The variation that exists within a population is heritable (i.e. genetic) and determined by the presence of alleles

• Alleles encode for the biological features of an organism and may be beneficial, detrimental or neutral
  

An adaptation is any feature that aids in the survival of an organism by allowing them to be better suited to their environment

• Structural adaptations describe any physical differences in biological structure (e.g. neck length of a giraffe)
  

• Behavioural adaptations refer to differences in patterns of activity (e.g. possums feigning death when threatened)
  

• Physiological adaptations relate to variations in the activity of internal organs (e.g. homeothermy, colour perception)

When exposed to a specific environmental agent (selection pressure):

• Organisms with beneficial adaptations will be more likely to survive long enough to reproduce and pass on their alleles
  

• Organisms without these beneficial adaptations will be less likely to survive long enough to reproduce and pass on their alleles
  

If environmental conditions change, what constitutes a beneficial or detrimental trait may change, and thus the allele frequencies in a population are constantly evolving

Natural Selection