That said, there is no proto-chicken that laid a chicken egg or a chicken hatching out of a proto-chicken egg. There was no "first chicken." Populations evolve, not individuals. Let's not confuse a de novo mutation with evolution. Evolution is a change in allele frequencies over time, and speciation is reproductive isolation of populations experiencing allele frequency shifts.

There are still population-wide polymorphisms in humans that are present in gorillas but have been fixed in chimpanzees. Populations of proto-chickens gradually evolved into chickens, but if there were a hard line to define the distinction (like the wavelength where orange ends), then there would have been a long time of proto-chickens and chickens interbreeding and their descendents oscillating back and forth between the two definitions over generations.

[–] bulwark 3 points 1 day ago

I also think the chicken or egg question isn't great and agree that it evolved over a long span of time, multiple times. The oscillation between the proto species and species is really interesting to think about and makes a lot of sense. Thanks for your insight.