[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.