A drug is any substance taken into the body that modifies or affects chemical reactions in the body.
Antibiotics are used for the treatment of bacterial infections.
Some bacteria are resistant to antibiotics, which reduces the effectiveness of antibiotics.
Antibiotics kill bacteria but do not affect viruses.
Using antibiotics only when essential can limit the development of resistant bacteria such as MRSA.
18 Variation and selection
18.1 Variation
Variation is the differences between individuals of the same species.
Continuous variation results in a range of phenotypes between two extremes; examples include body length and body mass.
Continuous variation is usually controlled by genes and the environment and can be represented by a histogram.
These characteristics are usually quantitative – they can be measured.
E.g. human growth is influenced by genes that influence protein synthesis in muscles, bone development, and production of hormones. But growth will only take place if an individual is taking in a balanced diet.
If we take height as an example, the heights of people in a population will look similar to this normal distribution where most people come in the middle of the range and fewer people at the lower and upper end.
Discontinuous variation results in a limited number of phenotypes with no intermediates; examples include ABO blood groups, seed shape in peas, and seed color in peas.
This variation shows no intermediate forms between the two extremes of phenotypes, or sometimes it shows few intermediate traits.
E.g. Sex in humans (Male/Female), Blood Group (A, B, AB, O), tongue rolling.
It is qualitative.
This kind of variation is represented by Bar charts.
Discontinuous variation is only affected by genes and is not affected by the environment.
Other examples of discontinuous variation: hemophilia, sickle cell anemia.
Discontinuous variation is usually caused by genes only, and continuous variation is caused by both genes and the environment.
Characteristics caused by the organism’s environment are called acquired characteristics e.g. skin color, they are not caused by genes so they cannot be passed on to the next generation
Mutation is a genetic change.
An Unpredictable change that occurs in the DNA of the cell it can happen in genes or in the whole chromosome.
This change leads to the appearance of different phenotypes.
Mutation is the way in which new alleles are formed.
It is defined as a random change in the base sequence of DNA.
Change in the sequence of bases in a gene this leads to the gene coding for a wrong protein.
Ionizing radiation and some chemicals increase the rate of mutation.
Mutations that occur in body cells, or somatic cells, often have no effects at all on the organism. Somatic mutations cannot be passed on to offspring by sexual reproduction.
However, mutations in cells in the ovaries or testes of an animal, or in the ovaries or anthers of a plant, may be inherited by offspring.
If a cell containing a mutation divides to form gametes, then the gametes may also contain the mutated gene. If such a gamete is one of the two which fuses to form a zygote, then the mutated gene will also be in the zygote. This single cell then divides repeatedly to form a new organism, in which all the cells will contain the mutated gene.
It is important to realize that mutations take place randomly, however a number of factors can increase the rate at which mutations occur. These factors are called mutagens and they include:
Ionizing radiation such as ultraviolet light (UV), X-rays, and gamma rays.
Many different chemicals e.g. tar in tobacco smoke and high concentrations of preservatives.
Both of these can cause damage to the structure of DNA.
Gene mutation is a random change in the base sequence of DNA.
Mutation, meiosis, random mating, and random fertilization are sources of genetic variation in populations.
18.2 Adaptive features
An adaptive feature is an inherited feature that helps an organism to survive and reproduce in its environment.
These features are inherited and passed from generation to generation in genes.
For example, fish have fins and a streamlined body that are adaptive features to allow them to move through water.
Reptiles lay hard-shelled eggs, which is an adaptive feature for reproduction (so that the eggs won’t dry out).
Different environments offer different challenges for survival and reproduction including:
Temperature: too hot like deserts or too cold like poles.
Water: some living organisms can have different adaptive features in regions where lack of water is a problem.
Lack of light: plants need light for photosynthesis and many animals also need light for vision although you won’t find plants in the dark cave, you can find animals that are adapted to living without light.
In the spring season, the mountain hare replaces the thick white winter fur with a thinner brown fur. The winter fur provides camouflage against snow and thick insulation against the cold, the brown fur provides camouflage against bushes and dead grass and is thinner to increase heat loss. The large ears and all-round vision help this animal see predators that are far. The large feet allow it to hold onto snow and rough mountain ground.
Plants adapted to dry environments are called Xerophytes.
Adaptive features of xerophytes:
Leaves have a thick waxy cuticle which reduces evaporation Longer distance for diffusion, not easy for water to pass through.
Leaves with few stomata that are often sunken below the level of the epidermis this maintains pockets of local high humidity around stomata so they are protected from wind or moving air. Usually, more stomata are found in the lower epidermis than on the upper because the lower is usually cooler than the upper one.
Hairy leaves or rolled-up leaves to reduce the surface area available for transpiration Air trapped inside rolled leaves has higher humidity This lowers the rate of transpiration by reducing the diffusion gradient. This reduces the surface area available for photosynthesis because less carbon dioxide and light can be absorbed.
These adaptations help to provide a static (Not moving) layer of air that is saturated with water vapor close to the leaf which will reduce diffusion gradient.
Fleshy, succulent leaves stores water to be used later for photosynthesis.
They usually have either deep roots to have access to water that drains deep, or they can have shallow roots that spread a long sideways from where the plant is growing to absorb water immediately after rainfall before it drains.
Plants that live in water are called Hydrophytes.
Its roots are beneath water, its leaves lie at the top where there is enough light for photosynthesis as well as carbon dioxide from the air.
