Before we get into the nitty gritty part of genetic change, you need to know some basic terms
Genetic biodiversity
A way of describing the number of differences in genetic combinations of a gene pool. The greater the genetic biodiversity of a population, the more variation it has. The less genetic biodiversity, the less variation it has, and the less likely the population will withstand an environmental change. |
Number of A's: 18
Number of a's: 16 Number of alleles in population: 34 Allele frequency for A: 18/34 = 0.53 Can also be written as 53% (it is found in the genotype 53% of the time) Allele frequency for a: 16/34 = 0.47 Can also be written as 47% (it is found in the genotype 47% of the time) |
Small changes to a population over time may result in
evolution
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The theory of evolution states that evolution is the change in characteristics (inheritable traits) of living things over long periods of time (through generations). There are two main ideas that are part of this theory (explained nicely in the video above on the left).
Idea 1. Organisms change over time
Diverse groups of animals are decedents of one (or few) common ancestor(s), who may be extinct. What was originally a finch 3000 years ago may not look like that any more.
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Idea 2. Evolution occurs due to natural selection
When traits are beneficial, the organism who has that trait is more likely to be successful. This means they will create more offspring compared to others. This results in a change of traits over time as the more beneficial ones will become the most common.
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8:08 is where Darwin's finches are explained!
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Evolution doesn't just happen. Things must drive it!
There are 4 main drivers/mechanisms for evolution
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1. Natural selection
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2. Mutations
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3. Migration (gene flow)
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4. Genetic drift
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The process by which phenotypes that prove beneficial tend to be preserved within the population (those alleles increase in frequency). Unfavourable phenotypes tend to be lost from the population (those alleles decrease in frequency and may disappear from the population altogether).
The process of NS occurs in the way described below:
1. Individuals in a population display genetic variation. There are differences in their DNA, making their phenotypes different. 2. Individuals with characteristics that are less favourable (in particular, detrimental) are killed due to selection pressures. 3. Individuals that are better adapted to the conditions survive, breed, and produce offspring. 4. Survivors continue to breed and pass their alleles on to the next generation. 5. Over time, 'successful' alleles become more common (their frequency increases) in a population and 'unsuccessful' alleles are removed. |
You need to be able to describe how natural selection acts on the genetic variation in a population - what it does to allele frequencies and how this happens. You must also be able to describe the consequences of reduced genetic variation in a population.
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Favourable phenotypes will always be more common in a population than unfavourable phenotypes, so the distribution of phenotypes is classed as 'normally distributed'. On a graph (x axis = continuum of phenotypes relating to one gene, y axis = number of individuals), this looks like a regular bell curve - the favourable phenotypes are the most common (in the middle) and the extremes on either side are not as favourable so are less common.
However, it is rare for an environment to be constantly stable - if the environment is dynamic then the population will be too. There are always selection pressures working on a population, creating different frequencies of particular phenotypes depending on the type of selection pressure. The three types of selection pressures are described below.
Stabilizing selection
Natural selection may favour the existing median (where phenotype is already most common).
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Directional selection
Natural selection may shift the phenotypic median in one particular direction.
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Disruptive selection
Natural selection may work against the phenotypic median.
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There's one more type of selection...
Artificial selection
This is when humans intervene in the breeding of organisms and select particular individuals from a population based on a particular trait that they have. This trait could relate to the colour, shape or size of something the animal has, or even the whole animal. Selective breeding is a common example of artificial selection. Many things we encounter today (types of food, breeds of dog/horse/cats/pretty much any domesticated animal) would not be found in the wild in its current form. We have literally bred the crap out of these organisms, choosing only the ones with 'desirable' traits and using them to fill out the gene pool. Another example of articficial selection is the impact humans have on the individuals on the population and thus their capacity to reproduce. For example, the tusk size of African elephants is way smaller than they used to be - around 150 years ago. This is thought to be a direct consequence of poaching... Poachers would target the elephants with the biggest tusks and kill them in order to get the ivory. This stopped those elephants from breeding and passing on those alleles. Only the elephants with smaller tusks were able to successfully reproduce and thus the allele with the highest frequency in the gene pool is the small tusk allele... over time African elephants have developed smaller tusks as a result of artificial selection (human poaching is not natural).
