6.2 Variation and evolution Flashcards
Variation
Variation is defined as differences between individuals of the same species.
Phenotypic variation is the difference in features between individuals of the same species.
Phenotypic variation can be caused in two main ways:
It can be genetic – controlled entirely by genes (this is called genetic variation).
It can be environmental – caused entirely by the environment in which the organism lives.
Or it can be due to a combination of genes and the environment.
Genetic variation
There is usually extensive genetic variation within a population of a species
All genetic variants arise from mutations
Mutations are random genetic changes that occur continuously
Most mutations have no effect on the phenotype as the protein that a mutated
gene produces may work just as well as the protein from the non-mutated gene
Rarely, mutations lead to the development of new alleles and so new phenotypes and if they do, most only have a small effect on the organism
Occasionally, the new allele gives the individual a survival advantage over other members of the species
If the new phenotype is suited to an environmental change it can lead to a relatively rapid change in the species
Environmental variation
Characteristics of all species can be affected by environmental factors such as climate, diet, accidents, culture and lifestyle.
Evolution
Evolution is a change in the inherited characteristics of a population over time.
Evolution occurs by a process known as natural selection.
During natural selection individuals with phenotypes that are best suited to their environment are more likely to survive and pass on their features to their offspring.
The theory of evolution by natural selection states that all species of living things have evolved from simple life forms that first developed more than three billion years ago.
Natural selection can lead to the development of new species by a process known as speciation.
Speciation is said to have occurred when two populations have phenotypes that are so different that they can no longer breed together to produce fertile offspring.
Impact of selective breeding
Selective breeding means to select individuals with desirable characteristics and breed them together.
It’s likely that not all of the offspring will show the characteristics you want so offspring that do show the desired characteristics are selected and bred together.
This process has to be repeated for many successive generations before you can definitely say you have a ‘new breed’ which will reliably show those selected characteristics in all offspring.
Artificial selection
Selective breeding (artificial selection) is the process by which humans breed plants and animals for particular genetic characteristics.
Selective breeding in animals
Animals are commonly selectively bred for various characteristics, including:
cows, goats and sheep that produce lots of milk or meat.
chickens that lay large eggs.
domestic dogs that have a gentle nature.
sheep with good quality wool.
horses with fine features and a very fast pace.
Selective breeding in plants
Plants are selectively bred by humans for development of many characteristics, including:
disease resistance in food crops.
increased crop yield.
hardiness to weather conditions (eg. drought tolerance).
better tasting fruits.
large or unusual flowers.
Problems with selective breeding
Selective breeding can lead to ‘inbreeding’.
This occurs when only the ‘best’ animals or plants (which are closely related to each other) are bred together.
This results in a reduction in the gene pool – this is a reduction in the number of alleles in a population.
As inbreeding limits the size of the gene pool, there is an increased chance of:
organisms inheriting harmful genetic defects.
organisms being vulnerable to new diseases (there is less chance of resistant alleles being present in the reduced gene pool).
Genetic engineering
Genetic engineering is changing the genetic material of an organism by removing or altering genes within that organism, or by inserting genes from another organism.
The organism receiving the genetic material is said to be ‘genetically modified’, or is described as a ‘transgenic organism’.
The DNA of the organism that now contains DNA from another organism as well is known as ‘recombinant DNA’
Plant genetic engineering
Genetically modified plants are plants that have had foreign DNA inserted into their genome.
This is usually done to improve food production in some way
For example:
Tomatoes have been genetically modified to make them grow larger fruit
Wild rice has been genetically modified to produce beta carotene (needed by humans to make vitamin A).
Crop plants have been genetically modified to be resistant to diseases or resistant to certain herbicides.
Genetic modification of bacteria to produce insulin
The gene for human insulin has been inserted into bacteria which then produce human insulin which can be collected and purified for medical use to treat people with diabetes.
The gene that is to be inserted is located in the original organism – the gene for insulin production is located within a human chromosome.
Restriction enzymes are used to isolate or ‘cut out’ the human insulin gene, leaving it with ‘sticky ends’ (a short section of unpaired bases).
A bacterial plasmid is cut by the same restriction enzyme leaving it with corresponding sticky ends.
The plasmid and the isolated human insulin gene are joined together by DNA ligase enzyme.
If two pieces of DNA have matching sticky ends (because they have been cut by the same restriction enzyme), DNA ligase will link them to form a single, unbroken molecule of DNA.
When the bacteria reproduce the plasmids are copied as well and so a recombinant plasmid can quickly be spread as the bacteria multiply and they will then all express the human insulin gene and make the human insulin protein.
The genetically engineered bacteria can be placed in a fermenter to reproduce quickly in controlled conditions and make large quantities of the human protein.
