11 Forces of Evolution

Ms. Sangeeta Dey and Prof. A.K. Kapoor

Contents:

1. Introduction

2. Hardy – Weinberg principle

3. Forces that affect Hardy – Weinberg principle

4. Mutation

5. Gene migration or gene flow

6. Genetic drift

7. Genetic recombination

8. Natural Selection

9. Evolution at a species level Summary

Learning Objectives:

To understand about evolution
To understand the genetic variation
To know the factors which affect Hardy – Weinberg equilibrium
To know about factors that affect the gene pool of a population by altering allele frequency.
To understand the impact of natural selection on organisms
To understand the importance of mutations, recombination, genetic drift on evolution.

1. Introduction

Evolution is an incredibly common phenomenon and may occur between generations for every group of organisms in the world, including humans. Over a period of time, the relative proportions of alleles in the population changes, some may increase and some may decrease and still other may remain the same. Over a short run of just a few generations, such changes in inherited traits may be very small, if further continued and elaborated, the results can and do produce spectacular kinds of adaptation and whole new varieties of life emerges due to these changes. These short term effects and variation in allele frequency in a population from one generation to the next is referred to as microevolution whereas the long term effects through fossil history are sometimes called macroevolution. However, the basic evolutionary mechanisms are similar.

Darwin described evolution as the gradual unfolding of new varieties of life from previous forms over long periods of time due to the evolutionary process. But these long term effects can only come out by accumulation of many small evolutionary changes occurring in every generation. Today we study evolutionary changes occurring between generations and are able to demonstrate how evolution works. We define evolution from the modern genetic perspective as a change in allele frequency from one generation to the next generation.

Allele frequencies are numerical indicators of the genetic makeup of a population, and the population is referred as an interbreeding group of individuals. An inherited trait may be present in slightly different form in different individuals. The variant genes that underlie these different forms in inherited trait are called alleles. The different expression of the inherited traits is the result of genetic variation within a population. The frequencies for combination of genes represent the proportions of a total and hence allele frequencies refer to only the whole groups of individuals i.e. populations. Individuals do not have the allelic frequencies; they have the genes or the combination of these genes. Therefore an individual cannot evolve, only a group of individuals can evolve over time.

Genetic variation is the morphological or physiological changes in an organism that takes place due to change in the genetic composition or in the environment. The environment never remains the same but it changes due to several factors. So, in order to adapt with the changing environment, the variations in an organisms are necessary to combat the harsh environmental conditions and thus survival. If there are no variations in an individual, then they will not be able to adapt accordingly with the changing environment and ultimately die. The presence of variability ensures the fitness of the individuals who are well adapted to the environment due to their heritable qualities in a population. The interaction of variation and natural selection causes evolution to occur.

How do allelic frequencies change? Or how does evolution occur? The modern theory of evolution isolates general factors that can produce alterations in allele frequencies. These factors are responsible for the introduction of variant genotypes within a population and due to this, different phenotypes evolve and depending on the suitable phenotype, the population adapt and compete for survival. Thus, survival of the fittest occurs and they are able to produce more offspring who possess adaptable phenotypes and thus those phenotypes are selected over generations by natural selection.

Evolution is a two stage process. The first stage is introduction of genetic variation within the population which are then distributed before it can be acted upon by natural selection which is the second stage.

2. Hardy-Weinberg principle

Hardy-Weinberg principle states:

Allele frequencies in a population are stable and is constant from generation to generation.
The gene pool (total genes and their alleles in a population) remains constant. This is genetic equilibrium.
Sum total of all the allelic frequencies is 1. For example, in a diploid individual, p and q represents the frequency of allele A and allele a. The frequency of AA individuals in a population is simply p2. Similarly, the frequency of aa is q2 and of Aa is 2pq. Hence, p2 + + 2pq + q2 = 1. This is an binomial expansion of (p + q)2.

When frequency, calculated or measured, differs from expected values, the difference (direction) indicates the extent of evolutionary change. Disturbance in genetic equilibrium or Hardy-Weinberg equilibrium that is change in frequency of alleles in a population would then be interpreted as resulting in evolution.

