Charles Darwin & the theory of natural selection
In 1856, a new the theory of natural selection was put forward by Charles Darwin and Alfred Russel Wallace. Darwin the most famous scientist of the two, perhaps because his publications developed his theory more and were widely read and discussed during the latter half of the 19th century.
Darwin was a thinker and experimenter. He made observations of the world around him, and then developed theories about how and why things happened.
His main theory is how living organisms may have evolved over time.
Darwin proposed a mechanism called natural selection to explain how organisms might change over time.
He made four observations and three deductions from his theory.
Observations
- All organisms over-reproduce, far more offspring are produced than are required to keep the population at a steady size.
- Population numbers tend to remain fairly constant over long periods of time.
- Organisms within a species vary.
- Some of these variations are inherited.
Deductions
- There is competition for survival- struggle for existence
- Individuals with characteristics that best adapt them for their environment are most likely to survive and reproduce.
- If these characteristics can be inherited, the organism will pass the characteristic on to their offspring.
Darwin argued that if this happened over a long period of time, then the characteristics of a species could gradually change, as better-adapted individuals were more likely to survive and pass on their adaptations to their offspring.
Gradually the species would become better and better adapted to its environment.
Overproduction
Almost all organisms have the reproductive potential to increase their populations.
In Australia in the 19th century, rabbit populations soared exponentially, as they found conditions there to their liking. Rabbits feed on low-growing vegetation such as grass, of which there was an abundance. There were also few predators to feed on them, so the number of rabbits increased. The increasing numbers affected the availability of grazing for sheep.
Such population explosions are rare in normal circumstances, although rabbit populations do have the potential to increase at a tremendous rate, they do not usually do so.
As a population of rabbits increases, various environmental factors act upon them in order to decrease their numbers.
These factors may be biotic factors – caused by other living organisms e.g. predation, competition for food, infection by pathogens.
Or the factors may be abiotic factors – caused by non-living components of the environment e.g. water supply or nutrient levels in the soil.
For example:
The increasing number of rabbits eats an increasing amount of vegetation, until food is short in supply.
The larger population may allow the populations of predators such as foxes, stoats to increase.
Overcrowding may occur, increasing the ease with which diseases such as myxomatosis can spread. Myxomatosis is caused by a virus which is transmitted by fleas. The closer together the rabbits live, the more easily fleas and therefore viruses will spread across the population.
These environmental factors reduce the rate of growth of the rabbit population.
Of all the rabbits born, many will die from lack of food, be killed by predators of die from viruses/diseases. Only a small proportion of rabbits will grow to adulthood and reproduce.
Natural Selection
What determines which rabbits will survive and which will die?
Some rabbits are born with a better chance of survival than other, due to inheritance of a particular allele that is useful for adapting to their environment.
Variation within a population of rabbits means that some will have characteristics that give them an advantage in the competition for survival.
The rabbits that are best adapted to their environment are most likely to survive and reproduce.
One characteristic that varies is coat colour. Most rabbits have alleles that give them the normal brown colour. However, few have darker coats.
Such darker rabbits will stand out from the others and are more likely to be seen by predators such as a fox.
They are less likely to survive than brown rabbits.
The chances of a dark rabbit surviving long enough to reproduce and pass on its genes for coat colour to its offspring are less then the chances for a normal brown rabbit.
Brown rabbits are better adapted to their environment.
Predation by foxes is an example of a selection pressure.
Selection pressures increase the chances of some genetic variations being passed on to the next generation and decreases the chance for others. The effect of this is natural selection.
Natural selection increases the frequency of certain characteristics within a population, at the expense of others.
The characteristics that best adapt an organism to its environment are most likely to be passed on to the next generation.
Antibiotic Resistance
The development of resistance to antibiotics and other medicinal drugs by bacteria is a good example of natural selection- and one that has great significance to us.
Antibiotics are chemicals produced by living organisms which inhibit or kill bacteria but do not harm human tissue.
Most antibiotics are produced by fungi. The first antibiotic discovered was penicillin.
Penicillin prevents cell wall formation in bacteria.
If someone takes penicillin to treat a bacterial infection, bacteria that are susceptible to penicillin will not be able to grow pr reproduce. In most cases, this will be the entire population of penicillin. However, by chance there may be one or more individual bacterium that are resistant (can withstand) penicillin.
One example is found in the populations of bacteria Staphylococcus, where some bacteria produce an enzyme, penicillinase, which inactivates penicillin.
These individual bacterium have a tremendous selective advantage. Bacteria that are not resistant are killed, while those with resistance survive and reproduce.
Bacteria reproduce very rapidly in ideal conditions, even if there was initially only one resistant bacterium, it might produce ten billion descendents in 24 hours. A large population of penicillin- resistant Staphylococcus would result.
