In the complicated processes of genetics described in blogpost 20, it is evident that there are multiple areas where even minor alterations in the function of genes, the production of proteins, and the influence of external factors can result in hereditary changes that are profound over time. Evolutionary changes take place over a wide range of time scales, from days to millennia. Macroevolution refers to large scale evolutionary changes that take place over long time periods, such as the splitting of one species into two. Although some processes of macroevolution are observable in human time frames, such as the hereditary increase in size of humans, biologists usually must make inferences about macroevolution based on fossil records or patterns in existing organisms. Microevolution refers to small survivable changes in the characteristics of a population–a group of individuals of the same species that live in the same geographic area and are therefore capable of interbreeding. Microevolution, which includes changes in a single gene, small alterations in the process of protein production and degradation, physical traits, or behavior, can occur over very short periods, especially in organisms with short gestation periods or brief life spans, and often can be observed by biological scientists. A regular example is seen in the rapid development of antibiotic resistance in bacteria.
Population geneticists examine a population’s gene pool (the total of all the different variations of genes that exist in a given population) and observe changes in the frequency of different alleles over time which can be documented using the Hardy-Weinberg Theorem as described in evolution blogpost 5. Any deviation from the Hardy-Weinberg Theorem suggests that the population is adapting and that the process of microevolution is taking place.
There are five basic causes of microevolution: 1. Mutation, 2. Genetic drift, 3. Gene flow, 4. Non-random mating, and 5. Selection. Any of the five processes may alter the genetic structure of a population, including allele frequencies, genotype frequencies, and genetic variation–the number of different alleles per gene; and, in sum, may lead to the eventual profound change of speciation.
The following is a very brief note on each of the five processes:
- Mutation is the random change in the DNA sequence of a gene–the only process that produces new alleles. As the number of alleles present in a gene pool increases, the likelihood of mutation and genetic variation in the population also increases. A number of events in DNA replication can bring about a mutation: changes in sequencing, positioning, loss or duplications, or insertion of a foreign substance into the gene. In general, because the rates of mutation are so low, they have a relatively minor effect compared with the other evolutionary processes. Genetic and molecular biologists estimate that one mutation occurs in every 100,000 genes per generation. Humans have 25-26,000 genes which means that approximately one in every four babies has a new mutant. The chances are slim that the mutation will have an effect on survival and will be selected as a trait contributing to fitness to produce a change that is reflected in the population. Nevertheless, mutation is the key to evolution, and without it, evolution would not take place. Because of the rarity of beneficial mutations, the process of evolution as dependent on mutation is understandably extremely slow.
- Genetic drift is an unpredictable, chance change in allele frequency that causes one allele to become more common in a population than another allele. Genetic drift can occur slowly over time, or it can occur as the result of a sudden decrease in population size due to bottlenecks and what is known as founder effects. In most instances, the larger a population is, the more stable its allele frequencies are, and the less likely it is to experience genetic drift.
Bottlenecks occur when a population undergoes a sudden and drastic reduction in size. Natural disasters can cause bottlenecks, and the rapid reduction in a population size may lead to the loss of alleles if they are not present in the surviving population. There are then fewer alleles available to be passed on to the next generation.
Founder effects occur when a few individuals are isolated from their original group and form a new population. When two individuals colonize an island, for example, they may not have all the alleles that were once present in the original population. The result is a change in allele frequency and a decrease in genetic variation, much like that seen in the more global effect of bottlenecks. Both bottlenecks and founder effects lead to changes in allele frequencies if the few surviving or colonizing individuals are not representative of the original population. Often rare alleles are lost during these events as individuals who possess them die off or leave, thereby lowering the genetic frequency of the entire population.
For example, northern elephant seals were hunted to the brink of extinction during the late nineteenth century. As a result, many alleles were lost. Although the population has recovered since that time owing to the intervention of international laws about seal hunting, the effects of the human-caused bottleneck are still present. Northern elephant seals today share many of the same alleles and possess extremely low genetic variation across the entire species. Similar effects are seen in families and cultures that favor consanguineous marriage (e.g. the retention of the hemophilia trait in the French hereditary monarchy).
3. Gene flow occurs when organisms migrate from one population to another. A population might lose some alleles when individuals leave the population and might gain new alleles when other individuals join the population.
4. Nonrandom mating occurs when mating takes place unequally between members of a population. Mates may match up due to individual choice, or selection, as members of a population favor 
certain characteristics possessed by some members over those possessed by others. Nonrandom mating is likely to alter both alleles and genotype frequencies in a population, since the alleles for the preferred characteristics will increase in frequency in the population. Other types of nonrandom mating, such as inbreeding, may alter genotype frequencies as well.
