Variation # Selection and Ovulation Summarized Biology Notes
Learning Objectives
● define variation as differences between individuals of the same species
● state that phenotypic variation is caused both by genetic and environmental factors
● describe continuous variation as influenced by the environment and genes as illustrated by height in humans
● state that discontinuous variation is mostly caused by genes alone, e.g. A, B, AB and O blood groups
● define gene mutation as a change in the base sequence of DNA
● outline the eftects of radiation and chemicals on the rate of mutation
● describe mutation as a source of variation, as shown by Down's syndrome
● describe the symptoms of sickle-cell anaemia
● explain the distribution of the sickle-cell allele in human populations with reference to the distribution of malaria
● define adaptive features as the inherited functional features of an organism that increase its fitness
● define adaptation as a process, resulting from natural selection, by which populations become more suited to their environment over generations
● describe the adaptive teatures ot hydrophytes and xerophytes to their environment
● define natural selection as the selection of only the best adapted organisms for survival and reproduction
● describe natural selection with reference to production of many offspring, variation within populations, struggle tor survival, competition tor resources, and reproduction by individuals that are better adapted to the environment and passing of their genes to the next generation
● define evolution as a process of change over a period of time
● describe evolution as the change in adaptive features of a population over time as the result of natural selection
● outline the importance of natural selection as the mechanism for evolution
● describe the development of strains of antibiotic-resistant bacteria as an example of evolution by natural selection
● define artificial selection as the modification of species by selective breeding
● describe the role of artificial selection in the production of varieties of animals and plants with increased economic importance.
Introduction
Your sex, skin colour, hair colour, height and blood group are all characteristics that are subject to variation. The human race shows all shades of skin colour and hair colour from very pale to very dark. Blood groups have no intermediate grades. In this post, you will learn why no two living things are ever completely identical. There are always some differences between them. The differences are called variation.
Variation: interaction between genotype and phenotyppe
Variation is the ditterences that exist between individuals belonging to the same species.
There are two types of variations:
● Continuous variation
● discontinuous variation.
Continuous variation
Continuous variation is a smooth gradation betweern individuals. In most characteristics variation is not so clear distinct. Examples include human height, body mass, hair colour and intelligence.
Discontinuous variation
Some people can roll their tongue into a tube and others cannot. A person is either male or female. There is no gradual change between the two extremes; you are either one or the other. These two characteristics show discontinuous variation that occurs when organisms either do, or do not, have a particular characteristic. Discontinuous variation is mostly caused by genes alone.
Blood groups
Another example of discontinuous variation is human blood groups. There are four human blood groups, A, B, AB, and O group. There are no intermediate groups. If you record the blood group of a large number of people, you will be able to plot a histogram. The exact shape of the histogram will vary according to the size of the population that you measure and where it is situated, for example, different parts of the world will have different distributions of blood groups.
What influences variation?
Variation is due to a combination of genetic and environmental factors.
Genetic variation
One reason for the ditference between individuals is that their genotypes are different. Tongue rolling, for example, is controlled by genes, as is hair colour, eye colour, blood
groups, height and many other characteristics. Genetic variation arises in several ways. It is partly caused by the reshufling of genes, which occurs when parents produce ottspring by sexual reproduction
Meiosis
During sexual reproduction, gametes are formed by meiosis. In meiosis, homologous chromosomes exchange genes, and separate from one another, so the gametes that are formed are not all exactly the same.
Fertilisation
Any two gametes of opposite types can fuse together at fertilisation, so there are many
possible combinations of genes that may be produced in the zygote. In an organism with
d large number of genes, the possibility of two offspring having identical genotypes is so
small that it can be considered almost impossible.
Environmental variation
Another important reason for variation is the difference between the environments of individuals. Scots pine trees posses genes that enable them to grow to a height of about 35 m. But it a Scots pine tree is grown in a very small pot, and has its roots regularly pruned, it will be permanently stunted, as shown in Figure below. The tree's genotype gives it the potential to grow tall, but it will not realise this potential unless its roots are given plenty of space and it is allowed to grow freely.
Characteristics caused by an organism's environment are sometimes called acquired characteristics. They are not caused by genes, and so they cannot be handed on to the next generation. Many of the differences between people, particularly in their behaviour, can be explained by the fact that they have been brought up in different environments. We still do not know for certain how important the environment is, compared with the genes, in making us different from one another. This particularly applies to features like intelligence and artistic ability.
