Small changes occur all the time in species as experimentally seen in micro-evolution of organisms. And fossils provide clear and unambiguous evidence of these small changes causing transformations like fins to limbs over time. And finally there is genetic and embroyological evidence that this transformation occured step by step.
Also there are plenty of organisms where fossil record is continuous enough to record how small changes lead to speciation over millions of years. An example below
Another example of a complete fossil record showing continuous evolution followed by speciation,
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Abstract.— Marine planktonic microfossils have provided some of the best examples of evolutionary rates and patterns on multi-million-year time scales, including many instances of gradual evolution. Lineage splitting as a result of speciation has also been claimed, but all such studies have used subjective visual species discrimination, and interpretation has often been complicated by relatively small sample sizes and oceanographic complexity at the study sites. Here we analyze measurements on a collection of 10,200 individual tests of the Eocene planktonic foraminifer Turborotalia in 51 stratigraphically ordered samples from a site within the oceanographically stable tropical North Pacific gyre. We use novel multivariate statistical clustering methods to test the hypothesis that a single evolutionary species was present from 45 Ma to its extinction ca. 34 Ma. After identification of a set of biologically relevant traits, the protocol we apply does not require a prior assignment of individuals to species. We find that for most of the record, contemporaneous specimens form one morphological cluster, which we interpret as an evolving species that shows quasi-continuous but non-directional gradual evolutionary change (anagenesis). However, in the upper Eocene from ca. 36 to ca. 34 Ma there are two clusters that persistently occupy distinct areas of morphospace, from which we infer that speciation (cladogenesis) must have occurred. (gradual evolution followed by speciation in late Eocene shown in figure below)
Thus:-
1) Where fossil record is complete (usually for marine invertebrates with hard exoskeletons) the fossil record captures the complete trend of small scale evolution causing large scale transformations and (in some cases) speciation (shown above).
2) For vertebrates where there are quite a number of fossils (like in the transformation of fish to amphibians), the fossils are numerous enough to show the trajectory of the small changes that led to transformation of fins to limbs (along with shoulder development etc.) I have already provided multiple lines of evidence to demonstrate how this supports evolution and refutes creationism. You are free to address any of the points I discussed:-
Evidence of Evolution that was presented but never addressed
3) The entire step-by-step genetic mutation pathway that resulted in the gradual transformation of fins to limbs have also been charted. The genetic pathways once again vindicates evolution demonstrating that nothing other than gradual step by step transformation through mutation and selection of a few of the fin generating genes is needed to transform the fin of a fish to limb of a land animal. I have presented the evidence which you are free to discuss,
Evidence of Evolution that was presented but never addressed
Evidence of Evolution that was presented but never addressed
A simple and clear exposition is linked below:-
So, given that all the extensive evidence supports the evolutionary explanation and none support creationism, rationality dictates that the evolutionary explanation is accepted as true.