[Music] There are almost 10,000 known bird species on the planet. How did they evolve into so many unique forms? Take the Birds-of-Paradise for example, with their dizzying array of colors, shapes, and sizes.
From genetic evidence, we know that the ancestor of all 39 species was a plain crow-like bird. And the process that caused this one species to split into many- is called speciation. Before we explain speciation, let’s talk about what we mean when we use the word species.
According to the classic definition, species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other groups. Speciation begins when groups become separated in space or become different enough in form and behavior that individuals from one group no longer regularly mate with individuals outside the group. One way for a new species to evolve starts when a few individuals colonize a new area that is outside their native range.
Take for example a volcanic island that rises out of the sea just off the coast of the mainland. Over millions of years, as the island grows larger and less volcanically active, plants begin to grow but there are no birds here yet. Then one day, birds from a common species on the mainland are blown over by a storm and start a new population.
If these birds move between the island and the mainland only rarely, the conditions for reproductive isolation are set and the brand new island population is on its way to becoming a new species. Over thousands of generations the two populations will be shaped by natural and sexual selection into separate forms, each with a unique genetic fingerprint. But at what point are we confident that this island population has really changed enough to become a new species?
Well let's put our definition to the test. If we introduce a female from the mainland species to potential mate from the island after two hundred generations and isolation (about four hundred years in bird terms), will they mate? It appears that these two [chirp, chirp] still recognize each other as members of the same species.
How about after another thousand generations of isolation? Now the female finds this island male’s song a bit strange, [chirp, chirp] but she still chooses to meet with him. What if we fast-forward again to 10,000 generations of isolation?
[tweetie, tweet] This time the female doesn't even recognize the male’s song and is completely uninterested. If this female’s mating preferences are widespread in the population, we now have two bona fide species: Mainland and Island. But these kinds of dating match ups don’t happen in reality.
Here because our birds avoid flights across the open ocean, Island birds do not regularly encounter Mainland birds. Because genes from the two populations no longer intermix, genetic differences accumulate and the two populations become different species. So in practice, scientists classify species separated in space by their genetic differences, not by observing blind dates.
[Music] But there is another kind of speciation that can occur when a habitat becomes divided and a once widespread bird population becomes fragmented. Imagine that our new island grows larger over time and the volcano develops a central mountain ridge while a valley forms along the Eastern Coast. Over time, the Eastern birds who have plentiful fruit resources and only need a single parent to successfully raise young evolve specialized mating systems.
Males compete for multiple mates, and and to get noticed evolve fancy plumage and display behaviors. On one side of the valley the males evolve elaborate head plumes and on the other it's the tales that get fancy. At the same time Western birds are consistently dealing with harsher food conditions and do not evolve a specialized mating system or fancy feathers.
Just as before early in the speciation process when individuals from isolated populations meet, our Eastern and Western birds readily mate. But as generations pass and Eastern males become fancier, the Western females begin to find the Eastern males’ habits foreign and surprising. But these traits are clearly not deal breakers!
Even after 10,000 generations of geographic isolation, some other Western females continue to find the male displays attractive. At this stage the eggs she lays aren’t viable because the separated populations have evolved too many genetic differences to be compatible. To review, in this kind of speciation, geographic barriers have split the habitat into three fragments and on each a unique species evolved.
What if the Southwestern slope erodes into a low peninsula, allowing Southeastern birds to mix with the Western birds? By this time, their mating systems are completely different and the two species can't create fertile offspring. Now related species can coexist, but because they no longer successfully interbreed, there's no going back.
So how did the 39 bird-of-paradise species evolve from one crow-like ancestor? New Guinea and the surrounding islands where birds- of-paradise evolved have changed dramatically throughout geologic time, repeatedly isolating populations. Over thousands of generations in isolation, natural and sexual selection have morphed the fragmented populations into a wonderful array of colorful species.
The process of speciation is ongoing and split after split new and unique organisms evolve. Incredibly, all those 10,000 bird species we see today evolved from one single bird ancestor. And it’s thanks to the evolutionary processes driving speciation that our planet is home to such impressive biodiversity.
