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Population Genetics and Evolution ( Population genetics deals with the…

- - - - Mutations
      - Accidents
      - Artificial selection
        
        It is the process in which humans purposefully change the allele frequency of a gene pool.
      - Natural selection
        
        Also known as survival of the fittest, those individuals that are most adapted to an environment survive, whereas less adapted do not.
        
        Factors that are not part of Natural selection are purpose, intention, planning, or voluntary decision making.
        
        Situations in which Natural selection does not operate
        
        It does not operate if all individuals of a population are identical genetically or if it is impossible to become adapted to a certain condition.
        
        Multiple selections pressure
- - - - The original species is physically divided into two or more populations that cannont interbreed: if speciation results, it is called allopatric or geographic speciation.
    - - Any biological phenomenon that prevents successful gene flow is a biological reproductive barrier.
      - When two groups become reproductively isolated even though they grow together, the result is sympatric speciation.
      - When pollens are prevented from moving one plant to another, neither pollination nor fertilization occurs. Consequently these are called Prezygotic isolation mechanisms.
      - The large number of unique, characteristic alleles, each may become sufficiently distinct phenotypically that the two subpopulations can no longer interbreed. Their genomes are two different called postzygotic internal isolation barriers.
        
        The earliest postzygotic barriers to arise is hybrid sterility.
        
        When zygote or embryo dies early in development , this is called hybrid inviability.