Term Paper on Adaptation in Crop Plants | Crops | Plant Breeding

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1. Term Paper on the Definition of Adaptation
2. Term Paper on the Types of Adaptation
3. Term Paper on the Causes of Adaptation
4. Term Paper on Factors affecting Adaptability

Term Paper # 1. Definition of Adaptation:

Adaptation refers to those changes in structure or function of an individual or population which lead to better survival or greater fitness in a given environment. Adaptability refers to the capacity of a genotype or population for genetic changes in adaptation. Adaptability is the ability of a genotype to exhibit relatively stable performance in different environments. Adaptability is measured in terms of phenotypic stability of a genotype over several environments.

The main features of adaptation are given below:

i. Adaptation is the process of adjustment of living organisms to the changes occurring in the environment.

ii. Adaptation favours those characters which are advantageous for survival and through which an individual acquires adaptive value or fitness in a given environment.

iii. In the process of adaptation survival is the main concern.

iv. Natural selection plays an important role in the process of adaptation.

v. Adaptable genotypes produce narrow range of phenotypes in different environments.

vi. Adaptability leads to stable performance of a genotype over a wide range of environments.

vii. General genotypic and general population adaptations are the examples of wide adaptability.

viii. Varietal adaptability is the result of genetic and physiological homeostasis.

ix. Productivity is the major concern in varietal adaptability. Human selection plays an important role in combining adaptability and productivity.

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Term Paper # 2. Types of Adaptation:

There are four types of adaptation, viz.:

i. Specific genotypic adaptation,

ii. General genotypic adaptation,

iii. Specific population adaptation, and

iv. General population adaptation.

i. Specific Genotypic Adaptation:

It is the close adaptation of a genotype to a limited environment. For the production of rice in a deep water area, a variety’s capacity for rapid inter-node elongation is an essential feature of its specific adaptation.

ii. General Genotypic Adaptation:

It refers to the capacity of a genotype to produce a wide range of phenotypes compatible with a range of environments. Semi-dwarf varieties of wheat and rice which can be grown over a wide range of environmental conditions are examples of this type of adaptation (i.e., adaptability).

iii. Specific Population Adaptation:

It refers to the capacity of a heterogeneous population to adapt to specific environment. A composite or a varietal mixture giving stable production is an example of this category. Here the competition is between the components of variety or mixture rather than adaptation of components themselves.

iv. General Population Adaptation:

It is the capacity of heterogeneous populations to adapt to a variety of environments. Synthetic varieties of forage crops are example of this type of adaptation. This property of adaptation is specific to an individual genotype or a group of genotypes and is termed as homeostasis.

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Term Paper # 3. Causes of Adaptation:

Varietal adaptability or stability is the result of homeostasis, which refers to the buffering capacity of a genotype to environmental fluctuations.

The homeostasis is of two types, viz.:

i. Genetic homeostasis, and

ii. Physiological homeostasis.

i. Genetic Homeostasis:

It refers to the genetic buffering capacity of a genotype to environmental fluctuations. In other words, it is the ability of a genotype to withstand environmental fluctuations. High buffering ability indicates consistent performance of a genotype or population over a wide range of environments.

A more homeostatic genotype is one which shows less variation in phenotypic values in different environments. Thus variability in performance over a wide range of environments can be used as a criterion for measure of phenotypic stability.

ii. Physiological Homeostasis:

It refers to physiological or developmental buffering capacity of a genotype to the environmental fluctuations. The internal self-regulatory mechanisms enable the individual to adjust to the fluctuating environments by resisting such changes. Physiological homeostasis is generally higher in heterozygous genotypes than in homozygous ones.

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Term Paper # 4. Factors affecting Adaptability:

There are four important genetic factors, viz. heterogeneity, heterozygosity, genetic polymorphism and mode of pollination, which affect the adaptability of a genotype over a series of environments.

These are briefly discussed below:

i. Heterogeneity:

The heterogeneous populations have broad genetic base. Such populations have greater capacity to stabilize productivity over a wide range of changing environments because such heterogeneity provides a higher degree of population buffering. Generally diverse (heterogeneous) populations give more stable yields over several environments than pure lines.

ii. Heterozygosity:

It has been observed that heterozygous individuals such as F₁ hybrids are more stable than their homozygous parents to environmental variation. The stability of heterozygous individuals seems to be related to their ability to perform better under stress conditions than homozygous plants.

Such property constitutes physiological homeostasis. In barley, F2 populations were more stable than their parents. Thus heterozygosity promotes the homeostasis. In maize, double cross hybrids are more stable than single crosses. This is probably due to more heterogeneity in double crosses than single crosses.

iii. Genetic Polymorphism:

The regular occurrence of several phenotypes in a genetic population is known as genetic polymorphism. The genetic polymorphism is usually maintained due to superiority of heterozygotes over both the homozygotes. When polymorphism is maintained as a result of heterozygote advantage,’ it is known as balanced polymorphism.

Sometimes, it is difficult to identify the polymorphic allelic forms by visual observations. The best way of detecting the polymorphic alleles is the isozyme studies or gel electrophoretic studies. It has been reported that two third of the loci in a population exhibit polymorphism.

Genetic polymorphism increases the adaptive value or buffering capacity of a population by providing increased diversity of genotypes in a population. Thus genetic polymorphism enhances the adaptability of a population, because heterozygotes are more adaptable than homozygotes.

iv. Mode of Pollination:

The cross-pollinated species have better buffering capacity than self- pollinated because of more heterozygosity in the former than latter.