5.2 NATURAL SELECTION
Nature of science: Use theories to explain natural phenomena—the theory of evolution by natural selection can explain the development of antibiotic resistance in bacteria. (2.1)
Understandings:
• Natural selection can only occur if there is variation among members of the same species.
• Mutation, meiosis and sexual reproduction cause variation between individuals in a species.
• Adaptations are characteristics that make an individual suited to its environment and way of life.
• Species tend to produce more offspring than the environment can support.
• Individuals that are better adapted tend to survive and produce more offspring while the less well adapted tend to die or produce fewer offspring.
• Individuals that reproduce pass on characteristics to their offspring.
• Natural selection increases the frequency of characteristics that make individuals better adapted and decreases the frequency of other characteristics leading to changes within the species.
Applications and skills:
• Application: Changes in beaks of finches on Daphne Major. • Application: Evolution of antibiotic resistance in bacteria.
• Natural selection can only occur if there is variation among members of the same species.
• Mutation, meiosis and sexual reproduction cause variation between individuals in a species.
• Adaptations are characteristics that make an individual suited to its environment and way of life.
• Species tend to produce more offspring than the environment can support.
• Individuals that are better adapted tend to survive and produce more offspring while the less well adapted tend to die or produce fewer offspring.
• Individuals that reproduce pass on characteristics to their offspring.
• Natural selection increases the frequency of characteristics that make individuals better adapted and decreases the frequency of other characteristics leading to changes within the species.
Applications and skills:
• Application: Changes in beaks of finches on Daphne Major. • Application: Evolution of antibiotic resistance in bacteria.
the mechanism for evolution
Besides providing evidence for evolution, Darwin and Wallace suggested a mechanism for evolution: natural selection. How does this work? It all starts with the overproduction of offspring and the presence of natural variation in the population; then there is a struggle between competing varieties that leads to survival for some and death for others. This section will look at how evolution works through natural selection.
variation within populations
Organisms such as bacteria reproduce simply by making a copy of their genetic information and then splitting into two using the process of binary fission. The result is that the second generation is identical to the first. In fact, many future generations will be identical or show very little change. There is little chance for the DNA to be modified.
The story is very different for species that reproduce sexually. When a cat has kittens, for example, each one is slightly different, or when a population of guinea pigs interbreeds there can be a wide variety of offspring
The story is very different for species that reproduce sexually. When a cat has kittens, for example, each one is slightly different, or when a population of guinea pigs interbreeds there can be a wide variety of offspring
variation and success
Variation is closely related to how successful an organism is. A baby bird that has pigments that give it a colour matching its surroundings will have a better chance of not being seen by a predator. A fish with a slightly different shaped mouth might be able to feed from parts of a coral reef that other fish are not able to access. A plant that produces a different shaped flower might have a better chance of attracting insects for pollination.
It might seem obvious that a young bird with a colour that makes it very conspicuous to predators has little chance of surviving to adulthood. On the other hand, it might be more attractive to mates. A fish with an oddly shaped mouth may, in fact, be incapable of feeding adequately and die of starvation. This means that if an adverse change happened in the environment, such as a change in pH, if one bacterium is susceptible to the change in pH and dies, they in fact all die because they all have the same vulnerability. In species where there is variation, a change in the environment will eliminate some but not all members of the population. This is why variation is a strength and not a weakness in a population. We will see how this works as this section continues
It might seem obvious that a young bird with a colour that makes it very conspicuous to predators has little chance of surviving to adulthood. On the other hand, it might be more attractive to mates. A fish with an oddly shaped mouth may, in fact, be incapable of feeding adequately and die of starvation. This means that if an adverse change happened in the environment, such as a change in pH, if one bacterium is susceptible to the change in pH and dies, they in fact all die because they all have the same vulnerability. In species where there is variation, a change in the environment will eliminate some but not all members of the population. This is why variation is a strength and not a weakness in a population. We will see how this works as this section continues
mutation, meiosis and sexual reproduction
There are three main mechanisms that give organisms in a species their variation:
• mutations in DNA
• meiosis
• sexual reproduction
• mutations in DNA
• meiosis
• sexual reproduction
mutation
Mutations can sometimes produce genes that lead to genetic diseases, and can have devastating effects on the survival of some individuals in a species. However, sometimes a mutation can produce a characteristic that is advantageous, perhaps a slightly faster growth rate for a tree or better frost resistance for a plant. A beneficial mutation for a bird or insect might result in a different camouflage that better matches a changing habitat. In each generation, only a few genes mutate, and most mutations produce effects that are neither useful nor harmful. As a result, sexual reproduction is a much more powerful source of variation in a population because thousands of genes are mixed and combined. But sexual reproduction is only possible thanks to meiosis.
