Here's a short story:

Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in heritable traits of a population over time. Charles Darwin popularised the term "natural selection", and compared it with artificial selection. 

Variation exists within all populations of organisms. This occurs partly because random mutations arise in the genome of an individual organism, and offspring can inherit such mutations. Throughout the lives of the individuals, their genomes interact with their environments to cause variations in traits. The environment of a genome includes the molecular biology in the cell, other cells, other individuals, populations, species, as well as the abiotic environment. Individuals with certain variants of the trait may survive and reproduce more than individuals with other, less successful, variants; therefore, the population evolves. Factors that affect reproductive success are also important, including sexual selection (now often included in natural selection) and fecundity selection. 

Natural selection acts on the phenotype, or the observable characteristics of an organism, but the genetic (heritable) basis of any phenotype that gives a reproductive advantage may become more common in a population. Over time, this process can result in populations that specialise for particular ecological niches (microevolution) and may eventually result in speciation (the emergence of new species, macroevolution). In other words, natural selection is a key process in the evolution of a population. Natural selection can be contrasted with artificial selection, in which humans intentionally choose specific traits, whereas in natural selection there is no intentional choice. (End of story)

Provide your best guess for the following question. Give ONLY the guess, no other words or explanation.

For example:

Guess: <most likely guess, as short as possible; not a complete sentence, just the guess!>

Here are a few examples of questions and answers:

Question: What is key to evolution?
Guess: Natural selection

Question: What is natural selection?
Guess: differential survival due to differences in phenotype

Question: Who made that wording popular?
Guess: Charles Darwin

Question: What did he compare it to?
Guess: artificial selection

Question: Is variation common in organisms?
Guess: yes

Question: Why does this happen?
Guess: because random mutations arise

Question: What happens to the offspring?
Guess: they can inherit mutations.

Question: Do genomes interact often?
Guess: yes

Now answer the following question in the same format.

Question: What type of biology is involved?

molecular biology