3. Forces that affect Hardy-Weinberg Principle

The various forces that are known to affect Hardy-Weinberg equilibrium and causes genetic variability within a population are:

Mutation: The pre-existing advantageous mutations when selected result in formation of new phenotypes.
Gene migration or gene flow: It is the transfer of genes between populations that differ genetically from one another but can interbreed. When migration of a section of population to another population occurs, gene frequencies change in the original as well as in the new population. New genes added to the new population and these are lost from the old population.
Genetic drift: It is the change in the gene frequency which is purely as a matter of chance. The smaller populations have greater chances for genetic drifts.
Genetic recombination: The exchange of genetic material provides new combination of genes, especially in large population with large gene pool. Thus, new combination of old genes produces new combination of characters in the organisms and adds to genetic variability.
Natural Selection: It is the process in which heritable variations enabling better survival to reproduce and leave greater number of progeny.

Thus, the occurrence of variations due to mutations or recombination during gametogenesis or due to gene flow or genetic drift results in changes in the frequency of genes and alleles in future generation.

4. Mutations

An actual alteration in the genetic material is called mutation. A mutation is a change in the basic sequence of DNA. For such changes to have evolutionary significance, they must occur in the sex cells, which are passed on between generations. Evolution is a change in the allelic frequencies between generations. If mutations do not occur in sex cells, either the egg or the sperm, they will not be passed to the next generation and no evolutionary change can result. If however a genetic change does occur in the sex cells, this change or mutation can change the allelic frequencies in the subsequent generation.

Geneticists often distinguish between two kinds of mutations, chromosomal aberrations, which may result in alterations in the amount of position of genetic material and point mutations, which are permanent, heritable changes within a gene. Mutations occur frequently in the nature and have been reported in many organisms. In man, mutations cause variations in hair colour, skin pigmentation and several malformations. Usually mutations bring about changes in the genetic structure at the gene level. All those factors which bring about a change at the gene level are termed as chromosomal aberrations. Chromosomal aberrations bring about a change in either the structural aspects of a chromosome or they may bring about a change in the number of the chromosomes present in the organism.

Actually, it would be rare to see evolution occurring by mutation alone. Mutation rates for any given trait are quite low, and thus, their effects would rarely be seen in small populations. In larger populations, mutations might be observed but have limited impact on shifting allele frequencies. However, when mutation is coupled with natural selection, evolutionary changes are quite possible.

Mutation is the basic creative force in evolution and in fact is the only way to produce “new” variation. Its key role in the production of variation represents the first stage of evolutionary process. Darwin was not aware of the nature of mutations. Only in the last century, with the spectacular development of molecular biology, the secrets of this phenomenal force have been revealed to evolutionary biologists.

5. Gene Migration or Gene Flow

Migration is the movement of genes from one population to another. If all individuals in a population do not choose their mates from within the group, significant changes in allele frequencies could occur. If a change in the allelic frequency does takes place, evolution will have occurred, this time by migration. Migration works both ways – changes in the genetic frequencies will be affected in both the cases of in-migration and out-migration. In humans, social rules, more than any other factor determine mating patterns, and cultural anthropologists must work closely with physical anthropologists in order to isolate and measure this aspect of evolutionary change.

6. Genetic Drift

The change in the frequency of a gene variant in a population due to random selection factor in evolution is called genetic drift . Since evolution occurs in populations, it is directly tied not only to the nature of the initial allele frequencies of the population, but to the size of the group as well. If, in a population of say 100 individuals, two blood group type O individuals are killed in a auto accident before completing reproduction, their genes would have been removed from the population. The frequency of the O allele would have been reduced in the next generation, and evolution would have occurred. In this case, with only 100 individuals in the population, the change due to the accident would have altered the O frequency in a noticeable way. If, however, the initial population had been very large then the effect of removing a few individuals will be very small indeed. In fact, in a large size population, random effects such as traffic accidents would be balanced out by the equal probabilities of such events affecting all the other individuals with different genetic combinations or different genotypes. Evolutionary change due to genetic drift is directly and inversely related to population size. It is evident that the smaller the population, the larger is the effect of genetic drift.

