What is Darwin’s theory of Evolution by Natural Selection? What is the evidence?

It describes the differential survival and reproduction of different organisms with varying phenotypes. These differences are at least partially heritable, thus over time, gradual changes accumulate in a lineage’s phenotype so that they appear better suited - or adapted - to their environment. This contributes to evolution over a long time frame. If two lineages’ phenotype diverge to a point they can no longer breed, this may result in speciation. Good evidence for evolution by natural selection occurring includes ‘missing link’ fossils, where intermediate organism forms between current organisms and what they supposedly evolved from have been found at relevant time periods, indicating gradual change occurring over time. Advanced students may discuss other extant organism features that back it up, such as the recurrent laryngeal nerve in giraffes, or forelimb bone structure in whales / humans / birds. Lenski's Long Term Evolution Experiment with E. coli is also a fantastic example of micro-evolution in real time.