● Continuous variation usually also has a strong environmental component, although reshuffling of genes may also be involved.
● Discontinuous variation has no environmental component, it is caused entirely by reshuffling of genes.
Mutation
As we have seen earlier, characteristiCS are passed on from one generation to the next in
a predictable way. The ability of a cell to survive depends on the instructions carried out by its DNA. If the DNA IS damaged, then the cell may not be able to make the correct proteins. Variation can occur but, again, the way in which this Occurs is governed by the rules of genetics. Sometimes, though, a gene may suddenly change. This is called mutation. It is defined as a sudden and unpredictable change in the genetic information. There are two types of mutation:
● gene mutation
● chromosome mutation.
A gene mutation is a change in the base sequence of DNA. This may result in a change in the sequence of amino acids. These mutations are small, and may involve just one base, for example, sickle-cell anaemia, cystic fibrosis, albinism and haemophilia. Such mutations produce new alleles, and these may have an influence on variation. Chromosome mutations occur during mitosis or meiosis. It results in a zygote with Too many, too few, or an abnormal mixture of chromosomes or chromosome fragments for example, Down's syndrome. All mutations confer variation in the organism. Single-base mutations are often insignificant, but if they cause phenotypic change, they are usually harmful or lethal. Occasionally a mutation may give an organism an advantage over other organisms of the species. This will be passed on to the next generation and may eventually lead to the mutant form becoming 'normal'. Mutations are a source of variation in the population. Usually, though, mutations are harmful and produce a characteristic or organism very different from the parents.
Deletion is when genetic material on the chromosome is lost. A few nucleotides could
be lost or a larger number are lost. Deletions can happen anywhere on the chromosome. In Figure above, B has been lost. Duplication occurs when a portion of the chromosome is copied and replicated, so that there is extra information. In Figure above, B has been duplicated. In inversion, the order of some of the genetic material is reversed or switched. A segment of the chromosome is reversed. No genetic intormation is lost, but if the break and reversal occurs at a crucial point, then there can be serious consequences and the inversion could be lethal. In Figure above, if A needs to be adjacent to B for the organism to survive, the inversion would be lethal. Translocation occurs when a piece of one chromosome breaks off and is transferred to a new location on another chromosome. In Figure above, the chromosome that originally began with ABC has lost DEF and gained JKL. DEF has been translocated to the second chromosome, so that it now reads GHIDEF. Down's syndrome is an example of a translocation mutation. A piece of chromosome 21 breaks off and attaches to chromosome 14.
Down's syndrome
Down's syndrome is a condition in humans caused by a chromosome mutation during meiosis. During the development of the ovum, two chromosomes fail to separate, so an extra chromosome is present in the ovum. Instead of the ovum having 23 chromosomes, it has 24. At fertilisation, the zygote has 47 chromosomes. The presence of this extra chromosome results in a baby with Down's syndrome. Children with Down's syndrome often have a flattened face and slanted eyes, and suffer from growth failure, mental retardation, unilateral or bilateral absence of one rib and diminished muscle tone. They are usually below average in height and intelligence but are affectionate and very contented. Their life expectancy may be reduced because of susceptibility to diseases, such as leukaemia and heart disease.
Causes of mutation
The major causes of mutation are certain chemical substances and radiation. These are called mutagens and they include ultra-violet radiation from the sun X-rays that can damage DNA gamma rays, alpha and beta particles from radioactive decay that can also damage DNA by altering the sugar-phosphate backbone in DNA molecules that it hits chemicals such as nitrous acid and tars. These chemicals can react with bases, changing their characteristics so that they pair incorrectly. Not all of these factors will cause mutations, but will increase the rate at which mutations occur.
An example of gene mutation-sickle-cell anaemia
Siecle-cell anaemia is a medical condition in humans. Sufferers of sickle-cell anaemia have abnormal haemoglobin in their red blood cells, which causes the cells to have a distorted shape. During exercise, the body needs increased oxygen, but the haemoglobin cannot carry much oxygen. The distorted cells get stuck inside capillaries and can cause great pain If not treated sensibly, sufferers from sickle-cell anaemia can die at a young age.
