Evolution and Environment
Evolution is a scientific explanation for the form, history, and diversity of life on Earth, which states that all organisms are connected by bonds of genealogy and have changed through time.  
Charles Darwin proposed a mechanism for evolution known as natural selection, by which individuals who are slightly better adapted to their environment, and therefore more reproductively successful, will leave more offspring in the next generation. If their higher fitness was caused by an inherited trait, this trait will also be passed down, and so the species will change through time as new traits are preserved generation to generation, based on what makes an individual most successful in the current environment.  
Nothing was known of genetics in Darwin’s day, and so the mechanism of inheritance and how it worked was unclear. At the same time as Darwin was working on The Origin of Species, an Austrian monk named Gregor Mendel was working out this very mechanism; he discovered that heredity is governed by discreet units, called genes, that are passed from generation to generation. Mendel’s work was not widely known in the scientific community until the early 1900s, when it was rediscovered. When combined with Darwin’s theory of natural selection (in the 1930s), the Modern Synthesis, or Neodarwinism, was born, a coming-together of ideas from various scientific fields into a coherent theory of what evolution is and how it works.  
Today, natural selection is widely accepted among scientists as being the primary mechanism that drives evolution, but other factors play a role as well, including genetic drift and chance.  
Resources on evolution and creationism