Variation is really important in a population. Variation is the small differences found between individuals, either of the same species or different species. Let's say you plant a rose garden using cuttings off one particular plant. This means all the roses in your new garden are genetically identical - there's a distinct lack of variation between them (although they may look slightly different if some get more nutrients/water than the others, but that's not going to cause genetic differences). So you plant them, they grow, and one night the temperature drops and you wake up in the morning to a huge frost. You go outside, to water your roses and they did not have any mechanism to help them survive the frost - not one of them - because they were all genetically similar, and they all died. If you had created your garden using cuttings off many different rose plants, they would have had small differences between them and it would be more likely that one or more of them had a characteristic to help them survive the change conditions. Overall, some of the roses may have died but some could have survived - meaning there are some individuals able to reproduce and keep the population alive. This is the idea of variation and why it is important in populations.
The more differences between individuals, the more variation. There are two 'types' of variation
continuous and discontinuous/fixed
This type of variation has a huge spread - individuals can be anywhere in between two extremes. For example, variation of height. Human height ranges from the shortest person in the world to the tallest person. We can be anywhere between those values.
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In contrast, this type of variation can be divided up into 'classes' where there is no intermediate between them. For example, variation of ear lobe attachment. Your ear lobes can either be 'attached' to the side of your head, or 'unattached'/'free'and dangle down. There is no in-between.
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Variation wasn't always around, it had to be created. There are four different things that contribute to the creation and increase of variation in a population. The following 4 things are all causes of variation (phenotypic differences) within a population.
Mutations
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Meiosis
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Fertilization
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The environment
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1. Mutations
The base sequence on a gene contains the code for a particular protein/allele. In order for new alleles to be created, a small (or large) section of DNA must undergo a change. This change results in a different protein being produced, creating a new phenotype which means more differences and thus variation is increased!
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Causes of mutations
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Effects of mutations
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Inheritable mutations
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Types of mutations
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All mutations come from a mistake made during the making of new DNA (all our cells contain DNA, so when we need to make new cells to replace old ones, or to make gametic cells, DNA replication must occur). Mutations occur naturally during the process of DNA replication. The most influential mutations occur during the DNA replication that precedes meiosis, where any mistakes here will be put into gametes and passed on! The rate at which mutations naturally occur can be sped up by mutagens.
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The rate of mutations (that occur during DNA rep) can be increased as a result of exposure to mutagens; things like chemicals (tobacco and in fatty foods), UV light (from the sun) and radiation from x-rays and radioactive materials. Mutagens make mutations more likely to occur.
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Silent/neutral
This type of mutation has no observable effect on the individual.
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Beneficial
May give organism survival advantage over other species members. For example, that organism may live on in conditions where the rest of their population die - such as when a common insecticide is used, there may be insects with a mutation that makes them resistant to it!
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Harmful
This type of mutation may affect the survival of an organism in it habitat. Either by harming the individual or making it more likely to die (e.g. having a colour that makes it easy for predators to see).
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Mutations can only be inherited if they occur in gametic cells!
Somatic cells
A somatic cell is any cell found in the body, excluding sperm and eggs. They are absolutely everywhere else. These cells are not passed on to offspring so any mutations originating in a somatic cell cannot be inherited by offspring.
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Gametic cells
A gametic cell is a cell that is used in reproduction. They are the cells that make eggs and sperm - ALL DNA inside these cells will be inherited, if that egg/sperm is fertilized.
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Gene mutations
These mutations are on a smaller scale compared to chromosomal mutations. Instead of 'blocks' of the chromosome being affected, individual bases within the gene gets changed. Bases can be substituted, inserted and deleted.
Chromosomal mutations
Chromosome mutations can alter the number of chromosomes in a cell, or the structure of any chromosomes. There are 4 types of chromosomal mutations - involving 'blocks' of the chromosome being altered.