Sickle-cell anaemia is caused by a recessive allele, h, of the gene that is responsible the production of haemoglobin. The 'normal allele of the gene for haemoglohin is labelled H. Only people who are homozygous recessive, hh, suffer from sickle-cell anaemia. Normally, one would expect the allele for sickle-cell anaemia to have been lot from the population, since most sufferers would die before being able to reproduce, In many parts of the world where malaria is common, it is actually an advantage to possess at least one sickle-cell allele.
Malaria
Malaria is a major cause of death in certain parts of the world, mainly in tropical Africa and India where it is spread by mosquitoes. There are three ways in which sickle-cell alleles affect whether or not people get malaria:
● Homozygous HH: these individuals have 'normal' haemoglobin and do not have sickle-cell anaemia, but can catch malaria
● Heterozygous Hh: these individuals do not have sickle-cell anaemia, but are less likely to catch malaria
● Homozygous hh: these individuals have sickle-cell anaemia, but do not suffer from malaria.
It is an advantage to carry a sickle-cell allele it one lives in a malarlal region. Fewer individuals will die of malaria than those with normal haemoglobin. Clearly, here is an example of a genetic disorder that does have a positive effect, but only in those parts of the world where other illnesses pose a greater threat to life.
Adapted features, selection and evolution
Adaptation is an important survival skill in all species. Adaptations are the changes an organism undergoes to fit different surroundings. If an organism is not able to evolve over time to suit its environment, it may eventually become extinct. The natural surroundings in which an animal or plant is adapted to live is called its habitat. When a habitat changes, the species that is able to adapt best is most likely to survive. Organisms have features that enable them to adapt to their environment and these are referred to as adaptive features.
Adaptive features
Fish are adapted to live in water because they have a streamlined body that cuts the water easily. They have fins to swim and for balance, gills to breathe under water, and a lateral line to detect vibrations in the water. Amphibians have a smooth, soft skin with mucus that prevents them from drying out. They have a thin transparent membrane used to cover the eyes when under water. This enables them to see even when they are under the water. Frogs' nostrils are mounted high up on the mouth so they are easily exposed to air when they are under the water. Reptiles have a hard, dry, tough skin that helps them to survive in harsh, dry conditions. They lay eggs with a leathery, tough skin that cannot easily be damaged. Birds have wings to fly, a beak to pick up bits of food, and feathers to keep the body warm. Mammals' bodies are covered with hair to keep them warm. Their ears have pinnae to catch sound waves. Females have mammary glands to nurse their young.
Adaptations of hydrophytes and xerophytes to their environments
Plants that are adapted for growing in dry places, and to withstand periods where water is unavailable, are called xerophytes. A major problem for xerophytes is that they lose water by transpiration and very little water is available where they grow. They have to adapt to conditions in which loss of water exceeds the amount of water taken in. The leaves of xerophytes are adapted to reduce transpiration rate in some or all of the following ways:
• a thick cuticle that is impermeable to water and reduces water loss
• relatively few stomata, which are often sunken; sunken stomata are buried deeper than the surface of the epidermis; this means that water vapour has further to diffuse and so slows down the rate of water loss by increasing the length of the diffusion pathway
• leaves reduced in surface area, sometimes to small spines or thorns; these reduced leaves do not transpire as they have no stomata and photosynthesis is carried out by the stems
• a hairy epidermis that traps water vapour and in this way reduces the water potential gradient
• leaves that fall off in the dry season
• leaves that curl up during the heat of the day.
In southern Africa, many places have a low or unpredictable rainfall, which explains why many plants in the region are xerophytes. Xerophytes that store water in their stems or leaves are called succulents. You can recognise them by their fleshy leaves or stems. Figures below shows two succulents.
• roots may be completely lacking or poorly developed
• root hairs are absent or poorly developed
• roots caps may be absent or root pockets are present
• roots are generally fibrous and adventitious, unbranched or sparsely branched
• the stem is long, slender, weak, spongy and flexible in submerged hydrophytes
• the stem is short, thick, and spongy, with extensive parenchyma in free-floating plants
• stomata are present on the upper epidermis, which is in contact with air, and gaseous exchange takes place through this stomata, the lower surface is in touch with water
• the upper leaf surface in floating leaves is coated with wax to prevent wilting
• the structure of many partially submerged plants show heterophylly (presence of different types of leaves)
• the entire plant body is covered with mucilage
• the flowers and seeds are less abundant
• reproduction is mainly by vegetative methods excessive development and elaborate system of parenchyma (air space)
• poor development of vascular and mechanical tissues
• cuticle absent or poorly developed
• stomata are completely absent in submerged leaves
• chlorophyll found in all the tissues
• mucilage canals and mucilage cells are present, which secrete mucilage to protect the plant body
• the reserve food is in the form of starch grains, which occur in the cortex and pith.
