1. The Environment and Phenotype
1.1 Variation is the Rule in Populations
We are very conscious of human diversity, but individuality in populations of other animals and plants may escape our notice. Nonetheless, some degree of individual variation occurs in all populations. So where does variation originate from?
Now, we come to the aspect of origins of variation. The phenotype is the cumulative product of a multitude of environment factors on an organism and an inherited genotype, which is produced by 2 random processes namely mutations and sexual recombination.
1.2 Environmental Effects and Gene Expression
The final appearance of an organism (phenotype) is the result of its genotype and the effect of the environment upon it. If organisms of identical genotype are subjected to different environmental influences, they show considerable variety. One of a pair of genetically identical plants is grown in nitrogen-deficient soil, will not attain height of others grown in normal conditions.
As environmental influences themselves vary and often form gradations, they are largely responsible for continuous variation within a population. Examples of some environmental factors affecting the phenotype are temperature, light and nutrition.
1.3 Variance and Heritability
1.3.1 Variance
The phenotype if any organism is determined by genetic and environmental factors. The phenotypic variance of a population of organism is the sum of the genotypic variance and environmental variance.
Genotypic variance which expresses phenotypic variance due to genotype includes the effect of additive genes, dominant genes and epistasis. The contributions of each to the phenotype can be estimated from crosses involving homozygous varieties and their F1, F2 and F1 backcross progeny. Variance due to the environment is also most easily determined when homozygous organisms are studied that is the genotypic variance is zero.
1.3.2 Heritability
This value expresses the degree to which a trait is influenced by the genotype. Heritability is a measure of the degree to which a phenotype is genetically influenced and the degree to which it can be influenced by phenotypic selection. If the environment is kept constant, an estimate of heritability can be obtained by dividing genetic variance by phenotypic variance.
A heritability of 1.0 indicates that the trait was produced solely by the action of the genotype. A heritability of 0.0 indicates that the phenotype is due entirely to the environment. Intermediate values estimate the relative contribution of heredity, as opposed to environment of a trait.
1.3.3 Conclusion
Both genetic and environmental factors are important in determining growth. An organism’s genotype does not specify absolutely the phenotype. Rather, the genotype determines the range of phenotypes that may develop. The range of phenotype that may develop is called a range, or norm of reaction of the genotype. Which phenotype actually develops will depend on the environment in which development takes place.
2. Variation and Mutations
Variation may be termed the ground plan for evolution. Without a constant pool of individuals within the same species of animals and plants that bear either marked differences or subtle ones, the status quo would remain indefinitely, providing the environment also remained static. If there were detrimental changes in the environment, the species would be wiped out and nothing would be there to replace them.
The number of members of the effected species would decrease and very soon there would be complete extinction. It this is true, all species of plants and animals then very soon, the number of different species would also decrease.
Variation thus provides heterogeneity among members of the same species which would help ensure the perpetuation of the species as well as safeguard the species from extinction.
2.1 Mutations
It was first proposed in 1901 as the cause of the sudden appearance of a new characteristic. It was also investigated that mutation forms the basis of discontinuous variations in populations. By definition, mutation is any sudden and stable inherited change in the structure or amount of DNA in an organism that leads to difference (variations) among the offspring by producing new characteristics. Thus, it provides variations which act as the raw material to be acted on by natural selection to bring about evolution.
Individuals that inherit these new characteristics are called mutants. Mutations are usually random and rare events. The vast majority of mutations will be harmful or at least neutral in their effect on the phenotype. Very few will have a beneficial effect.
2.2 How do mutations arise?
Mutations can arise spontaneously or they may be induced. Spontaneous mutations result from errors in replication of DNA. Induced mutations result from the effects of various mutagens.
There are environmental mutagens. Firstly, there are ionizing radiations which comprise of x-rays and gamma rays. They cause gene and chromosomal aberrations by breaking up the genetic material directly. There are also non-ionizing radiations which comprise of UV light (causes structural distortions in DNA) and heat / high temperature.
There are chemical mutagens. Colchicine prevents spindle formation in mitosis and so doubles chromosome number. Cyclamate causes chromosome aberrations. Mustard gas causes G in DNA to be replaced by other bases. Nitrous acid causes A in DNA to be deaminated so it behaves like G. Acridine orange causes the addition or removal of bases in DNA.
2.3 Classification of Mutations
According to the level of changes of the genetic material, there are 2 types of mutation. They are chromosome mutations and gene mutations (point mutation).
