Biology

–– Justify the effect of the environment on the phenotype of plants and animals

The earth is inhabited by billions of organisms, every one of which is unique. Individuals belonging to different species are usually easy to distinguish. Members of the same species may differ only in small ways; but even clones (such as identical twins) show some differences. The differences between individuals of the same   species are called variation. These differences between cells, individual organisms, or groups of organisms of any species are caused either by genetic differences (genotypic variation) or by the effect of environmental factors on the expression of the genetic potentials (phenotypic variation). Variation may be seen in; physical appearance (phenotype of individuals), metabolism, fertility, mode of reproduction, behavior, learning and mental ability, and other obvious or measurable characters

Genotypic variations are caused by differences in number or structure of chromosomes or by differences in the genes carried by the chromosomes. Eye color, body form, and disease resistance results by genotypic variations. Individuals with multiple sets of chromosomes are called polyploid. Many common plants have two or more times the normal number of chromosomes and new species may arise by this type of variation.

Variation may be due to either environmental factors or genetic disorders. For example, the action of sunlight on a light- colored skin may result in its becoming darker. Such changes have little evolutionary significance as they are not passed from one generation to the next. Much more important to evolution are the inherited forms of variation which result from genetic changes. These genetic changes may be the result of the normal and frequent reshuffling of genes which occurs during sexual reproduction, or as a consequence of mutations.

15.1.2 Importance of variation
Variation plays different roles such as:
–– Make some individuals better fitted in the struggle for existence.
–– Help the individuals to adapt themselves according to the changing environment.
–– Produce new traits in the organisms.
–– Allow breeders to improve races of useful plants and animals for increased resistance, better yield, quicker growth and lesser input.
–– Constitute the raw material for evolution.
–– Give each organism a distinct individuality.
–– Species do not remain static. Instead, they are slowly getting modified forming new species with time.
–– Pre-adaptations caused by the presence of neutral variations are extremely
useful for survival against sudden changes in environment, e.g., resistance against a new pesticide or antibiotic.

Variation does occur into two categories namely; genetic and phenotypic as described in detailed below.

Genetic differences reflect the genotype (the genetic make-up of an individual organism, an individual ‘s genotype functions as a set of instructions for the growth and development) of an organism, that is, its genetic make-up. A diploid organism has two sets of chromosomes and two forms (alleles) of each particular gene. These alleles may be the same (the organism is homozygous for that gene) or different (the organism is heterozygous for that gene). If different, one of the alleles (the dominant allele) may mask the other allele (the recessive allele). The dominant allele is therefore expressed in either the heterozygous or the homozygous condition. If an organism is haploid (that is, it has only one set of chromosomes), all its alleles will be expressed and will be reflected in its observable or measurable characters (the features or traits transmitted from parent to offspring).

There are three primary sources of genetic variation:

1. Mutations are changes in the DNA. A single mutation can have a large effect, but in many cases, evolutionary change is based on the accumulation of many mutations.
2. Gene flow is any movement of genes from one population to another and is an important source of genetic variation.
3. Sex can introduce new gene combinations into a population. This genetic shuffling is another important source of genetic variation.

Variation is one of the main things that drive evolution. First, there are limited resources available, and there is just not enough; food, water, shelter, etc. available for all organisms. Second, to make matters worse, most species have many offspring that can possibly survive. Just think of how many insect eggs are laid compared to the number that make it to adulthood. This leads to competition for the limited resources.

Not all individuals in a species are the same. There are variations in; size, speed, coloration, etc. These small variations can help or hinder individuals in their survival. These variations are caused by small differences in genes. Organisms that have helpful variations are more likely to survive. On average, they get more food, get better shelter, etc.  Coloration can help a predator get closer to prey and eat better. Or, for the prey species, coloration can make it harder for predators to find and eat it. So, organisms that have helpful variations tend to survive better, and reproduce more. As they reproduce, their genes (including the helpful genes) become more common in the gene pool, and these variations spread out more and more.

The measurable physical and biochemical characteristics of an organism, whether observable or not, make up its phenotype (observable physical or biochemical characteristics of an individual organism, determined by both genetic make-up and environmental influences, for example, height, weight and skin color). The phenotype results from the interaction of the genotype and the environment. The genotype determines the potential of an organism, whereas the environment factors to which it is exposed determine to what extent this potential is fulfilled. For example, in humans the potential height of a person is genetically determined, but a person cannot reach this height without an adequate diet. Phenotypic variation is of two main types: continuous and discontinuous.

