3.7.3 Evolution may lead to speciation

• Individuals within a population may show a wide range of variation in phenotype. This is due to genetic and environmental factors.
• Causes of Genetic Variation:

  1. Mutation: Changes in DNA that introduce new alleles.

  2. Meiosis: Processes like crossing over and independent assortment during gamete formation.

  3. Random Fertilisation: Adds variation by producing unique combinations of alleles in offspring.
• Variation can also result from environmental factors (e.g., diet, climate), although only genetic variation is heritable.
• Predation, disease and competition for the means of survival result in differential survival and reproduction.
• Organisms with phenotypes providing selective advantages are likely to produce more offspring and pass on their favourable alleles to the next generation. This is reproductive success.
• Th niche of a species is its role within the environment. Species which share the same niche compete with each other and a better adapted species survive.
• Natural selection is the process by which individuals with advantageous traits are more likely to survive and reproduce, passing on their alleles to their offspring.

  • It causes a change in allele frequencies over time.
• The process of natural selection:

  1. Variation exists within a population due to mutations and genetic differences.
  2. An environmental change occurs which changes the selection pressures (e.g. predation, disease, competition).
  3. Some individuals possess advantageous alleles which give them a selective advantage, meaning they are likely to survive, reproduce, and pass on their alleles.
  4. Over generations, the frequency of alleles changes, this leads to evolution.
• The types of selection include stabilising selection, directional selection and disruptive selection. They affect allele frequency.
• Selection is the process by which individuals that are better adapted to their environment are more likely to survive and reproduce. This means they can pass on their advantageous alleles.
• Stabilising selection favours average phenotypes and reduces variation.

  • Occurs in stable environments.
  • Example: Human birth weight – babies with extreme weights (very low or very high) have lower survival rates.
• Directional selection favours phenotypes at one extreme, leading to a shift in the population’s traits.

  • Occurs when environmental conditions change.

  • Example: Antibiotic resistance in bacteria – resistant strains survive and reproduce.
• Disruptive selection favours extreme phenotypes at both ends of the range.

  • Can lead to speciation.

  • Example: Beak sizes in finches – birds with small or large beaks are more successful than those with intermediate sizes in environments with only small and large seeds.
• Evolution is the change in the allele frequencies in a population over time.
• Speciation is the formation of a new species. This occurs as populations become separated and cannot interbreed. Allopatric speciation and Sympatric speciation are the types of speciation.
• Stages of Speciation:

  1. Isolation: Populations of the same species become separated, preventing gene flow.
  2. Different Selection Pressures: Each population experiences unique environmental pressures, leading to different adaptations.
  3. Genetic Divergence: Over time, allele frequencies change in each population due to natural selection, mutation, and genetic drift.
  4. Reproductive Isolation: Populations can no longer interbreed to produce fertile offspring.
• Allopatric speciation is caused by geographical isolation.

  • Populations are physically separated by a barrier (e.g., mountains, rivers).
  • The reproductive isolation reduces the gene flow
  • Each group experiences a different selection pressure as the environment they live in is different
  • Over time, the frequency of alleles changes through natural selection and the two populations can no longer interbreed (speciation).
• Sympatric Speciation (Reproductive Isolation) is when populations remain in the same area but become reproductively isolated due to:

  • Temporal Isolation: Differences in breeding times.
  • Behavioural Isolation: Differences in mating behaviours or courtship.
  • Mechanical Isolation: Incompatible reproductive structures.
  • Hybrid Sterility: Offspring are sterile (e.g., mules from a horse-donkey cross).
• Genetic drift is random changes in allele frequencies in small populations. It occurs due to the bottleneck effect and founder effect.
• Bottleneck Effect:

  A sharp reduction in population size (e.g., due to a natural disaster) leads to reduced genetic diversity.
• Founder Effect:

  A small group of individuals becomes isolated and forms a new population with limited genetic variation.
• In directional selection, the recessive allele gives a disadvantage while the dominant allele gives an advantage, meaning individuals with this allele are more likely to survive and reproduce.