Darwin's four postulates
Postulate 1: Trait variation
Darwin's first observation involving evolution is that there will be different traits (IE: height, strength, intelligence) shown in a population. This just means that in a community of badgers, not all of them will show the same traits.
Postulate 2: Inherited traits
Another observation that Darwin made when he was in some islands in the pacific was that animals with certain traits sometimes passed them on to their offspring. A woodpecker with a large beak is more likely to give birth to another large beaked bird than a short beaked bird.
Postulate 3: Things die
As is the case with cell death, populations grow, but a percentage of it will die before it gets to reproduce. This is a natural process and is observed in all species. Populations grow quickly so that more organisms survive and get to reproduce than the ones that die off. Also, the ones that die off most likely do so because their traits were not the best for their habitat.
Postulate 4: Natural selection
Darwin's 4th postulate is likely the most widely known. It states that organisms that reproduce the most do so because they have certain desirable traits. It is not left to chance to see what genes will be passed on. Instead, the nature-given traits of each animal determine if they, or their offspring, will survive and procreate.
Freeman, S., Quillin, K., & Allison, L. (2014). Biological Science (5th ed., Vol. 1). Pearson.