Evolutionary processes give rise to diversity at every level of biological organisation, including the levels of species, individual organisms, and molecules.
All life on Earth shares a common ancestor known as the last universal ancestor, which lived approximately 3.5–3.8 billion years ago, although a study in 2015 found “remains of biotic life” from 4.1 billion years ago in ancient rocks in Western Australia. According to one of the researchers, “If life arose relatively quickly on Earth … then it could be common in the universe.”
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 fossilized multicellular organisms. Existing patterns of biodiversity have been shaped both by speciation and by extinction. More than 99 percent of all species that ever lived on Earth are estimated to be extinct. Estimates of Earth’s current species range from 10 to 14 million, of which about 1.2 million have been documented.
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 demonstrated by the observation that more offspring are produced than can possibly survive, along with three facts about populations:
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.
This teleonomy is the quality whereby the process of natural selection creates and preserves traits that are seemingly fitted for the functional roles they perform. Natural selection is the only known cause of adaptation but not the only known cause of evolution. Other, nonadaptive causes of microevolution include mutation and genetic drift.