Reproductive Isolation (Edexcel A (SNAB) A Level Biology)

• Organisms that belong to the same species share the same characteristics and are able to produce fertile offspring
• Reproductive isolation occurs when changes in the alleles and phenotypes of some individuals in a population prevent them from successfully breeding with other individuals in the population that don't have these changed alleles or phenotypes
• Examples of allele or phenotype changes that can lead to reproductive isolation include:
  • Seasonal changes - some individuals in a population may develop different mating or flowering seasons (becoming sexually active at different times of the year) to the rest of the population (i.e their reproductive timings no longer match up)
  • Mechanical changes - some individuals in a population may develop changes in their genitalia that prevent them from mating successfully with individuals of the opposite sex (i.e. their reproductive body parts no longer match up)
  • Behavioural changes - some individuals in a population may develop changes in their courtship behaviours, meaning they can no longer attract individuals of the opposite sex for mating (i.e. their methods of attracting a mate are no longer effective)
• These changes could be brought about due to geographical barriers isolating populations or random mutations producing new, different alleles in a population

Speciation

• Speciation can occur when populations of a species become separated from each other by geographical barriers
  • The barrier could be natural like a body of water, or a mountain range
  • It can also be man-made, like a motorway
• This creates two populations of the same species who are geographically isolated from each other, and as a result, no gene exchange can occur between them
• If there are sufficient selection pressures acting to change the gene pools (and allele frequencies) within both populations then eventually these populations will diverge and form separate species 
  • The changes in the alleles/genes of each population will affect the phenotypes present in both populations
  • Random mutations within each population will also change allele frequencies in each
  • Over time, the two populations may begin to differ physiologically, behaviourally and anatomically (structurally)

Speciation occurring due to geographical isolation of two populations of the same species