The Theory of Evolution – 4.2, 4.3

So the way that evolution by natural selection works,

you start off with a population of organisms and there's gonna

be genetic diversity in there and phenotypic diversity,

so they'll have different genes and different characteristics.

And at steady state, so if nothing changes,

then that population just continues to be the way that it

is. But every now and then,

you'll have something that changes in the environment and

whether it's food becomes less available

there's some kind of volcanic eruption or something that

changes the conditions that the animals live in or plants as

well that can also evolve.

So, what this creates is something called selection pressures.

And so, when you have this change in the environment,

some animals will be more able to adapt to these new

conditions based on the characteristics that they have.

But others won't handle it so well. So,

it might be that they their characteristics aren't as well

suited to this change.

It might be that there's a new predator comes into an

environment and some animals bigger leg muscles and so can

run away from the predator more quickly,

whereas others with small leg muscles are slow and get caught

and very quickly die out as they're eaten by the new predator.

So, these animals, let's say,

in this hypothetical situation with the larger leg muscles,

they're going to survive, they're not eaten by the

predator. And so, they are able to reproduce

because you can't reproduce if you've been eaten by a predator.

And so by being able to reproduce,

they can pass on their traits and they'll do so in their

genes and we'll talk more about that in a separate lecture.

And in doing so,

they'll produce offspring who have the bigger leg muscles.

So, This is the selection pressure on the animals

and those were the advantageous traits considered the most fit

for the new conditions. And so survival of the fittest

happens, those animals with the optimal traits survive,

have offspring, and offspring will have those traits.

So if the conditions persist over a long period of time,

then eventually you'll end up with a population that all have

this characteristic.

And so are more suited to their environment than perhaps the

more diverse population

before the change to the environment happened.

So it does tend to be a slow process.

You think a multicellular organism has lots of different

cells, lots of different cell types,

And so even if there's a change in one cell type in the body,

it's not gonna change the whole organism.

And you think there's

lots of characteristics in people,

but you don't see babies born with wings or anything like

that, they're never massive changes.

It tends to be smaller changes over time.

And we'll talk in the next talk about how human evolution has

occurred and the kind of gradual change in our

characteristics that have led to how humans look today.

So in the short term,

when you've got these different characteristics between

organisms in a population,

the organisms are still able to interbreed with other members

of that population. So, the large leg muscled animals can

still reproduce with the small leg muscled animals.

That's still possible.

If you think about how animals change over time,

so they'll change genetically and originally if two animals

that have changed slightly genetically meat,

they're able to produce offspring and because they're

not so genetically diverse from each other. So,

if you think about breeds of dog,

then there's lots of different sizes and shapes of dogs,

but they are still genetically related enough to each other that's

a labrador and a poodle can breed and you get a labradoodle.

But if you think about animals that have had millions of years apart,

they've become so genetically different that they become

separate species.

And that's one of the defining characteristics of species is that they can't

interbreed with individuals of another species.

And that's a process called speciation,

and it's important for the genetic diversity on earth.

So,

thinking about how the science of evolution and the survival

of the fittest and natural selection happened.

Initially,

scientists just had to observe species,

so it meant traveling to all over the world.

Observing these species in the world,

perhaps catching some and dissecting them to see how they

were structurally beneath the surface.

And so,

it relied on the ability of those scientists to visually

observe differences

and commonalities between these different species. So,

not the most accurate thing in the world.

There are lots of species that look quite similar to each

other, behave quite similarly,

but are genetically from different ancestors and so are

completely different.

So, in modern genetic science,

we can trace back the ancestry of different species by looking

at their genomes and working out where they diverged from their ancestors.

So, the more diverse, the more different two genomes are

between two species.

The longer ago it would have been that the two species

separated, and then the more changes have accrued since.

So, it is a really important process.

You might be wondering why we bother to kind of establish

these family trees of species.

But it's useful for understanding the

classification of different organisms.

So we've got the system of kingdoms and classes and we use

genetics to help identify what species are related to one

another, what their common genetic traits are,

even if they're physically very different.

And it's also given us a lot of insight into the importance of

maintaining genetic diversity in the world as well.

So in another talk,

we'll speak about conservation and how important that is.

One of the main points is to maintain this genetic diversity

because if you think about natural selection and survival

of the fittest, that relies on that diversity existing.

