Microevolution is the change in the genome, or gene pool, for a given species in a relatively short period of geologic time by the alterations of successfully reproducing individuals within a population. Evolution is the change in the gene pool of a species such that the organism becomes a new species. Environmental pressure favors changes that allow an organism to reproduce more successfully in that environment. Some environmental conditions are more harsh than others, and organisms may have to adapt more to survive—areas where the environmental pressures are stable, or the organisms have adapted to it, exhibit nonevolving populations. Interestingly, in a nonevolving population, the allele frequency, genotype frequency, and phenotype frequency remain in genetic equilibrium. In other words, the random assortment of genes during sexual reproduction does not alter the genetic makeup of the gene pool for that population. This phenomenon was illuminated by the mathematical reasoning of a German physician, Weinberg, and a British mathematician, Hardy, both working independently in 1908. Their combined efforts are now known as the Hardy-Weinberg equilibrium model.
To demonstrate the Hardy-Weinberg equilibrium, assume G and g are the dominant and recessive alleles for a trait where GG = green, gg = yellow, and Gg = orange. In our imaginary population of 1,000 individuals, assume that 600 have the GG genotype, 300 are Gg, and 100 are gg. The allele and genotype frequency for each allele is calculated by dividing the total population into the number for each genotype:
After the allele frequency has been determined, we can predict the frequency of the allele in the first generation of offspring.
First, determine the total number of alleles possible in the first generation. In this imaginary case, because each organism has 2 alleles and there are 1,000 organisms, the number of possible alleles in the first generation of offspring is:
Next, examine the possibility of each allele. For the G allele, both GG and Gg individuals must be considered. Taken separately,
The letter p is used to identify the allele frequency for the dominant allele (.75) and q for the recessive allele (.25). Note that p + q = 1.
The frequency for the G allele is therefore:
For the g allele, the calculation is similar:
The frequency for the g allele is therefore:
Hardy-Weinberg can also predict second-generation genotype frequencies. From the previous example, the allele frequencies for the only possible alleles are p = .75(G) and q = .25(g) after meiosis. Therefore, the probability of a GG offspring is p × p = p2 or (.75) × (.75) = 55 percent. For the gg possibility, the allele frequencies are q × q or (.25) × (.25) = 6 percent. For the heterozygous genotype, the dominant allele can come from either parent, so there are two possibilities: Gg = 2pq = 2(.75)(.25) = 39 percent.
Note that the percentages equal 100, and the allele frequencies (p and q) are identical to the genotype frequency in the first generation! Because there is no variation in this hypothetical situation, it is in Hardy-Weinberg equilibrium, and both the gene and allele frequencies will remain unchanged until acted upon by an outside force(s). Therefore, the population is in a stable equilibrium with no innate change in phenotypic characteristics. As mentioned in Historical Development and Mechanisms of Evolution and Natural Selection, populations tend not to stay in Hardy-Weinberg equilibrium for very long because of environmental pressures.
The Hardy-Weinberg equation highlights the fact that sexual reproduction does not alter the allele frequencies in a gene pool. It also helps identify a genetic equilibrium in a population that seldom exists in a natural setting because five factors impact the Hardy-Weinberg equilibrium and create their own method for microevolution. Note that the first four do not involve natural selection:
Refer to Historical Development and Mechanisms of Evolution and Natural Selection for a more in-depth coverage of the natural selection model.
A mutation is an inheritable change of a gene by one of several different mechanisms that alter the DNA sequencing of an existing allele to create a new allele for that gene.
A primary mechanism for microevolution is the formation of new alleles by mutation. Spontaneous errors in the replication of DNA create new alleles instantly while physical and chemical mutagens, such as ultraviolet light, create mutations constantly at a lower rate. Mutations affect the genetic equilibrium by altering the DNA, thus creating new alleles that may then become part of the reproductive gene pool for a population. When a new allele creates an advantage for the offspring, the number of individuals with the new allele may increase dramatically through successive generations. This phenomenon is not caused by the mutation somehow overmanufacturing the allele, but by the successful reproduction of individuals who possess the new allele. Because mutations are the only process that creates new alleles, it is the only mechanism that ultimately increases genetic variation.
Gene migration is the movement of alleles into or out of a population either by the immigration or emigration by individuals or groups. When genes flow from one population to another, that flow may increase the genetic variation for the individual populations, but it decreases the genetic variability between the populations, making them more homogeneous. Gene migration is the opposite effect of reproductive isolation, which tends to be genetically near the Hardy-Weinberg equilibrium.
Genetic drift is the phenomenon whereby chance or random events change the allele frequencies in a population. Genetic drift has a tremendous effect on small populations where the gene pool is so small that minor chance events greatly influence the Hardy-Weinberg arithmetic. The failure of a single organism or small groups of organisms to reproduce creates a large genetic drift in a small population because of the loss of genes that were not conveyed to the next generation. Conversely, large populations, statistically defined as greater than 100 reproducing individuals, are proportionally less affected by isolated random events and retain more stable allele frequency with low genetic drift.
Two types of genetic drift act when a large population is modified to be considered statistically as a small population: fragmentation effect and pioneer effect.
The fragmentation effect is a type of genetic drift that occurs when a natural occurrence, such as a fire or hurricane, or man-made event, such as habitat destruction or overhunting, unselectively divides or reduces a population so it contains less genetic variability than the once-large population. A large population may become fragmented when a man-made dam creates a large lake where once an easily forded river provided no obstruction of movement. Likewise, a new highway can isolate species on either side. The net result is a small fragment that becomes reproductively separated from the main group. The fragmented group did not become isolated because of natural selection, so it may contain a fragment or all of the genetic variation of the larger population.
Even when small populations recover, their genetic variability is still so low that they remain in danger of extinction from a single catastrophic event.
Zoos spend a great deal of time, money, and energy in an effort to increase their genetic diversity by locating new breeding organisms for mating, usually from other zoos.
The pioneer effect occurs whenever a small group breaks away from the larger population to colonize a new territory. Like the fragmentation effect, the pioneers, which may consist of only a single seed or mating pair, remain an extinction threat because they do not have the genetic diversity of the main body and therefore are less likely to produce offspring capable of surviving changes in the environment. Even though a pioneer population reproduces successfully, the gene variation has not increased. So the danger involved with survival in a changing environment still exists!
The Hardy-Weinberg equation assumes that all males have an equal chance to fertilize all females. However, in nature, this seldom is true because of a number of factors such as geographical proximity, as is the case in rooted plants. In fact, the ultimate nonrandom mating is the act of self-fertilization that is common in some plants. In other cases, as the reproductive season approaches, the number of desirable mates is limited by their presence (or absence) as well as by their competitive premating rituals. Finally, botanists and zoologists practice nonrandom mating as they attempt to breed more and better organisms for economic benefit.
Excerpted from The Complete Idiot's Guide to Biology © 2004 by Glen E. Moulton, Ed.D.. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.