Such antibiotic-resistant strains of bacteria are constantly appearing. One important one is MRSA- methicilin-resistant Staphylococcus aureus.
MRSA bacteria normally harmless is capable of infecting people whose immune systems are not strong- perhaps because they have another illness.
Many people who were already ill have picked up MRSA infections whie in hospital and have dies as a result.
MRSA bacteria has become resistant to almost all antibiotics, thus infection are very difficult to treat.
By using antibiotics, we change the environment in which species of bacteria are living.
We change the selection pressures.
Individual bacteria that have genes that make them better adapted to the new environment win the struggle for existence, and pass on their advantageous genes to their offspring.
The more we use antibiotics, the greater the selection pressure we exert on bacteria to evolve resistance to them.
Alleles for antibiotic resistance often occur on the plasmid- small circles of DNA other than on a “chromosome.” The plasmids can thus be transferred from one bacterium to another, and even between different species.
As a result, it is possible for resistance to a particular antibiotic to arise in one species of bacterium and be passed to another.
Insecticide Resistance
Just as natural selection has led to the development of populations of bacteria that are resistant to antibiotics, so it has led to the development of resistance to insecticides in some populations of insects.
There are many kind of insects that we would like to see less of. E.g. mosquitoes transmit malaria which is a major cause of death in tropical and subtropical parts of the world.
Insects also eat our food stores.
They damage crops.
A wide range of insecticides has been developed in an attempt to keep insect populations to a reasonably low level.
A main insecticide used in the world is aimed at eradicating insects that damage cotton plants.
The major pest of cotton is the cotton boll worm. This is a species of moth and it is the caterpillars that cause all the damage.
They feed not only on cotton, but also on crops of maize, groundnuts (peanuts) and sorghum.
Resistance to many different insecticides has developed, and growers have often fallen back on using highly toxic chemicals that not only destroy boll worms but also other beneficial or harmless organisms.
Case study (may arise as stimulus in exam) GM cotton to beat the boll worm
There is a lot of pressure on growers to use fewer chemicals. However, they still have to control cotton boll worms, if they did not they could lose their entire crop. This happened in Mississippi in 1999.
In an attempt to get around this problem and to make large profits a company produced a genetically engineered variety of cotton plants. The cotton plants had genes inserted into them that enabled them to make a protein. The gene came from a bacterium.
The protein is a toxin which destroys insects that eat the cotton, by attaching to the receptors on the plasma membranes of the cells lining the insect’s gut and destroying the gut wall.
This particular variety of the toxin only affects the butterflies and moths (and their caterpillars) because only they have the receptors to which the toxin can bind.
The cotton GM cotton seeds are expensive, more expensive than unmodified cotton- however, growers should still gain profit, because they should not need to use insecticide, and should get high yields because less of the crop will be lost. The GM cotton could also be good for the environment, because it should only harm the caterpillars that eat the cotton plants.
However, natural selection is at work. Cotton boll worms are becoming resistant to the toxin in the Bt cotton.
An experiment was carried out by researches in Australia to investigate how resistance could arise. They found that after many generations, resistance would occur due to the lack of the binding site on the plasma membrane of the boll worm’s cells- the toxin could not bind to the cells, and so could not harm them. The resistance levels decreased later, researchers believed this may have been because lacking the receptor disadvantages them in some way.
At the moment, growers should use at least two different weapons against boll worms – e.g. two different insecticides. It is unlikely that a boll worm will have genes that make it resistant to both.
Speciation
To produce a new species we need to produce a group that can no longer breed with the original species. The population has to become reproductively isolated.
The production of a new species is called speciation.
It is a difficult event to study, as it takes a long time to happen.
An easier method of studying this would be to look at populations that exist now, and use the patterns identified to suggest what might have happened in the past.
One idea is that geographical isolation often plays a role in speciation.
Two populations of the same species may become separated by a geographical barrier.
Because the environment in which each population lives is different, they will have different selection pressures acting on them, and so different adaptations will be selected for.
Over time this may cause heritable changes in the characteristics of one or both populations.
Eventually, these changes might become so great that the two populations are no longer able to interbreed.
They have become two different species.
This is called allopatric speciation.
However, a new species can sometimes evolve without being geographically separated. This is called sympatric speciation.
The inability of two populations to interbreed to produce fertile offspring is called reproductive isolation.
There are many reasons for it e.g.
- They may have different courtship behaviour
- Their sperm and eggs may be incompatible
- They may have different chromosome numbers, so cells of a hybrid (offspring from parents from two different species) cannot undergo meiosis, because not every chromosome will have a partner to make a pair.
Definition of a species- a group of organisms with similar morphological, physiological and behavioural features, which can interbreed to produce fertile offspring, and are reproductively isolated from other species.
