Sheep - AgRIS

on at the present time among these different blood groups, and the frequency of the A and O
genes might be gradually decreasing unless, of course, there are other factors that have opposite
effects and have brought the gene frequencies into equilibrium.
Natural selection is a very complicated process, and many factors determine the proportion
of individuals that will reproduce. Among these factors are differences in mortality of the
individuals in the population, especially early in life; differences in the duration of the period of
sexual activity; the degree of sexual activity itself; and differences in degrees of fertility of
individuals in the population.
It is interesting to note that in the wild state, and even in domesticated animals to a certain
extent, there is a tendency toward an elimination of the defective or detrimental genes that have
arisen through mutations, through the survival of the fittest.
6.2.1.2 Artificial Selection
Artificial selection is that which is practiced by man. Under this man determines to a great
extent which animals to be used to produce the next generation of offspring. Even in this
selection seems to have a part. Some research workers have divided selection in farm animals
into two types one known as automatic and the other as deliberate selection.
Litter size in swine can be used as an illustration to define these two terms. Here, automatic
selection would result from differences in litter size even if parents were chosen entirely at
random from all individuals available at sexual maturity. Under these conditions, there would
be twice as much chance of saving offspring for breeding purposes from a litter of eight than
from a litter of four. Automatic selection here differs from natural selection only to the extent
that the size of the litter in which an individual is reared influences the natural selective
advantage of the individual for other traits. Deliberate selection, in this example, is the term
applied to selection in swine for litter size above and beyond that which was automatic. In one
study by Dickerson and coworkers involving selection in swine most of the selection for litter
size at birth was automatic and very little was deliberate; the opportunity for deliberate
selection however among pigs was utilized more fully for growth rate.
Definite differences between breeds and types of farm animals within a species is proofthat
artificial selection has been effective in many instances. This is true, not only from the
standpoint of color patterns which exist in the various breeds, but also from the standpoint of
differences in performance that involve certain quantitative traits. For instance, in dairy cattle
there are definite breed differences in the amount of milk produced and in butterfat percentage
of the milk.
6. 2.1. 2.1 Basis of selection
a) Individuality
Selection on the basis of individuality means that animals are kept for breeding purposes
on the basis of their own phenotype. Selection may be made for several traits, such as coat
color, conformation, perforrnance, or carcass quality. In the past, the emphasis in selection
probably was based on coat color and conformation, although performance and carcass quality
have received more attention in recent years.
Most of the breeds of livestock are characterized by a particular coat color or color pattern,
and this is one of the requirements for entry into the registry associations. Selection for coat
color has been practiced because of its aesthetic value rather than its possible correlation with
other important economic traits.
Attempts to relate variations of coat color to performance within a breed have not met with
success although many livestock men feel that there is a relationship. There is a strong belief of
horse breeders that there is a strong relationship between color and temperament which has no
basis as per the evidence. There is, however evidence that animals of some colors are better
able to cope with certain environmental conditions, such as high temperatures and intense
sunlight in some regions of the tropics or in the South and the Southwestern portions of the
United States. Coat color in some instances is clot ralated to lethal and undesirable genes in
farm animals. Further other species, such as the mouse, dog, cat, mink, and fox, also show such
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