[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]