Variation and Evolution

Variation and Evolution

Genetic Variation

  • Genetic variation in a population increases the chance that some individuals will survive under changing environmental conditions.
  • Variation arises from mutagenic agents, which can alter the DNA base sequence, and from sexual reproduction, through independent assortment and crossing over in meiosis.
  • Independent assortment occurs during meiosis I, where homologous chromosome pairs align randomly before being separated, creating a mix of paternal and maternal chromosomes in gametes.
  • Crossing over involves the exchange of genetic material between non-sister chromatids during prophase I of meiosis, resulting in recombinant chromosomes.

Hardy–Weinberg Principle

  • The Hardy–Weinberg principle states that allele frequencies in a population remain constant from generation to generation in the absence of other evolutionary influences.
  • The principle is represented by the equation p^2 + 2pq + q^2 = 1, wherein p and q are the frequencies of the two alleles (A and a respectively) and p^2, 2pq, and q^2 the frequencies of the genotypes (AA, Aa, and aa respectively).

Natural Selection and Evolution

  • Natural selection involves individual organisms with advantageous traits surviving and producing more offspring than those without these traits.
  • Over time, advantageous traits (adaptive traits) become more common in the population, resulting in evolutionary change.
  • Some selection pressures may favour one extreme of a trait (directional selection), both extremes (disruptive selection), or intermediate values (stabilising selection).
  • Speciation can occur as a result of geographic isolation (allopatric speciation) or reproductive isolation (sympatric speciation).

Revision summary: Variation within a population enhances its ability to adapt and respond to environmental changes. The Hardy–Weinberg principle describes how, barring other influences, allele frequencies remain constant across generations. Natural selection works on this variation to ensure survival of the fittest, potentially leading to speciation over generations. Bell-ringer activity: Compare and distinguish between independent assortment and crossing over during meiosis.