Maybe you have a favorite recipe, something you've experimented with over the years, trying

different seasonings, different quantities, different cooking methods and times and temperatures,

until you've arrived at what you think is the ideal

version of this dish. At the point you've cracked it, you know it's time to stop tweaking.

But evolution doesn't work that way. From KERA in Dallas, this is Think. I'm Chris Boyd.

Even when a species seems ideally adapted to its environment, there is no mechanism to shut down

the processes of evolution.

Mutations keep happening as if a cook with a great recipe just can't stop adding or subtracting

spices.

And because nature can't leave well enough alone, while our success as a species might make

us feel like the winners in the biological lottery. We humans are not and never

will be perfect. Lawrence D. Hearst is professor of evolutionary genetics in the Milner Center for

Evolution at the University of Bath. His book is called The Evolution of Imperfection, the Science of Why

We Aren't and Can't Be perfect. Lawrence, welcome to think.

Well, my pleasure.

Lovely to be here.

How are we defining perfect from an evolutionary perspective?

Well, in many regards, you do it backwards.

That's to say, imagine that view of evolution, that many of us have that view of a progressive progress.

So, you know, the classic march of progress, the chimp, slowly coming upright, turning into the human,

what have you. So that gives you an idea of that which is really well adapted, that which is

really well functioning. But then there are a number of

...