Continuous variation is variation which does not show clear cut differences i.e. it shows a gradual change from one extreme to another. Characteristics such as; human height and weight show continuous variation, and are usually determined by a large number of genes (i.e. polygenic) and/ or considerable environmental influence. Some examples of continuous variation are: Height, weight, heart rate, finger length, and leaf length. They are also called fluctuating variations because they fluctuate on either side (both plus and minus) of a mean or average for the species. Continuous variations are typical of quantitative characteristics. They show differences from the average which are connected with it through small intermediate forms. If plotted as a graph, the mean or normal characteristic will be found to be possessed by maximum number of individuals. The number of individuals will decrease with the increase in degree of fluctuation. The graph (figure 15.1) will appear to be bell shaped. The variations are already present in different organisms or races of a species.

Continuous variations are produced by:
–– Segregation of chromosomes at the time of gamete or spore formation.
–– Crossing over or exchange of segments between homologous chromosomes
during meiosis.
–– Chance combination of chromosomes during fertilization.

Therefore, these variations are also known by the name of re-combinations. They make an organism better fitted to struggle for existence in a particular environment. They also enable human beings to improve the races of important plants and animals. However, they are unable to form a new species.

Continuous variations are of two types:

a. Substantive: They influence appearance including; shape, size, weight and color of a part or whole of the organism, for example., height, shape of nose, skin color, color of eyes, hair, length of fingers or toes, yield of milk, eggs, etc.

b. Meristic: They influence the number of parts, for example, number of grains in an ear of wheat, number of epicalyx segments in Althaea, tentacles in Hydra or segments in earthworm, etc.

Discontinuous variation is variation where there is a clear cut difference with no intermediates between individuals e.g. blood groups (A, B, AB, or O), Rhesus factor (+ve or –ve), mice coat colour, gender, eye colour in drosophila, haemophilia, tongue rolling, flower colour, seed shape, pawpaw tree sex (male or female) etc. Such variations are represented in a bar graph as shown in Figure 15.2. Such variations are controlled by a single gene or many alleles of the same gene. Continuous variations are usually quantitative (they can be measured) whereas discontinuous variations are qualitative (they tend to be defined subjectively in descriptive terms). Thus height in humans is a continuous variation given a value in meters, whereas height in sweet peas is a discontinuous variation described as tall or dwarf. Such discontinuous variations are not changeable and neither can environment change them

Discontinuous variations are caused by:
–– Chromosomal aberrations like; deletion, duplication, inversion and translocation,
–– Change in chromosome number through aneuploidy and polyploidy,
–– Change in gene structure and expression due to addition, deletion or change in nucleotides.

It is caused by the substitution of a single amino acid in molecular structure of RBCs. When the oxygen content of an affected individual is low (at high altitude or under physical stress), the sickle cell Hb deforms the RBCs to a sickle shape. Sickling of the cells, in turn, can lead to other symptoms.

Individuals who are heterozygous (having a single copy of the allele) for the sicklecell allele are said to have sickle-cell trait. They carry a normal life but suffer some symptoms of sickle-cell disease when there is an extended reduction of blood oxygen. Although the sickle-cell anaemia is lethal for homozygous, the sicklecell trait (heterozygous) is sometimes considered as an advantage. People who are heterozygous are resistant to malaria. Thus, in tropical Africa, where malaria is common, the sickle-cell allele is both beneficial and an afflicition.

Most genes, including the β-globin polypeptide gene, have several different alleles. For the moment, only the two alleles of this gene are considered. For simplicity, the different alleles of a gene can be represented by symbols. In this case, they can be represented as follows:

The letters Hb stand for the locus of the haemoglobin gene, whereas the superscripts ^A and ^S stand for particular alleles of the gene. In a human cell, which is diploid, there are two copies of the β-globin polypeptide gene. The two copies might be: Hb^AHb^A or Hb^SHb^S or Hb^AHb^S. The alleles that an organism has form its genotype. In this case, where we are considering just two different alleles, there are three possible genotypes.

