Microevolution Research Paper

The primary factors which determine evolution are mutation (genetic change) and selection (in Charles Darwin’s words ‘The preservation of favorable individual differences and variations, and the destruction of those which are injurious’ (Darwin 1859)). The two, however, can be influenced by population structure. Among the variables that should be considered in this latter context, mention should be made of population size, mobility and life histories of its members, sex and the genetic system, and assortative mating.

The dazzling array of organic diversity present on earth has always fascinated mankind. An important property of this variability is that it is discontinuous. Organic matter is organized in discrete individuals and groups of individuals. Moreover, these groups may be isolated reproductively from other similar ensembles. When this occurs it is said that they are different species.

Microevolutionary processes are those that occur within a species. Homo sapiens is a particularly favorable subject for the investigation of these processes because, of course, humans know much more about themselves than is true for any other organism. On the other hand, humans are unique among other forms of life because they developed culture. This species-specific trait varies according to its own laws, and largely independently of biological factors. The reciprocal, however, is not true; the organic evolution in humans may be strongly influenced by cultural processes.

2. Mutation

The information needed to assure the transmission of the biological inheritance is given by a molecule, deoxyribonucleic acid (DNA). The particular sequence of its component units (nucleotides) is important because it codes (through another molecule, ribonucleic acid, or RNA) for the sequences of amino acids, the protein units that combined in arrays make up physical bodies. The unit of heredity is the gene (present in a locus), composed of coding (exons) and noncoding (introns, that probably have regulatory functions, i.e., they may modulate gene expression). The transmission takes place in DNA-protein arrays called chromosomes. The genetic material can occur either in the nucleus of the cells or in the cytoplasm, in an organelle called the mitochondrion.

Detrimental agents or problems during the replication process may lead to mutations, changes in the previously existent DNA sequence. Advances in the molecular study of these structures revealed that many different types of change may occur (alteration in just one nucleotide, deletions or insertions of many of them, triplet (the coding unit is made of trios) expansions, variations in nucleotide position, changes in the process of DNA RNA protein information transfer, unequal recombination or gene conversion between different DNA strands. Moreover, these variations do not occur at random. There are changes that are allowed and others that are forbidden, depending on the importance of the particular DNA region for cell physiology. Their fate will also be much different if they occur in noncoding or coding regions. A special type of change is that which occurs due to horizontal DNA transfer, that is, pieces of DNA (transposons) move from one species to the other, or between chromosome regions of the same species, helped by infectious agents.

3. Natural Selection

Natural selection is also a name that involves many different processes. For instance, selection may be directional, when it changes the adaptive norm of a population; stabilizing, when this norm is protected; or balancing, which includes all those factors that maintain genetic heterogeneity or polymorphisms (common variants). Selection occurs at all levels of the biological hierarchy (molecular, cellular, tissue, organ, individual, population, species, community of different taxonomic entities), and its agents accordingly diverse (Salzano 1975, Williams 1992).

Selection can act through differences in mortality or fertility, and a dialectic dilemma that faces any organism is how much energy it should invest in individual survival, as opposed to reproductive efforts. Throughout human history significant changes have occurred in the emphasis given to these two processes, and they have been connected with subsistence patterns greatly influenced by culture. Among hunters and gatherers, mobility was very important, and the nomadic way of life would disfavor large numbers of offspring. Several measures were then developed to assure a small number of children (herbal contraceptives, mating taboos, abortion induction by mechanical means, infanticide). Variance in number of children, a key evolutionary parameter, was thus restricted. Since population numbers were small, infectious agents would not be very important, and mortality would be mainly determined by other environmental factors or by violent deaths determined by rival groups.

The situation changed with the domestication of plants and animals. The premium now was to have a large number of children, to help in the cultivation and harvest of plants, or to take care of the animals. The possibility of storing large quantities of food made life more independent of the environment, and there was a concomitant increase in population size. Higher mortality levels then developed, due to epidemics and also due to the fact that it is easier to take care of two as compared to, say, 10 children.

Modern technology made possible a better control of number of children and of infectious agents. The action of natural selection, therefore, became more restricted.

This does not mean that the human species is no longer evolving. Our interaction with infectious agents and other parasites can be compared to an arms race, in which development of a given trait in the host may be followed almost immediately by a corresponding change in the attacking organism. Fascinating cases of coevolution can then emerge; some of these were reviewed in Levin et al. (1999). As an example of the possibilities of analytical studies in this area, mention can be made of Andrade et al.’s (1999) work. Using a specific molecular technique, Andrade and colleagues were able to detect Trypanosoma cruzi (the protozoan responsible for Chagas’ disease) directly on tissues of affected organisms, and confirmed experimentally the clear differential tissue distribution of diverse clones. This, of course, is important information for the understanding of the disease’s pathogenesis, with concomitant implications for therapy.

Indirect evidence for the action of natural selection can be obtained using homozygote frequencies at the allele level, and the ratio of synonymous to nonsynonymous DNA changes (that is, modifications that lead to different amino acids as contrasted to those that result in the same amino acid). Salamon et al. (1999) considered these points in relation to three loci of the human leukocyte antigen system, which regulate human immune response against pathogenic agents. For all of them they found strong indication of balancing selection at the amino acid level.

