18. Variation and Selection Flashcards
What is variation?
Variation is defined as differences between individuals of the same species
What is phenotypic variation?
Phenotypic variation is the difference in features between individuals of the same species
What is genetic variation?
Some of these differences are caused by differences in genes, which is genetic variation
What two types of phenotypic variation are there and how are they divided?
Phenotypic variation can be divided into two types depending on how you are able to group the measurements:
continuous and discontinuous variation
What is continuous variation?
Continuous Variation is when there are very many small degrees of difference for a particular characteristic between individuals and they are arranged in order and can usually be measured on a scale
What are examples of continuous variation?
Examples include height, mass, finger length etc. where there can be many ‘inbetween’ groups
What is discontinuous variation?
Discontinuous Variation is when there are distinct differences for a characteristic
What are examples of discontinuous variation?
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’
What two things can cause phenotypic variation?
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
What are examples of genetic variation in humans?
blood group
eye colour
gender
ability to roll tongue
whether ear lobes are free or fixed
What are examples of environmental variation?
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
What environmental factors may affect variation?
Characteristics of all species can be affected by environmental factors such as climate, diet, accidents, culture and lifestyle
What type of variation is discontinuous variation caused by?
Discontinuous variation is usually caused by genetic variation alone
What types of variation cause continuous features?
Continuous features often vary because of a combination of genetic and environmental causes
What is an example of a combination of genetic and environmental variation which leads to continuous variation?
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
What are mutations?
Mutations are random genetic changes
Do mutations affect the phenotype?
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
Can mutations lead to development of new alleles? What effect do they have?
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
Can mutations be useful? How?
Occasionally, the new allele gives the individual a survival advantage over other members of the species
For example:
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
Can mutations be harmful? How?
Mutations can also lead to harmful changes that can have dramatic effects on the body – for example, sickle cell anaemia in human
What two words can describe mutations?
Mutations happen spontaneously and continuously
What can increase the frequency of mutations?
their frequency can be increased by exposure to the following:
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
Why are increased rate of mutations bad?
Increased rates of mutation can cause cells to become cancerous, which is why the above are linked to increased incidence of different types of cancer
What type of mutation results in sickle cell anaemia?
Sickle cell anaemia was the first genetic disease to be described in terms of a gene mutation
What is a gene mutation?
A gene mutation is a change in the base sequence of DNA
How does the mutation change cells in sickle cell anaemia?
The mutation changes the molecule haemoglobin
What effects do the mutations have on the red blood cells?
causing the red blood cells (RBC’s) to become stiff and sometimes sickle-shaped when they release oxygen to the body tissues
What can happen to the red blood cells due to their shape?
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
What things may people with SCA suffer from?
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
How long do people with SCA live with?
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
What two versions of the gene are important for sickle cell anaemia?
There are two versions or alleles of the gene important for the inheritance of sickle cell anaemia : A and S
Are the A and S alleles codominant?
The two alleles are codominant, meaning there is no ‘dominant’ or ‘recessive’ version of the gene
How would you represent someone having normal haemoglobin?
Individuals with two A alleles (HbAHbA) have normal haemoglobin, and therefore normal RBCs
How would you represent someone having SCA?
Those with two S alleles (HbSHbS) develop sickle cell anaemia
How would you represent someone being heterozygous for SCA?
Those who are heterozygous for sickle cell (HbAHbS)
What type of RBC’s do people who are heterozygous for SCA have?
produce both normal and abnormal haemoglobin (as the alleles are codominant)
What symptoms do people who are heterozygous for SCA have?
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
What are people who are heterozygous for SCA called?
Heterozygous individuals are said to be ‘carriers’ of the sickle cell gene and are said to have ‘sickle cell trait’
What other illness is SCA related with?
malaria
How many people in USA are affected by SCA?
In the United States, about 1 in 500 African-Americans develops sickle cell anaemia
How many people in Africa are affected by SCA?
In Africa, about 1 in 100 individuals develops the disease
What is malaria?
Malaria is a disease spread by mosquitoes that are endemic in many areas of Africa
How many deaths does malaria cause?
causes over 1 million deaths per year
What relation is there between malaria and anaemia?
