Final answer:
Characteristics of a species can evolve over time due to genetic mutations, environmental changes, natural selection, and migration, all of which can introduce new traits or affect the prevalence of existing traits in a population.
Step-by-step explanation:
The characteristics of a species can change over time due to several factors, including genetic mutations, environmental changes, natural selection, and migration (also known as gene flow). Genetic mutations introduce new genetic variations into a population, which can be passed on to subsequent generations. Environmental changes can affect which traits are advantageous, thereby influencing natural selection. Natural selection is the process by which individuals with favorable traits are more likely to survive and reproduce. Migration allows for the exchange of genes between populations, which can introduce new traits into a gene pool. Therefore, all of the listed options can contribute to the evolution of a species.