E.g. although donkeys and horses look and behave similar and can breed with each other, their offspring (mules) are infertile- thus donkeys and horses belong to a different species.
For one species to form two different species, they must be reproductively isolated. This may happen because of reason such as:
Isolating mechanisms-
A population becomes physically separated by a barrier that prevents them from mixing. E.g. a stretch of water or a mountain range.
Geographical- In the two areas there could be very different selection pressures, resulting in different alleles being advantageous and thus increasing frequency.
Overtime, the morphological, physiological and behavioural differences are so great that they can no longer interbreed.
Habitat- A population becomes separated because two groups may live on the same mountain but at different altitudes, or in the same area but different types of soil. The important thing to remember in this topic is that there are different selection pressures acting upon a species which causes them to become a completely different species, because they have evolved very differently, form living in different areas.
Seasonal- A population becomes separated because two groups breed at different times of the year.
Behavioural- A population becomes separate because two groups behave differently.
The evidence for evolution
Evolution by natural selection is a very convincing theory.
There is a huge range of evidence that suggest that it has happened and is still happening now.
Evidence that supports the theory includes fossils and molecular evidence including that from DNA.
Fossils
Fossils are the preserved remains of organisms that lived and died long ago.
Many fossils form from hard parts of organisms e.g. bones and shells, that have gradually become mineralised over time.
However, soft parts are often also preserved as fossils e.g. dinosaur droppings and worm burrows.
Only a tiny proportion of organisms that die will be preserved as fossils, and we only come across small proportions that have been formed.
As a result, fossils only provide a glimpse into what organisms were like long ago. There are large gaps in the fossil record.
Nevertheless, enough fossils have been found of a particular organism to suggest how one might have evolved into another over time, e.g. horses.
Fossils of different ages have different bone structures and arrangements.
If the fossils are organised according to their ages, it can be seen how one structure could have changed o produce another, over time.
There appears to be a sequence of changes over time, in the current species of horses.
Fossil sequences provide us with evidence that evolution of one species into another has happened, and also suggest how it may have happened.
From the rocks in which they have been found, we can deduce the kind of environment in which each fossil species of horse lived.
The changes that we can see in the fossil horses can be explained by considering how natural selection might have favoured one characteristic over another, and how features that provided successful adaptations to the environment become more and more common in succeeding generations.
As the environment changed, so did the selection pressures, and this could have increased the succession of different species over time.
We cannot ever say that “this species evolved into that species.” All we can say is that they appear to be related- they share a common ancestor.
They may have been other species that existed, for which no fossils have ever been found.
It is possible that the fossils we have do not include the common ancestor.
Molecular Evidence
Structural similarities and differences between fossils that lived at different times can suggest to us how they might be related to each other.
These features were caused by genes.
By looking at the molecular structure of genes, rather than the features that they produce, we can find even more evidence for evolution.
Often ancient bones are found in such good condition that DNA can be extracted from them. E.g. DNA has been found in the mitochondria of bones of woolly mammoths.
The degree of similarity between the base sequences of the mammoth DNA and the base sequences of the DNA of modern elephants suggest that they had a common ancestor that lived 6 million years ago.
At that point, African elephants developed a separate species.
The DNA evidence suggests that mammoths and Asian elephants diverged around 440 000 years later.
As a result, mammoths are more closely related to Asian elephants than to African elephants.
We can also find out a lot about the evolutionary relationships between organisms that are alive today by comparing their DNA.
There are great similarities between the DNA base sequences of chimpanzees and in ourselves. The simplest explanation for this is that we have both evolved form a common ancestor.
However, DNA analysis can often give provide unexpected results.
E.g. many different animals have eyes, however they can be so different in structure that it has been assumed that eyes have evolved separately in different groups.
Eyes of insects seems to have little in common with the structure of the eyes of molluscs or vertebrates.
The simplest explanation is that eyes of one type evolved in an ancestral line leading to insects, whereas eyes of a different type evolved separately in the line leading to vertebrates.
The fruit fly has a gene called ey that controls the development of eyes.
It has been realised that the base sequence of this genes is similar to that of a gene called Pax-6 which controls the development of eyes in vertebrates.
Researchers tried out the effects of Pax-6 from a mouse in fruit fly cells. They found that the fruit fly grew and eye in its wing (where the gene was injected) and it was a Drosophila (fruit fly) eye not a mouse eye.
It has been found that Pax-6 can also bring about the development of an eye in other organisms such as squid (mollusc)
It seems clear that there is a common origin of eyes in all animals, despite their very different features.
The fact that such different organisms all have a similar sequence of DNA bases that controls the development of their eyes is very strong evidence for an evolutionary relationship between them.
This is an example of an ancient gene- it has been recycled and re-used in many different organisms, continuing to control the development of strikingly different features.