meiosis
Meiosis enables the production of haploid cells to make gametes (sperm cells and egg cells). At the end of meiosis, four cells are produced that are genetically different from each other and only contain 50% of the parent cell’s genome. An individual that reproduces sexually can produce huge numbers of possible combinations of half the genetic material it possesses, thanks to meiosis. For example, in a woman’s lifetime, it is nearly impossible for her to produce the same egg
twice. This is why, no matter how many pregnancies she has, she will never have the same child twice from two different pregnancies. The only way identical humans have ever been formed is when two embryos are formed from a single egg, i.e. identical twins, and even then there are slight genetic differences between the siblings.
The variety in gametes comes mainly from the process of random orientation during metaphase I. The lining up of chromosomes in a random order is like shuffling a deck of cards, and it greatly promotes variety in the egg cells or sperm cells produced. In addition to this, the process of crossing-over contributes to the shuffling of genetic material and further increases the genetic variety.
twice. This is why, no matter how many pregnancies she has, she will never have the same child twice from two different pregnancies. The only way identical humans have ever been formed is when two embryos are formed from a single egg, i.e. identical twins, and even then there are slight genetic differences between the siblings.
The variety in gametes comes mainly from the process of random orientation during metaphase I. The lining up of chromosomes in a random order is like shuffling a deck of cards, and it greatly promotes variety in the egg cells or sperm cells produced. In addition to this, the process of crossing-over contributes to the shuffling of genetic material and further increases the genetic variety.
sexual reproduction
As we have seen, asexual reproduction such as binary fission in single-celled organisms does not promote variety in the population. Generally speaking, in an asexually reproducing population, all the members of the population are identical.
Part of what determines whether or not a female animal becomes pregnant is that all the conditions must be right inside her body, and that sperm cells must be present at the opportune moment when an egg is ready. Of the many sperm cells that may be present, only one will penetrate the egg. In determining exactly which sperm cell and egg will meet and fuse together, a certain amount of chance and luck are involved.
In non-human primate species, such as chimpanzees, for example, when a female is fertile, many males may copulate with her to try to impregnate her. In such a scenario, it is impossible to guess which male’s sperm cells will successfully fertilize her egg. It is largely up to chance. In flowering plants, which bees will land on which flower of a population, with what pollen from another flower in that population, is also a matter of chance.
Part of what determines whether or not a female animal becomes pregnant is that all the conditions must be right inside her body, and that sperm cells must be present at the opportune moment when an egg is ready. Of the many sperm cells that may be present, only one will penetrate the egg. In determining exactly which sperm cell and egg will meet and fuse together, a certain amount of chance and luck are involved.
In non-human primate species, such as chimpanzees, for example, when a female is fertile, many males may copulate with her to try to impregnate her. In such a scenario, it is impossible to guess which male’s sperm cells will successfully fertilize her egg. It is largely up to chance. In flowering plants, which bees will land on which flower of a population, with what pollen from another flower in that population, is also a matter of chance.
to adapt or not to adapt?
The adjective adaptation and the verb to adapt are freely used when talking about evolution.
For example, humans can consciously decide to adapt to a situation: think of a student learning the language of a country he or she has just moved to, or of a person who is used to driving his or her car on the right-hand side of the road and rents a vehicle in a country where driving is done on the left-hand side and so adapts very quickly to left-hand driving. These are conscious adaptations made by individuals. In nature, the vast majority of adaptations referred to in evolution and natural selection are unconscious adaptations made by populations rather than by individuals.
