evolution

• phenotype


  observable characteristics of organism
• phenotype


  observable characteristics of organism
• phenotypic variation


  difference in phenotypes between organisms of same species
• some phenotypic variation is by genetic factors:

  such as blood groups an individual has two alleles of three possible ones (ABO)
• some phenotypic variation by environmental factors


  such as plants having same gene but will grow to different heights depending on the conditions of environment
• phenotypic variation can be also by combination of genetic and environmental
• genetic variation


  small differences in DNA base sequences between individual organisms
• The phenotypic variation of the individuals in a population is determined by the genetic variation within the population and the interaction of the environment on the individuals:

  Phenotypic variation = Genetic variation + Environment
• genetic variation transferred from one generation to next and generates phenotypic variation within species population
• mutation is major cause of genetic variation

  mutation results in generation of new alleles

  • may be advantageious, disadvantageous or gave no effect on phenotype
  • new alleles not always seen in individual first occurs in
  • can be not expressed for several generations before contributing to phenotypic variation
• other causes of genetic variation:

  • crossing over of non-sister chromatids during meiosis (prophase I)
  • independent assortment of homologous chromosome during meiosis (metaphase I)
  • random fusion of gametes during fertilisation
• Crossing over


  The process by which non-sister chromatids exchange alleles
• Crossing over

  1. During meiosis I homologous chromosomes pair up and are in very close proximity to each other

  2. The non-sister chromatids can cross over and get entangled

  3. These crossing points are called chiasmata (singular = chiasma)

  4. The entanglement places stress on the DNA molecules

  5. A section of chromatid from one chromosome may break and rejoin with the chromatid from the other chromosome
• Crossing over can result in a new combination of alleles on the two chromosomes
• There is usually at least one chiasma present in each bivalent during meiosis but often there are multiple chiasmata
• Crossing over is more likely to occur further down the chromosome away from the centromere
• crossing over

  A) centromere

  B) bivalent

  C) chromosome

  D) chromatid

  E) chiasma

  
• Independent assortment


  The production of different combinations of alleles in daughter cells due to the random alignment of homologous pairs along the equator of the spindle during metaphase I
• Independent assortment


  • Increases genetic variation between gametes
• Independent assortment

  1. In prophase I homologous chromosomes pair up

  2. In metaphase I they are pulled towards the equator of the spindle

  3. Each pair can be arranged with either chromosome on top, this is completely random

  4. The orientation of one homologous pair is independent / unaffected by the orientation of any other pair

  5. The homologous chromosomes are then separated and pulled apart to different poles

  6. The combination of alleles that end up in each daughter cell depends on how the pairs of homologous chromosomes were lined up
• Formula to work out number of different possible chromosome combinations

  2^n, where n corresponds to the number of chromosomes in a haploid cell
• Random fertilisation of gametes

  • Meiosis creates genetic variation between the gametes produced by an individual through crossing over and independent assortment
  • This means each gamete carries substantially different alleles
  • During fertilization any male gamete can fuse with any female gamete to form a zygote
  • This random fusion of gametes at fertilization creates genetic variation between zygotes as each will have a unique combination of alleles
  • There is an almost zero chance of individual organisms resulting from successive sexual reproduction being genetically identical
• Sources of genetic variation

  • Independent assortment of homologous chromosomes during metaphase 1
  • Crossing over of non-sister chromatids in prophase 1
  • Random fertilisation
  • Mutation
• Independent assortment of homologous chromosomes during metaphase 1

  1. Random alignments of chromosomes

  2. Results in different combination of chromosomes and different allele combination in each gamete
• Independent assortment of homologous chromosomes during metaphase 1

  • Results in genetic variation between gametes produced by an individual
• Crossing over of non-sister chromatids in prophase 1


  1. Exchange of genetic material between non-sister chromatids

  2. Leads to new combination of alleles on chromosomes

  3. Can also break linkage between genes
• Crossing over of non-sister chromatids in prophase 1


  • Results in genetic variation between gametes produced by an individual
• Random fertilisation


  Male gamete fusing with female gamete in random mating in species of a population
• Random fertilisation


  • Results in genetic variation between zygote and resulting individuals
• Mutation


  • Random change in DNA base sequence
  • Results in generation of new allele
  • Mutation within gametes so can be passed onto future generation
• Mutation


  • Results in genetic variation between individuals within a species population
• Environmental factors


  Factors that limit population size as they result in differential survival and reproduction
• Environmental factors

  • Can be biotic
  • Can be abiotic
• Biotic factors


  Involve other living organisms
• Biotic factors

  • Predation
  • Competition for resources
  • Disease
• Abiotic factors

  Involve the non-living parts of an environment
• A combination of biotic and abiotic factors means that not all individuals within a population will survive
• Natural selection


  The process by which individuals with a fitter phenotype are more likely to survive and pass on their alleles to their offspring so that the advantageous alleles increase in frequency over time and generations
• Variation exists within a species population
• Phenotype


  The observable characteristics of an organism that result from the interaction of its genotype with the environment