variation

• Visible differences in characteristics between individuals are called phenotypic variations. Phenotypic variation is caused by a combination of genetic variation and the environment.
• Genetic variation

  • There is huge variation between individuals in the same species.
  • E.g. A golden retriever looks completely different from a pug but they are both part of the species Canis familiaris.
  • This variation is caused by differences in the genetic code. This is called genetic variation.
  • Genetic variation happens when individuals in the same species have different alleles.
  • Alleles are different versions of the same gene.
• Environmental variation

  • Environmental factors can also contribute to phenotypic variation.
  • The genotype of an individual interacts with the environment.
  • E.g. If an individual has the alleles for being tall but is fed a nutrient-poor diet, they will not grow as tall as an individual with the same alleles and a more nutrient-rich diet.
• Causes of Genetic Variation

  Mutation

  • Mutation is the main source of genetic variation.
  • Mutations are changes in the base sequence. Changes may cause the structure of proteins encoded by DNA to be altered.
  • Some mutations are harmful (e.g. the mutation that causes disease), some are beneficial and some have no effect on survival.
• Causes of Genetic Variation

  Recombination

  • Meiosis is the process that produces haploid gametes from diploid cells.
  • During meiosis, the genes on homologous chromosomes are reshuffled through a process called recombination or crossing over.
  • Recombination produces new combinations of genes. This increases genetic variation.
• Causes of Genetic Variation

  Independent segregation

  • Meiosis also involves the independent segregation of chromosomes into haploid cells.
  • The chromosomes in a pair of homologous chromosomes are randomly separated and this creates genetic variation.
• Causes of Genetic Variation

  Random fertilisation

  • During sexual reproduction, the gametes of two unrelated individuals are fused to produce a diploid individual.
  • This process is called random fertilisation.
  • Random fertilisation produces genetic variation because pairs of chromosomes from two separate individuals are combined.