Contents
From genetics to ecology, Biology Topics cover a vast array of life sciences.
A Detailed Overview of Organisms and Population Attributes
Life on Earth has evolved from a single cell to the most complex of organisms with millions of cells. The diversity among organisms ranges from single-celled bacteria to huge redwood trees, elephants, and whales. The basis of life in all these organisms is the nucleic acid which undergoes variations and develops successive changes forming new species. The development of new species is the first step toward evolution. Another important factor contributing to the process of evolution is Natural Selection.
Nature selects the variations which prove to be the fittest to survive in the world. Such variation can be slowly adapted or can again be due to mutations. The selected variations then get perpetuated through the generations and the specific organism gets established in nature. The same process has led to the establishment of the most superior and intelligent organism, humans.
Concept of Population
It is, therefore, true that evolution is promoted through the origin of variation. As a result of evolution, we get some new species that usually were absent on Earth in the previous period. In the beginning, the earth was only inhabited by the single-celled anaerobic prokaryotic form of organisms. With the emergence of an aerobic atmosphere, a simplified aerobic organism formed. In the course of time gradually multicellular forms of animals and plants took their existence. The most complex forms of organisms such as vertebrates and land-dwelling tetrapods appeared much later in time and space.
However, behind the origin of all different forms, variations appeared to be the ultimate source of evolution. The species are the units of evolution and by the process of evolution, one species may give birth to another species. The difference between species is measured through the variation they possess. Variation may appear in various ways and that the individual level. But individualistic variation is not very important for the process of evolution. The same variation when appears in a population of species, is considered to be an important factor for evolution. A population represents a group of interbreeding organisms of a species inhabiting a particular geographical area. During evolution, a whole group belonging to a species accumulate variations in such a way that the group may be reproductively isolated from such group.
The genes present in a population of organisms determine the variation available in organisms. Hence, the genetic structure of a population is important in consideration to the process of evolution. In the context of the present state of knowledge, gene pools differ from species to species. Sum total of genes present in a species is its gene pool and all the members of a species share a common gene pool. The structure of the gene pool and frequency of each gene in a species population is always under consideration for assessing the process of evolution. With the change in gene frequency, a population becomes unstable promoting evolution.
The Mendelian concept provides an idea of the principles of genetics involving the inheritance of genes in individuals. But the fate of these genes in the population is not known. Inheritance of individual genes may be governed by Mendelian principles, but the frequencies of individuals carrying these genes may depend on several factors, the frequency of a particular gene in the population, the size of the population, and other factors. The distribution of a particular gene and its alleles in time and space is not dependent on individuals carrying this gene but is governed by the properties of the population.
A population consists of a community of sexually interbreeding organisms inhabiting a particular geographical region. This population may consist of humans or any group of animals or plants carrying one or more genes. These populations are called ‘Mendelian Populations’ by Sewall Wright because individuals of these populations follow the Mendelian principle of inheritance.
Charles Darwin considered evolution as Descent with Modification. The present concept of evolution has regarded it as a change in the genetic composition of the population rather than the change at the individual level. Therefore, the unit of evolution is the population. A population is any group of interbreeding organisms. The human population is large population. A large population comprises a number of local populations. Local population may be defined as The individuals of a given locality that potentially form a single interbreeding community or Deme. Deme, therefore, is a local population of a species, the community of potentially interbreeding individuals at a given locality.
Among geneticists and evolutionists, a population is usually defined as a community of sexually interbreeding or potentially interbreeding individuals. Since Mendelian laws apply to the transmission of genes among these individuals, such a community has been termed by Wright as a Mendelian population. The size of the population may vary, but it is usually considered to be a local group (also called deme), each member of which has an equal chance of mating with any other member of the opposite sex.
Population Attributes
The populations can be said to have two important attributes gene pool and gene frequencies.
A. Gene Pool
The genes in a population comprise its gene pool. The gene pool is defined as the sum total of genes present in a Mendelian population. The genes are embodied in the individuals and are passed on to the next generation in the reproductive gametes of a population. Therefore, the gene pool can also be considered a gametic pool. The gene pool is the totality of the genes of a given population existing at a given time. The study of the gene pool of a population reveals
- the total number of genes
- their kinds and variety present in the population
- the proportion of different kinds of genes
- also, the way these kinds are distributed among the individuals of the population.
If the two genes are present in equal proportions and these are represented by A and a, the gene pool in the state of equilibrium will contain \(\frac{1}{4}\)AA, \(\frac{1}{2}\)Aa and \(\frac{1}{4}\)aa and this will be maintained as long as random mating occurs. The proportion of alleles of a particular gene in a population is represented by gene frequency and their distribution among the individuals by genotype frequency.
Gene Flow:
The movement of alleles from one population to the other is gene flow.
The gene pool of a population maintains its integrity as long as there is no interbreeding between populations. Due to the migration of some members of one population to the other, gene transfer or gene flow from one population to the other may be possible. Gene flow leads to the mixing and reshuffling of the gene pool. Transferring from one population to another usually occurs through migration.