Process of genetic engineering
The main steps in the process of genetic engineering:
Enzymes are used to isolate (cut out) the required gene.
This gene is inserted into a vector.
The vector is usually a bacterial plasmid (a piece of circular DNA found inside bacterial cells) or a virus.
The vector is used to insert the gene into the required cells of the target organism.
Genes are transferred to the cells of animals, plants or microorganisms at an early stage in their development so that they develop with desired characteristics.
Curing disease with genetic engineering
Modern medical research is exploring the possibility of genetic modification to overcome some inherited disorders.
As these inherited genetic diseases are caused by faulty genes, it may be possible to treat these by inserting working versions of these genes into people with the genetic disease.
This is called gene therapy.
GM crops
Crops can be genetically modified (they are known as GM crops).
Crop plants, such as wheat and maize, have been genetically modified to contain a gene from a bacterium that produces a poison that kills insects, making them resistant to insect pests such as caterpillars. This can improve crop yields.
Crop plants have also been genetically modified to make them resistant to certain herbicides (chemicals that kill plants), meaning that when the herbicide is sprayed on the crop it only kills weeds and does not affect the crop plant.
Some crops have been genetically modified to produce additional vitamins and improved nutritional value, eg. ‘golden rice’ contains genes from another plant and a bacterium which make the rice grains produce a chemical that is turned into vitamin A in the human body, which could help prevent deficiency diseases in certain areas of the world.
Some have been genetically modified to be drought-resistant (to grow better in very dry conditions). This can also improve crop yields
Concerns about GM crops include the effect on populations of wildflowers and insects.
Some people feel the effects of eating GM crops on human health have not been fully explored.
Advantages and disadvantages of GM crops
Advantages - Increased yield of crops as they are not competing with weeds for resources and suffering from pest damage.
Reduces the need to use chemicals making farming cheaper.
Disadvantages - Risk of inserted genes being transferred to wild plants through pollination which could reduce the usefulness of the GM crop.
Some research shows that plants that have had genes inserted into them do not grow as well as other plants.
Tissue culture
Tissue culture is a process in which very small pieces of plants (tissue) are grown (cultured) using nutrient media.
Because they are initially grown in petri dishes on nutrient agar we say they are grown ‘in vitro’ – outside a living organism.
How to propagate plants in vitro:
Cells are scraped from the parent plant (these cells are known as explants).
The cells are transferred to a sterile petri dish containing nutrient agar.
Hormones (eg. auxins) are added to encourage plants to grow into small masses of tissue (callus tissue).
Tissue continues to grow and forms plantlets that can be transferred to individual potting trays and develop into plants.
Plant clones
Clones are genetically identical individuals.
The cloning of plants has many important commercial uses.
It allows a variety of a plant with desirable characteristics to be produced: cheaply with a greater yield, quickly and at any time of the year.
It can also ensure diseases prevalent in other areas of the world are not imported and spread by ensuring native varieties of plants are produced in large enough quantities to supply demand in one country without importing plants from abroad.
Tissue culture can also be an important process in preserving rare plant species.
Cutting
An older and more simple method to clone plants (mainly used by gardeners) is by taking cuttings.
Gardeners take cuttings from good parent plants.
These cuttings are then planted and grow into genetically identical versions of the original plant.
Plants cloned by taking cuttings can be produced cheaply and quickly.
Embryo cloning
It is possible to clone animals using embryo transplants.
For example, if a farmer wants the best cattle, they must first create an offspring from the best bull and best cow, and then clone this offspring to create many genetically identical copies (clones).
This process is known as embryo cloning and is achieved in the following way:
Egg cells from the best cow are artificially fertilised using sperm cells taken from the best bull.
This forms an embryo.
The developing animal embryo is then split apart many times before the cells of the embryo become specialised.
This forms many separate embryos that are genetically identical.
These cloned embryos are then transplanted into host mothers.
The calves born from these host mothers are all genetically identical.
Adult cell cloning
Adult cell cloning is achieved in the following way:
The nucleus is removed from an unfertilised egg cell.
The nucleus from an adult body cell, such as a skin cell, is inserted into the egg cell.
A very small electric shock stimulates the egg cell to divide (by mitosis) to form an embryo.
These embryo cells contain the same genetic information as the adult skin cell.
When the embryo has developed into a ball of cells, it is inserted into the womb of an adult female (known as the surrogate mother) to continue its development until birth.
Advantages and disadvantages of cloning
Advantages - cloning can be used to help preserve endangered species of plants and animals.
Cloning makes it possible to quickly and cheaply produce commercial quantities of high quality plants at any time of the year.
Disadvantages - Cloning can result in a reduced gene pool. this can lead to the population having a smaller chance of having resistance to a disease.
There are some evidence that cloned animals may not be as healthy as normal ones.
