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Microbial populations in nature generally inhabit extended environments with substantial spatial variation in ecological factors: light intensity in the ocean, temperature in geothermal hot springs, or a variety of chemical concentrations including salt and pH. In such continuously varying environments, it remains unclear why a finite number of subpopulations form and how this number is set. Here we show that a model of asexual evolution in a gradient maps onto a no-gradient neutral model, and by mapping this model to a gas of kinks and antikinks, we derive the full distribution of the number of coexisting lineages, and their correlation functions. Testing these predictions in controlled laboratory experiments would provide valuable insights into many real-world microbial communities.