It's all about mating and genetic lines, rather than babies being born that are spectacularly different. Such infants would noit be very likely to procreate so would be at an evolutionary disatvantage.
So what happens with the Heidelbergensis. The Homo Sapiens and the Neanderthaler are meant to have evolved from that.
There's a period of 50000 years or more of when the H.S's 'forefather' dies out and when the H.S comes.
50000 years to me doesn't seem gradual. After all the HS has already been around for 250000.
That's the first question.
And the second is what I've been on about the whole time so I'll use the example of the Neanderthaler and the Heidelbergensis, since the N's forefather was still around when the N was around. At some point in a relatively short period of time, the N. evolves from the H. at some point there's a breakoff and the H. becomes a different species.
I understand that the environment over time can change the charactereristics of a certain species(black, white, yellow, large, small)but even then there must be a breakoff point(no matter how gradual) when parents and children are different(species).