4. Population Size

Natural selection can only operate on the material that is available to it. There are several factors, therefore, that can also influence the destiny of a population or of its gene pool. Size is one of the most important. In a small population, random factors can determine the fate of a given variant independently of its adaptive value. Wright (1978) discussed in detail the several misunderstandings that developed in the adoption and discussion of the concept of random drift. It is important to specify, for instance, whether the phenomenon arose from accidents of sampling, or from fluctuations in the systematic (mutation, selection) pressures. These stochastic events, in combination with the deterministic factors indicated above and migration rates, may lead in a species with multiple partially isolated local populations to what this author calls the shifting balance process of evolution.

Also important, in human populations, are unique events such as the adoption of a given invention or spread of a major technological advance. Thus, the population expansion of Homo sapiens that seems to have occurred about 50,000 years ago is generally associated with the ‘creative explosion’ of the upperPaleolithic-type technology.

Given the importance of population size for evolution, it may be asked what numbers have prevailed for the most part of human history. Of course, only inferences can be made for prehistoric populations, but a series of statistical or mathematical methods have been developed which relate present genetic diversity or the overall branch length of a genealogical tree to a parameter called effective size. The latter is the breeding size of an abstract population in which the effects of population size and subdivision are taken into consideration. It has been suggested that the effective size of human populations is about one-half of their census size.

Harpending et al. (1998) contrasted two hypotheses about past population sizes. The ‘hourglass’ hypothesis proposed that there was a contraction (bottleneck) in the number of human ancestors at some time before the last interglacial in the Pleistocene, but that the previous population was large and distributed over a large part of the Old World. The ‘long-neck’ hypothesis, on the other hand, postulated that the human ancestral population was small during most of the Pleistocene. Data they assembled on the variability of the mitochondrial DNA and Alu insertions (transposition-type elements) suggested that the second hypothesis was the correct one, and that the human population in that period would have had an effective population size of 10,000. Jin et al. (2000), using two sets of dinucleotide repeats (28 and 64, respectively), obtained even lower numbers (2,301–3,275).

5. Migration

Besides population size, the amount of migration among groups is critical in the interpretation of genetic variability. At the tribal level, what happens can be described broadly by the fission–fusion model proposed by Neel and Salzano (1967). Hunter-gatherer groups experience cyclic events of fissions (mainly due to social tensions, the migrating units being composed of lineal relatives) and fusions, as convenience dictates. This demographic pattern changes dramatically as socioeconomic conditions lead to larger agricultural and urban populations. Large-scale migrations also conditioned the mixing of diverse continental groups. The genetic consequences of this population amalgamation were considered by Chakraborty et al. (1988). They concluded that the treatment of these agglomerates as panmictic (random mating) populations can lead to erroneous estimates of mutation rates, selective pressures, and effective population sizes.

A special aspect of the consequences of migration is the ‘founder effect’ (Mayr 1942). This term designates the establishment of a new population by few founders, who carry only a small fraction of the total genetic variability of the parental population. Distinct alleles or gene arrangements can become more prevalent in different regions due to this phenomenon. For instance, in African-derived Americans, the beta-S haplotypes (distinct allele arrangements present in the hemoglobin gene) Benin and Bantu show clearly different prevalences in Brazil and Mexico, as compared to other nations of the continent. Bantu is the most frequent in these two countries, while Benin is the most prevalent haplotype in North America and the Caribbean area (review in Bortolini and Salzano 1999). This difference probably arose due to the diverse source of African slaves that came to the New World during the sixteenth to nineteenth centuries.

6. Assortative Mating

Mating choice is a complex behavior characteristic, which involves psychological, cultural, socioeconomic, and biological variables. Its main evolutionary influence is on the distribution of genotype frequencies. In multiethnic communities there is a clear preference for homogamic matings. Recently, with the use of exclusive matrilineal (mitochondrial DNA) or patrilineal (Y chromosome) genetic markers, it is possible to evaluate the influence of sex when heterogamic unions occurs, in historical perspective. Thus, in Latin Americans of mixed ancestry, the European component was mainly contributed by males, while the Amerindian fraction is mostly derived from females. This is a reflection of unions that occurred in the Colonial period and that could be detected independently of the demographic, cultural, and biological changes that occurred afterwards (Salzano and Bortolini 2001).

7. Intra- and Interpopulation Variability

Studies at the blood group, protein, and DNA levels, all indicated that in humans the intrapopulation variation is far higher than the interpopulation variation. An overall evaluation of this question was made by Barbujani et al. (1997), who indicated that the intrapopulation variability, independently of the markers used, is of the order of 85 percent, that which occurs among populations within a continent is of 5 percent, and that among continents of 10 percent. Intercontinental differences, therefore, are small and do not support the concept of continental ‘races.’ This does not mean that, using a convenient array of genetic markers, we cannot establish with complete confidence the continental origin of a given population or of the ancestors of a given individual (Salzano 1997). Most of the interpopulation variability, also, consists of gradients of allele frequencies (see, for instance, Rothhammer et al. 1997), and not of abrupt discontinuities. The biological data, therefore, is in complete agreement with the ethical concept of the brotherhood of humankind.

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