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
What is an adaptive feature?
An adaptive feature is an inherited feature that helps an organism to survive and reproduce in its environment
( inherited functional features of an organism that increase its fitness)
What does fitness mean?
Fitness is the probability of an organism surviving and reproducing in the environment in which it is found
What are hydrophytes?
Plants adapted to live in extremely wet conditions
What are 3 common adaptations of hydrophytes?
large air spaces in their leaves
small roots
stomata mainly found in the upper epidermis
Why do hydrophytes have large air spaces in their leaves ?
Large air spaces in their leaves to keep them close to the surface of the water where there is more light for photosynthesis
Why do hydrophytes have small roots?
Small roots as they can also extract nutrients from the surrounding water through their tissues
Why do hydrophytes have stomata mainly found in the upper epidermis ?
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
What are xerophytes?
Plant adapted to live in extremely dry conditions
What are 6 adaptations of xerophytes?
thick waxy cuticle
sunken stomata
leaf rolled with stomata inside
small leaves
extensive shallow roots
thickened leaves or stems
Why do xerophytes have a 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
Why do xerophytes have sunken stomata ?
stomata may be sunk in pits in the epidermis; moist air trapped here lengthens the diffusion pathway and reduces evaporation rate
Why do xerophytes have leaf rolled with stomata inside ?
Leaf rolled with stomata inside and an inner surface covered in hairs – traps moist air and prevents air movement across stomata which reduces transpiration
Why do xerophytes have small leaves ?
many xerophytic plants have small, needle-shaped leaves which reduce the surface area and therefore the evaporating surface
Why do xerophytes have extensive shallow roots ?
allowing for the quick absorption of large quantities of water when it rains
Why do xerophytes have thickened leaves or stems ?
contain cells that store water
What does selective breeding mean?
Selective breeding means to select individuals with desirable characteristics and breed them together
Why is selective breeding not only done once?
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
At what point can one determine that selective breeding is successful?
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
What is the difference between natural and artificial selection relating to how it occurs?
NS - occurs naturally
AS - only occurs when humans intervene
What is the difference between natural and artificial selection relating to how it develops populations and their features?
NS - results in development of populations with features that are better adapted to their environment and survival
AS - Results in development of populations with features that are useful to humans and not necessarily to survival of the individual
What is the difference between natural and artificial selection relating to how long it takes?
NS - takes a long time to occur
AS - takes less time as only individuals with the desired features are allowed to reproduce
For what reasons may crops be selectively bred? 5
disease resistance in food crops
increased crop yield
hardiness to weather conditions (e.g. drought tolerance)
better tasting fruits
large or unusual flowers
What is an example of a selectively bred plant and what has it produced?
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:
How is selective breeding of animals carried out?
Individuals with the characteristics you want are bred together
Offspring that show the desired characteristics are selected and bred together
Why, during selective breeding, are several different parents often 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
How may cows be selectively bred?
cows that produce lots of milk or meat
How may chickens be selectively bred?
chickens that lay large eggs
How may domestic dogs be selectively bred?
domestic dogs that have a gentle nature
How may sheep be selectively bred?
sheep with good quality wool
How may horses be selectively bred?
horses with fine features and a very fast pace
What is an example of a selectively bred animal that has led to many different characteristics?
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
- In any environment, which individuals are likely to survive?
What does this result in?
In any environment, the individuals that have the best adaptive features are the ones most likely to survive and reproduce
This results in natural selection:
- Are all individuals in a species the same?
What is this caused by?
NO
Individuals in a species show a range of variation caused by differences in genes
- How much offspring do organisms produce?
When organisms reproduce, they produce more offspring than the environment is able to support
- What does organisms having a large number of offspring result in?
This leads to competition for food and other resources which results in a ‘struggle for survival’
- What individual will survive this?
Individuals with characteristics most suited to the environment have a higher chance of survival and more chances to reproduce
- Which type of individual will increase in number and why?
This means that in the next generation, there will be a greater number of individuals with the better adapted variations in characteristics
- How are these adaptations passed down?
the alleles resulting in these characteristics are passed to their offspring at a higher rate than those with characteristics less suited to survival
- What is this theory called? (NS)
This theory of natural selection was put forward by Charles Darwin and became known as ‘survival of the fittest’
What is chance mutation?
