(6) WJEC A2 Biology 4.4 Isolation and speciation

Species

Group of phenotypically similar organisms that can interbreed to produce fertile offspring.

Deme

Subgroup in a population that breed more often with each other than with the rest of the population.

Allopatric speciation

The evolution of new species from demes isolated in different geographical locations.

Sympatric speciation

Evolution of new species from demes sharing a geographical location.

What happens when a deme becomes isolated?

• Can’t breed with members of other demes.

  • Gene flow in and out of deme is prevented. The mechanism preventing this is an isolating mechanism and the demes are reproductively isolated.

• If demes isolated for many generations, accumulation of changes in allele frequency and many different mutations mean they are no longer able to interbreed successfully with initial population. Speciation has occurred. Separate species now have own gene pool.

Reproductive isolation can be: (2 types)

• Pre-zygotic → gametes prevented from fusing, zygote is never formed.

• Post-zygotic → gametes fuse, zygote formed. Organism develops and grows but it is sterile. Genes of parent species are kept separate and species don’t merge.

Check notes for the types of pre- and post-zygotic isolation.