One example of adaptation of the peppered moth populations over time before and after the industrial revolution. On light-coloured backgrounds, the grey moths were better adapted, whereas on dark-coloured backgrounds, the black moths were better adapted.
An organism that has characteristics that are well adapted for its environment is said to be fit. The characteristics it possesses fit well into its environment.
Natural selection tends to eliminate from a population individuals that show low fitness, whereas the fittest individuals in a population have a higher likelihood of surviving. Although there are rare exceptions, individuals are usually incapable of changing themselves to adapt. For example, a giraffe born with a short neck cannot stretch its neck to get a longer one. Rather, because it will have difficulty feeding itself and surviving, the chances are very low that it will find a mate and reproduce to be able to pass on its genes to the next generation. Hence the alleles for making a short neck are not found in the giraffe population.
For example, humans can consciously decide to adapt to a situation: think of a student learning the language of a country he or she has just moved to, or of a person who is used to driving his or her car on the right-hand side of the road and rents a vehicle in a country where driving is done on the left-hand side and so adapts very quickly to left-hand driving. These are conscious adaptations made by individuals. In nature, the vast majority of adaptations referred to in evolution and natural selection are unconscious adaptations made by populations rather than by individuals.
One example of adaptation of the peppered moth populations over time before and after the industrial revolution. On light-coloured backgrounds, the grey moths were better adapted, whereas on dark-coloured backgrounds, the black moths were better adapted.
An organism that has characteristics that are well adapted for its environment is said to be fit. The characteristics it possesses fit well into its environment.
Natural selection tends to eliminate from a population individuals that show low fitness, whereas the fittest individuals in a population have a higher likelihood of surviving. Although there are rare exceptions, individuals are usually incapable of changing themselves to adapt. For example, a giraffe born with a short neck cannot stretch its neck to get a longer one. Rather, because it will have difficulty feeding itself and surviving, the chances are very low that it will find a mate and reproduce to be able to pass on its genes to the next generation. Hence the alleles for making a short neck are not found in the giraffe population.
too many offspring
Darwin noticed that plants and animals produce far more offspring than could ever survive. Plants often produce hundreds or thousands more seeds than necessary to propagate the species. Mushrooms produce millions more spores than ever grow into new mushrooms. A female fish lays hundreds or thousands of eggs but only a handful survive to adulthood.
This seems paradoxical, because the production of seeds, spores, and eggs involves using energy and nutrients that also are vital to the parents’ survival. Why are such valuable resources squandered on so many excess cells that are never going to give rise to viable offspring? The answer is to maximize the chances of some offspring surviving, even if the survival rate is less than 1%.
Having too many offspring and not enough resources is a problem of supply and demand. There is high demand for water, space, nutrients, and sunlight, but there is a limited supply. The consequence is competition for these resources in order to stay alive. This is called the struggle for survival.
This seems paradoxical, because the production of seeds, spores, and eggs involves using energy and nutrients that also are vital to the parents’ survival. Why are such valuable resources squandered on so many excess cells that are never going to give rise to viable offspring? The answer is to maximize the chances of some offspring surviving, even if the survival rate is less than 1%.
Having too many offspring and not enough resources is a problem of supply and demand. There is high demand for water, space, nutrients, and sunlight, but there is a limited supply. The consequence is competition for these resources in order to stay alive. This is called the struggle for survival.
adaptation and survival
Evolution is not just based on chance. In a situation where there are too many organisms for limited resources, it is obvious that some individuals will succeed in accessing those resources and the rest will fail. In other words, there is a selection. Exactly which individuals survive and which ones do not is not based on chance alone but determined by their surroundings and the compatibility of their characteristics with those surroundings. The steps of evolution by natural selection are outlined below.
• Overproduction of offspring and, in those offspring, natural variation as a result of genetic differences (e.g. body size, morphology, pigmentation, visual acuity, resistance to disease). In the offspring:
– useful variations allow some individuals to have a better chance of survival (e.g. hiding from predators, fleeing danger or finding food)
– harmful variations make it difficult to survive (e.g. inappropriate colour for camouflage, heavy bones for birds, having such a big body size that there is not enough food to survive).