When considering genetic drift, the genetic makeup of individuals is in no way related to the chance happenings that would affect their lives. If the individual, because of some such hereditary trait, dies early and produces fewer offspring than the other individuals, then this is not random genetic drift but a natural selection.

7. Genetic Recombination

Parents contribute genes to the offspring in any sexually reproducing species and the genetic information is inevitably reshuffled every generation. Such recombination does not in itself change allele frequencies. However, it does produce the whole array of genetic combinations, which natural selection can then act upon. In fact, the reshuffling of chromosomes during meiosis can produce literally trillions of gene combinations, making every human being genetically unique.

The phenomena and the process by which the genetic combination of the offspring becomes different from the parental combination is called recombination or genetic recombination. Compared to mutation, recombination has got the greatest role to produce variability. Mutation and gene flow or migration can produce variability in a population with respect to single gene. Recombination can combine the novel alleles, which at first are likely to be carried out by different individuals in a single genotype. Furthermore, recombination can multiply the different gene types in the population. It converts a small initial stock of multiple gene variation into a much greater amount of genotype variations.

Adaptation is the attribute of the assembly of beneficial genes rather than a single gene. These assemblies are harmonized in the covers of evolution to suit best in the environment and obviously, such assembly is essential for the evolution. Recombination is the only process which can form such assembly. Sexual reproduction is the chief mechanism of recombination in higher eukaryotes which have high chromosome number, have high gene number, and reproduce sexually. Hence these organisms have maximum recombinant forms. This is the reason why no individuals are alike, which develop from different zygotes (exception being the identical twins which develop from the same zygote).

The commencement of evolution is from mutation in the form of multiple gene variation. Later, through recombination the assembly of adaptive gene is over and they are competent enough to be selected i.e., attainment of requisite adaptation. Thus, recombination is the mid-point in the process of evolution.

8. Natural Selection

The evolutionary forces such as mutation, gene flow, genetic drift and recombination interact to produce variation and to distribute genes within and between populations. But there is no long term direction to any of these factors. What then does enable populations to adapt to changing environments? The answer is of course natural selection – the process so well elucidated by Darwin more than 125 years ago. Genetic variations among individuals within a population results in variations which influence reproductive success which ultimately leads to increased number of offspring. This is natural selection which selects the fittest genetic variant which is able to contribute more offspring to the succeeding generations. In fact, natural selection is defined as differential net reproductive success.

Natural selection leads to change in allele frequency relative to specific environmental factors. If, environment changes the selection pressures changes as well. Such a functional shift in allele frequencies is referred as adaptation. If there are long term environment changes in a consistent direction then allele frequencies should also change gradually in each generation. If sustained for many generations, the results may be quite dramatic.

Unit of Selection: Selection acts on individual. It is an individual who reproduces or do not reproduce and who continually attempt to maximize their own reproductive success. Thus the individual, not the group, is the unit of selection. If the total reproductive success of all members of a population continuously falls below replacement value that is where more individuals die than are born in a generation, the population will become extinct. Individuals will attempt to maximize their own reproductive success even in the face of such impending extinction.

Overpopulation is the result of individuals maximizing their reproductive success. Even if such behaviour means the whole group will perish, no special evolutionary mechanisms exist to keep individuals from reproducing at their greatest capacity. Of course, humans have the potential to manipulate their numbers, and the entire world now faces the problem of controlling an exploding population. Evolution, therefore, has no built-in mechanisms to guard against extinction. Indeed, extinction is actually the rule – not the exception – in evolution. Of all the species that have ever lived on earth, it is estimated that less than 0.1% are now currently living; the remaining 99.9% met their almost inevitable evolutionary fate.

Unit of Evolution: While selection acts on the individual, changes in allele frequency occur between generations for an entire population; that is, an inbreeding group of organisms. The net result of all individuals’ reproductive success (natural selection) – in addition to the possible effects of mutation, migration and genetic drift – will affect the entire population. The population, then, is the unit of evolution, and it is the population that changes from generation to generation.