The process by which DNA undergoes a permanent change in its base sequence. The effect of this change can be unnoticeable, neutral (neither helpful or harmful), harmful or beneficial.
Keep in mind that mutations aren't what conveniently happen to an organism when they 'need' a new trait (like if the environment underwent a change), mutations happen independently of the organism's needs! Its completely random whether or not, or when a mutation occurs. The effect of the mutation (i.e. if its harmful or beneficial) doesn't dictate whether or not the mutation will occur - so there's no way that due to the mutation being beneficial, that it will occur more often or something like that... mutations don't work this way. They are RANDOM! Mutations don't enter a gene pool unless individual carrying the mutation has offspring and the mutation is passed on. |
The process by which individuals move into a population (immigration) or out of a population (emigration) - taking their alleles with them.
Let's say you own 2 houses and 20 rabbits. One of your houses is in Auckland, the other in Wellington. You keep 10 rabbits at each house. This means you have two populations of rabbits, one in Auckland, one in Wellington, 10 rabbits in each population. Every population has their own gene pool, so you've got 2 gene pools, one in Auckland, one in Wellington. Those gene pools are going to be different from each other, because your two rabbit populations are not identical. Now, let's say: |
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The implications go further...
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The process by which chance events change the allele frequency within a population, rather than a selection pressure.
Genetic drift is when the allele frequency in a population can change as the result of chance, rather than a selection pressure acting on the population. The smaller the population, the bigger the impact on allele frequency. The larger the population, the more stable it (typically) is, and the less dramatic the impact on the population. Things such as the introduction of pests (i.e. possums to NZ) can remove individuals from a population, thus removing alleles from the gene pool and reducing their frequency. Also events such as a couple of birds from one population flying across to a nearby island and establishing a new population over there. The removal of their alleles from the gene pool isn't a result of a selection pressure acting on the population, but the event of the moving house!
2 birds moving from one established population to a new environment where there is no established population is genetic drift. This is not the same as 2 birds moving from one established population to a new environment where there is already a population of the same species found. This is gene flow. Emigrating and then immigrating to a new population is gene flow. Genes are flowing between two established populations. Genetic drift is where genes drift from one population and are used to start up a new population. Genetic drift decreases the variation within a population. However, if you compare the old population to the new population, there is greater variation between them. Over time, this can lead to speciation (where the differences between the two populations become so great that they become separate species and can no longer breed together).
Genetic drift is more likely to occur following a 'founder effect' - this is where a new population is established from a couple of 'founders' who moved to the new location - and following a genetic 'bottleneck' - where there is a sharp decline in population size due to an environmental effect.
2 birds moving from one established population to a new environment where there is no established population is genetic drift. This is not the same as 2 birds moving from one established population to a new environment where there is already a population of the same species found. This is gene flow. Emigrating and then immigrating to a new population is gene flow. Genes are flowing between two established populations. Genetic drift is where genes drift from one population and are used to start up a new population. Genetic drift decreases the variation within a population. However, if you compare the old population to the new population, there is greater variation between them. Over time, this can lead to speciation (where the differences between the two populations become so great that they become separate species and can no longer breed together).
Genetic drift is more likely to occur following a 'founder effect' - this is where a new population is established from a couple of 'founders' who moved to the new location - and following a genetic 'bottleneck' - where there is a sharp decline in population size due to an environmental effect.
The founder effect
This occurs when a small number of individuals leave a population and find (founder) a new location to establish a population. The founder effect can also occur as a result of geographical isolating mechanisms separating a population, such as the formation of a river. The one population is now divided into two smaller populations which are geographically isolated from each other (i.e. they can't get together to make babies). The variation of alleles in the new population is much smaller than the original. The founder effect reduces genetic variation!
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The bottleneck effect
A genetic bottleneck is when the size of a population is depleted due to an external factor. This can occur in many different ways, such as due to environmental effects like bush fires, floods, volcanic eruption, drought, hurricane, etc. It can occur as a result of humans, such as hunting, habitat destruction, introduction of new species to an area, etc. The population then has to basically start from scratch, from a few remaining individuals. The variation of alleles in that resulting population is much smaller than the original. Bottleneck events reduce genetic variation!
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