Evolution happens very slowly - it takes many generations to become visible.
Small things must happen for large-scale evolution to occur.
Small things must happen for large-scale evolution to occur.
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What do changes in the allele frequency of a population look like?
These changes are the result of one or more of the following processes:
- some alleles becoming more common
- some alleles becoming less common
- new alleles being added into the gene pool
- some alleles being removed from the gene pool
These changes are the result of one or more of the following processes:
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Non-random mating
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Mutations
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Genetic drift
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Gene flow
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Natural selection
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Non random mating is when individuals select mates based on their phenotype. 'Desirable' traits are favoured - these can be anything. In humans, we could pretend that a desirable trait is the combination blonde hair and blue eyes. So if mostly blonde-haired, blue-eyed males were chosen to mate with, blonde hair and blue eyes would become more common. This doesn't mean the population is better off - perhaps some predators prefer blonde hair!
The fact that particular alleles (desirable ones) are becoming more common means the allele frequency is changing - this is small scale evolution!
The fact that particular alleles (desirable ones) are becoming more common means the allele frequency is changing - this is small scale evolution!
Mutations result from mistakes made during DNA replication. Some mistakes change the amino acid chain - causing a different protein to be made by the cells in the organism. When a new protein is made and the DNA is passed on to the next generation, we can say a new allele has been formed and has entered the gene pool.
Ways mutations alter allele frequencies:
Beneficial allele formed |
New allele neither beneficial or harmful |
Harmful allele formed |
If allele improves survival and/or reproductive success of individual, allele will likely become more common. |
New allele will likely remain in the population but will is not likely to become the most common as it offers no real advantage. If the environment was to change, the allele may prove beneficial (thus become more common) or harmful (thus become less common). |
If allele reduces the chance of survival or reduces the reproductive success of individual, allele will likely not become established in a population. |
Genetic drift is when a chance or random event drastically changes allele frequency of a population. It is more likely to have a significant effect in smaller populations.
Allele frequencies can change due to:
Allele frequencies can change due to:
- the bottleneck effect - when majority of the population is wiped out (not because of their phenotype). Surviving individuals will pass on alleles but the frequency of these alleles will be different to the original population.
- the founder effect - when a few individuals leave an existing population and start a new population elsewhere. New population will have a different allele frequency compared to the original population.
For speciation to occur,
GENE FLOW MUST STOP
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because populations must be able to accumulate differences in order to undergo speciation and not share genetic information with another population.
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Gene flow is when genes flow between populations (you couldn't say that in the exam, though, but it helps to understand what it means). Basically, alleles move between populations if an individual emigrates from (leaves) one population and immigrates into (joins) another one and then breeds with individuals of the newly joined population. If alleles can be shared between populations like this, then those populations do not become very different - which is essential for them to become different species (speciation). Migration will hinder the speciation process.
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Natural selection causes alleles to become more common (if they are 'selected for') or less common (if they are 'selected against'). When populations are separated (i.e. no gene flow) then different alleles are likely to become more common in each population, making them more different over time.
Not only do some of the above 5 things need to happen to each population, but lots of
time
is needed for these differences to become significant
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Before we can say 'evolution has occurred and a new species has formed' one more thing must happen - reproductive isolation. This means that an individual from one population is unable to successfully breed with an individual from a different population due to all the differences they have accumulated over time.
There are different ways that populations can become reproductively isolated.
There are different ways that populations can become reproductively isolated.
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Pre-zygotic reproductive isolating mechanisms
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Post-zygotic reproductive isolating mechanisms
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Before a zygote can form (before fertilization) - mechanism is an obstacle/barrier that prevents mating or successful fertilization
Geographic isolation
Species do not come into contact with one another as they occupy different geographic locations - therefore they cannot mate |
Behavioural isolation
Species perform different behaviours and do not respond to mating behaviour of other type - i.e. different courtship ritual, song, dance, call, colouration - therefore they do not mate |
Temporal isolation
General activity or mating behaviour (such as flowering or being 'in heat' etc.) occur at different times of the day/month/year - therefore they do not mate |
Ecological isolation
Populations occupy different habitats within the same geographic location but do not interact - therefore they do not mate |
Mechanical isolation
Reproductive structures are incompatible and do not allow the successful transfer of gametes - therefore they do not mate successfully (although they might try) |
Gametic isolation
Gametes are unable to form a zygote due to incompatibility issues - therefore mating is not successful |
After a zygote has formed (after fertilization has occurred)
Hybrid inviability
Zygote forms but embryo fails to develop properly due to genes from the two different parental species being incompatible. In some species the hybrid offspring is born but does not develop properly |
Hybrid sterility
Offspring are formed but are unable to have offspring themselves as they are sterile. This is because they have an uneven number of chromosomes (due to different parental species) and are unable to undergo meiosis to form gametes |
Hybrid breakdown
Hybrid offspring are fine but subsequent generations have defects that result in reduced genetic fitness, or they may be sterile. Over time a new generation is unable to form |