Modes of Selection: Depending upon the environmental changes i.e., the rate at which the changes occurs like slow change, abrupt change or change in the ecological diversity, the population is thrown to different environments and is hence selected accordingly. This sets particular pattern of selection which is termed as mode of selection. There are three modes of selection.

Directional Selection: Selection pressures shifts as environmental pressures change gradually. If the environmental pressure is directional, selection should also be directional. For example, in the last one million years, there have been long periods of gradual cooling in the earth’s climate. One means of coping with cold among mammals has been increase in body size. If we assume animals, such as mammoths, to be descendents from smaller ancestors, how did they gradually get bigger in size? The answer lies in understanding natural selection. Those animals with genotypes with larger overall size perhaps resisted cold better, lived longer, nourished their offspring better and mate more often and so forth. In any case, the result was that they reproduced more successfully than other smaller individuals. As the climate continued to grow progressively colder, allele frequencies also continued to shift and average mammoth size gradually increased.
Stabilizing Selection: If, on the other hand, environments remain relatively stable, there should be selection for those genotypes already established within an “adaptive plateau”. In other words, those phenotypes in the centre of the population distribution or “modal” varieties should have higher reproductive success, and those at either extremes will be selected against, as long as the environment remains stable. If we again choose size as a characteristic, we can note that some varieties of turtles have not changed much from that of previous generations. Therefore, the “optimal” phenotype is probably an average-sized turtle and those much larger or much smaller should be selected against.
Diversifying Selection: In this type of selection, phenotypic variants at both extremes are favoured and those in the centre of distribution are selected against. For example, in the case of baboon size relative to predation pressure, it may be advantageous to be small and less conspicuous or very large for active defense. Over time, such selection pressure would act to create a twin peak which is bimodal phenotypic distribution in nature. In fact, such a process would not have long-term effects in a fully reproductive population, since individuals of all sizes would be mating with each other, thus producing individuals with intermediate body size. In order to let the diversifying selection pressures to be maintained, some degree of reproductive isolation between segments of the population would be required. Another possibility is that phenotypic variation can be partitioned and maintained by other genetic differences for example sex. In fact, baboons do differ markedly in size with males averaging about twice the size of the females.

9.Evolution at the Species Level

A species is defined as a group of interbreeding organisms that are reproductively isolated and, therefore, cannot successfully interbreed with other species. The capacity to reproduce is the critical factor in defining species. Theoretically, one can test whether two kinds of organisms are members of different species by observing their reproductive behaviour under natural conditions that is who mates with whom and by observing the results of the offspring which are fertile.

Actually, a species is composed of subunits that are breeding communities which are called as populations. All members of a species potentially interbreed, and some degree of migration or interbreeding is theoretically possible between all populations of that species. The ultimate result of all forces of evolution acting on all populations determines the fate of the species as a whole. If sustained over a long period of time, gradual changes in allele frequencies between member populations can eventually lead to sufficient genetic differences, so that fertile reproduction is no longer possible. We then may recognize a new form of life having arisen from one species “splitting” and producing new species, a process called speciation. If on the other hand, total reproductive success is so low that all or even most populations become extinct, the whole species will be doomed and will disappear from the earth forever.

Summary

Evolution is an incredibly common phenomenon and may occur between every generation for every group of organisms in the world, including humans. Over a period of time, the relative proportions of alleles in the population changes, some may increase and some may decrease and still other may remain the same. Darwin saw evolution as the gradual unfolding of new varieties of life from previous forms over long periods of time due to the evolutionary process. But these long term effects can only come out by accumulation of many small evolutionary changes occurring in every generation. The various forces that are known to affect Hardy-Weinberg equilibrium and causes genetic variability within a population are mutation, gene migration, genetic drift, genetic recombination and Natural Selection. Thus, the occurrence of variations due to mutations or recombination during gametogenesis or due to gene flow or genetic drift results in changes frequency of genes and alleles in future generation.

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