evolution

• Evolution


  A process of change by which new forms of life arise from pre-existing forms. It occurs slowly since changes accumulate over many generations. For evolution to take place, variation is crucial and any changes that occur must be inherited. Therefore, it is the population that evolves not the individual.
• Genotypes influence the phenotype

  The phenotype is also influenced by the environment
• Continuous variation is the cumulative effect of varying environmental factors acting on a variable genotype

  
• No two organisms will possess identical phenotypes, even identical twins, because even though their genotype is the same, their interaction with the environment will be different
• Microevolution


  Evolution on a small scale over a short period of time, e.g. mutations which give rise to different alleles for eye colour
• Macroevolution


  Evolution on a very large scale over a long period of time, e.g. the development of all organisms from single-celled protoctista OR the development of man from ape-like hominids
• A population evolves when individuals with different genotypes survive or reproduce at different rates. A change in the ratio of alleles from one generation to the next is considered evolution
• Phenotypes

  • 1st generation: 1 brown eyed individual; 9 blue eyed individuals. Allele ratio 1:9

  2nd generation: 5 brown eyed individual; 30 blue eyed individuals. Allele ratio 1:6
• Discontinuous variation


  Produces very clear-cut phenotypes with no intermediates. Such characteristics are usually controlled by one or two genes which show two or more allelic forms in the population and are rarely affected by the environment.

  
• Continuous variation


  Produces a complete range of possible phenotypes. Such characteristics are usually under the control of many genes (polygenes) with each individual gene contributing a very small effect to the observed phenotype (combined effect is significant). They are also affected by environmental factors.

  
• Studying population variation requires large numbers of individuals
• Continuous variation


  Results are plotted graphically into a number of categories (showing all possible measures), and histogram patterns reveal frequency distributions. A normal distribution is typical of such variation (i.e. most individuals fall in the middle and less individuals occur at the extremes).

  
• Discontinuous variation


  Bar graphs are used and these show a few data sets e.g. 4 in the case of blood groups.
• Crossing-over


  Chiasma formation between homologous chromosomes during prophase I. Exchange of genetic material is a major source of genetic recombination.

  
• Independent assortment


  Random orientation of chromosomes and chromatids during metaphase I and II respectively, which leads to them being assorted independently in the subsequent anaphase step.

  
• Random fertilisation


  Fusion of sperm and ovum is random; any male gamete can fertilise any female gamete.

  
• Mutation


  A change in the amount, arrangement or structure of the DNA of an organism. Mutations occurring in gametes are inherited whilst those in somatic cells can only be inherited by mitotically produced daughter cells.

  
• Sexual recombination (crossing-over, independent assortment, random fertilisation) simply mix-up existing phenotypes and are the basis for continuous variation
• Mutations possibly generate major changes (new phenotypes) that can result in the formation of new species
• Types of mutations


  • Gene mutations: Deletion, Insertion, Inversion, Substitution

  Chromosome mutations: Changes in chromosome number (Aneuploidy, Polyploidy), Changes in chromosome structure (Inversion, Deletion, Duplication, Translocation)
• Spontaneous mutations occur without outside influence due to DNA polymerase errors or non-disjunction during meiosis
• Induced mutations are caused by mutagens (UV radiation; Pesticides; chemicals in tobacco) causing (a) changes in nucleotide bases, (b) adding groups to bases or (c) damaging genetic code due to radiation
• Silent mutations


  An alteration in a DNA sequence that does not result in an amino acid change in a polypeptide

  
• Frameshift mutations


  Cause the mRNA to be misread during translation due to a deletion or insertion of nucleotides which shifts the codon

  
• Aneuploidy


  Loss or gain of a single chromosome
• Aneuploidy arises from a failure of a pair or pairs of homologous chromosomes to separate during anaphase I of meiosis. Hence, both chromosome sets are passed to the same pole of the cell. This is known as non-disjunction.

  
• Polyploidy


  The increase in entire haploid sets of chromosomes

  
• Polyploidy is typically more common in plants since they undergo propagating vegetation whilst meiosis in animals would be more prone to errors

  
• Autopolyploidy


  Derived from a single species, arises naturally or artificially as a result of an increase in the chromosome number

  
• Allopolyploidy


  Arises when chromosome number in a sterile hybrid (hybrid sterility) becomes doubled (producing a new species) and produces fertilise offspring with polyploids like itself but is infertile with parental species

  
• Polyploids with an odd number of chromosome sets are usually sterile due to the production of aneuploid gametes

  
• Pros of sterility in polyploids include seedless fruits like bananas and watermelon
• Population genetics


  The study of the genetic composition of a population, genetic inheritance along generations and the factors which affect both
• Population


  A group of individuals of the same species co-existing in the same habitat
• Allopolyploidy is RARE in animals since there are less instances for cross-breeding between species
• Most allopolyploid plants


  • Have different characteristics from either parental species
  • Include many economically significant plants e.g. wheat (Triticum aestivum)
• Population


  A group of individuals of the same species co-existing in the same habitat at the same time and which therefore all have a chance to interbreed
• Gene pool


  The sum total of all the different genes and alleles which are present in a sexually reproducing population and that can be passed on to the next generation. It contains the variation that produces the differing phenotypes on which agents of evolution act
• The gene pool determines the characteristics of the population as a whole
• Allele frequency


  The proportion of one allele in relation to all the alleles found in the population