Natural selection emerged in 1859 from the work of Darwin and Wallace, and (since the work of Mendel was not known) genetics only emerged around 1900 as a result of various more-or-less forgotten biologists such as de Vries and William Bateson who soon agreed to award retrospective credit to Mendel.
It is generally forgotten that for several decades after its discovery, genetics was regarded as a new science, a rival to Darwinian natural selection and having - pretty much - superseded natural selection.
It was only in the mid-twentieth century that natural selection, genetics and (also forgotten) embryology were synthesised in the Neo-Darwinian synthesis by a variety of workers such as Fisher, Julian Huxley, Mayr, Dobzhansky and others.
But why was genetics seen as having superseded natural selection?
Nowadays the link seems obvious but it was not at all obvious - forging the new synthesis between genetics and natural selection (and embryology) required the joint work of several geniuses spread across several nations over a few decades.
Why were genetics and natural selection for so long seen as rivals rather than parts of a greater whole?
My guess is that genetics was seen as superseding natural selection mostly because genetics was more useful.
i.e. Genetics was more useful to the people who used biological theories.
Who was genetics useful to? - why, exactly the same people who had initially found natural selection useful - that is mostly breeders: animal and plant breeders in agriculture, horticulture, and for hobbies (e.g pedigree dog showers, fancy pigeon showers).
Genetics enabled breeders to do their job better - genetics enabled more rapid, reliable, precise creation of functionally useful new breeds that would themselves breed true.
By contrast, pre-genetics Darwinian natural selection was vague and incoherent ('gemmules' and all that stuff) exactly where genetics was most exact - in relation to the business of how characteristics were inherited.
This comes back to my point about real science being rooted in common experience - in this instance in the experience of animal breeders.
By 'common experience' I mean the individual personal experiences of people engaged in devising practical applications of scientific ideas.
In other words, common experience means that non-scientists should - ultimately - evaluate science.
That is, stock breeders evaluated biological science, doctors evaluated medical science, engineers evaluated physical science - and so on.
The work of these practical and applied 'technologists' itself being evaluated in its turn by more general users of the technology - the farmers and animal fanciers who bought from breeders, the patients who chose which doctors to consult and pay, the people who bought or depended on the products of engineers - and so on.
It is (or was) the independence and practical nature of this evaluation of science by 'common experience' which was the underpinning validation of real science - without which science becomes a fake.
(For example, when science 'captures' its own evaluation system it becomes fake; as has happened in medicine, where treatments are now supposedly evaluated by the people who do clinical trials - rather than by the experience of doctors, and the experiences of their patients.)
So... who was the neo-Darwinian synthesis useful to?
I mean, aside from professional biologists.
One suggestion: it neatly explained the emergence of resistance to antibiotics.
REF: Science as a Process - by David L Hull, 1987.