Variation, classification and natural selection

• Evolution suggests that all organisms are descended from a common ancestors as a result of the process of mutation and natural selection, leading to speciation
• Phylogenetic groups

  • organisms grouped according to their evolutionary history
  • those who interbreed and produce fertile offspring are grouped as a species
  • species with features in common are grouped to form a genus
  • organisms grouped together in progressively larger groups create a hierarchy
• Taxonomic groups (descending order)

  • kingdom
  • phylum
  • class
  • order
  • family
  • genus
  • species
• Features of taxonomic classification

  • A hierarchy i.e. large groups containing smaller groups
  • no overlap between groups
  • organisms share a common evolutionary history
• Characteristics used to define a given group are homologous features which are used to put organisms into their appropriate groups:

  • similar biological structures
  • details of embryonic developments
  • shared biochemistry e.g. metabolic pathways, protein structure/ amino acid sequence
  • genetic sequencing
• Binomial systems

  • Genus species
  • organisms belonging to different species are not able to interbreed to produce fertile offspring
• 5 kingdoms

  • all organisms can be placed into a kingdom
  • 4 contain eukaryotic organisms whilst the 5th contains prokaryotes- bacteria

  1. animalia
  2. plantae
  3. fungi
  4. protocista
  5. prokaryotea
• Kingdom Animalia

  • include 5 classes of vertebrates (mammals, birds, reptiles, amphibians, fish) and invertebrates (insects, arthropods, molluscs)
  • animals are eukaryotic and multicellular
  • no cell walls
  • cannot make food (heterotrophs)
  • have muscles and nervous systems
• Kingdom plantae

  • includes flowering plants, conifers, ferns, mosses, liverworts
  • eukaryotic and multicellular
  • cells have cell walls made of cellulose
  • most have stem, leaves and roots
  • most possess chlorophyll and photosynthesise (autotrophs)
  • a few are modified to a parasitic way of life
• Kingdom fungi

  • includes filamentous fungi and yeast
  • single celled fungi called yeast
  • eukaryotic but multicellular fungi don't have separate cells, nuclei are dotted around in the tissue
  • walls of fungal cells contain chitin
  • all fungi feed by extracellular digestion, secreting enzymes onto food and absorbing the soluble products
  • most feed on dead material (saprophytes)
  • some are parasitic
• Kingdom protocista

  • includes single cells protozoans e.g. amoeba, plasmodium, algae and sponges
  • taxonomic dustbin
  • group is based on ancestry and evolutionary relationships
  • cells are eukaryotic but show diversity of cell types
  • feed in a variety of ways
• Kingdom prokaryotea

  • includes bacteria and single celled prokaryotic organisms
  • cells are smaller than eukaryotes
  • most feed by extracellular digestion, secreting enzymes and absorbing soluble product
  • most feed on dead material
  • cyanobacteria are photosynthetic
  • reproduce by binary fission but can reproduce sexually
• Types of variation

  1. continuous
  2. discontinuous
• Continuous variation

  • controlled by 2 or more separate gene pairs
  • polygenic
  • e.g. height, eye colour, intelligence

  1. polygenic traits are not expressed as absolute or discrete characters
  2. show a range of values
  3. traits are quantitative
  4. frequencies of measured traits form normal distribution
  5. environmental factors may have a major impact on the phenotype
• Discontinuous variation

  • characteristics that show separate/discrete categories or classes
  • normally shown on a bar chart or pie chart
  • e.g. ABO blood groups
  • no intermediate types
  • cannot be measured in units, all qualitative
  • strong genetic factor, the environment will have no effect
  • normally coded for by one or two genes
• Evolution and natural selection

  1. organisms generally produce a large number of offspring of which only a few survive to reproduce
  2. individual members of the species and their offspring differ from each other
  3. individuals who do survive are those which posses the genes providing greatest adaptation to the environment i.e. survival of the fittest
• Natural selection operates on a population level; there may be a range of alleles within a population
• If changes in the environment provides a selection pressure in favour of certain alleles then natural selection can occur:

  • variation of phenotypes will be present in the population as a result of random mutations
  • organisms which posses these advantageous alleles or genotypes may be more likely to survive
  • surviving organisms can reproduce and pass on their advantageous alleles to their offspring
  • there is an increase in frequency of the alleles for the beneficial characteristics
• If enough genetic changes occur then a group of organisms may become reproducibility isolated: i.e. no longer reproduce to produce fertile offspring thus becoming a separate species
• • Cause of variation: independent assortment
  • How variation is produced: produces different combinations of maternal and paternal chromosomes
• • Cause of variation: crossing over
  • How variation is produced: produces new combinations of alleles
• • Cause of variation: gene mutation
  • How variation is produced: a change in the order of the nucleotide bases of a gene may alter the amino acid sequence in the protein
• • Cause of variation: random fertilisation
  • How variation is produced: which pair of gametes fuses during fertilisation is a matter of chance and totally random
• • Cause of variation: environmental factors
  • How variation is produced: the expression of genes may be affected by factors such as diet, disease or temperature during development. mutagenic agents may cause gene mutations in tissues which grow abnormally
• Different types of natural selection

  1. stabilsing selection
  2. directional selection
• Stabilising selection

  • occurs when the environment is not changing
  • natural selection favours organisms with alleles for characteristics towards the middle of the range
  • organisms with alleles for the extremes are less likely to survive
  • a similar range of characteristics are passed down
• Directional selection

  • occurs when the environment is changing; selects for organisms with alleles for an extreme phenotype
  • overtime preferable allele is selected and increases
  • directional selection results in a change in the range of phenotypes until a new optimum is establishes
• Disease

  • introduction of an unfamiliar pathogen
  • devastating effects on populations with little variety in the population (e.g. monocultures)
  • e.g. potato blight
  • populations normally contain varied organisms which may survive the pathogen due to the alleles they posses
  • organisms survive, reproduce and pass alleles to future generations
• Competition

  • within a species (intraspecific) or different species (interspecific)
  • plants: competition for light, CO2, space, water
  • animals: competition for food, water, sexual partners, territory
• Speciation

  • development of a new species
  • occurs when a gene flow stops between 2 populations where it previously existed
  • results from natural selection affecting a population to such an extent that the varieties produced can no longer interbreed to produce fertile offspring
• Allopatric speciation

  • occurs when a population is separated geographically e.g. different islands or impenetrable barriers e.g. mountains/rivers
  • different populations may be exposed to different selection pressures
  • different alleles may be selected for different environments slowly leading and eventually to the development of different species
• Sympatric speciation

  • occurs when a population forms a new species in the same area as the parent
  • results from a change in habitat or a partioning of resources that are essential and may limit survival
  • in plants, changes in ploidy (chromosome number) may result in instant speciation as gametes are incompatible