Lec 17

• Natural Selection


  Population of individuals, groups, gene pool, gene frequencies, genetic diversity, existing genetic variation, environment, peculiar characteristics, individuals evolved to fit the changing environment, adaptation to environment
• Natural Selection


  • Favouring individuals with desirable genes most adapted to the environment, more chance to survive, more offspring
• Natural selection and the diversity of species and their genomes and Theory of Natural Selection is due to Charles Darwin's study in the late 19th century
• Theory of Natural Selection


  One of the prime motives for all species is to reproduce and survive, passing on the genetic information of the species from generation to generation. When species do this they tend to produce more offspring than the environment can support. The lack of resources to nourish these individuals places pressure on the size of the species population, and the lack of resources means increased competition and as a consequence, some organisms will not survive. The organisms who die as a consequence of this competition were not totally random. Those organisms more suited to their environment were found more likely to survive.
• Theory of Natural Selection


  This resulted in the survival of the fittest - organisms most suited to their environment had more chance of survival if the species falls upon hard times. Those who are better suited to their environment exhibit desirable characteristics, which is a consequence of their genome being more suitable to begin with. This 'weeding out' of less suited organisms and the reward of survival to those better suited led to the deduction that organisms had evolved over time, where the most desirable characteristics of a species are favoured and those organisms who exhibit them survive to pass their genes on.
• As a consequence of this, a changing environment would mean different characteristics would be favourable
• It is believed that organisms had 'evolved' to suit their environments, and occupy an ecological niche where they would be best suited to their environment and therefore have the best chance of survival.
• As the above indicates, those alleles of a species that are favoured in the environment will become more frequent in the genomes of the species, due to the organisms higher likeliness of surviving as part of the species at large.
• Mutation always introduces new genetic materials for better Adaptation and survivality
• Genetic Variation


  As measured by genetic variance/additive genetic variance, contributed to the superiority of selected individuals as parents over the population/herd average, Selection Differential (SD), how well offspring will survive in the environment
• Need genetic variation for selection to show response
• Mutation


  Changes in genetic materials - chromosomal abnormalities, aneuploidy, polyploidy, mainly involving DNA changes such as nucleotide substitutions
• Rate of mutation

  Very small, 10^-4 to 10^-6 mutations per locus per generation, recurrent rate of mutation is very small, but cumulative effect of mutation can be appreciable
• Migration


  Individuals leaving the population - emigrants, Individuals joining the population - immigrants, Contribution to genetic variation depends on the difference between gene frequencies and rate of migration
• Hardy-Weinberg Equilibrium


  In the absence of selection, mutation and migration and in an infinitely large population, gene frequencies remain stable over generations. In realities, especially in practical livestock breeding, selection (artificial selection), migration and small populations occur >>> changes in gene frequencies
• Artificial selection


  Allowing some individuals more opportunities than others to reproduce, Choice of individuals to become parents for the production of the next generation of offspring
• Selection remains the only means of directing genetic improvement in closed populations
• Result of selection is gradual but consistent
• Improvement of livestock by selection

  Made through increase in frequency of desirable genes, With quantitative traits the no. of genes can be large, so gene frequency at any one locus may not be fixed or reaching 1.0, Result of selection is predictable in magnitude and direction
• Progress from selection


  Progress of a selection programme is measured as Response to Selection, Response is of interest because of genetic change expected in the population, Change in population mean, Selection of desirable genes tends to increase their frequency - more homozygous individuals will be selected
• Genetic value and environmental effects


  If Environment is uniform, then Phenotypic value reflects largely Genetic value of an individual, P = G + E = A + D + I + E, Very often environmental conditions differ from year to year then it is difficult to ascertain changes in P is due to A or favourable E, However environmental influences tend to cancel each other >>> ΣE = 0
• Genetic components of variance


  Value G = A + D + I, Variance VG = VA + VD + VI, VA variance of breeding values, A major cause of resemblance between relatives, Chief determinant to response to selection
• Response to Selection

  Very much dependent on additive genetic variance, VA, Also determine the magnitude of h^2, With a high h^2, VP is a good indicator of VA, Response to selection R = h^2 x (Selection Differential), Response to selection (offspring generation) is a reflection of the transmittable component/breeding value contributed by the parents, Superiority of parents
• Limit to Selection

  Change in gene frequency >> change in genetic properties of population >> change in magnitude of genetic variation, When selection is directed for a specific trait/alleles over a long term period, Will response to selection continue for how long, Population mean changes by how much, Will genetic limit be reached when there is no more progress, Could result from opposing forces of natural selection and artificial selection and may not be due to loss of additive genetic variance, Artificial selection goes for extreme types, Natural selection favours intermediate expression of metric traits, heterozygotes or homozygotes suited for fitness, One explanation why limit to selection is seldom attained: VA contributes maximally to h^2 >> h^2 = VA/VP, Response to selection relies on VA >> R = h^2 (SD), Assuming (P - P̄) is the fitness difference between selected parents P and population mean P̄, with the overdominant model the heterozygotes will have higher fitness compared to homozygotes. Thus genetic variation is maintained in natural and artificial selection (with favoured heterozygotes) > high h^2, Response to selection will eventually diminish when all favourable alleles at most loci become fixed in the homozygous type. Genetic variation will decline when fixation is complete and response to selection will cease >> be at the Selection Limit. This depends much on the initial genetic variation in the founding population. With no new mutation to add to the gene pool, selection limit may quickly be reached. In livestock breeding, new genetic materials may constantly be introduced from outside the herd and this will maintain genetic variability in a herd >> therefore it is seldom that the selection limit be attained.
• Variable response to selection


  Response from 1 generation to another is variable which could be due to: Genetic drift, Restricts no. of parents, Sampling error, Population mean is poorly estimated due to small sample size, Variation in Selection Differential, Individuals to be parents not correctly identified
• Selection Methods


  Set selection objectives, Decide selection criteria, Evaluate genetic variation and genetic correlation associated among the traits to be used as the selection criteria
• Selection objective


  A goal set to attaining some production target through selection, What is it that we want to improve through selection?
• Selection criteria


  Traits selected to achieve the selection objective, What traits which can be used to represent the traits mentioned in the selection objective?
• Selection objective and selection criterion can have different traits
• Some traits in objective might be difficult to measure, expensive to measure, strongly associated with other traits
• For selection criteria choose traits which are directly or closely or indirectly related with the traits mentioned in the selection objective
• Traits are genetically correlated (positively)
• Selection objective


  The traits that are intended to be improved
• Selection criteria


  The traits that are actually evaluated and selected for improvement
• Selection objective and selection criteria can have different traits
• Some traits in the selection objective might be difficult to measure, expensive to measure, or strongly associated with other traits
• For selection criteria, choose traits which are directly or closely or indirectly related with the traits mentioned in the selection objective
• Selection objective and criteria in different livestock

  • Goats: Objective - Improve mature size of Katjang goats, Criteria - Average daily gain, Yearling weight
• Factors to consider when deciding what traits to include

  • Economic importance
  • Heritability
  • Phenotypic standard deviation
  • Genetic correlation among traits
  • Phenotypic correlation among traits
  • Ease or expense of measurement
• Single trait selection


  Can produce rapid genetic change in the trait, but can lead to undesirable changes in correlated traits and selected individuals may not be suited to many environments