If you've got a population where they all have the same characteristics,

they're at risk of something coming along that affects

organisms with that characteristic and the whole

population dying out.

So really key that we have that genetic diversity on Earth as

much as possible.

So, an example of survival of the fittest and evolution

is antibiotic resistant bacteria. So, Whereas most

species take millions of years to evolve,

antibiotic resistant bacteria is kind of very quick very sped up evolution.

And so, we invented,

as a species humans invented antibiotics to try and fight

bacterial infections.

So before that,

if you had a scratch and it got infected with bacteria.

If your body wasn't able to fight that,

you're at risk of dying of very minor infection.

Because it would just spread and become a very severe infection.

But antibiotics became a drug tool that we could use to fight

bacterial infection.

And kind of tip the tables in our favor as a species in that

battle against bacteria.

One of the problems is though that bacteria and reproduce

very quickly. So,

they're prokaryotic organisms and they reproduce asexually. So,

Each bacterial cell can divide to produce two daughter cells

that are genetically identical to it

in theory. But if you think about the number of bacteria,

so each bacteria will double and so you go from two to four to eight.

And very quickly,

you end up with a huge population of bacteria.

And the mechanisms that they use because they don't have

nuclei within their cells is a little bit more clumsy

than mitosis in animal cells. So it's more at risk of having

little errors sneaking into the genetic code of the bacterial

cells. So when these errors occur, sometimes they're

just a problem for the bacteria and it might kill the bacterial

cell because they don't function properly anymore.

But every now and then,

a change in the genetic code will give the bacteria a

survival advantage just like the genetic diversity of

sexually reproducing organisms does.

If it gives them a survival advantage in terms of resisting antibiotics.

So it might be that they are able to break down the

antibiotic chemically, so it no longer works.

It might be that they're able to pump antibiotic drugs out of

their cells so they don't affect them,

then that bacterial cell is gonna be able to survive in the

presence of antibiotics.

It's gonna clear out the other bacterial cells,

the ones that are sensitive to antibiotics still.

And so that's gonna kind of free up a lot of space lot of

nutrients, lot of resources for this bacteria to then feed on,

reproduce and produce genetically identical copies of itself.

So you can see that even if you only have a very small

proportion of bacteria that survive antibiotics,

if they've got antibiotics resistance genes, very quickly,

they'll replace that lost population and you won't be

able to use that antibiotic then to treat the infection

because the bacteria will be resistant to it.

So, with the invention of multiple different antibiotics,

bacteria have sometimes been able to become resistant to

more than one type of antibiotic and we call these

multi drug resistant bacteria.

So we're creating this artificial selection pressure

with all these different drugs that we're using.

And bacteria are evolving and becoming resistant to each of

the drugs as we use them.

And the more commonly we use antibiotics

and the less responsibly we use them.

So if we're using antibiotics to treat things like cough and

colds which are caused by viruses and so not affected by antibiotics.

If we're using them in animal feed in agriculture,

then some of they'll be exposed to lots of bacteria,

and some of those bacteria will become resistant to the antibiotics.

So,

the problem then is if we have a patient,

someone comes in with an infection,

and the bacteria that they're infected with that's causing the infection,

is resistant to all of the antibiotics that we have,

then we don't have any drug treatment for that infection. And so,

we're relying on the person's body to fight that infection.

Sometimes they'll be able to clear an infection without the

help of antibiotics,

but quite often in patients who are very ill,

then they're not gonna be able to survive an infection without

that extra help from antibiotics.

So the problem is that the more antibiotic resistance we

create by using antibiotics irresponsibly,

then the more chance that we create these kind of superbugs

as they're known,

where they have resistance to multiple or all of the antibiotics.

So there are strategies at the moment to try and reduce it,

things like antibiotic stewardship

and it's hoped that by using antibiotics as responsibly and

appropriately as possible,

we can kind of nip this evolution in the buds and make

sure that our antibiotics are as useful for as long as possible.

And it's not just limited to antibiotic resistant bacteria as well. So,

there's a drug called warfarin that used to be used to poison rats,

but it was found that rats that were resistant to warfarin

treatment would survive, and so they create lots of offspring.

And so lots of rats now are resistant to warfarin and it

means can't really use that poison anymore because it's not

effective in the vast majority of rats.