The true brilliance of Darwin's theory of
evolution by natural selection is its simplicity.
There is nothing complicated in the way that
many of the other basic pillars of modern
scientific understanding are difficult for
the non-scientist--like gravity or particle
physics both of which require a lot of math.
In order to understand living systems, natural
selection is the most basic of all pieces
of fundamental knowledge because it explains
the way in which organisms adapt to the demands
of their habitats over a period of generations.
As I said, the basic mechanism of natural
selection is very simple.
Only three things are required--call them
"necessary conditions" in order for natural
selection to operate.
These three things are:
1) variation--the population cannot be all
the same.
They must vary in some phenotypic trait so
that some phenotypes can be "favored" with
higher reproductive success.
Obviously this can't happen if every individual
were identical.
2) selection--there must be an association
between certain phenotypes and higher "fitness"--here
meaning success in survival and reproduction.
You can have variation in the population and
no selection.
If everybody in the varying population has
the same fitness, then--again I think this
is obvious--there is no systematic favoring
of one phenotype over others and so natural
selection will not be causing any evolutionary
change in the next generation.
3) inheritance--there must be an association
between the phenotype of parents and the phenotype
of offspring--this is basically a fancy way
of saying that offspring must resemble parents.
We may take this for granted and say "of course--the
offspring are inheriting their genes from
their parents."
But the reality is that there's more than
just genes that determine phenotype.
Maybe you've heard of "nature vs. nurture"--the
crux of that issue is that part of who we
are as individuals is genetically determined
(nature) while other parts are the result
of some aspect of our upbringing (nurture)--everything
from conditions in the womb--how many cigarettes
mom smoked when she was pregnant--to the teachers
and role models you have had over the course
of your life.
Only the nature part is inherited.
If a phenotypic trait is primarily ore entirely
determined by nurture rather than nature,
then the offspring won't necessarily resemble
the parent, and so the next generation won't
be any different from this generation, and
so evolutionary change won't be happening.
Variation, selection, and inheritance.
Three necessary conditions for evolution by
natural selection.
I'm going to add now that if you have all
three of these conditions--if you meet these
three requirements--then it is a fact that
the next generation will be different from
this generation, being a little bit more similar
in phenotype to the successful parents of
this generation.
This is evolutionary change at a micro-scale.
We even refer to this as microevolution.
From one generation to the next, the amount
of evolutionary change will be miniscule,
something that might not even register in
your awareness.
But if similar miniscule microevolutionary
changes were to occur generation after generation
for thousands of years, the population of
descendants could look noticeably different
than the population of its ancestors.
And over tens of thousands of years, they
might even become dramatically different.
Life has been on the planet Earth for at least
3.5 billion years.
That's time for an awful lot of change to
transpire.
Evolutionary biologists like myself are concerned
with the history of change and the mechanisms
of change (because there's more to the story
than just what Darwin brought to the table).
If you want to follow up on this, take the
biology majors' class BIO202.
For now, we are going to run a simulation
exercise in which students take the role of
a population of predators that exploit a population
of a prey species (this might be toothpicks,
beans, or beads depending what the class you're
in).
For the predator population (again, that's
you all), the first necessary condition of
variation will be in the kind of "mouthpart"
you have.
Please don't put any of the implements we
give you in your mouths.
They are just surrogates for what would be
mouthparts in a real predator like a bird
or something.
You will be operating your implement with
your hand.
Initially the population of predators will
consist of about an equal frequency in each
of a few different types of mouthparts.
You'll be assigned to one of the mouthpart
groups randomly with each generation of the
simulation.
After you have your mouthpart (in your hand)
you'll go out into nature and live a full
life of exactly 60 seconds, collecting as
many prey items as you can.
After your minute of glory, we'll sit down
and assess how successful we were in amassing
resources needed for our reproduction.
The offspring each of us leaves will be proportional
in number to the amount of food we collect.
This can mean that the next generation of
predators may not have equal amounts of the
different kinds of mouthparts.
We'll run a calculation of what the new frequencies
of the mouthparts are for the next generation,
and I'll assign students to be predators again
with numbers of each mouthpart type corresponding
to the new frequencies.
This continues for five generations.
Okay.
I said that the necessary condition of variation
was met by the existence of different kinds
of mouthparts in our population of predators.
Will there continue to be variation in our
population for the entire five generations?
Well, yes.
Unless one of the mouthpart types completely
outdoes all the others such that by the end
of five generations it's the only kind that's
left, there will continue to be different
types of mouthparts in our population and
thus variation will be there.
In this simulation, is the second necessary
condition of selection being met?
How?
Hit pause and think about this for a minute
before moving on with the playback.
Okay, I hope you decided that selection here
would be met if the different mouthpart types
had different rates of success in collecting
prey.
Darwin's observations of finches on the Galapagos
islands were that the finches' beaks varied
in ways that would make individuals better
suited to one type of food over another.
If a finch population found itself in a place
where only large, hard seeds were available,
then a thicker, more massive beak was better
because it could break the hard shells of
the seeds.
In a different location where only small seeds
were available, smaller beaks did better--they
were more maneuverable and also less costly
to produce compared with larger beaks, which
require more material investment.
Why did the finch decide for the smaller beak?
Because it couldn't afford to pay a large
bill.
The fact that one beak type does better than
a different beak type is exactly the selection
needed as condition #2 for evolution by natural
selection.
Here's a related question--a serious one this
time--about our lab activity this week.
We might anticipate that as successful mouthparts
do better, they become more common over five
generations, and thus first condition of variation
is actually changing over the course of our
five simulated generations.
Is selection also changing over the five generations
of our simulation?
I'll leave this as something to talk about
during the lab activity.
The third necessary condition for evolution
by natural selection is inheritance.
How is inheritance incorporated into our simulation
activity?
Again--hit pause and think about this for
a bit.
Okay, the fact that we use the success of
the different mouthpart types in the calculation
to determine their relative frequencies in
the next generation means that individuals
with a successful kind of mouthpart re more
likely to be represented in the next generation
than individuals with lousier mouthparts.
This is like saying "I have this really good
mouthpart, collected tons of food, and so
I had a lot of offspring, and so there are
more mouthparts like mine in the next generation."
Because if mouthparts were not heritable,
then even if I were successful and had a lot
of babies, those babies wouldn't look like
me and that would be the absence of inheritance--condition
#3.
But since our calculation makes the offspring
more closely resemble the successful parents
of the previous generation, this is the way
inheritance is incorporated into our activity.
With all three of the necessary conditions
met by our activity, the expectation is that
evolutionary change will occur--each generation--and
these smaller changes will add up over five
generations into something larger.
Moreover this change will be for the better--the
population we have at the end will be better
adapted for consuming the available prey.
In our simulation, the prey items will also
be evolving.
I'm not addressing this dimension of the simulation
here, but you should start thinking about
the three conditions needed for evolution
by natural selection to occur as it pertains
to evolution of the prey species.
Is there variation?
In what way is variation manifest for the
prey species?
Is there selection?
What is the cause for the differential success
in survival and reproduction?
Is there inheritance?
Because if you can establish that all three
of these things are met, then you should be
expecting evolution by natural selection will
occur in the prey as well.
