The appearance of a new species is not so dramatic. The first members of a new species will typically be indistinguishable — to us — from the species they have evolved from. And while extinction has a clear final moment — the last member of a species dies — the formation of a new species does not usually happen in a single recognizable instant. Which is why we haven’t yet raised our glasses to celebrate, say, Rhagoletis pomonella, the apple maggot fly.
The most common way to define a species is a group of individuals that breed with each other successfully. For example, dogs, despite their vastly different looks, can breed with each other, so they are are considered one species. Horses and donkeys are counted as different species because their offspring (mules and hinnies) are sterile. For individuals to be considered as belonging to separate species thus means that they are “reproductively isolated”: they can’t, won’t, or don’t breed with each other.
I can sense your excitement. And perhaps that’s the real reason we don’t celebrate apple maggots, or any of the other new species (and there are many we know about) that are in the process of evolving. For when a new species does appear, it’s just not that different from the old species. To evolve the flamboyant differences that distinguish a swan from a duck, or a human from a chimpanzee — that takes thousands, even millions, of years.
That is what we lose with extinction.
Photograph: Rhagoletis pomonella, the apple maggot fly. (Wikimedia