variation and selection Flashcards
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 is the difference in features between individuals of the same species
Some of these differences are caused by differences in genes, which is
genetic variation
Phenotypic variation can be divided into two types depending on…: …
- how you are able to group the measurements:
Continuous Variation is when there are very many small degrees of difference for a particular characteristic between individuals and they are arranged in order
Examples include height, mass, finger length etc. where there can be many ‘inbetween’ groups
Discontinuous Variation is when there are distinct differences for a characteristic
For example, people are either blood group A, B, AB or O; are either male or female; can either roll their tongue or not – there are no ‘inbetweens’
When graphs of Continuous Variation are plotted, they give … (a result of all the small degrees of difference), whereas discontinuos variation …
-smooth bell curves
-gives a step-like shape
is it a type of …variation
height is an example of continuous variation which gives rise to a smooth bell-shaped curve when plotted as a graph
what type of variation
Blood group is an example of discontinuous variation which gives rise to a step-shaped graph
Phenotypic variation can be caused in two main ways:
It can be genetic – controlled entirely by genes
Or it can be environmental – caused entirely by the environment in which the organism lives
Examples of genetic variation in humans include:
blood group
eye colour
gender
ability to roll tongue
whether ear lobes are free or fixed:
Environmental Variation
Characteristics of all species can be affected by environmental factors such as climate, diet, accidents, culture, lifestyle and accidents during lifetime
In this instance ‘environmental’ simply means ‘outside of the organism’
Examples include:
An accident may lead to scarring on the body
Eating too much and not leading an active lifestyle will cause weight gain
Being raised in a certain country will cause you to speak a certain language with a certain accent
A plant in the shade of a big tree will grow taller to reach more light
Genetic and Environmental Causes
Discontinuous variation is usually caused by genetic variation alone
Continuous features often vary because of a combination of genetic and environmental causes, for example:
tall parents will pass genes to their children for height
their children have the genetic potential to also be tall
however if their diet is poor then they will not grow very well
therefore their environment also has an impact on their height
Another way of looking at this is that although genes decide what characteristics we inherit, the surrounding environment will affect how these inherited characteristics develop
Mutations are
random genetic changes
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 have a small effect on the organism
Occasionally, the new allele gives the individual a … over other members of the species
for example:…
Mutations can also lead tos that can have dramatic effects on the body…for example:…
Mutations happen… and .. but their frequency can be …by exposure to the following: …
Increased rates of mutation can cause cells to become …, which is why the above are linked to increased incidence of different types of
- survival advantage
- A bird develops a mutation leading to a change in feather colours
This makes it more attractive to birds of the opposite sex
Which causes the bird to breed more frequently and have more chances of passing on the mutated phenotype to the next generation
- harmful change
- sickle cell anaemia in humans
- spontaneously
- continuously
- increased
-
Gamma rays, x – rays and ultraviolet rays – all types of ionising radiation which can damage bonds and cause changes in base sequences
Certain types of chemicals – for example chemicals such as tar in tobacco
- cancerous
- cancer
Sickle Cell Anaemia
Sickle cell anaemia was the first genetic disease to be described in terms of a gene mutation
A gene mutation is a change in the base sequence of DNA
The mutation changes the molecule haemoglobin, causing the red blood cells (RBC’s) to become stiff and sometimes sickle-shaped when they release oxygen to the body tissues
The sickled cells tend to get stuck in narrow blood vessels, blocking the flow of blood
As a result, those with sickle cell disease suffer painful “crises” in their joints and bones
They may suffer strokes, blindness, or damage to the lungs, kidneys, or heart. They must often be hospitalized for blood transfusions and are at risk for a life-threatening complication called acute chest syndrome
Although many sufferers of sickle cell disease die before the age of 20, modern medical treatments can sometimes prolong these individuals’ lives into their 40s and 50s
Inheritance of sickle cell
There are two versions or alleles of the gene important for the inheritance of sickle cell anaemia : A and S
The two alleles are codominant, meaning there is no ‘dominant’ or ‘recessive’ version of the gene
Individuals with two A alleles (HbAHbA) have normal haemoglobin, and therefore normal RBCs
Those with two S alleles (HbSHbS) develop sickle cell anaemia
Those who are heterozygous for sickle cell (HbAHbS) produce both normal and abnormal haemoglobin (as the alleles are codominant)
Heterozygous individuals are usually healthy, but they may suffer some symptoms of sickle cell anaemia under conditions of low blood oxygen, such as high altitudes or during exercise
Heterozygous individuals are said to be ‘carriers’ of the sickle cell gene and are said to have ‘sickle cell trait’
Inheritance of sickle cell trait: (punnett square)
Inheritance of sickle cell disease:(disease)
Sickle cell anaemia & natural selection
