"A change in the heritable characteristics of a species" is evolution.
Most frequently this is a gradual cumulative change.

Changes to individual organisms during their lifetime are not evolution, as the change has to be heritable.
Evolution occurs a the species level because individuals survive or die which changes the composition of the members of the species.

Evidence for evolution in this list comes from;

• The fossil record.
• Selective breeding of food crops and domesticated animals.

If the are different selection pressures in the two environments, then snails will be affected by different natural selection. This is divergent selection.

Continuous variation across the geographical range of related populations matches the concept of gradual divergence.

All the insects share the feature of having 2 pairs of 'wing-like' structures although there are differences in each animal.

Selective breeding provides evidence for evolution because it shows that when a breeder chooses the animals for the breeding programme which have specific features, the offspring have more of these features.

Natural selection causes an increase in the number of dark moths because the birds which eat the moths cannot see the dark forms so easily, as they have better camouflage in polluted environments.

All the answers have some relevance but the most compelling is that they document the existence of extinct organisms that show a relationship to present day species, often illustrating the cumulative changes which have occurred.

Homologous structures evolved from a common ancestor and have a similar structure but this structure may be modified to adapted to a different function.

Homologous structures have a similar structure but not identical structures, and this allows them to adapt to new functions. Legs and arms of animals are sometimes examples of homologous structures but, not always, e.g. insect legs and human legs are not homologous.

Adaptive radiation occurs when different selection pressures lead to the evolution of a range of different adaptations. This may occur at first in a single species but will eventually be seen in several related species. They may be in different isolated locations or in organisms exploiting different niches in a single environment.

Adaptive radiation occurs when different selection pressures lead to the evolution of one or more species which are exploiting different niches in an environment. Mutations bring about new alleles in several genes and different selection pressures in each niche lead to the natural selection to favour different adaptations.

Dog breeds are developed by humans artificially selecting certain traits in dogs and breeding only the animals with these desired traits.

As evolution occurs, the heritable characteristics (genes) in the gene pool of a species change to those which are the most successful for survival.

For two groups of one species to form two species, they must be reproductively isolated, so they cannot interbreed and then different mutations will allow different genetic variation in each of the isolated populations, leading to adaptation to their respective environments and phenotypic differences between the groups.