Random mutation that leads to a change in the features of the snail
Will selection change if the environment doesn’t change?
If the environment does not change, selection does NOT change
- What individuals will be favoured if the environment doesn’t change?
(evolution)
This will favour individuals with the same characteristics as their parents
- What will happen if the environment changes or a chance mutation occurs?
(evolution)
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
- What will be passed onto the offspring?
evolution
So the individuals that survive and reproduce will have a different set of alleles that they pass on to their offspring
- What will the offspring having a different set of alleles result in?
(evolution)
Over time, this will bring about a change in the characteristics of the species – it will produce evolution
What is evolution defined as?
Evolution is defined as the change in adaptive features of a population over time as a result of natural selection
What process does natural selection result in and what is it?
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
What does process of adaptation overall do?
This means populations of organisms become better suited to their environment
What is an example of natural selection?
A good example of this is the development of antibiotic resistance by bacteria
- What is an antibiotic?
antibiotic resistance
An antibiotic is a chemical that can kill or inhibit the growth and reproduction of bacteria
- Why do we need antibiotics?
antibiotic resistance
They are extremely useful to humans as some bacteria are pathogenic and can cause life-threatening disease
- How often do bacteria reproduce and what does this aid/result in?
(antibiotic resistance)
Bacteria reproduce, on average, every 20 minutes and therefore evolution occurs in a much shorter time span
- What process will be found in this population of bacteria?
(antibiotic resistance)
Like all other organisms, within a population there will be variation caused by mutation
- What may cause antibiotic resistance?
antibiotic resistance
A chance mutation might cause some bacteria to become resistant to an antibiotic (eg penicillin)
- What happens when a population of bacteria with non/antibiotic-resistant bacteria is treated with an antibiotic?
(antibiotic resistance)
When the population is treated with this antibiotic, the resistant bacteria do not die
- What happens as a result of the non-antibiotic-reistant bacteria dying?
(antibiotic resistance)
This means they (antibiotic-reistant bacteria) can continue to reproduce with less competition from non-resistant bacteria, which are now dead
- What amount of bacteria in the next generation will be antibiotic-resistant and why?
(antibiotic resistance)
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
- Why is antibiotic-resistance considered natural selection?
(antibiotic resistance)
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
- In what ways have humans helped create antibiotic-resistant bacteria?
(antibiotic resistance)
for treatment of non-serious infections
routine treatment to animals in agriculture
failure to finish prescribed course of antibiotics
- Why is antibiotic resistance a problem?
antibiotic resistance
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
Adding to these difficulties, the number of new antibiotics discovered has slowed significantly
- What is an example an antibiotic-resistant bacteria?
antibiotic resistance
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
Define variation
Differences between individuals of the same species
What is phenotypic variation?
The difference in features between individuals of the same species and some of these differences are caused by genes which is genetic variation
What is phenotypic variation caused by?
Genetic and environmental factors
What is continuous variation?
A quantity usually measured eg. height
What is discontinuous variation?
Limited number of phenotypes with no intermediates eg tongue rolling and blood group
Define mutation
Genetic change
Define gene mutation
A change in the base sequence of a DNA
How are new alleles found?
By mutation
What increases the rates of mutation?
Ionising radiation and gamma rays and certain chemicals which can cause cells to become cancerous
What is sickle cell anaemia?
A mutation that changes the molecule haemoglobin causing the RBC’s to become stiff and sometimes sickle shaped when they release oxygen and these tend to get stuck in narrow blood vessels blocking the flow of blood. They may suffer strokes, blindness or damage to the heart, lungs and kidneys.
Which alleles give normal haemoglobin?
HbA HbA
Which alleles give sickle cell anaemia?
HbS HbS
Which alleles produce normal and abnormal haemoglobin?
HbA HbS
Who are resistant to malaria?
People who are heterozygous for sickle cell anaemia HbS HbA
Why is there a strong correlation between the prevalence of sickle cell anaemia and in areas where malaria is more common?
If a heterozygous individual is 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 then the affected allele could be more common.