In the United States, about 1 in 500 African-Americans develops sickle cell anaemia
In Africa, about 1 in 100 individuals develops the disease
Why is the frequency of such a serious disease so much higher in Africa? The answer is to do with malaria
Malaria is a disease spread by mosquitoes that are endemic in many areas of Africa and causes over 1 million deaths per year
In the late 1940s, studies of diseases in populations suggested a connection between African populations, malaria and sickle cell disease
A theory was suggested; if the heterozygous individuals (HbAHbS) are protected from malaria, and the negative effects (of sickle cell) are only present in the small proportion of people who are homozygous for the affected allele (HbSHbS), then the affected allele could become more common
Later studies supported this theory, showing that African children who are heterozygous for the sickle cell allele have a ten-fold reduction in their risk of getting malaria
This means that there is a strong correlation between the prevalence of sickle cell anaemia in areas of the world where malaria is common
You should be able to explain how these maps support the idea that having…
You should also be able to use numerical data and graphs given in exam questions to explain this.
a sickle cell allele gives resistance to malaria.
An adaptive feature is an
inherited feature that helps an organism to survive and reproduce in its environment
You should be able to interpret images or other information about a species in order to describe its adaptive features, for example:
A typical question might be to explain how the leaf area and distribution and density of stomata help different species of plant survive in their different habitats
Adaptations & Fitness
Adaptive features are the inherited functional features of an organism that increase its fitness
Fitness is the probability of an organism surviving and reproducing in the environment in which it is found
Hydrophytes
Plants adapted to live in extremely wet conditions
Common adaptations include:
Large air spaces in their leaves to keep them close to the surface of the water where there is more light for photosynthesis
Small roots as they can also extract nutrients from the surrounding water through their tissues
Stomata usually open all the time and mainly found on the upper epidermis of the leaf where they can exchange gases much more easily with the air
Xerophytes
Plant adapted to live in extremely dry conditions
Common adaptations include:
Thick waxy cuticle – the cuticle cuts down water loss in two ways: it acts as a barrier to evaporation and also the shiny surface reflects heat and so lowers temperature
Sunken stomata: stomata may be sunk in pits in the epidermis; moist air trapped here lengthens the diffusion pathway and reduces evaporation rate
Leaf rolled with stomata inside and an inner surface covered in hairs – traps moist air and prevents air movement across stomata which reduces transpiration
Small leaves: many xerophytic plants have small, needle-shaped leaves which reduce the surface area and therefore the evaporating surface
Extensive shallow roots allowing for the quick absorption of large quantities of water when it rains
Thickened leaves or stems which contain cells that store water
this:…results in natural selection
In any environment, the individuals that have the best adaptive features are the ones most likely to survive and reproduce
Natural Selection
ndividuals in a species show a range of variation caused by differences in genes
When organisms reproduce, they produce more offspring than the environment is able to support
This leads to competition for food and other resources which results in a ‘struggle for survival’
Individuals with characteristics most suited to the environment have a higher chance of survival and more chances to reproduce
Therefore the alleles resulting in these characteristics are passed to their offspring at a higher rate than those with characteristics less suited to survival
This means that in the next generation, there will be a greater number of individuals with the better-adapted variations in characteristics
This theory of natural selection was put forward by Charles Darwin and became known as ‘survival of the fittest’
An example of natural selection
explain it
Within the population of snails there is variation in shell colour
Normal varieties of shell colours in this snail species is black or grey (as evidenced by the first picture)
Chance mutations lead to a small number of snails / one snail having a white shell
This ‘small number’ is shown in the second diagram where there are less white shelled snails than black or grey shelled snails
The white shelled snail(s) survive longer
This is the ‘survival of the fittest’, a term used to explain why some organisms succeed in the competitive struggle for survival against other members of their population
The reason the white shelled snail(s) survive longer is because they are better camouflaged
This means that they are less likely to be seen by predators and eaten
As they survive longer they get more opportunities to reproduce
And so the allele for white shells is passed on to offspring more frequently than the alleles for black or grey shells
Over generations, this is repeated until the majority of snails in the population have white shells
There are hundreds of thousands of examples of natural selection and you cannot possibly be familiar with all of them, however, they ALL follow the same sequence described above
Based on the idea that within a species there is always variation and chance mutations, some individuals will develop a phenotype (characteristic) that gives them a survival advantage and therefore will:
live longer,
breed more
and be more likely to pass their genes on.
