variation and evolution

variation and evolution

1. variation - the differences between organisms in the same species

   • in Asexually reproducing organisms, heritable variation can only be increased by mutation
     • in sexual reproduction, has many generatable heritable variation -
       • random distribution of chromosomes during metaphase 1 of meiosis,
         • the random distribution of chromatids at metaphase 2 of meiosis,
           • them mixing of 2 different parental genotypes at fertilisation
       • cross over between homologous chromosomes during prophase 1 of meiosis,

2. organisms differ in their phenotypes can

   • have different genotype
   • have the same genotype but different epigenetic modifcations
   • they have different environments
   • reproductive isolation; prevention of reproduction and so gene flow between breeding grps within a species
     • demes; sub grps of a population may interbreed more often with each other than with the rest of the population
     • pre-zygotic
       • gametes are prevented from fusing and so a zygote is never formed
         • geographical isolation; when a population become separated by a physical barrier into separate demes
           • allopatric speciation; the evolution of new species from demes isolated in different geographical locations
         • behaviour isolation; courtship (mating rituals) recognised by other members of the species
           • individuals share a habitat so this is sympatric speciation; the evolution of new species from demes sharing geographical locations
         • morphological isolation; when genitalia of males and females are not complimentary but they are in the same habitat so they diverge into different species as they are unable to reproduce together so this is sympatric speciation
         • gametic isolation; gametes in open environments e.g water often meet gametes of other species, in third of the cases they make viable inter species hybrids but barriers prevent the fusion of gametes of different species so the populations are isolated
         • seasonal isolation; reproductive organs of different demes mature at different times of the year
     • post zygotic
       • gametes fuse and zygote forms. even if the organism grows, it is sterile and so the genes of the parent species are kept separate and the species do not merge
         • hybrid inviability; fertilisation occurs but incompatibility between the gene of the parent prevents the development of an embryo
         • hybrid sterility; in some cases an emryo is formed from the gametes of 2 different species can develop. but if the gametes are not sufficiently similar, they are unable tp pair at prophase 1 of meiosis and so gametes cannot be formed. this means the hybrid is sterile

3. selective pressure; an environmental factor that can alter the frequency of alleles in a population, when it is limiting

   • availability of nesting sites
   • day length affects reproductive behaviour
   • overcrowding allows diseases to spread
   • predation

4. Natural seslection; the increased chance of survival and reproduction of organisms with phenotypes suited to their environment, enhancing the transfer of favourable alleles from one generation to the next

5. gene pool; all the alleles present in a population at a given time

6. genetic drift; chance variation in allele frequencies In a population , most significant in small isolated population

7. the hardy Weinberg equation allows allele frequencies to be calculated from genotype frequency frequencies and vice versa

   • f(AA)=p^2
     • f(Aa)=2pq
     • f(aa)=q^2
   • p+q=1
   • hardy Weinberg principle In a large, randomly mating population the frequency of alleles (both recessive and dominant) and genotype remains the same.

8. evolution; a chance in the average phenotype of a population

9. speciation; the formation of a new species

10. founder effect;the loss of genetic variation in a new population established by a very small number of individuals from a larger population