B. Gene Frequencies
Gene frequency is defined as “The percentage of a given gene in a population”. Gene frequencies are simply the proportions of the different alleles of a gene in a population. To obtain these proportions, count the total number of organisms with various genotypes in the population and estimate the relative frequencies of the alleles involved. For example, if we consider the human MN blood group, there are a total of 200 genes in a population that contains 50 MM, 20 MN, and 30 NN individuals. Of these, 100 + 20, or 0.6 of the total, is M, and 20 + 60, or 0.4 of the total, is N. The same gene frequencies can also be calculated from frequencies of the three genotypes, MM, MN, and NN according to the formula:
Frequency of a gene = frequency of homozygotes for that gene + \(\frac{1}{2}\) frequency of heterozygotes
Thus, frequency M = 0.5MM + (\(\frac{1}{2}\)) 0. 2MN = 0.6
and frequency N = 0.3NN + (\(\frac{1}{2}\)) 0. 2MN = 0.4
Gene frequency refers to the percentage of a certain allele among all alleles for that gene in a population.
For example, if an allele occurs as 12 in every 50 individuals in a population, its frequency is 12%.
Changing Gene Frequencies Underlie Evolution:
Microevolution occurs when the frequency of an allele in a population changes. This happens when any of the following conditions are met:
- Mutation introduces new alleles into a population.
- Individuals migrate between populations.
- Individuals remain in groups, mating among themselves.
- Some genotypes tolerate specific environments.
- Certain genotypes do not produce fertile offspring eliminating their genes from the population.
Population and its Genetic Structure
When population in terms of genetic structure is considered it entails all the genes present in a population. To consider all the genes of a population at a time it becomes a very tough and tedious job. Hence, to consider the genetic structure of a population it is better to consider such structure in terms of a specific gene. In the human population, one type of blood group belongs to the ABO system which is expressed through A, B, AB, and O blood types among the human individuals in a particular population. These phenotypes are controlled by three different allelic genes namely, IA, IB, and i.
However, never a particular blood type may carry all the 3 alleles together. But, if we consider all the individuals of a population together we may get the total picture of all these three alleles in the population. The total number of all these alleles in the population in consideration to the ABO system determines the gene pool of the population for a particular gene-determining ABO phenotypes. The genes from this population may be transmitted to the individuals of the next generation through the gametes they produce. Thus, the gene pool of a preexisting population may be carried into the population in the next generation. This is the way by which the stability of a species is maintained in nature. Any change in the structure of the gene pool may affect this stability promoting evolutionary changes.
Measuring the Allelic Frequencies in a Population
The genetic structure of a population may be expressed by the allelic frequencies for a gene present in a population. Therefore, the method for measuring the allelic frequencies is the primary task of a student of population genetics. The population is always considered a dynamic structure because it moves through various sorts of changes. One such change is the change in the gene frequencies or the allelic frequencies. The frequency refers to the proportional representation of a particular component or event within the total. Hence, the allelic frequencies mean a proportion of different types of alleles of a gene present in a population.
Say, there are two alleles of a gene as A and a, if we consider the frequencies of these two alleles are p and q, then p + q = 1. With this idea we may say p represents the frequency of the allele A and q represents the frequency of the allele a. According to Mendelian’s principles of inheritance, the distribution of these two alleles in the population may represent the presence of three genotypes AA, Aa, and aa. If the frequencies of these three genotypes are known then the frequency of the individual allele may be calculated. Say the frequencies of the three genotypes are p2 : AA, 2pq : Aa, and q2 : aa, then the frequency of A may be denoted as p2 + \(\frac{1}{2}\)(2pq) or p2 + pq or p (p + q) = p and that of a may be denoted as q2 + \(\frac{1}{2}\)(2pq) or q2 + pq or q(p + q) = q, p + q = 1.
The frequency of the alleles of a gene may also be determined if the total number of individuals for different genotypes for the alleles is known. For example, in the human population, there is a blood group known as the MN blood group which is determined by two different alleles namely LM and LN. For these two alleles, there are three possible genotypes and corresponding phenotypes. Namely, the M blood group (genotype LMLM), the N blood group (genotype LNLN), and the MN blood group (genotype LMLN). Suppose the number of this individual in a population of 1000 shows 540M, 260 MN, and 200N, then the frequency of two alleles in the population may be calculated as under.
No. of homozygotes for the concerned genes × 2 + no. of heterozygotes
Total number of individual × 2 = \(\frac{400+260}{2000}=\frac{660}{2000}\) = 0.33
Hence, the frequency of LM + the frequency of LN = 0.67 + 0.33 = 1
This method of calculation for measuring allelic frequencies of any gene is applicable in any situation.
Example 1.
Suppose in a population there are two alleles as R1 and R2 with the phenotypic expressions corresponding to different genotypes are R1R1, R1R2, and R2R2. The numbers of three different genotypes in the population are 250, 60, and 30 respectively. To find out the frequency of the two alleles (R1 and R2) the following calculation may be applied-
Solution:
Let, the frequency of R1 = p and that of R2 = q
Therefore, frequency of p + q = 0.82 + 0.18 = 1
Example 2.
In Mirabilis jalapa colour of the flower may be red or white. The colours are determined by R and r, but R is incompletely dominant over r. Therefore Rr genotype gives a different and intermediate phenotype, pink coloration. In a garden, there are 1000 plants with the distribution of the different plant varieties as 400 red variety, 400 pink variety, and 200 white variety plants. Find out the frequency of the two alleles in this population of plants.
Solution:
Let, the frequency of R is p and that of r is q, then p + q = 1.
Therefore, the three genotypes RR, Rr, and rr represent Red, Pink, and White variety plants (R is incompletely dominant over r).
In this condition, 400 red variety plants represent the RR genotype, and 400 pink variety plants represent the genotype.
