Variation and evolution

Variation and evolution

1. Population genetics

   • determining the relative proportions of the various genotypes present in a population
   • gene pool is stable if environment is stable
   • if environment changes some may be advantageous/disadvantageous
   • Certain factors can act upon a genetic equilibrium and bring about significant changes to the frequency of the alles an change the compositions of the gene pool, these include gentic drift, mutations and natural selection.
   • genetic drift = chance variations in allele frequencies in a population
   • founder principle = individuals become isolaed and start a new population, may have different allele frequency so undergoes genetic drift and becomes even more different from large paretn population

2. Evoution and Selection

   • evolution = process by which newspecies are formed from pre existing ones over a period of time
   • natural selection = a process that encourages the transmission of favourable alleles and hinder the trasnsmission of unfavourable ones and contributes to evolution
   • theory: in any popualtion there is variation, indivuduals within a population have the potential to produce large numbers of offsrping, yet the number of adults tends to stay the same from one generation to the other
   • there is a stuggle for survival with only the fittest surviving, the individuals that survive and reproduce pass on to their offspring the characteristics that enable them to succeed (selective advantage)
   • Three types of natural selection: stabilising selection, directional selection and disruptive selective
     • natural selection doesn'yt always result in change, if organism has favourable phemotype more likely yo produce offspring similar to itself, if environment stable and doesn't change extreme variations eliminated - stablising selection
     • directional selection is a type of natural selection in which an extreme phenotype is favored over other phenotypes, causing the allele frequency to shift over time in the direction of that phenotype.

3. Isolation and Speciation

   • speciation - within a population there are sub groups of interbreeding individuals - demes, an isolating mechanism is something that prevents gene exchange between demes, if separation is long term separate species are formed - speciation
   • two main forms of isolating mechanisms - geographical, reproductive isolation
     • reproductive isolation = barriers to breeding including behavioural isolation, mechanical isolation (genitals incompatible), gametic isolation if gamete cannot reach each other, hybrid inviability (if chromosomes no longer match each other), hybrid sterility (individuals of different species chromosomes can't pair up)