Define Adaptive features
The inherited functional features of an organism that increase its survival
Define fitness
The probability of an organism surviving and reproducing in the environment in which it is found
What are hydrophytes?
Live in wet conditions
What are xerophytes?
Live in dry conditions
Describe the adaptations of Hydrophytes
Have large air spaces to keep them close to the surface to get more light
Small roots so that can also extract nutrients from the surrounding water
Stomata found in the upper epidermis in order to exchange gases more easily with the air
Describe the adaptations of Xerophytes
Thick cuticle to prevent water loss as it acts as a barrier to evaporation and the shiny surface reflects gear so temperature goes down
Thickened leaves contain cells that store water
Extensive shallow roots allow for quick absorption of large quantities of water
Sunken stomata trap moist air and lengthens diffusion pathways and reduces evaporation rate
Small leaves reduce SA therefore the evaporating surface
Define the process of adaptation
The process resulting from natural selection by which populations become more suited to their environment over many generations.
What is evolution?
The change of adaptive features of a population over time as the result of natural selection
What does natural selection lead to?
Competition for food
Competition for survival as the individuals with characteristics most suited to the environment have a higher rate of survival and more chances to reproduce therefore these alleles are passed to their offspring at a higher rate.
Give an example of evolution by natural selection
Development strains of antibiotic resistant bacteria
What is selective breeding?
It is done by humans of individuals with desirable features. They cross these individuals to produce the next generation until the new breed will reliably show those characteristics in all offspring
How does artificial selection take place?
Individuals with the characteristics you want are bred together
Offspring that show the desired characteristic are selected and bred together
This process is REPEATED FOR MANY SUCCESSIVE GENERATIONS before you have a new breed
What is a mutation (general)
A rare random change in genetic material that can be inherited.
The theory of evolution by natural selection
Evolution happen by natural selection. Since there is competition for resources in the natural world only those most suited to their environment can survive and pass their genes onto the next generation. Mutations in genetics can also lead to population variation giving some individuals an advantage or disadvantage.
What are the 4 bases of a DNA nucleotide
Adenine
Guanine
Cytosine
Thymine
Process of bacteria becoming antibiotic resistant
Bacteria reproduce very frequently. During reproduction mutations may occur and some mutations can cause bacteria to become resistant to antibiotics, if the full course of antibiotics are not taken resistant bacteria will survive. These resistant bacteria will multiply until the whole population is resistant to antibiotics.
How can the likeliness of mutation be increased
Exposure to ionising radiation (gamma, X-ray, ultraviolet) and some chemical mutagens (chemicals in tobacco)
Variation
difference between individuals of the same species
Mutation
genetic change
Gene mutation –
A change in the base sequence of DNA
Adaptive feature
-an inherited feature that helps an organism to survive and reproduce in the environment
-inherited functional features of an organism that increase its fitness
Fitness
The probability of an organism surviving and reproducing in the environment in which it found
DNA controls cell function by
controlling the production of proteins (some of which are
enzymes), antibodies, and receptors for
neurotransmitter
Each amino acid is
coded by a sequence of 3 bases on DNA.
how many amino acids
20
how many bases are in DNA
Adenine (A)
thymine (T)
cytosine (C)
guanine (G)
HOW MANY BASES IN mRNA
adenine (A)
Thymine (T)
Uracil (U)
Cytosine (C)
-guanine (G)
Protein Synthesis
❖ Gene coding remains in the nucleus
❖ A copy of the DNA is made by transcription
❖ mRNA molecules carry a copy of the gene to the cytoplasm
❖ mRNA passes through ribosomes
❖ Ribosomes assemble amino acids into protein molecules by translation
❖ The specific order of amino acids is determined by the sequence of bases in the mRNA. Each
amino acid is joined to the other by a peptide bond
All body cells in an organism contain the same genes, but
but many genes in a particular cell are not
expressed because the cell only makes the specific proteins it needs.
how many sets of chromosomes in body cells
23 pairs of chromosomes
mitosis uses
in the growth, repair of damaged tissues, replacement of cells, and asexual reproduction.
