I would start with the requirements for natural selection.
- Individuals in a population possess varied traits.
- Traits can be passed from parents to offspring
- More offspring are produced than an environment can support, leading to competition for resources.
- Evolution occurs through long periods of time either through punctuated equilibrium or gradualism.
An ancestral species is a common ancestor that species have. A living species is an extant species (the opposite of extinct). Adaptations can either increase or decrease an organism's chance of survival (and therefore lesser or greater reproductive success). It is important to remember that if a mutation occurs in an organism but it is not passed down to a new generation it is not evolution.
If organisms have a common ancestor, you could infer that they have similar DNA or traits.
