[Music] 
Why do some birds spend time and 
energy frantically dancing? 
Why do some have oversized ornaments 
making it harder to fly and forage? 
How can this be natural selection? 
Well, natural selection isn’t 
actually the competition to survive: 
it’s the competition 
to pass on genes. The battle to breed 
generates a particular kind of 
selection called sexual selection.
[Music]
Sexual selection is about out-competing, often 
out-attracting competitors to 
get more and better mates, 
and that’s where the dancing and 
ornaments come in. 
To understand how sexual selection 
can make an animal like this, 
we need to simplify things. 
First we’ll show how natural selection 
favors some traits over others 
and how that results in changes 
to a population. 
Then we’ll show how sexual selection 
can change the same population 
in unexpected ways. Let’s start 
with a representative  
everyday bird from a small population 
living on an island.
[Music]
In every generation,
due to recombination or mutation, 
some chicks are born with changes in their
genes that cause them to grow up 
with traits that are a little different.
Here are three hypothetical new traits 
that might appear: Awkward, Armored Shield,
and Long-bill.
Let’s take Awkward first.
Like most new traits 
Awkward isn’t helpful. It’s hard to get 
around and forage.
A baby born with the Awkward trait 
won’t grow up as strong as other chicks 
and might even die before it has a 
chance to breed. So the genes that caused
Awkward
don’t make it into the next generation.
Armored Shield seems at first to be 
beneficial.
This chick is totally safe from predators 
and it lives twice as long 
as everyone else.
But suppose the shield gets in the 
way and makes it physically impossible to 
breed.
Even though the individual survives 
to a ripe old age, 
the Armored Shield genes die off. Every 
once in a while, 
a new trait pops up that provides an 
advantage.
Long-bill allows this bird 
to eat fruit that was too hard to handle 
before, enabling it to access a
nutritious
and plentiful resource. He grows up 
stronger and is able to gather more food 
and help raise more young.
These young, both male and female, 
inherit both the trait and the advantage,
so everyone with Long-bill,
generation after generation, is more
successful
until everyone on the island eventually
has a long bill.
What we've just examined
is classic adaptation by natural
selection:
the process by which traits become more
for less common
depending on an individual's ability to
survive and gather resources.
So what then is sexual selection?
Sexual selection is the process by which
traits
become more or less common depending on an
individual's ability to mate with more
or better partners.
To really understand how this works,
let's revisit our island population,
now full of long billed birds.
These new bills 
have improved their diets so much that
it’s easier to raise young.
Females can raise chicks on their own,
freeing up males for the mating game.
And in this food rich environment, sexual
selection can become a more dominant
evolutionary force. How food is
distributed,
whether it's clustered or dispersed, can
have a dramatic effect
on what kinds of traits will be favored
by sexual selection. Let's look at two
situations.
One that leads to male-male competition,
and one
that leads to female choice. First
male-male competition—
which we’ll demonstrate with the trait
Burly. When a primary food source
like fruit trees are clustered, males can
defend territories around them
to gain exclusive access to the females who
come there to eat.
A male who was born burly is better able
to defend a territory
and will be able to mate with more females.
Thus burly males will have more
offspring,
and their genes will become more common.
Each generation of males will get burlier
and burlier
to the point of impacting their survival.
Females who mate with burly males
benefit in two ways:
they have access to the food they need
and their sons are more likely to be
burly and successful.
But females don't get burly since they
were already the right size for survival
and there's no advantage for them to
carry this extra weight.
Natural selection and sexual selection
are pushing females and males into
very different forms
and this difference between the sexes is
a telltale
indicator of sexual selection at work.
Now let's look at mate attraction,
specifically female mate choice. This
kind of selection
happens most dramatically when bountiful
resources like fruit
are spread out and impossible for males
to defend.
Instead males must try to attract
females,
convincing them that they are the best
possible mates.
A male who was born Fancy
might have an ornament that he uses to
attract females.
If the ornament works and more females
choose to mate with him,
the genes for fancy will be passed on.
Fancy traits
maybe innately appealing to females, or
they may reveal something about the
male’s underlying health.
In either case, these genes give rise to
fancy males
and females who prefer mates with those
particular genes.
Each generation males become more
elaborate until the traits significantly
decrease their chances of survival.
And just like our first situation, the
females don't change because
natural selection still favors their
camouflage coloration.
[Music]
To sum up, classic natural selection
leads to adaptations for gathering
resources and surviving.
Sexual selection
leads to adaptations for gathering mates
and breeding.
these two kinds of selection may seem
different,
but the mechanism behind them is
actually the same:
the competition to leave more copies of the
genes
in the next generation. Evolution is
never ending,
with recombination and mutation
creating new traits every generation.
Most of these will die out, but those traits
that provide an advantage
in the competition to breed inevitably
become more common over time—
sometimes transforming the drab
into the magnificent.
[Bird song]