Selection and competition
We have seen that sexual eproduction produces individuals that differ genetically from one another. This produces a range of phenotypes in the population something we refer to as Variation. In any given environment, some variations may be more advantageous and better suited to successful survival than others. For the majority of species, there are more members of a species living in a particular area than the erea can support. This results in members of the species struggling to obtain sufficient foood and space. This Situation is called competition. For Example, if you scattered two hundred maize seeds over an area one metre by One metre and watered them. you would eventually see that, as the plants grew, they were severely overcrowded. Not every plant would grow healthily and survive. Possibly only those at the edges of the plot would grow and reproduce. When a population of organisms competes for resources in short supply.
• those organisms better suited to obtain those resources Will survive and reproduce
• those organisms less well suited to the environment may not survive long enough to reproduce.
This is called differential survival. Only those organisms best fitted to the environment will survive and reproduce and pass on their genes to the next generations. This principle forms the basis of a process called natural selection.
The competition for resources, such as food, light, oxygen, and space to breed, which is imposed by the environment, acts as a pressure on the organism. We call these pressures selection pressures, because they act as agents determining which organisms will survive to reproduce. Micro-evolution is the occurrence of small changes in allele frequency in a population over a few generations. This may happen due to mutations, natural selection or artificial selection.
Natural selection and evolution
Natural selection is the selection of only the best-adapted organisms for survival and reproduction. Natural selection eliminates those individuals that cannot meet the challenges of their environment. Those organisms that can meet these challenges pass their genes on to the next generation. Here, the new offspring are tested by the environment. Through this process continuing over many generations, the genotype and phenotype of the species can change considerably. Figures below shows how natural selection can occur in a population of cacti.
The effect of this process gradually reduces the occurrence of particular forms of genes from the population. Those alleles that produce less favourable characteristics will gradually disappear from the population. We say that selection pressure has changed the allele frequency in the population. Eventually the genotype of the entire population changes to such an extent that organisms cease to retain the characteristics and features of their ancestors. Finally, the point comes when these organisms can no longer interbreed with the ancestral type of organism. When this point is reached, a new species has appeared. We call this process evolution. Evolution is defined as a process of change over a period of time. Evolution is the change in adaptive features of a population over time as the result of natural selection, which can lead to the evolution of new species.
Let us look at the steps that lead to evolution:
• in any population there is variation
• there is also overproduction of offspring; far more produced than just replacement numbers
• as a result, there is competition for resources and mating partners among these offspring, and between them, and other life forms.
The only individuals that survive are the fittest and strongest. They can run the fastest to either escape predators or catch their prey and withstand the effects of cold or drought. The most successful individuals are likely to pass on their genetic make-up to the next generation. In the next generation, more of the population will consist of the 'successful type. So there will be a gradual change in the character of the population. As a result of natural selection, those alleles that produce more advantageous phenotypes are passed on to the next generation more frequently than those alleles that produce less advantageous phenotypes. This can lead to different reproductive rates.
Antibiotic-resistant bacteria
One example of natural selection that is causing great concern to doctors at the momen is the way in which bacteria are becoming resistant to antibiotics. Antibiotics are drugs that kill bacteria. Before the introduction of the earliest antibiotic, penicillin, in the 1940s, many people died from bacterial infections, such as tuberculosis (TB). Antibiotics changed the nature of the fight against bacterial diseases However, for over 50 years the tuberculosis bacteria have been exposed to antibiotics designed to kill them. This has allowed the selection of bacteria that were slightly different from the normal. These 'odd' bacteria, which were not killed by antibiotics, were able to reproduce and continue to produce generations of resistant bacteria. Over the years, these have gradually replaced the bacteria that were destroyed by antibiotics. The only way to combat these diseases has been to develop new and stronger antibiotics to kill off the resistant bacteria. In all these bacterial diseases, antibiotics have been misused and this has given the bacteria an improved chance of developing resistance. The problem is that although people start treatment, they very often fail to finish the course of antibiotics. They stop when they feel better and if just some of the course is taken, then only the weak bacteria are killed. In any population of bacteria, a small percentage will have considerably higher resistance to the antibiotic than the rest. A partial course will kill only the non-resistant bacteria. Resistant bacteria then multiply to take their place. This results in a larger gene pool of more resistant bacteria. These are now passed on. The next person may also tail to complete the course. Now the bacteria have had another round of selection. Antibiotics may also select bacteria that have a high reproductive rate and, as you know, a high rate of cell division increases the opportunity for mutation. This may give increased resistance to certain bacteria. Repeated exposure of huge numbers of bacteria to sub-lethal doses of antibiotics are likely to produce resistant strains. This is evident in the treatment of gonorrhoea, syphilis and TB. There is novwa type of TB that is resistant to most antibiotics. Fifty per cent of people who become infected with this strain die.
