The Natural World Series Written and performed by Stephanie Poppins This sleep series has been written to provide an educational transition into deep sleep.
Having taught in high schools for over 20 years,
I have come to understand learning happens in a myriad of ways,
And sometimes when you least expect it.
It is my pleasure to present to you one of those ways.
But before we begin,
Let's take a moment to focus on where we are now.
Take a deep breath in through your nose,
And let it out on a long sigh.
It is time to relax and fully let go.
Feel your shoulders melt away from your ears,
And feel yourself relaxing to the support beneath you,
As you let all the worries of the day seep away.
You are safe here.
We are all safe here.
Happy listening.
Episode 3 Natural Selection All species are capable of producing more than enough offspring to replace themselves.
But the environment may affect an individual's chances of survival.
There are competitors and predators,
So in any population,
Not all individuals survive to reproduce.
Perhaps there is not enough food.
Perhaps one does not find a mate.
Or fails to find somewhere comfortable enough for a nesting place.
So why is it some individuals survive and not others?
In some cases it's a matter of chance.
If every one of the species were identical,
Survival would always be a matter of chance.
But no two individuals from one species are exactly alike,
And sometimes the variations between them can affect their chances of survival.
For example,
With mice.
Not all mice are the same colour.
Some are darker than others.
This means it is easier for them to blend into their environment and not be as conspicuous to their predators.
The individuals that are best adapted to their environment are the ones most likely to survive and produce offspring.
Some characteristics are passed on to the next generation.
Some of these variations between individuals are inherited.
In the next generation,
There may be a higher proportion of one variation than another,
According to the percentage of either variation that survived.
Over the course of many generations,
The proportion of well-adapted individuals in a population is likely to increase.
And Darwin called this process natural selection.
Natural selection provides an explanation of how the characteristics of a population can change as individuals become better adapted to their environment.
As an environment changes,
So do the species who frequent it.
For example,
Towards the end of the 20th century,
Certain smoke from factories would blacken the bark of the trees in many industrial areas.
The darker insects that lived on these trees were therefore more likely to survive.
Over the course of time,
Therefore,
This would cause the proportion of darker insects in that population to increase.
Another example of natural selection we can see when looking at the whitebark pines that live on the slopes of the Sierra Nevada mountains in America.
There are two types,
Low bushes and tall trees.
These different appearances are mainly inherited.
The two types can interbreed as well to produce hybrids,
But hybrids are very rare.
On the Sierra Nevada mountains,
The upper and lower slopes provide completely different environments for the pine trees.
The upper slopes are cold and windy,
And frozen ground often prevents the roots from taking in water from the soil.
The lower slopes,
Meanwhile,
Are mild and sheltered,
So mild conditions allow the plants to get a regular supply of water from the soil.
We can see,
Therefore,
The lower plants are well adapted to the upper slopes,
And the tall trees are well adapted to the lower slopes,
But the hybrids are not well adapted to either.
Low bushes and tall trees have a good chance of survival and a good chance of passing on their characteristics to the next generation,
But the hybrids do not.
This natural selection can divide a population into two different types.
Other factors such as climate also influence the survival of a species.
In some populations,
For example of snails,
Those with dark shells are better adapted to cold conditions,
And paler shells better adapted to hot conditions.
The different colours absorb heat differently.
But we must remember it's nearly impossible to predict the effects of natural selection on all species,
Especially those in nature.
The results depend so much on the context because each environment is unique,
And each population has a different range of characteristics.
What we can say is that natural selection is one of the many factors that must be taken into account when we look at survival of a species.
What we do know is,
As a result of natural selection,
Populations become better adapted to their environment.
If the environment changes,
The characteristics of the population will change,
But natural selection does not always lead to change.
Sometimes in an unchanging environment,
Natural selection ensures a well-adapted population remains that way,
And sometimes when the environment varies,
Natural selection may divide a population into two or more distinct types,
According to where in the environment they are living.
In all these cases,
The effects of natural selection are easy to see.
But in most living populations,
So many different factors have to be taken into account that it is in fact much harder to spot the effects of natural selection than we first suppose.
As Darwin himself suggested,
Each new species is formed over many thousands of years by a process involving natural selection.
But that is only one part of the story.
More recently,
Scientists have recognised three distinct stages in the formation of a new species.
There may be a barrier to breeding,
And therefore the continuation of a species.
The barrier may often be geographic,
Such as a mountain range.
Sometimes populations become so well-adapted to their local environment,
Over many generations,
They gradually become different.
These two populations may eventually become so different,
They can no longer interbreed.
Even in a small area,
There may be such differences in the environment that the two populations may become adapted to a completely different ecosystem.
For example,
There may be different levels of rainfall.
There may be different temperatures,
Different altitudes.
Perhaps the terrain is different between the two areas.
Or perhaps each population is now existing at a different depth.
As the environment changes,
So do the populations within it,
Even if they are geographically quite close together.
When this happens,
The two populations may eventually become so different,
They can no longer interbreed.
In England,
For example,
There are two species of hawthorn,
Which can easily be distinguished by their leaves.
Common hawthorns live in open places,
And Midland hawthorns live in dense woods.
As woodlands are cleared for agriculture,
The habitat of the Midland hawthorns is greatly being destroyed,
But the common hawthorns can grow in the clearings.
The two species can now,
As a result of an adaptation to their environment,
Now interbreed.
And as a result,
These new hybrids are better adapted to the new habitats than the other hawthorns are.
So the hybrids are becoming more and more widespread.
One day they may entirely replace the Midland hawthorns altogether.
Either way,
It takes a very long time for one species to become two,
But sometimes the hybrid offspring can become a new species almost at once.
So can natural selection explain the evolution of a new species?
Darwin thought that it could,
And it certainly seems to offer a convincing explanation for the origin of species.
Today it is generally accepted that other mechanisms,
Some not yet fully understood,
May also have played their part in the evolution of new species.
The theory of natural selection and the debate surrounding it have stimulated an enormous amount of research and raised a great many questions.
Some of which are as follows.
What factors can influence the changing patterns of variation amongst living things?
What role can fossils play in helping us interpret evolutionary change?
Does chance play an important part in evolution?
These questions are yet to be answered,
But the theory of natural selection remains central to any study of evolution and is one of the keys to our understanding of the diversity of life.
