Long before breed registries existed, dog populations were distinct without being standardised. Those populations still exist, and comparing them with modern breeds is informative.
What a landrace is
A landrace is a population shaped by its environment and by the work it does, maintained without a written standard or a closed studbook.
Selection operates on outcomes. A herding dog that cannot manage stock is not bred from, regardless of its coat colour or ear carriage.
Appearance varies widely within such a population, because nothing selects for uniformity and nothing penalises variation that does not affect function.
Why variation persists
With no closed registry, animals from neighbouring populations enter continually, which introduces genetic material rather than only losing it.
Selection also acts on many traits at once and only weakly on each, which prevents any single characteristic being driven to an extreme.
The result is a population with substantially more genetic diversity than a comparable pedigree breed, and correspondingly fewer concentrated recessive conditions.
What standardisation changes
When a landrace is formally recognised as a breed, a subset is registered as founders and the population is closed behind them.
Selection then shifts from function to conformity, since the standard describes appearance in far more detail than it describes working ability.
Within a few generations the registered population becomes visibly uniform and genetically narrower, while the unregistered original often continues alongside it.
The behavioural consequences
Landrace populations selected for independent work tend to produce dogs that make decisions without direction, which is valuable in the field and demanding in a house.
Livestock guardian types are the clearest example, having been selected for centuries to act alone and to treat unfamiliar approaches with suspicion.
These traits do not disappear when such a dog is kept as a companion, and they account for a large share of the difficulties owners report with them.
What this means for mixed-breed dogs
Free-breeding dog populations across much of the world are effectively landraces, and dogs from them tend to converge on a similar medium size and build.
Their genetic diversity is generally higher than that of pedigree dogs, which reduces the likelihood of the concentrated inherited conditions seen in closed populations.
This is an advantage on average rather than a guarantee for any individual, and a mixed-breed dog still needs the same veterinary attention as any other.