Preventing antibiotic resistance
Three things can be done to slow down or stop the development of antibiotic resistance:
• Antibiotics should be used sparingly and only for serious conditions.
• People must be educated to complete the course of antibiotics. This means taking all the doses of antibiotics prescribed by the doctor. If not, they remain infected and are able to pass on the infection to other people.
• The need for antibiotics must be reduced by improving general health and living conditions.
• People should be made aware of preventive methods of avoiding disease, such as eating more fresh fruit and vegetables, and by engaging in safer sex practices.
Artificial selection
Those characteristics that allow an organism to fit into its environment are described as adaptations. In the adaptions. In the case of certain plants and animals that are valuable to humans, the breeding of organisms in terms of producing materials with increased yield and quality. This process is showing these desirable characteristics has important commercial called artificial selection and it is used to produce varieties of plants and animals with increased economic importance. Artificial selection is the selective breeding of organisms to emphasise their desirable features. These features are not necessarily good for the organisms, but they may result in:
• increased milk or meat yield
• resistance to high or low temperature
• resistance to drought.
For thousands of years, farmers have selected from the herd the cow that produces the best yield of good-quality milk and mated it with a bull known to produce cows with excellent yields ofgood-quality millk. The offspring of such a mating are likely to have equally good characteristics when it comes to producing milk. This is artificial selection. It is a practice used by farmers all over the world to produce plants and animals of economic importance.
figure below shows the varieties of cabbage that have been bred by artificially crossing wild cabbage plants that have particular characteristics useful to humans.
Artificial selection has produced the following types of crops and animals:
• climate-adapted crops, for example, lettuce for cool and hot conditions
• disease-resistant crops, for example, strawberries resistant to viruses
• high-yielding crops, for example, rice plants that do not blow over in the wind, so spoiling the rice grains
• nutritious crops, for example, maize strains containing all the essential amino acids helping to fight kwashiorkor
• purpose-selected organisms, for example, yeast for brewing
• cattle that are resistant to disease or producers of high milk yields.
Summary
• Variations within a species may be inherited or caused by the environment.
• Inherited variations arise from different combinations of genes or trom mutations.
• In meiosis, the maternal and paternal chromosomes are randomly distributed between the gametes.
• Because the gametes do not carry identical sets of genes, new combinations of genes may arise at fertilisation.
• Discontinuous variation results, usually, from the effects of a single gene pair. It produces distinct and consistent differences between individuals.
• Discontinuous variation is not affected by the environment.
• Continuous variation is usually controlled by a number of genes affecting the same characteristic.
• Continuous variation can be influenced by the environment.
• Mutations are sudden and unpredictable changes in genetic information.
• Two types of mutations exist: gene mutations and chromosome mutations.
• Chemical substances that cause mutations are called mutagens.
• Adaptive features are common in a population since they provide some improved function.
• Adaptations are produced by natural selection and are well suited to their function. An adaptation is a characteristic that improves the survival or reproduction of organisms, for example, the Welwitschia mirabilis' leaves have waxy Coatings to stop moisture escaping, or they can have surface hairs that trap still air and reduce any water loss through evaporation.
• Natural selection involves the elimination of the less well-adapted varieties by pressure of the environment.
• Artificial selection is used to improve commercially useful plants and animals.
• Natural selection is a possible mechanism for evolution.
• Overuse of antibiotics has acted as a means of selecting resistant strains of bacteria.
• Members of a species may have variations that enable them to compete better.
• These variants will probably live longer than the others and leave more offspring. If the beneficial variations are inherited, the offspring will also be likely to survive longer.
Posted by Mr Steven Artssen
BIOLOGISEDTEAM™
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