Question: How to explain the diversity of living things on earth?
Answer: Assumptions are (almost) everything.
One explanation is that they were created different.
A second explanation is that there are different forms.
A third explanation is evolution by natural selection.
The first and third explanations are well known - the second idea (that things are different due to having different intrinsic forms) is a very old one, as still going, but not well known as such.
Well-known exponents of the idea that things differ due to there being different underlying forms include Aristotle, Aquinas, Goethe, D'Arcy Thomson, Conrad Waddington, Steven J Gould (sort of), Stuart Kauffman, Brian Goodwin and Rupert Sheldrake.
In general, these theoreticians tend towards a mathematical - often geometric - understanding of form. This is, indeed, the dominant stream in British theoretical biology (as exemplified by the elite Cambridge-based Theoretical Biology Club).
How to decide between Creation, Forms and Natural Selection as the cause of the multiplicity of 'species'?
All these can explain enough, in principle 'everything' - so how to choose?
Well, there is really no way of choosing except on the basis of assumptions. Each body of work makes different assumptions.
Creation assumes things like that species were intentionally created with some kind of plan and purpose - however general this plan and purpose might be.
Forms assume that underlying reality is structured, and our job is to discover the identity and nature of these formal structures, and perhaps the mechanisms of how they impose form, or how appearances relate to underlying form.
Natural selection assumes that all significant and lasting variation can be explained by selection processes. In other words, it seeks to explain significant variation using selection processes: undirected variation with replication and competition leading to differential replication.
All three types of explanation find what they look for - and only what they look for.
So why has Natural Selection carried the day in professional biology? - when results are determined by assumptions, and these assumptions cannot in principle be proven to be more (nor less) true than the rival assumptions?
The answer is essentially because NS is currently, has led to, and so far proved more amenable to, professional success: to the business of being a professional scientist - getting jobs and funding, researching, teaching and the rest of it.
Professional success may be related to pragmatic or rhetorical usefulness, or it may have nothing to do with such things - after all, ideas of form have considerable prestigious elite influence - for example among mathematicians and computer scientists working in chaos and complexity theory (which are modern variants of ideas of form); and ideas of creation have massive support among religious people.
And, of course, in general culture the implications of Natural Selection - especially as applied to humans - are accepted and promoted or ignored and suppressed purely according to their political or religious convenience - entirely regardless of reason or evidence.
But within biology as a professional activity there is much more scope deriving from the assumptions of Natural Selection - and that is why it carries the day.
That is the only reason why it carries the day.
[Note: my main job title is Reader in Evolutionary Psychiatry and I have been reading, publishing and teaching about natural selection for more than 15 years.]