Variations arising during the process of reproduction can be inherited. These variations may lead to increased survival of the individuals. Sexually reproducing individuals have two copies of genes for the same trait. If the copies are not identical, the trait that gets expressed is called the dominant trait and the other is called the recessive trait.
Traits in one individual may be inherited separately, giving rise to new combinations of traits in the offspring of sexual reproduction. Sex is determined by different factors in various species. In human beings, the sex of the child depends on whether the paternal chromosome is X (for girls) or Y (for boys).
Variations in the species may confer survival advantages or merely contribute to the genetic drift. Changes in the non-reproductive tissues caused by environmental factors are not inheritable.
Speciation may take place when variation is combined with geographical isolation. Evolutionary relationships are traced in the classification of organisms. Tracing common ancestors back in time leads us to the idea that at some point of time, non-living material must have given rise to life.
Evolution can be worked out by the study of not just living species, but also fossils. Complex organs may have evolved because of the survival advantage of even the intermediate stages. Organs or features may be adapted to new functions during the course of evolution. For example, feathers are thought to have been initially evolved for warmth and later adapted for flight.
Evolution cannot be said to progress from lower forms to higher forms. Rather, evolution seems to have given rise to more complex body designs even while the simpler body designs continue to flourish.
Study of the evolution of human beings indicates that all of us belong to a single species that evolved in Africa and spread across the world in stages.