2. Meiosis
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Meiosis overview
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Independent assortment
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Crossing over
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Segregation
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Diagram
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Initially, each chromosome will replicate (make a copy) itself and its copy will remain attached to the original chromosome by a centromere (they will be separated the second time the spindle forms, preceding the second cell division). The homologous pairs (with their copies attached) will line up randomly (independent assortment), may undergo crossing over and eventually move to opposite ends of the cell and segregated into two different cells.
Then, the chromosomes within these two cells (one of each in the pair) line up and the spindle attaches to the chromatids (the two parts attached by the centromere), pulling apart the chromatids and segregating them into two new cells. The result of this is four cells, each are haploid - meaning they have half of what the initial cell had - they have one of each chromosome.
Then, the chromosomes within these two cells (one of each in the pair) line up and the spindle attaches to the chromatids (the two parts attached by the centromere), pulling apart the chromatids and segregating them into two new cells. The result of this is four cells, each are haploid - meaning they have half of what the initial cell had - they have one of each chromosome.
Continuations of the diagrams below will be used in the explanations of independent assortment, crossing over and segregation.
This is a picture of a cell before independent assortment has happened - it contains three sets of homologous pairs, each of which have undergone DNA replication to produce an identical copy of each chromosome (identical copy stays attached by centromere).
Independent assortment is when homologous chromosomes line up during meiosis (at equator) they do so randomly (i.e. in no particular order). This means it is completely random which combination of alleles end up in a particular gamete. |
Below are all the possible assortments of the 6 chromosomes we have in our cell
After they line up, the homologous pairs are separated from each other and end up in two different cells (segregation). Each picture represents the two cells each assortment can make. The two cells will again divide to make four cells, each containing one single chromosome from each homologous pair. The numbers below each diagram match up to the diagrams above.
The below pictures, the 4 cells, are the gametes, for each possible assortment. Remember that each colour represents identical chromosomes - the result of DNA replication. So two of the four cells contain the same colour - the identical chromosome. Also, keep in mind that the assortment for meiosis is different each time it happens, so overall there will be gametes containing many of the possible allele combinations! If independent assortment didn't happen, each time meiosis occurred there would be the allele combinations created in each gamete every time. This means there isn't much variation.
Notice that each set of gametes is different to the others - this is all due to independent assortment!
Crossing over happens at the same time as independent assortment - when the chromosomes line up in their homologous pairs, after DNA replication (so there are two chromatids, attached by a centromere, forming a chromosome, which is paired up with its homologous pair). At the time they line up, chromosomes are able to swap sections of their DNA. This is random and doesn't happen every time meiosis happens. Crossing over is a major cause of variation, particular with linked genes.
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With crossing over, you can see that we get more different allele combinations in our 4 gametes - there is significantly more variation. This is just in one cycle of meiosis! Remember crossing over doesn't happen every time, and when it does it may affect only one homologous pair, or two, or 17 - the pairs may differ each time and this makes for so many more opportunities to mix up alleles!
During meiosis, pairs of alleles (i.e. Bb) are segregated (separated) when the homologous chromosomes are divided up into two cells during the first cell division of meiosis. In the second cell division of meiosis, the chromatids are also segregated and end up in different cells. The alleles that were once paired will never be paired again. The consequence of this is each gamete will only receive one allele for each pair. When two gametes fuse (sperm+egg) alleles find new pairs. If allele pairs were never separated to begin with, there would not be this opportunity for variation to occur.
3. Fertilization
During the process of sexual reproduction, once the gametes are made, two gametes (male and female) come together to form a zygote. This is fertilization. What has actually happened is the genetic information from two individuals has joined up to form a new, completely unique individual. The more completely unique individuals in a population, the more variation that population has. The combination of different varieties/mixtures of alleles from two parental genotypes is what causes variation.
Fertilization, in conjunction with the processes involved in making gametes (independent assortment, crossing over, segregation), creates literally trillions of possible genotypes within the human species. Crazy! |
4. The environment
Environmental factors can cause organisms to look different without changing the DNA of the organism. For example
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Environmental factors can also induce change to organism's DNA - mutagens. Any environmental factors that make mutations more likely to occur are called mutagens. Mutagens can cause new alleles to form, increasing variation.
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