BIOLOGYINDEEP

Variation

• Variation is the presence of any differences (genotypic and phenotypic), between individuals in a population, or between parents and offspring.
• Genotypic variation can be due to:
  • Mutation
  • Random assortment during Meiosis,
  • Crossing over during Meiosis,
  • Random fertilisation of gametes.
• Phenotypic variation can be due to:
  • Genetic Factors,
  • Environmental Factors,
  • Combination of both.
• Variation provides organisms with a means of producing new phenotypes.
• Variations to the structure or behaviour of an organism may confer a selective advantage to survive and reproduce

Evolution

• Evolution is the sum of changes of heritable characteristics in population.
• It is the development of life in geological time
• Natural selection is the process where organisms that are better adapted to their environment survive and breed.
• Those that fail to adapt will not survive. 
• The environment therefore exerts a selection pressure on the population.
• Allele frequency is the commonness of the occurrence of any particular allele in a population.
• Evolution changes allele frequency in population because:
  • Organisms with a selective advantage:
    • more likely to survive  →  reproduce  →  pass on genes to next generation.
    • This will increase the frequency of the allele that corresponds to the adaptation.
  • Organisms with a selective dis-advantage:
    • less likely to survive  →  die  → fail to pass on genes to next generation.
  • This will decrease the frequency of alleles possessed by these individuals.
• A population is a group of individuals of a species, living close together, and able to interbreed.
• Populations reproducing in isolation can result in the formation of new species
• This is known as Speciation.

Allopatric Speciation:

• Geographically separated sister species reproduce in isolation.
• Natural and man-made barriers may arise rapidly.
• For example: a population may be divided if a river breaks it's banks and takes a new route, or a new road may separate a population.
• Many generations may elapse.
• Sister species are now so different - no interbreeding is possible.

Sympatric Speciation:

• Genetic variation occurring in same geographical location.
• Reproductive isolation due to:
  • Temporal mechanisms
  • Behavioural mechanisms
  • Polyploidy
• Temporal (time) mechanisms may occur when two very closely related species occupy the same habitat, differing only in the time of year that they complete their life cycles.
• Reproductive isolation may develop within the population so that some members produce gametes at distinctly different times of the year from others.
• Thus, two distinctive gene pools start to evolve.
• Behavioural mechanisms results when members of a population acquire distinctive behaviour routines in their growth and development, courtship or mating process that are not matched by all individuals of the same species.
• For example: imprinting behaviour of birds.
• Polyploidy is a change in structure or number of chromosomes.
• Applicable mostly to Plants.
• This chromosomal mutation may instantly give rise to a new species.
• Human activities have, and continue, to alter the environment of many organisms.
  • Changes selection pressures
  • May effect evolution of species.
• Artificial Selection is an intentional selection process by humans,
• selecting certain organisms based on their phenotype, specifically for breeding purposes,
• In the hope offspring inherit the desired characteristics. (NOT a guarantee).
• Usually carried out by animal and plant breeders during Selective Breeding.
• Selective Breeding is use of artificial selection to modify and improve quality/yield
• In plants and animals.
• Eg. cross-fertilisation of maize variates to produce plants that:
  • mature faster,
  • larger fruit size,
  • higher yield,
  • resistant to disease.
• Eg. All modern race horses have been artificially selected.
  • Horses are selected for breeding on basis of success in races.
  • Gene pool is constantly improved.
• Humans are the selective agents.