Mitosis involves:
❖ DNA replication in the cell
❖ The 2 copies separate so each new cell gets a copy of each chromosome
❖ As a result of mitosis , each daughter cell has the same chromosome number as the original
parent cell.
during the development of an embryo, most cells become
specialized cells.
stem cells.
-embryos contain a special type of cell
- These can grow into any type of cell found in the body. Stem cells divide by mitosis to produce daughter cells that can become specialized for specific functions.
meiosis is used for the production of
gametes
how does meiosis produce variation?
by forming new combinations of maternal and paternal
chromosomes.
difference between mitosis and meiosis
mitosis-
one division
- the number of chromosomes remains the same
- the daughter cells are genetically identical
- two daughter cells formed
- 2 daughter cells are formed
meiosis
- 2 divisions
- the number of chromosomes is halved
- daughter cells are genetically different
- 4 daughter ceells formed
In pure breeding, both parents are
homozygous individuals
A heterozygous can not take part in pure-breeding, as there is the likelihood of the offspring gaining a
new phenotype
Pedigree diagrams
Symbols:
➔ Square = male
➔ Circle = female
➔ Shaded circle or square = individual has the condition
➔ Unshaded circle or square = individual not showing the condition
why test cross?
used to determine the genotype of an individual to find out whether it is homozygous or heterozygous dominant.
only possible genotype in test cross
homozygous recessive
The results from a test cross show the
ratios expected in the
offspring, not the actual numbers
codominance.
both alleles are expressed and neither is dominant
In the case of human blood groups, the four different blood groups are determined by the gene
, with three different alleles I A , I B,
and I O . I A and I B are codominant, and code for different molecules on the surface of RBCs.I O is recessive and doesn’t code for these molecules.
I A I A
blood group: A
IA IO
blood group A
I B I B
blood group B
I B I O
blood group B
I A I B
blood group AB
I O I O
blood group O
The genes located on the X chromosome are described as
sex-linked.
The genes located on the X chromosome are involved in
controlling vision and blood clotting
Males have an X and a Y chromosome, and if the X they
possess is
recessive, they will have the diseases
The disease is less common in females as they have
another X chromosome which may still be dominant
Color blindness
A gene on the X chromosome controls the ability of the cones in the retina to see red and green. There is
a recessive allele that doesn’t produce the protein necessary for color vision. If a male gets the recessive X
with the Y, he will have the disease. Males cannot be carriers .
phenotypic variation
Variation in morphology or anatomy
can be influenced by both genetic and environmental factors.
Genetic variation
differences between the genotypes of individuals. This means the genotype or alleles of the genes inherited are different.
There are two types of phenotypic variation within a species:
1)Continuous variation
2)Discontinuous variation
Continuous variation
In this type of variation, there is a range along
which individuals can be placed in terms of
how they display a trait. For eg: height of 16
years olds: Ranges from 120-210cm. Height is
affected not only by genes but also by the food
consumed, exercise done, and growth spurt
times. Therefore this type of variation can be
shown on a frequency histogram.
-A histogram showing the number of people at
each height range
Discontinuous variation
In this type of variation, there are a few
possible phenotypes and no intermediates.
Discontinuous variation is caused by genes
alone and the environment does not affect
them. For eg: Blood group - everyone is A, B,
AB, or O, and there are no intermediates.
Therefore this type of variation can be shown
on a bar chart.
-A bar chart showing the number of people with
each type of blood group
Gene mutation is the only way
alleles are formed.
Mutations are caused by
damage to DNA or a failure in the copying process that occurs before nuclear division.
The rate at which they occur can be increased by:
● Exposure to ionizing radiation - UV, X-rays, ɣ
● Chemicals
a greater dose of chemicals means
greater chance of mutation.
Hydrophytes
plants that grow submerged or partially submerged in water. Eg: water lilies, hydra
adaptive features of hydrophytes
● These plants are buoyed up in the water, so plant cells save energy as they don’t produce
xylem cells.