• Individuals with genetic characteristics that are poorly adapted for their environment tend to be less successful at accessing resources and have less chance of surviving to maturity.
• Individuals with genetic characteristics that are well adapted for their environment tend to be more successful at accessing resources and have a better chance of surviving to maturity. Such individuals are said to have better fitness.
• Because they survive to adulthood, the successful organisms have a better chance of reproducing and passing on their successful genetic characteristics to the next generation.
• Over many generations, the accumulation of changes in the heritable characteristics of a population results in evolution: the gene pool has changed.
• Overproduction of offspring and, in those offspring, natural variation as a result of genetic differences (e.g. body size, morphology, pigmentation, visual acuity, resistance to disease). In the offspring:
– useful variations allow some individuals to have a better chance of survival (e.g. hiding from predators, fleeing danger or finding food)
– harmful variations make it difficult to survive (e.g. inappropriate colour for camouflage, heavy bones for birds, having such a big body size that there is not enough food to survive).
• Individuals with genetic characteristics that are poorly adapted for their environment tend to be less successful at accessing resources and have less chance of surviving to maturity.
• Individuals with genetic characteristics that are well adapted for their environment tend to be more successful at accessing resources and have a better chance of surviving to maturity. Such individuals are said to have better fitness.
• Because they survive to adulthood, the successful organisms have a better chance of reproducing and passing on their successful genetic characteristics to the next generation.
• Over many generations, the accumulation of changes in the heritable characteristics of a population results in evolution: the gene pool has changed.
passing on successful characteristics
It should be obvious that an individual that never reaches maturity will not be able to pass on its genes to the next generation. An individual that is poorly adapted to its environment, such as an insect with deformed mouthparts that make it impossible to feed, is not likely to survive to adulthood and be able to reproduce.
On the other hand, an individual showing high fitness has a better chance of surviving until adulthood and reaching maturity. Individuals that reach maturity have the possibility of reproducing and passing on their genetic material.
On the other hand, an individual showing high fitness has a better chance of surviving until adulthood and reaching maturity. Individuals that reach maturity have the possibility of reproducing and passing on their genetic material.
natural selection and the frequency of characteristics
Pesticide resistance in rats and multiple antibiotic resistance in bacteria are both carefully studied modern examples of natural selection. What is striking about these examples is their rapidity. Although evolution is generally considered to be a long-term process, the mechanism of natural selection can sometimes be quick, taking place over months, years or decades, rather than millennia. As you read the descriptions below, see if you can identify the main features of how natural selection works: variation in the population making some individuals better suited for their environment than others, overproduction of offspring leading to a struggle for survival, differentiated survival because some die and some live, and, finally, the passing on of successful traits to the next generation.
Pesticide resistance in rats
Pesticides are chemicals that kill animals that are regarded as pests. Farmers use them to eradicate pests, such as rats that eat their crops. Consider the following scenario.
1 Once applied in the fields, pesticides kill all the rats ... or so the farmer thinks.
2 As a result of natural variation, a few rats from the population on the farm are slightly different and are not affected by the poison.
3 The resistant rats are better adapted to survive in the presence of the pesticides and now, thanks to the farmer’s actions, have no other rats to compete with for a food supply. Hence, they thrive and reproduce, making a new population in which some or all of the members possess the genes that give resistance to the pesticide.
4 Seeing rats again, the farmer puts out more of the original poison; this time fewer rats die. Because the characteristic of poison resistance was favoured in the rat population, it is now much more common in the population.
5 To kill the resistant rats, a new pesticide must be used.It is important to note that, in this example, we cannot say that the rats become immune to the poison. Although the term ‘immunity’ is sometimes interchangeable with the term ‘resistance’, that is not the case here. Immunity develops within the lifetime of an individual; pesticide resistance is a change that evolves in a population from one generation of rats to the next generation. The evolution happened in the population, not in any single rat. A rat is either born with a susceptibility to be killed by the pesticide or is born with resistance to it. An individual rat cannot adapt and evolve into a resistant rat.