Repeated over generations, the ‘mutated’ phenotype will become the norm.
Evolution
If the environment does not change, selection does not change
This will favour individuals with the same characteristics as their parents
If the environment changes, or a chance mutation produces a new allele, selection might now favour individuals with different characteristics or with the new allele
So the individuals that survive and reproduce will have a different set of alleles that they pass on to their offspring
Over time, this will bring about a change in the characteristics of the species – it will produce evolution
Evolution is defined as the change in adaptive features of a population over time as a result of natural selection
Natural selection results in a process of adaptation, which means that, over generations, those features that are better adapted to the environment become more common
This means populations of organisms become better suited to their environment
A good example of this is the development of antibiotic resistance by bacteria
Antibiotic Resistance in Bacteria
An antibiotic is a chemical that can kill or inhibit the growth and reproduction of bacteria
They are extremely useful to humans as some bacteria are pathogenic and can cause life-threatening disease
Bacteria reproduce, on average, every 20 minutes and therefore evolution occurs in a much shorter time span
Like all other organisms, within a population there will be variation caused by mutation
A chance mutation might cause some bacteria to become resistant to an antibiotic (eg penicillin)
When the population is treated with this antibiotic, the resistant bacteria do not die
This means they can continue to reproduce with less competition from non-resistant bacteria, which are now dead
Therefore the genes for antibiotic resistance are passed on with a much greater frequency to the next generation
Over time the whole population of bacteria becomes antibiotic-resistant because the bacteria are best suited to their environment
This is an example of natural selection that humans have helped to develop due to overuse of antibiotics in situations where they were not really necessary, for example:
for treatment of non-serious infections
routine treatment to animals in agriculture
failure to finish prescribed course of antibiotics
Increases in the population of antibiotic-resistant bacteria cause infections and diseases which are harder to control as it is difficult to find antibiotics that certain strains of bacteria are not resistant to
An example of this is MRSA, a very dangerous bacterial strain that is resistant to most antibiotics
If someone gets infected with MRSA they cannot be treated easily
Adding to these difficulties, the number of new antibiotics discovered has slowed significantly
Artificial Selection
Selective breeding means to select individuals with desirable characteristics and breed them together
The process doesn’t stop there though because 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
Natural vs Artificial Selection
Selectively Breeding 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
better tasting fruits
large or unusual flowers
An example of a plant that has been selectively bred in multiple ways is wild brassica, which has given rise to cauliflower, cabbage, broccoli, brussel sprouts, kale and kohlrabi:
Selectively Breeding Animals
Selective breeding of animals has been carried out by humans for thousands of years
It takes place in the same way as selective breeding of plants
Individuals with the characteristics you want are bred together (often several different parents all with the desired characteristics are chosen so siblings do not have to be bred together in the next generation)
Offspring that show the desired characteristics are selected and bred together
This process is 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
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
An example of an animal that has been selectively bred by humans in many ways to produce breeds with many different characteristics is the domestic dog, all breeds of which are descended from wolves
Make sure that you include the need to repeat the selective breeding for many generations in any exam answer you give
Selecting two parents with desired characteristics, breeding them and stopping there is not selective breeding and will not give rise to a new breed.
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