Explain the biological basis for assigning all human populations to a single species. can you think of a scenario by which a second human species could originate in the future?

Molecular biology (DNA sampling) is the best proof.

Explanation:

An study published in the journal of science found that 99.9% of humans are quite similar. The  scientific team analysed DNA from 1,056 people from 52 populations in Africa, Eurasia (Europe, the Middle East, Central and South Asia), East Asia, Oceania and the Americas.

The phenotype ( visible appearance) results from a very small proportion of  differences in genetic traits, hence the concept "race" is more a cultural concept than a biological concept.

1: most common garden plants prefer loam — soils with a balance of different-sized mineral particles (approximately 40% sand, 40% silt, and 20% clay) with ample organic matter and pore space. however, some plants grow better in sandy soils, while others are well-adapted to clay soils.

2: available water capacity is the maximum amount of plant available water a soil can provide. it is an indicator of a soil's ability to retain water and make it sufficiently available for plant use. available water capacity is the water held in soil between its field capacity and permanent wilting point.

3: it is the same for all living things (plants and animals). it also includes very tiny organisms that our eyes cannot see. they are known as microorganisms, and include viruses, bacteria and fungi.