In sexual reproduction, haploid gametes are made, following meiosis, from diploid body cells. Each gamete contains one of each pair of chromosomes. Therefore, each gamete contains only one copy of each gene. Think about what happens when sperm are made in the testes of a man who has the genotype HbAHbS. Each time a cell divides during meiosis, four gametes are made, two of them with the HbA allele and two with the HbS allele.

Of all the millions of sperm that are made in his lifetime, half will have the genotype HbA and half will have the genotype Hb^S. Similarly, a heterozygous woman will produce eggs of which half have the genotype HbA and half have the genotype Hb^S. This information can be used to predict the possible genotypes of children born to a couple who are both heterozygous. Each time fertilisation occurs, either an Hb^A sperm or an Hb^S sperm may fertilise either an Hb^A egg or an Hb^S egg.

As there are equal numbers of each type of sperm and each type of egg, the chances of each of these four possibilities are also equal. Each time a child is conceived, there is a one in four chance that it will have the genotype Hb^AHb^A, a one in four chance that it will be Hb^SHb^S and a two in four chance that it will be Hb^AHb^S. Another way of describing these chances is to say that the probability of a child being Hb^SHb^S is 0.25, the probability of being Hb^AHb^A is 0.25, and the probability of being Hb^AHb^S is 0.5. It is important to realize that these are only probabilities. It would not be surprising if this couple had two children, both of whom had the genotype Hb^SHb^S and so suffered from sickle cell anaemia.

The major distinctions between continuous and discontinuous variations in inheritance are as follows:

Continuous variations have the following characteristics:
–– The variations fluctuate around an average or mean of species.
–– Direction of continuous variations is predictable.
–– They are already present in the population.
–– Continuous variations are formed due to chance segregation of chromosomes during gamete formation, crossing over and chance pairing during fertilization.
–– They can increase adaptability of the race but cannot form new species.
–– Continuous variations are connected with the mean or average of the species by intermediate stages.
–– The continuous variations are also called fluctuations.
–– When represented graphically, continuous variations give a smooth bell shaped curve
–– They are very common
–– Continuous variations do not disturb the genetic system. Discontinuous variations have the following characteristics:
–– A mean or average is absent in discontinuous variations.

–– The direction of discontinuous variations is unpredictable.
–– Discontinuous variations are new variations though similar variations might have occurred previously.
–– Discontinuous variations are produced by changes in genome or genes.
–– Discontinuous variations are the fountain head of continuous variations as
well as evolution
–– These variations are not connected with the parental type by intermediate stages.
–– Discontinuous variations are also known as mutations or sports.
–– A curve is not produced when discontinuous variations are represented graphically.

                              

–– These variations appear occasionally.
–– They disturb the genetic system of the organism

Genes are interchanged resulting in new chromosomes (recombinants), different from the parental combination. Chromosomal crossover (or crossing over) is the exchange of genetic material between homologous chromosomes that results in recombinant chromosomes during sexual reproduction. Crossing over and random segregation during meiosis can result in the production of new alleles or new combinations of alleles. Portions of paired chromosomes may be exchanged to form new chromosomal and gene combinations in gametes resulting into new trait combinations in offspring.

Non-disjunction results into doubling of the chromosome number due to failure of chromosomes to segregate during meiosis. This leads to increase in cell size and subsequent increase in size of various parts of the organism, hence variation.

Random fertilization that results during the fusion of the gametes also contributes to variation. Gametes are the egg and sperm, or pollen, produced by meiosis. Each gamete has a unique set of combination of genes. A male gamete can fertilize any of the female gametes. The fertilization between a male gamete and a female gamete occurs randomly in the fallopian tube. As a result, each zygote is unique and hence variation occurs due to the different combination of genes from the male and female gamete.

The random fusion of gametes is a source of genetic variation in offspring (with the same parents). For example, a litter of puppies or kitten (bred) by the same father will show variation between individuals as shown below.

Random mating involves individuals pairing by chance, not according to their genotypes or phenotypes. Random mating is a source of variation in a population. For example, a population in which mating only occur between organisms of similar phenotypes, such as red beetles mating with red beetles and yellow beetles mating with yellow beetles, will tend to show less variation than a population where crosses are random. For example, red beetles mating with yellow beetles.

