What is evolution?
Evolution does not refer to a change an individual organism undergoes during its own lifetime, but rather the process by which the proportion of a genetic trait within a population changes across multiple generations. In biology, evolution is a different concept from an individual's growth, aging, or changes brought about by training. The core of evolution is that, across successive generations within a population (a group of the same species), the proportion of individuals carrying a particular genetic trait increases or decreases. Such changes usually accumulate and become apparent only over a very long period of time.
Variation β the raw material natural selection acts on
Even within the same species, individuals differ genetically from one another to some degree, and these differences are continually generated anew through mutation and the genetic recombination that occurs during sexual reproduction. Without variation, there would be nothing for natural selection to act on, so evolution could not occur at all. Mutations that arise by chance during DNA replication, and the process by which sexually reproducing organisms newly combine genes from both parents, are considered the main sources continually supplying a population with diverse variation.
How natural selection works
This is the process by which individuals carrying traits that are relatively advantageous for survival and reproduction in a given environment leave more offspring, causing that trait to be passed on to an increasingly larger share of the next generation. Natural selection is not a process of deliberate choice by anyone β rather, it is a phenomenon that emerges as a statistical result: individuals that happen to carry traits advantageous for survival and reproduction under given environmental conditions statistically tend to survive and leave offspring at higher rates. When this process repeats over many generations, the genetic makeup of the population as a whole gradually shifts.
Darwin, Wallace, and On the Origin of Species
Charles Darwin systematically laid out the concept of evolution by natural selection in his 1859 book On the Origin of Species, and Alfred Russel Wallace independently arrived at similar conclusions, which were presented together with Darwin's. Darwin spent years refining his theory of natural selection based on the diverse organisms he observed during the voyage of the HMS Beagle across multiple continents. Around the same time, however, Wallace arrived at a similar idea through his own independent observations, and the two men's work was first jointly presented at a scientific society meeting in London in 1858, before becoming widely known the following year with the publication of Darwin's On the Origin of Species.
The difference between natural and artificial selection
Natural selection is a process in which the environment filters traits, whereas artificial selection (selective breeding) differs in that humans deliberately choose and breed individuals carrying desired traits. Darwin actually drew on the long-standing human practice of selectively breeding livestock and crops to help explain the concept of natural selection. The underlying principle is similar: when individuals carrying a particular trait are made to reproduce more, that trait becomes more pronounced in later generations β in this sense, artificial selection can be seen as a case of humans deliberately applying the principle of natural selection.
Speciation β the process by which one species splits into two
When two populations are separated geographically for a long time and accumulate variation as they adapt to their respective different environments, they can eventually split into separate species that can no longer interbreed. When a geographic barrier such as a mountain range or an ocean separates two populations for a long time, each population accumulates variation independently under different environmental pressures. If this difference becomes large enough, the two populations may reach a point where, even if they later come back into contact, they can no longer naturally interbreed and produce fertile offspring β this point is considered the moment they have split into two distinct species.
Common misconceptions
It is often misunderstood that evolution is a process of progress moving toward a better direction or a goal, when it is in fact merely a result produced by relative fitness to a particular environment. The ideas that "an evolved organism is superior" or that "evolution has a predetermined goal" are classic misconceptions. Natural selection has no direction or intention, and which trait is advantageous depends entirely on the environmental conditions of the moment. If the environment changes, a trait that was previously advantageous can become disadvantageous instead.
Evidence supporting evolution
The sequential changes seen in the fossil record, similar skeletal structures shared by different species (homologous structures), similarities in DNA sequences, and cases actually observed in real time are together presented as evidence supporting evolution. Leading pieces of evidence include the fossil record, which shows form gradually changing in sequence with rock layers; homologous structures, such as the human arm and a bat's wing, which look different on the outside but have a similar underlying bone structure; and DNA analysis results showing genetic similarity between species. In addition, cases observed over relatively short periods of time β such as bacteria developing antibiotic resistance, or changes in the beak size of GalΓ‘pagos finches β are frequently cited as evidence that natural selection is actually at work.