Theories of Evolution of Class 12


For evolutionary processes to occur it needs to break the inertia of genetic equilibrium which is connected with large population.  Any factors  that tend to break large population in smaller one would promote evolution and Isolation is one such factor.


Geographic isolation : The most easily visualized type occurs when a population is split by barriers like water bodies, high mountain ranges, deserts, open plains (for forest dweller) forests (for open plains  dwellers).  Such isolated populations are out of touch with each other and hence the occurrence of new mutations, genetic drift and action of natural selection in one population has no effect on the other  population.  Ultimately this results in reproductive isolation.

Reproductive Isolation

Reproductive isolation is the main factor to divide the two populations to develop as different species. Members of different species do not interbreed
Many inbuilt (or genetic) factors prevent them to interbreed. Sometimes they interbreed but they produce only sterile hybrids.Barriers to hybridization
Operate at various levels :

  • Prezygotic and
  • Post-zygotic.

Pre-zygotic barriers:
Prevent mating or formation of zygotes by following ways :

  • Ecological Isolation : Two species living in different habitat cannot meet, e.g. fresh water fish cannot breed with sea fish.
  • Temporal Isolation : Breeding season of animals differ and so is the flowering time in plants.
  • Behavioural Isolation : Different animals have their own kind of courtship behaviour which is recognised by the fair sex of the same species only.
  • Mechanical Isolation : There are structural differences in the genitalia of animals of different species and therefore copulations is not possible among the individuals of different species
  • Gametic Isolation : Sperm and ovum of the same species recognise each other through fertilizin-antifertilizin reaction, hence are unable to fuse if are from different species

Post-zygotic mechanism :

If all above barriers are removed, fertilization results in hybrid zygote but then development does not proceed smoothly causing.

  • Hybrid inviability : In most cases hybrid zygotes either fail develop or fail to survive later.
  • Hybrid sterility : If the hybrid survive and lead normal life they are sterile, can not reproduce,  Ex.Mules and henny are such hybrids of horse and donkey.
  • Hybrid breakdown : Sometimes hybrids are fertile and give rise to offspring which do not continue to survive in F2  generation. Eg. Tigon between tiger & lioness.


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