How do species change over time?

Species change over time by a process called natural selection. This is commonly associated with Darwin, as he wrote about it in his book "On the origin of species". Evolution (the change of a species over time) works through the process of natural selection. Natural selection happens when an individual gets a mutation (random change) in their DNA sequence, which results in a change in the organism's behaviour or morphology. Some mutations will be advantageous, and some will be detrimental. Advantageous mutations increase the organism's chances of survival (fitness), and therefore increases the chance that that individual will reproduce to create offspring. In theory, the individual's offspring inherits this changed DNA, which then increases their fitness. This process repeats itself until a significant majority of the species have this new DNA and its associated physical change.