Mutations are sudden and permanent changes in the genes and chromosomes which are then passed on from cell to cell during mitosis. Such changed genes or chromosomes will produce offspring that differ from parents.

A mutation is also a change in the amount or the chemical structure of DNA. If the information contained within the mutated DNA is expressed, it can cause a change in the characteristics of an individual cell or an organism. Mutations in the gametes of multicellular organisms can be inherited by offspring. Mutations of the body cells of multicellular organisms (somatic mutations) are confined to the body cells derived from the mutated cell; they are not inherited. Mutations can happen spontaneously as a result of errors in DNA replication or errors during cell division, or they can be induced by various environmental factors (such as certain chemicals, X-rays, and viral infection). Factors that induce mutations are called mutagens.

This occurs at the time of gamete formation. At the time of gamete formation during meiosis, the parental chromosomes separate at random hence forming different gametes with different chromosomes. This independent assortment gives a wide variety of different gametes and hence individuals.

These variations are not inherited but are due to environmental factors. The environmental factors bring about only slight modifications in animals but in plants the modifications are much more conspicuous. This is due to the environmental effect on the meristems of various parts. A slight change in the meristematic activity can have permanent effect on the plant. Environment can also change the amount of flowering and bring about non- inheritable changes in the floral parts.

In the absence of light, the plants remain etiolated. Shade produces elongated internodes and thinner and broader leaves. It increases the succulence of many vegetables. Strong light, on the contrary, helps in the production of more mechanical tissue and smaller and thicker leaves. The effect of light has also been observed by Cunningham in flat fish Solea. The fish habitually rests on left side. It develops pigmentation and eyes on right side, the side exposed to sun. If left side is exposed to sunlight in the young fish, both eyes and pigmentation develop on that side.

Temperature directly affects the metabolic activity of the organisms and rate of transpiration in plants. Plants growing in hot area show stunted growth of the aerial parts and greater growth of the root system. Strong sunlight and high temperature bring about sun-tanning of human skin by production of more melanin for protection against excessive insulation and ultraviolet radiations.

The individual provided with optimum nutrition grows best while the under nourished shows stunted growth. The abundance or deficiency of a mineral salt produces various types of deformities in plants. A larva of honey bee fed on royal jelly grows into queen while the one fed on the bee bread develops into worker.

Plants growing in soils deficient in water or in areas with little rainfall show modifications in order to reduce transpiration and retain water, e.g., succulence, spines, reduced leaves, thick coating, sunken stomata, etc. Those growing in humid and moist area show luxuriant growth.

The t- test is used to test the statistical significance of continuous variables. The t-test therefore has less application in genetics and far more in other areas of biology, such as ecology. The t- test is used when a sample size is relatively small, e.g. Under 30 readings/ figures. The mean and standard deviation of these small samples are prone to error since a single extreme reading will have a disproportionate effect. The t- test accounts for this error. For the t-test to be of use, the data used have to conform to certain conditions, namely they must be related to one another, normally

distributed, have similar variances and the sample size must be small. The t-test can be expressed as:

Standard deviation is calculated as follows:

To take an example. A farmer wishes to decide which of two fertilizers gives the best yield for her crop of wheat. She divides one of her fields into 16 plots, eight of which she treats with fertilizer 1 and eight with fertilizer 2.  The number of tons of wheat obtained from each plot is given in this table

The first stage of t-test is to calculate the standard deviation for each sample. It is calculated from the mean of each sample (see table 15.3), the deviation of each reading from the mean (see table 15.4) and the square of this deviation and the sum of squares (see table 15.4)

It is now substitute in the equation:

Finally, to discover whether value of 3.68 indicates whether the different readings are significant, or merely due to chance, we need to look up 3.68 on a statistical table called t-table. To do this we need to know the degrees of freedom. This is calculated according to the formula:

Degrees of freedom (v) = (n₁ + n₂) – 2. In our example:  v = (8+8) -2 =14

It is found that looking along the row for 14 degrees of freedom values of 3.68 lies between 2.98 and 4, 14, which corresponds to a probability value of between 0.01 and 0.001. This refers to the probability that chance alone is the reason for the difference between the two sets of data. In this example, the probability that the different wheat yields when using our two fertilizers was pure.