In a species of organism, there is always some variation, i.e. slight differences between members of a family and within the population in an area.
Most of this simple variation is a natural consequence of sexual reproduction.
Other more marked variation can occur due to very rare changes (mutations) in the genetic material of certain individuals (mutants).
Certain individuals within the population will (by chance) have characteristics which make them more successful (fit) in their environment.
e.g. faster-running cheetahs (to chase and catch prey), better camouflaged predators (can get closer to prey), monkeys with longer arms and tails (for balance)
(many more examples — add your own)
These individuals are more likely to survive and have offspring (which have more or less the same characteristics)
Other – less fit – individuals do not survive, breed etc, and become extinct.
Most organisms can potentially produce many offspring.
For many species except humans, the population in an area usually remains fairly constant. Since some may die, there is said to be a struggle for survival. This is usually not a literal struggle between individuals, but competition for resources, although animals may fight for access to mates and space.
The struggle usually involves intraspecific competition, because each member of a species needs practically the same resources, such as food, water, territory, and mates, as other members of the same species - and these needs overlap much more than with other species, but interspecific competition also has a part to play.
The principle operating here is "survival of the fittest".
Evolution is a gradual change (or an accumulation of many changes) over many generations resulting in the formation of new and different species. In fact the time scale over which it occurs is of the order of thousands of years. By comparison, changes within species due to selective breeding have mostly taken a few hundred years, and changes due to genetic modification have taken place in the last 10 years.
The result is the existence of distinct species, each of which seem to be well suited to living in their own environment. Of course environmental change can be a major factor in determining which individuals are successful and which ones are at a disadvantage. There have been many examples of this in the past and it looks likely that it will be happening again in the not-too-distant future.
Some people explain this in other ways. Some attribute the creation of all living things to a God, who made them to be specialised according to some divine plan, which ought to be unchanging.
They find it difficult to accept that random processes and chance can change the characteristics of a species, or an explanation that contradicts statements in the Bible.
More recently, others have tried to promote an alternative argument ("intelligent design") to contradict the broadly accepted scientific explanation above.
This topic has connections with other units on this site:-
EVOLUTION and NATURAL SELECTION