Evolution is change
in the heritable characteristics
of biological populations over successive generations. Evolutionary processes give rise to biodiversity
at every level of biological organisation, including the levels of species, individual organisms, and molecules.
Repeated formation of new species (speciation), change
within species (anagenesis), and loss of species ( extinction ) throughout the evolutionary history of life on Earth
are demonstrated by shared sets of morphological and biochemical traits, including shared DNA sequences. These shared traits are more similar among species that share a more recent common ancestor
, and can be used to reconstruct a biological " tree of life " based on evolutionary relationships (phylogenetics), using both existing species and fossils. The fossil record includes a progression from early
biogenic graphite, to microbial mat fossils, to fossilised multicellular organisms. Existing patterns of biodiversity
have been shaped both by speciation and by extinction .
In the mid- 19th century , Charles Darwin
formulated the scientific theory of evolution by natural selection, published in his book On the Origin of Species (1859). Evolution by natural selection is a process first demonstrated by the observation that often, more offspring are produced than can possibly survive. This is followed by three observable facts about living organisms: 1) traits vary among individuals with respect to morphology, physiology, and behaviour (phenotypic variation), 2) different
traits confer different
rates of survival and reproduction (differential
fitness), and 3) traits can be passed from generation to generation (heritability of fitness). Thus, in successive generations members of a population are replaced by progeny of parents better adapted to survive and reproduce in the biophysical environment
in which natural selection takes place.