● No root hair: easy absorption when in water
● No cuticle on leave: no need to conserve water
● CO 2 and O 2 diffuse slowly in water, so an extensive system of air spaces in the stem and
leaves allow gases to diffuse
○ Provide buoyancy to keep the plant afloat
● Wide flat leaves to absorb as much light as possible
● Many stomata always open on the upper surface
Xerophytes
plants which can exist in conditions where water is scarce. Eg: Cactus
adaptive features of xerophytes
● Leaves reduced to spines and round, compact shape
○ Reduces surface area exposed
○ Less water lost
● Waxy cuticle
○ Reduced transpiration
● Swollen stems with water storage tissue
● Shallow, root spreading system
○ Quick absorption of water from rain and overnight condensation
● Shiny surfaces
○ Reflect heat and light
● Stomata closed during the day
○ Reduced water loss
○ CO 2 for photosynthesis collected and stored at night
describe natural selection:
Within all populations, there exists variation, which is a result of gene mutations, meiosis, and
thus fertilization. Organisms of the same species reproduce, and there is an overpopulation as a
result of a large number of offspring being born. Predators eat seeds and eggs, and disease and
starvation can kill the young. Therefore, populations remain stable. In the competition for
resources, the organisms with helpful mutations can survive the struggle for survival. The
organisms best adapted to survive to emerge on top and then reproduce, passing on their
favorable alleles to future generations. (
Variation
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.
Discontinuous variation
results in a limited number of phenotypes with no
intermediates; examples include ABO blood groups, seed shape in peas and seed
colour in peas
Discontinuous variation is usually caused by
genes only and continuous variation is
caused by both genes and the environment.
Mutation
genetic change
The mutation is the way in
which new alleles are formed?
Ionising radiation and some chemicals increase the
rate of mutation.
Gene mutation
random change in the base sequence of DNA.
sources of genetic
variation in populations
Mutation, meiosis, random mating and random fertilisation
An adaptive feature
the inherited feature that helps an organism to survive and
reproduce in its environment, i.e. increase fitness.
Adaptive features of hydrophytes:
1 submerged leaves are divided ;
2 providing large area for, photosynthesis
/ absorption ;
3 leaves have large surface area ;
4 to float ;
5 (floating leaves so) little xylem/ little
lignin ;
6 water provides support ;
7 little/ no roots /root hairs ;
8 roots for anchorage only / no need for
roots to absorb water or mineral ions ;
9 little/ no, cuticle ;
10 no need to conserve water ;
11 stomata only on upper surface ;
12 only upper surface exposed to air/ to
allow diffusion of gases ;
13 lots of air spaces (between cells) ;
14 for flotation/ buoyancy ;
15 floating leaves ;
16 to allow, diffusion/AW of (named)
gas(es) ;
17 aerial roots ;
18 to allow roots to receive oxygen ;
Adaptive features of xerophytes:
few stomata / low stomatal density ;
sunken stomata ;
stomata close during the day and
open at night ;
rolled leaves ;
thick epidermis / thick cuticle ;
few / no / small, leaves ;
hairs on leaves ;
low rates of transpiration ;
deep roots / AW ;
to absorb water from the water table /
AW
long and spread out below the
surface ;
to absorb water when it rains ;
root cells have low water potential ;
to absorb water by osmosis ;
from (very) salty soils / AW ;
roots branch many times ;
have many roots hairs ;
to give a large surface area (for
absorption of water) ;
Natural selection:
genetic variation within populations
production of many offspring
struggle for survival, there is competition for resources
a greater chance of reproduction by individuals that are better adapted to the
environment than others
these individuals pass on their alleles to the next generation
Selective breeding:
selection by humans of individuals with desirable features
crossing these individuals to produce the next generation
selection of offspring showing the desirable features.
Selective breeding by artificial selection is carried out over
many generations to
improve crop plants and domesticated animals
Adaptation
process, resulting from natural selection, by which populations
become more suited to their environment over many generations.
example of natural
selection
The development of strains of antibiotic-resistant bacteria
Difference between natural and artificial
selection:
features are adaptive
caused by environment
slower changes
increase in fitness
explained: ability to survive AND
reproduce (in natural environment)
maintain (genetic) variation/ less
(genetic) variation in selective breeding
random mating
Difference between artificial and
natural selection:
human choice (rather than
environmental pressures) /
AW ;
less, diversity / variation ;
faster change ;
AVP ; e.g. mating is not
random
