Genetics, Evolution and Ecosystems

• Genotype: describes all the alleles that an organism has. determine the limits characteristics can vary
• phenotype: the set of observable characteristics of an individual resulting from the interaction of its genotype with the environment.
• homologous pairs: a pair of chromosomes that carry the same genetic inform
• Population: a group of organisms of the same species occupying a particular space at a particular time.
• species: a group of similar organisms that can reproduce together to create fertile offspring
• gene pool: the term used to describe all of the alleles within a population. The higher the genetic diversity, the larger the gene pool. When organisms migrate into the population (gene flow), the gene pool gets bigger.
• Basis of evolution: Over time, the frequency of different alleles changes in response to changes in the environment – this is the basis of evolution
• evolution:

  change in allele frequency over time
• allele frequency:

  prevalence of an allele in a population
• Hardy-Weinberg principle:

  allele frequency will not change between generations but makes the following assumptions:

  1. large population
  2. no immigration or emigration
  3. mating is random
  4. no new mutations
  5. no natural selection
• Hardy-Weinberg Equations:

  p + q = 1

  p2 + 2pq + q2 = 1

  p = frequency of dominant allele

  q = frequency of recessive allele

  p2 = homozygous dominant genotype

  2pq = heterozygous genotype

  q2 = homozygous recessive genotype
• Variation:

  Individuals within a population of a species may show a wide range of variation in phenotype due to having different alleles and environmental influences. Most phenotypic variation is affected by both genes and the environment.
• genetic variation causes:

  • mutations
  • crossing over
  • independent assortment
  • random fertilisation of gametes
• natural selection and evolution:

  the process by which species evolve.

  1. there is variation within the population, gene mutations cause new alleles to appear
  2. organisms with the most suited characteristic survive and pass on the alleles to their offspring
  3. a greater proportion of the next generation have the beneficial allele so will reproduce and pass on the allele
  4. increasing the frequency of the advantages allele
• Types of selection:

  • Directional selection - extreme phenotype selected, triggered by change in the environment
  • Stabilising selection - average phenotype selected, occurs in stable environments
  • Disruptive selection -alleles for extreme phenotype is selected and the mean are against, occurs when the environmental conditions are changing, leads to speciation
• speciation:

  the development of a new species
• how speciation occurs: when two groups of a population become reproductively isolated from each other. Reproductive isolation prevents the transfer of genes between the two groups so natural selection acts separately on the two sub-populations. The frequency of alleles within their gene pools will change differently. Eventually the two groups will become so different that they will no longer be able to breed with each other - they are now classed as separate species.
• how reproductive isolation occurs:

  • behavioural changes
  • Mechanical changes
  • temporal changes
• allopatric speciation:

  • organisms become geographically separated
  • the groups become reproductively separate so can't exchange alleles
  • different groups will experience different selection pressures so the allele frequencies will change creating two distinct species
• Sympatric Speciation:

  • occurs in the absence of a geographical barrier
  • random mutations occur within the population
  • this results in behavioural, mechanical or temporal changes making organisms reproductively isolated
• Genetic drift:

  Genetic drift describes the random changes in allele frequencies that occur due to chance. strongest in smaller populations.
• alleles:

  a version of a gene - we possess two alleles for each gene - one on each chromosome
• genotype:

  the combination of alleles we possess
• phenotype:

  the visible characteristics genes produce
• dominant allele:

  • decides what characteristic is expressed
  • always expressed even with one copy
• recessive allele:

  • 'weaker' than the dominant allele
  • require both copies to be present in order to express
• codominant alleles:

  both alleles contribute to the phenotype
• monohybrid inheritance:

  • inheritance of characteristics controlled by a single gene
• multiple alleles:

  more than two alleles for the same gene
• dihybrid cross:

  how two separate phenotypes are inherited

  usually a 9:3:3:1 ratio of offspring
• linked genes:

  two different genes are on the same chromosomes = inherited together and linked - offspring with express both phenotypes
• sex linkage:

  genes which are located on one of the sex chromosomes. their expression will depend on whether the offspring is male or female. men inherit one allele while women Inherit two
• Epistasis:

  Epistasis is when the allele of one gene masks the expression of the alleles of other genes.
• null hypotheis:

  no significant difference between the observed and expected results
• population:

  all the members of a single species living in the same area
• Habitat:

  the area where an organisms lives
• community:

  all the populations of different species which interact together in the same habitat
• Ecosystem: a community of organisms and the non-living components of their environment. Ecosystems can range in size from very small to extremely big.
• Niche:

  A niche is the role that an organism plays in the ecosystem

  • includes biotic and abiotic interactions
  • in the same habitat, different organisms have different Niches. if two species try to occupy the same niche one is out-competed
• abundance/population size:

  The number of individuals of one species in a particular area.