ka7: evolution

• Evolution — the changes in organisms over generations as a result of genomic variations.
• Natural selection is the non-random increase in frequency of DNA sequences that increase survival and the non-random reduction in the frequency of deleterious sequences
• The changes in phenotype frequency as a result of stabilising, directional and disruptive selection.
• • In stabilising selection, an average phenotype is selected for and extremes of the phenotype range are selected against.
• In directional selection, one extreme of the phenotype range is selected for
• In disruptive selection, two or more phenotypes are selected for.
• Horizontal gene transfer is where genes are transferred between individuals in the same generation
• Natural selection is more rapid in prokaryotes. Prokaryotes can exchange genetic material horizontally, resulting in faster evolutionary change than in organisms that only use vertical transfer.
• • Vertical gene transfer is where genes are transferred from parent to offspring as a result of sexual or asexual reproduction.
• Speciation is the generation of new biological species by evolution as a result of isolation, mutation and selection.
• A species is a group of organisms capable of interbreeding and producing fertile offspring, and which does not normally breed with other groups.
• The importance of isolation barriers in preventing gene flow between sub-populations during speciation.
• Geographical barriers lead to allopatric speciation and behavioural or ecological barriers lead to sympatric speciation.
• Evolution is the changes in organisms over generations as a result of genomic variations
• Evolution accounts for the origin of existing species from ancestors that lived long ago & were often very different from present day species.
• It involves the processes of gene transfer, selection, genetic drift & speciation.
• 1. Vertical transfer : Genes (sequences of protein coding DNA) are transferred from parents down to their offspring (one generation to another/inheritance).

   

  This can happen by:

  i) Sexual reproduction.

  This involves two parents who differ from one another genetically.  Offspring inherit different combinations of genes from each parent.

   

  ii) Asexual reproduction.

  This is reproduction from a single parent. It produces offspring who are genetically identical to the parent.
• Horizontal transfer


  Genetic material can be transferred from one cell to another through horizontal gene transfer (individuals of the same generation)
• Prokaryotes


  • Reproduce using asexual reproduction
• Early evolution of prokaryotes

  1. A lot of horizontal gene transfer (HGT) occurred

  2. Obtaining a gene from a neighbour is much faster than waiting for one to evolve

  3. Allowed rapid evolution of prokaryotes
• Horizontal gene transfer (HGT) in early evolution of prokaryotes

  A risky strategy as there are no guarantees the transferred genetic material will give an advantage
• A significant amount of HGT still occurs in modern day prokaryotes
• Resistance to antibiotics has occurred through

  • Transfer of plasmids carrying antibiotic resistance genes from bacteria to bacteria
• • Eukaryotes: Although less common, horizontal gene transfer can occur in eukaryotes.

   

  From viruses - Some viruses can integrate their DNA into the host’s genome. Where they remain dormant (this is called a provirus) until they reproduce and destroy the cells, e.g. HIV and Herpes virus.
• Charles Darwin and Alfred Wallace presented a theory suggesting that the main driving force for evolutionary change is natural selection


  1858
• Natural selection

  The non-random increase in frequency of DNA sequences that increase survival and the non-random reduction in frequency of deleterious sequences
• Natural selection

  Acts as a sorting mechanism that eliminates less well adapted organisms, usually by preventing their growth or killing them
• Certain characteristics become more or less common in a population


  Due to factors within the environment (selection pressures)
• Selection pressures

  Factors within the environment that lead to the non-random reduction in frequency of deleterious sequences (genetic sequences which code for inferior versions of a characteristic which leave the individual poorly adapted to the environment)
• Types of natural selection


  • Directional selection
  • Disruptive selection
  • Stabilising selection
• Directional selection

  • Favours a single extreme phenotype
  • Common during periods of environmental change
• Directional selection

  • European black bears increased in mass during each ice age as larger bodies lose relatively less heat than smaller ones
• Disruptive selection

  • Favours two or more extreme forms within a population
  • Driving force behind sympatric speciation
• Disruptive selection


  • Darker and lighter mussels. Dark mussels are well camouflaged on rocks and light mussels against sand. Mid-colour mussels are not camouflaged on either & are not selected for
• Stabilising selection

  • Favours the average phenotype
  • Extremes of the phenotype range are selected against
  • Gives a decrease in variation as more extreme traits do not survive
• Stabilising selection

  • Human birth mass remains in range of 3-4 kg. Babies with lower mass are more susceptible to disease, higher mass have difficulties during birth
• Speciation is the generation of a new biological species by evolution as a result of isolation, mutation and selection.
• A species is a group of organisms capable of interbreeding and producing fertile offspring, and which does not normally breed with other groups.
• • Allopatric speciation

  This occurs when gene flow between two (or more) populations is prevented by a geographical barrier, e.g. rivers, mountain ranges, desert, sea.
• • Sympatric speciation

  Two (or more) populations live in close proximity in the same environment but still become genetically isolated. Behavioural barriers, such as breeding patterns or rituals, and ecological barriers, such as food availability, are important in sympatric speciation. Sympatric speciation is the evolution of new species in populations that live in the same geographic area.