No two environments are ever exactly alike. Some regions of the earth are harsh waterless deserts, whereas others, such as oceans are totally liquid and rich in nutrients. A very large number of distinct environments exist on earth today, and over its long history many have changed, often quite dramatically.
Yet, for millions of years in every age, some living creatures have been able to make their homes in almost every available corner of this planet regardless of the prevailing conditions.
The ability of life to exist in all these environments and to adjust to all changes that have taken place is called adaptation, another critical sign of life.
Each special set of environmental conditions, which can be defined in terms of temperature, water availability, food resources, danger, predators, landscape, etc., is called an ecological niche.
This is an "address" at which an organism is found most often. Within each niche, some form of life has undergone a series of adaptations so that it can survive, grow, and reproduce under the conditions found in that niche.
Cacti, for example, are plants well adapted to dry desert areas. They have evolved waterproof outer surfaces that cut down on water loss and rigid body structures that hold them above the ground and toward the sunlight.
Seaweeds, on the other hand, are adapted to life in the sea. They do not have to protect themselves against water loss, and, because of the sea's natural buoyancy, they do not need elaborate supporting structures.
Cacti and seaweeds are two different types of organisms, but each is very well adapted to its own particular environment.
Adaptation and homeostasis are not the same. Homeostasis is a property of individual organisms and enables each individual to adjust to rapid, passing changes in the immediate environment.
Adaptation is a process whereby permanent changes accumulate in the whole populations, and it takes place over many generations and much longer periods of time.
Small changes in the genetic-hereditary material of an organism cause changes in either its outward appearance or inner workings. These changes are permanent and can be passed along from one generation to the next.
Some modifications are beneficial and help individuals fit better into their own or even a new niche. A better adapted organism has a higher chance of survival and thus a greater chance of reproducing successfully. During every successful reproductive event, the modified genetic instructions are passed on. Gradually an entirely new population appears in which most individuals are carrying the new instructions.
The process of genetic change followed by reproductive success within a niche is called natural selection, first proposed by Charles Darwin as a means of explaining how living things adapt to changing circumstances.
Over long periods of time, adaptation and natural selection lead to evolution and the generation of new species of organisms.