AGRIC/5: BASIC SCIENCE I: Selection

Selection

Selection is the preferential survival and reproduction or preferential elimination of individuals with certain genotypes (genetic compositions), by means of natural or artificial controlling factors.

Selection comes in two types:

1. Natural selection: the process by which plants and animals that can adapt to changes in their environment are able to survive and reproduce while those that cannot adapt do not survive. It gives the greater chance of passing on of genes by the best adapted organisms.
    

2. Artificial selection: a method used by humans to produce varieties of animals and plants which have an increased economic importance. People use selective breeding to produce new varieties of a species, so that certain desirable traits are represented in successive generations.

NATURAL SELECTION

The theory of evolution by natural selection was proposed by Charles Darwin and Alfred Russel Wallace in 1858. They argued that species with useful adaptations to the environment are more likely to survive and produce progeny than are those with less useful adaptations, thereby increasing the frequency with which useful adaptations occur over the generations.

The limited resources available in an environment promotes competition in which organisms of the same or different species struggle to survive. In the competition for food, space, and mates that occurs, the less well-adapted individuals must die or fail to reproduce, and those who are better adapted do survive and reproduce.

In the absence of competition between organisms, natural selection may be due to purely environmental factors, such as inclement weather or seasonal variations.

Darwin’s process of natural selection has four components.

1. Variation.  Organisms (within populations) exhibit individual variation in appearance and behavior.  These variations may involve body size, hair color, facial markings, voice properties, or number of offspring.  On the other hand, some traits show little to no variation among individuals—for example, number of eyes in vertebrates.
2. Inheritance.  Some traits are consistently passed on from parent to offspring.  Such traits are heritable, whereas other traits are strongly influenced by environmental conditions and show weak heritability.
3. High rate of population growth.  Most populations have more offspring each year than local resources can support leading to a struggle for resources.  Each generation experiences substantial mortality.
4. Differential survival and reproduction.  Individuals possessing traits well suited for the struggle for local resources will contribute more offspring to the next generation.

In natural selection, those variations in the genotype that increase an organism’s chances of survival and procreation are preserved and multiplied from generation to generation at the expense of less advantageous ones. Evolution often occurs as a consequence of this process.

Natural selection may arise from differences in survival, in fertility, in rate of development, in mating success, or in any other aspect of the life cycle. All such differences result in natural selection to the extent that they affect the number of progeny an organism leaves.

Gene frequencies tend to remain constant from generation to generation when disturbing factors are not present. Factors that disturb the natural equilibrium of gene frequencies include mutation, migration (or gene flow), random genetic drift, and natural selection.

MUTATION

A mutation is a spontaneous change in the gene frequency that takes place in a population and occurs at a low rate. Migration is a local change in gene frequency when an individual moves from one population to another and then interbreeds.

RANDOM GENETIC DRIFT

Random genetic drift is a change that takes place from one generation to another by a process of pure chance. Mutation, migration, and genetic drift alter gene frequencies without regard to whether such changes increase or decrease the likelihood of an organism surviving and reproducing in its environment. They are all random processes.

Natural selection moderates the disorganizing effects of these processes because it multiplies the incidence of beneficial mutations over the generations and eliminates harmful ones, since their carriers leave few or no descendants.

Natural selection enhances the preservation of a group of organisms that are best adjusted to the physical and biological conditions of their environment and may also result in their improvement in some cases.

Some characteristics, such as the male peacock’s tail, actually decrease the individual organism’s chance of survival. To explain such anomalies, Darwin posed a theory of “sexual selection.” In contrast to features that result from natural selection, a structure produced by sexual selection results in an advantage in the competition for mates.

ARTIFICIAL SELECTION.

Artificial selection (or selective breeding) differs from natural selection in that heritable variations in a species are manipulated by humans through controlled breeding. The breeder attempts to isolate and propagate those genotypes that are responsible for a plant or animal’s desired qualities in a suitable environment.

These qualities are economically or aesthetically desirable to humans, rather than useful to the organism in its natural environment.

A variety is a type of a particular species that is different in some clear way from other varieties of that species. The different breeds of domestic dogs and large ears of maize corn are products of artificial selection.

E.g Selective breeding of cows

Suppose you wanted a variety of cow that produced a lot of milk. This is what you could do:

• choose or select the cows in your herd that produce the most milk
• let only these cows reproduce
• select the offspring that produce the most milk
• let only these offspring reproduce
• keep repeating the process of selection and breeding until you achieve your goal.

In mass selection, a number of individuals chosen on the basis of appearance are mated; their progeny are further selected for the preferred characteristics, and the process is continued for as many generations as is desired.

The choosing of breeding stock on the basis of ancestral reproductive ability and quality is known as pedigree selection.

Progeny selection indicates choice of breeding stock on the basis of the performance or testing of their offspring or descendants.

Family selection refers to mating of organisms from the same ancestral stock that are not directly related to each other.

Pure-line selection involves selecting and breeding progeny from superior organisms for a number of generations until a pure line of organisms with only the desired characteristics has been established.

Darwin also proposed a theory of sexual selection, in which females chose as mates the most attractive males; outstanding males thus helped generate more young than mediocre males.

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