There is always a difference, or a variation between animals in one species, due to genetic mutations, like the spikes on an iguana
In the harsh and cruel environment, having a genetic mutation can give one an edge, and allow one to better survive in their surroundings, which is also commonly known as "survival of the fittest"
Without the spikes that warded off predators, the other iguanas have been unable to survive in the harsh environment, whereas this iguana has been able to survive due to it's genetic mutation. This is otherwise known as natural selection
Around 500 years later, the spiked iguana reproduces, and the genetic mutation is carried throughout the species
This allows all the iguanas to have spikes in order to ward off predators; giving the species a better survival rate in order to thrive
Now, not only was that one iguana with a genetic mutation the fittest in the environment, but all of the iguanas are the fittest, and can survive with this genetic mutation passed down in their species