Cichlids, a type of fish, were introduced to a lake around 200 years ago. Over the years, the fish have developed into two distinct groups. One group stays in the main lake and the other group resides in the streams that flow into it. Both groups of cichlids breed in the same streams at the same time of year. However, the groups are splitting into two genetically and physically different types of fishes. How would you classify this type of speciation

sympatric speciation

Explanation:  

Different sympatric species have the same closest ancestor and live in the same area but inhabiting different niches.

The steps for sympatric speciation are:

An ancestral species inhabit a geographic area.With time, different populations of the same species occupy different niches or microhabitats in this general area. As the microhabitats are different, they have different environmental pressures that are acting on each population.These pressures lead to the origin of differences between groups, which need to adapt to each environmentEach population suffers genetic changes due to environmental conditions. These changes are inheritable.With time, these differences drive to the final complete divergence of populations, becoming different species and reproductively isolated.

In sympatric speciation, the interruption of genetic flow must be fast, and there must be differences in niches so no species can compete and displace the other species. Speciation occurs in the same distribution area of the original species. The new species originate in a place with no physical barriers but different environmental pressures.

In the exposed example,

The introduced Cichlid fishes 200 years ago → Original species and the common ancestor between the new derivated speciesThe lake → general geographic areaThe main lake and the streams → Microhabitats or niches with different pressures eachThe groups are splitting into two genetically and physically different types of fishes. They do not mate. There is no genetic flow between groups. → Speciation process.