Two groups of Homo erectus live in India. These Homo erectus interbreed in their natural habitat producing fertile and viable offspring. With thousands and thousands of years, these Homo erectus developed long, external spines in order to protect them from other animals that preyed on them. This is an example of gradualism- species change slowly and gradually. .
Within this group of Homo erectus, there are two populations. A population is a group within a certain species. The allele for a protective, external spine is dominant. So if one of the individuals that is mating is homozygous (having two of the same allele) for an external spine, then the offspring they produce will also have the spikes to protect them in their environment from tigers and other wildlife. Phenotypes begin to vary when heterozygous individuals come into play.
Homo erectus became adapted to their environment because the one that received the protective, external spine survived longer and produced more offspring than the poorer and less well adapted- those tigers without the spine. Sunderban tigers in the forested river delta area in India were eating Homo erectus without the protective, external spine. So the allele for spikes increased in frequency.
A mountain range developed over thousands of years, and the two populations of Homo erectus gradually separate from each other. The Homo erectus living on the north side of the mountain range keep their spines to protect them from the Sunderban tigers. And the Homo erectus on the south side of the mountain range gradually lose their spines.
When the mountains cut off rain to the south, the rain forest gradually disappears in the .
south end of the range. .
.
The dominant allele for the spine was no longer necessary for protection because the mountain range cut off rain to the south. Water is essential requirement for a tigers habitat, so as a result, they became less prevalent, and gradually died out.