A dramatic example of this phenomenon is found in a question that has puzzled zoologists for more than a century: How did the Peacock get his tail? Charles Darwin first noted that the very choosy peahen plays a crucial role in the evolution of this extravagant sexual display. It is safe to conclude, as he did “that…those males which are best able by their various charms to please or excite the female, are under ordinary circumstances, accepted. If this be admitted, there is not much difficulty in understanding how male birds have gradually acquired their ornamental characters.” However, the magnificent tail comes at a large energy cost: it must be regrown every year; and the resplendent colors of the large tail attract predators. Hamilton and Zuk (1982) first suggested that more showy males were signaling to females that they were, if not parasite free, then at least had a reduced load of parasites. Until recently, there has been little evidence to support this hypothesis. Anders Pape Moller and Marion Petrie, Condition Dependence, Multiple Sexual Signals, and Immunocompetence in Peacocks, Behavioral Ecology 13, No. 2: 248-253, 2002, from Laboratoire d’Ecologie Evolutive Parasitaire, Université Pierre et Marie Curie, and Evolution and Behavioral Research Group, Department of Psychology, University of Newcastle, UK, now suggest that the plumage of the male may specifically convey the strength of the male’s immune system and therefore his desirability as a mate. They took blood samples from male Blue Peafowl (Pavo cristatus) and recorded the numbers of B and T cells, and also measured the peacocks’ tails and counted the number of eye spots. The researchers discovered that the condition and length of the peacock’s tail was related to the production of B cells, and the size of the eye spots was related to the male’s T cell production. That is, females look at aspects of the males’ immune competence as reflected in the degree of elegance of the males’ tails. In effect, the males strut about advertising their health and fitness. Other research in chickens and quails had demonstrated that the immune system is under genetic control; so, offspring will inherit their parents’ ability to fight parasites. The enhanced immunity from parasites for offspring trumps the problems of the energy costs and attractiveness for predators because of enhanced reproductivity. Therefore, the ostentatious adaptation persists.
5. Selection, both natural and sexual, occurs when some individuals leave more progeny than others, resulting in dramatic changes in allele frequencies in a population. Selection is often classified according to those individuals favored, or more fit, to issue more offspring. There are three basic kinds of selection: stabilizing, directional, and disruptive.
Stabilizing selection occurs when individuals with average phenotypes are favored, and those with phenotypic extremes are selected against.
Directional selection occurs when individuals of one phenotypic extreme are favored, and those at the other extreme are selected against.
Disruptive selection occurs when individuals of both phenotypic extremes are favored, and those with intermediate phenotypes are selected against.
An example of disruptive selection can be observed in populations of sticklebacks, a species of fish found in freshwater lakes of Canada. These fish often have two distinct phenotypes: a variation which specializes in feeding at the bottom of the lake, and one that specializes in feeding at the top of the lake. These morphs represent two phenotypic extremes demonstrated in the number of spiny plates they possess. More plates yields more protection against predators. Alterations in the availability to obtain food in the water depth to which evolution has placed them may alter survival. Individuals with intermediate phenotypes are selected against because they cannot feed efficiently at either the top or the bottom of the lake.
The evolution of stickleback fish is more complicated that just being directed by disruptive selection, however. The geologic/fossil record indicates that the oceanic threespine stickleback fish that invaded the lakes and streams created by melting glaciers at the end of the ice age became isolated as the glaciers disappeared. The original ocean form of stickle-backs has a continuous row of more than thirty armor plates running from head to tail. In many fresh water populations, this number has been reduced to a range of from 3 to 9 plates. The selective advantage of plate reduction in freshwater lakes and streams may be due to greater body flexibility and maneuvering while swimming. The evolutionary effect was seen dramatically in Loberg Lake, Alaska when oceanic sticklebacks colonized the lake after a chemical eradication program exterminated the resident populations in 1982. Over a span of just twelve years, from 1990-2001, regular sampling revealed that the frequency of the oceanic form dropped steadily, from 100% to11%, while a form with low plate number rose to 75%, an example of directional selection and a striking example of very rapid microevolution which has occurred during the lifetime of most of the adults of the world.
The evolution of populations proceeds by rules that have been found by researchers over the many years that population genetics have been studied. The rules for constructing phylogenetic trees are:
- Maximum parsimony—A phylogenetic tree is designed to demonstrate the fewest possible evolutionary events–changes in characters–and the fewest instances of homoplasy–shared character states not inherited by a common ancestor; parallel evolution or convergence–between branches. Trees constructed using the rule of maximum parsimony seek to simplify relationships by requiring the fewest assumptions about relatedness as possible.
- Maximum likelihood—Evolutionary events are weighted according to the likelihood of occurrence. As genomic research advances, biologists are finding that DNA characters evolve at different rates, which suggests that homoplasy may happen more often than the rule of maximum parsimony would allow. continued…