Pesticide resistance in rats
Pesticides are chemicals that kill animals that are regarded as pests. Farmers use them to eradicate pests, such as rats that eat their crops. Consider the following scenario.
1 Once applied in the fields, pesticides kill all the rats ... or so the farmer thinks.
2 As a result of natural variation, a few rats from the population on the farm are slightly different and are not affected by the poison.
3 The resistant rats are better adapted to survive in the presence of the pesticides and now, thanks to the farmer’s actions, have no other rats to compete with for a food supply. Hence, they thrive and reproduce, making a new population in which some or all of the members possess the genes that give resistance to the pesticide.
4 Seeing rats again, the farmer puts out more of the original poison; this time fewer rats die. Because the characteristic of poison resistance was favoured in the rat population, it is now much more common in the population.
5 To kill the resistant rats, a new pesticide must be used.It is important to note that, in this example, we cannot say that the rats become immune to the poison. Although the term ‘immunity’ is sometimes interchangeable with the term ‘resistance’, that is not the case here. Immunity develops within the lifetime of an individual; pesticide resistance is a change that evolves in a population from one generation of rats to the next generation. The evolution happened in the population, not in any single rat. A rat is either born with a susceptibility to be killed by the pesticide or is born with resistance to it. An individual rat cannot adapt and evolve into a resistant rat.
antibiotic resistance in bacteria
Antibiotics are medications such as penicillin that kill or inhibit the growth of bacteria. They are given to patients suffering from bacterial infections. They are also sometimes given to people who are suffering from something else and, because their immune system is weak, are at a greater risk of a bacterial infection. However, overuse of antibiotics can lead to the production of resistant strains of bacteria.
Antibiotic resistance in bacteria develops in several steps. Consider the following scenario.
1 A woman gets a bacterial infection such as tuberculosis.
2 Her doctor gives her an antibiotic to kill the bacteria.
3 She gets better because the bacteria are largely destroyed.
4 By a modification of its genetic makeup, however, one bacterium is resistant to the antibiotic.
5 That bacterium is not killed by the antibiotic and it later multiplies in the patient’s
6 She goes back to the doctor and gets the same antibiotic.
7 This time, no result: she is still sick and asks her doctor what is wrong.
8 The doctor prescribes a different antibiotic that (hopefully) works. But if the population of bacteria continues to acquire mutations, new strains could show resistance to all the antibiotics available body to make her sick again.
Because bacteria reproduce asexually, genetically they generally do not change very often. However, there are two sources of possible change in the genetic makeup of bacteria:
• mutations
• plasmid transfer.
Plasmid transfer involves one bacterium donating genetic information to another in a ring of nucleotides called a plasmid. Both the donating and receiving cells open their cell walls so that the genetic material can pass from the donor to the receiver.
Antibiotic resistance in bacteria develops in several steps. Consider the following scenario.
1 A woman gets a bacterial infection such as tuberculosis.
2 Her doctor gives her an antibiotic to kill the bacteria.
3 She gets better because the bacteria are largely destroyed.
4 By a modification of its genetic makeup, however, one bacterium is resistant to the antibiotic.
5 That bacterium is not killed by the antibiotic and it later multiplies in the patient’s
6 She goes back to the doctor and gets the same antibiotic.
7 This time, no result: she is still sick and asks her doctor what is wrong.
8 The doctor prescribes a different antibiotic that (hopefully) works. But if the population of bacteria continues to acquire mutations, new strains could show resistance to all the antibiotics available body to make her sick again.
Because bacteria reproduce asexually, genetically they generally do not change very often. However, there are two sources of possible change in the genetic makeup of bacteria:
• mutations
• plasmid transfer.
Plasmid transfer involves one bacterium donating genetic information to another in a ring of nucleotides called a plasmid. Both the donating and receiving cells open their cell walls so that the genetic material can pass from the donor to the receiver.
Theory of knowledge:
Natural Selection is a theory. How much evidence is required to support a theory and what sort of counter evidence is required to refute it?
Natural Selection is a theory. How much evidence is required to support a theory and what sort of counter evidence is required to refute it?