2.3.1 Chromosome Mutation
It involves changes in the number of chromosomes per cell or changes in the gross structure of a chromosome. These changes are generally observable under the microscope
2.3.1.1 Changes in Chromosome Number
These changes involve the number of chromosomes per cell caused by non-disjunction. Non-disjunction is the non-separation of chromosomes during mitotic or meiotic division. It may occur during meiosis I so that a homologous pair does not separate. It may also occur during meiosis II when sister chromatids do not separate. It often results in one cell receiving 2 of the same type of chromosomes and the other receiving no copy. This may occur on the whole set of chromosomes and give rise to a triploid (3n), tetraploid (4n) and so on; a term called euploidy a.k.a. polyploidy. It may occur on a particular pair or pairs of chromosomes and give rise to monosomic (2n-1) trisomic (2n+1) and so on; a term called aneuploidy.
2.3.1.1.1 Euploidy / Polyploidy
Polyploidy is a situation where the chromosome is more than 2 complete sets. The changes involve the entire set of chromosomes.
It is a result of mitotic non-disjunction of a diploid zygote that doubles the chromosome number. This can occur naturally or artificially induced with the chemical colchicines that destroys the spindle fibre formed during cell division and thus prevents anaphase movement.
It is also a result of production of unreduced diploid gametes due to meiotic non-disjunction of all chromosomes. A triploid organism will be produced if a diploid gamete (2n) fertilizes with a normal haploid gamete (n). A tetraploid organism will be produced if a diploid gamete (2n) fertilizes with another diploid gamete (2n). Tetraploids can self fertilize of mate with other diploids or tetraploids to produce fertile and viable hybrids. Triploid hybrids are sterile because they have problems in pairing and distribution of chromosomes at meiosis.
2.3.1.1.1.1 Autopolyploid
Autoployploids are polyploids with more than 2 sets of chromosomes from within a single species. It arises by spontaneous chromosome doubling due to spindle failure at meiosis, giving unreduced gametes. It can be artificially induced with the chemical colchicine that destroys the spindle fibre formed during cell division and thus prevents anaphase movement.
2.3.1.1.1.2 Allopolyploid
Allopolyploids are polyploids with more than 2 sets of chromosomes from 2 different species / genomes. When 2 different species interbreed, a hybrid is produced. Such interspecific hybrids are usually sterile, because the haploid set of chromosomes for 2 species are not homologous and they cannot pair during meiosis with the 2 haploid set of chromosomes from the second species.
2.3.1.1.1.3 Significance of Polyploidy
Polyploidy is common in plants but rare in animals. Polyploids are usually larger because they have larger nuclei than their diploid relatives and this in turn gives rise to an increased cell size and an altered pattern of development and morphology. Leaves tend to be larger and thicker, the growth rate is slower and there may be physiological differences in response to certain environmental factors (temperature, rainfall).
Polyploids are usually stringer than their diploid relatives, especially allopolyploids which develop from different parents and have some beneficial characteristics that are not possessed by either of the parents. This is known as hybrid vigor.
In the case of allopolyploids, entirely new species may be created.
Polyploids have more genetic variations. For these reasons, they differ in their adaptation from diploids and may be able to colonize and to occupy new and different habitats to those of their diploid relatives.
2.3.1.1.2 Aneuploidy
This is a change that involves addition or loss of 1 or more chromosomes. It causes unbalance in the chromosome complement and results in an abnormal phenotype. The degree of abnormalities depends on which particular chromosome is missing or additional. In general, an extra chromosome is less deleterious than a missing one.
During meiosis anaphase I, non disjunction occurs at a certain pair of homologous chromosomes and gives rise to half the gametes possessing 2 chromosomes of the same type while the other half have no such type of chromosome at all.
Aneuploid offspring may result if a normal gamete unites with an aberrant one produced as a result of non-disjunction. An aneuploid cell that has a chromosome in triplicate is said to be trisomic for that chromosome and it is monosomic if there is a lack of one chromosome. When an aneuploid zygote divides by mitosis, it transmits the chromosome abnormally to all subsequent embryonic cells.
There is aneuploidy in men, mostly involving sex chromosomes and some autosomes. The unbalance results in abnormalities in development but it is not generally lethal. Abnormal gene dosage causes characteristic symptoms in survivors.
Down’s syndrome is trisomy 21. Edward’s syndrome is trisomy 18. Patan’s syndrome is trisomy 13. Turner’s syndrome is monosomy X. Klinefelter’s syndrome is XXY or XXXY. Finally there is the XYY syndrome.
2.3.1.2 Alterations of Chromosome Structure
These are changes involving the gross structure of the chromosome. It is caused by the breaking and rejoining of chromosomes during meiosis prophase I. It may be deficiency (deletion) that causes loss of genes, duplication that causes the addition of genes, inversion that results in the reverse of the normal sequence of certain genes and translocation which changes chromosome segments between the non-homologous chromosomes.
There are various effects that occur. Homozygous deletions, including a single X in a male human, are usually lethal. Duplications and translocations tend to have a deleterious effect. Even if all genes are present in normal dosages, reciprocal translocations and inversions can alter the phenotype because of subtle position effects.