Instead of having stomata on the underside of the leaf, most plants have them on the upper surface because this is where they can be in contact with air.
Their stomata are open all the time because there is no problem getting enough water for photosynthesis and transpiration because they live in water.
They have very small roots because they can also extract nutrients from the surrounding water through other tissues.
Water will also support the plant, so the roots will have very little role in keeping the plant in place.
Their roots also need oxygen for growth that’s why the stem contains air spaces so that oxygen can diffuse down from the leaves to the roots.
The stems have large air spaces which allow the plant to float on the surface of water where they can get plenty of light for photosynthesis.
There is no need for a thick waxy cuticle, because there is no need to prevent water loss from leaves.
If these plants grow excessively in water, they become a serious problem because they block out light, oxygen, and carbon dioxide from plants growing in the water.
18.3 Selection
Natural selection:
Many gene mutations are harmful, some mutations are considered neutral, and if they occur in the gametes they might be passed to the offspring without affecting it.
On the other hand, few mutations are considered beneficial to a living organism and are considered as a source of variation, that can increase the chances of survival of living organisms.
Most populations of living organisms have individuals that are slightly different from one another. Some of these variations may cause the organism to better adapt to the environment.
Most organisms produce many offspring but many of them fail to survive to adulthood.
Only the organisms which are really well adapted are more likely to survive.
Advantageous characteristics are passed on to the offspring.
The well-adapted individuals are more likely to breed than those that are less well adapted so they will pass on their genes to the next generation, with time the population gradually loses all the poorly adapted individuals and will become better adapted.
Natural selection: the greater chance of passing on genes by the best-adapted organisms.
Development of strains of antibiotic-resistant bacteria as an example of natural selection.
Adaptation is a process, resulting from natural selection, by which populations become more suited to their environment over many generations.
A drug is any substance taken into the body that modifies or affects chemical reactions in the body.
Antibiotics are drugs that are produced by microorganisms and are used to kill, stop the growth of bacterial cells without affecting our cells.
e.g. penicillin works by stopping the bacteria from forming cell walls, as a result, the bacteria will burst open and die.
In a population of bacteria in an infected person’s body, there will be millions of bacterial cells, there will be a chance that one of them has a mutated gene and this makes it resistant to the effect of penicillin this means that penicillin doesn’t kill them as quickly as the other bacteria (variation).
This is considered an advantageous characteristic that allows it to adapt more than the other non-resistant bacteria when penicillin is used.
The resistant bacterium will survive (survival of the fittest) and will reproduce and will pass on the gene of antibiotic resistance for the next generations (advantageous characteristics are passed on to offspring).
So over time there will be an increase in the frequency of the resistance allele and this is how new resistant bacteria evolve.
Over time, the bacteria have developed resistance to a large number of antibiotics.
This is a reason why there are so many different antibiotics available so that if bacteria become resistant to one, they may be treated with another. Many species also show resistance to many kinds of antibiotics (multiple resistance).
An example of Antibiotic-resistant bacteria is MRSA. MRSA stands for Methicillin-resistant Staphylococcus aureus. It has been called a “super Bug” because it is resistant to many antibiotics including methicillin, a type of penicillin that is no longer used. It is a major problem in hospitals, where it is responsible for many infections that are difficult to treat.
Unless the full course of the antibiotic is taken the few bacteria that are more resistant will survive and reproduce.
Why antibiotics should not be used very often: The more we use antibiotics, the more we are introducing a strong selection pressure that allows the resistant bacteria to be the fitter to survive and reproduce more and eventually form populations of bacteria that are resistant to a certain antibiotic antibiotic will no longer be effective because the antibiotics offer strong selection pressures this can increase the rate at which the evolution of antibiotic resistance occurs.
Selective breeding It is the way humans make use of variations between living organisms, this is done by selecting those individuals with the most useful characteristics and allowing only these individuals to breed this is called artificial selection.
Steps:
Selection by humans of individuals with desirable features.
Crossing these individuals to produce the next generation.
Selection of offspring showing the desirable features.
Artificial selection in plants:
Farmers choose two individuals with two different desirable characteristics and breed them together.
For example, a variety of tomatoes with sweet taste but deformed shape, and another variety with large size and bad taste, the farmer then grows large numbers of offspring and chooses the ones with the best combination (large and sweet taste) and breeds them together. If he keeps doing so for many generations, eventually, he will get a variety of tomatoes with good taste and large size.
The bad genes will be lost over time.
The same process can also be done with domesticated animals:
For example, a farmer may have a variety of cows that have different qualities and quantities of produced milk. The farmer might choose some cows that are good milk producers and breed them with a bull whose mother or sister are good milk producers by transferring sperm to the cow through artificial insemination, the offspring (cows) with the improved milk yield are chosen, these cows are then used to breed the next generation The farmer can continue this selective breeding for many generations each time picking only the best milk producers to be the mothers of the next generation. The milk production of this herd will gradually increase. Other applications
It can also take place to develop the breed of dogs that make good pets.
Increase the size of chicken eggs.
Increase the size of farm animals kept for meat.
This can take a longer than plants, as animals generally take longer to become mature enough for reproduction.
Plants of the same species but with different characteristics are called varieties.
Animals of the same species with different characteristics are called breeds.
Differences between artificial and natural selection:
In artificial selection, humans are the agents of selection.
Artificial selection is much quicker.
Artificial selection offers no advantage to the animal or plant in its environment.