themselves or with the local population, thereby introducing genetic variation into it.