In biology, evolution is the process by which novel traits arise in populations and are passed on from generation to generation. Its action over large stretches of time explains the origin of new species and ultimately the vast diversity of the biological world. Contemporary species are related to each other through common descent, products of evolution and speciation over billions of years. The phylogenetic tree represents these relationships for the three major domains of life. 
The understanding of evolution is based on the theory of natural selection, which was first set out in a joint 1858 paper by Charles Darwin and Alfred Russel Wallace, and achieved a wider readership in Darwin's 1859 book, On The Origin of Species. Natural selection is the idea that individual organisms which possess variations giving them advantageous heritable traits are more likely to survive and reproduce and, in doing so, increase the frequency of such traits in subsequent generations.
In the 1930s scientists combined Darwinian natural selection with the theory of Mendelian heredity to create the modern evolutionary synthesis (often simply called the modern synthesis). The modern synthesis understands evolution to be a change in the frequency of alleles within a population from one generation to the next. The mechanisms that 
produce these changes are the basic mechanisms of population genetics: natural selection and genetic drift acting on genetic variation created by mutation, sex, and gene flow. This theory has become the central organizing principle of modern biology. It helps biologists understand topics as diverse as the origin of antibiotic resistance in bacteria, eusociality in insects, and the staggering biodiversity of the living world.
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Because of its potential implications for the origins of humankind, the evolutionary theory has been at the center of many social and religious controversies since it was first introduced.
EVIDENCE OF EVOLUTION
The process of evolution has left behind numerous records which reveal the history of different species. While the best-known of these are the fossils, fossils are only a small part of the overall physical record of evolution. Fossils, taken together with the comparative anatomy of present-day 
In biology, evolution is the process of change in the inherited traits of a population of organisms from one generation to the next. The genes that are passed on to an organism's offspring produce the inherited traits that are the basis of evolution.
Mutations in genes can produce new or altered traits in individuals, resulting in the appearance of heritable differences between organisms, but new traits also come from the transfer of genes between populations, as in migration, or between species, in horizontal gene transfer. In species that reproduce sexually, new combinations of genes are produced by genetic recombination, which can increase the variation in traits between organisms. Evolution occurs when these heritable differences become more common or rare in a population.
There are two major mechanisms driving evolution. The first is natural selection, a process causing heritable traits that are helpful for survival and reproduction to become more common in a population, and harmful traits to become more rare. This occurs because individuals with advantageous traits are more likely to reproduce, so that more individuals in the next generation inherit these traits.
Over many generations, adaptations occur through a combination of successive, small, random changes in traits, and natural selection of those variants best-suited for their environment. The second is genetic drift, an independent process that produces random changes in the frequency of traits in a population. Genetic drift results from the role probability plays in whether a given trait will be passed on as individuals survive and reproduce. Though the changes produced in any one generation by drift and selection are small, differences accumulate with each subsequent generation and can, over time, cause substantial changes in the organisms.
One definition of a species is a group of organisms that can reproduce with one another and produce fertile offspring. When a species is separated into populations that are prevented from interbreeding, mutations, genetic drift, and natural selection cause the accumulation of differences over generations and the emergence of new species The similarities between organisms suggest that all known species are descended from a common ancestor (or ancestral gene pool) through this process of gradual divergence.
Evolutionary biology documents the fact that evolution occurs, and also develops and tests theories that explain its causes. Studies of the fossil record and the diversity of living organisms had convinced most scientists by the mid-nineteenth century that species changed over time. However, the mechanism driving these changes remained unclear until the 1859 publication of Charles Darwin's On the Origin of Species, detailing the theory of evolution by natural selection. Darwin's work soon led to overwhelming acceptance of evolution within the scientific community.
In the 1930s, Darwinian natural selection was combined with Mendelian inheritance to form the modern evolutionary synthesis, in which the connection between the units of evolution (genes) and the mechanism of evolution (natural selection) was made. This powerful explanatory and predictive theory directs research by constantly raising new questions, and it has become the central organizing principle of modern biology, providing a unifying explanation for the diversity of life on Earth.
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