Differences/variation in phenotype can be inheritable or non-inheritable
In other words, traits/characteristics can be inherited (passed down via DNA) or acquired (picked up/learned).
Inheritable variation |
Non-inheritable variation |
Differences between us that can be passed down through generations via. gametes. |
Differences between us that cannot be passed down through generations via. gametes. |
Coded for in your DNA. |
Not coded for in your DNA |
You received them from your biological parents. |
Develop during life (not born with it). |
Often things you inherit (e.g. eye colour, hair colour, skin colour, blood type, hitchhiker's thumb, attached/unattached earlobes, widow's peak, etc.). |
Often things you learned (e.g. riding a bike, reading, writing) or things that happened to you (e.g. broken bone, lost a limb, dyed your hair). |
Inherited traits are coded for in the DNA, they can be passed on to the next generation. |
Acquired traits cannot be passed on to offspring as they are not coded for by the DNA. |
Inheritable traits can become more common in a population if they give the individual an advantage over others. They can also become less common if they cause the individual a disadvantage. This is called natural selection.
Natural selection
The process whereby the organisms that are better adapted to their environment tend to survive and produce more offspring. The traits/characteristics that allowed them to survive and reproduce will therefore be passed down throughout the generations as long as the trait remains beneficial (and is inheritable). The more variation (different alleles) a population has, the more likely the population will survive an environmental change.
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Unicorn example
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Scenario
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Environmental change 1
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Environmental change 2
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Environmental change 3
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Summary
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You have a population of unicorns. There is a high amount of genetic variation within their population. In particular, there are differences in horn length, tail length and leg length. These three traits are all inheritable.
Your population is living happily in their habitat - lots of lush grass to eat, a river near their paddock ...they have all they need.
Your population is living happily in their habitat - lots of lush grass to eat, a river near their paddock ...they have all they need.
However, there hasn't been much rainfall this year and as a consequence, the unicorns' habitat has experienced a drought. This means that the usually luscious grass is very short and dry. The unicorn have nothing else to eat except the short grass.
The unicorns with the long horns are unable to get to the short grass because their horns get in the way. However, there are some unicorns with short enough horns that allow them to eat enough grass to survive the drought, and reproduce. |
As time passes, the unicorns that survived the drought (short horns) continued to reproduce, as did their offspring, and their offspring. Short horns became quite common. And then the swarms came. Disease-carrying mosquitoes passed through the paddock where the unicorn population lives. They are big mosquitoes and if they bite the unicorns, can infect them with a disease which will kill them.
The unicorns with the longest tails are able to use them to swish the mosquitoes out of the way, meaning they won't get bitten. However, the unicorns with the shorter tails cannot swish the mosquitoes away effectively and as a consequence, ended up contracting the disease and dying. These unicorns were unable to reproduce and pass on their short tail alleles, but the unicorns with the longer tails could. |
As time passes, the unicorns that survived the swarms of mosquitoes (long tails) continued to reproduce, as did their offspring, and their offspring. Longer tails became quite common. Until the flood. Over 24 hours there was a huge amount of rainfall,, flooding the nearby river and causing it to burst its banks. It flooded the paddock, with the water level reaching over 1m high.
The unicorns with the longest legs were able to stand up in the flood. However, the unicorns with the shorter legs could not stand up in the flood and were forced to swim to stay afloat. Many could not tread water for very long and drowned. These unicorns were unable to reproduce and pass on their alleles, only the unicorns with longer legs could. |
Over time, the frequency of particular alleles in a population changed. Initially, there was a large number of alleles in the population, many different horn sizes, tail lengths and leg lengths. However due to changing conditions, the alleles for long horns, short tails and short legs became a lot less frequent. They were selected against by the environment. On the other hand, short horns, long tails and long legs became more frequent, as they were selected for by the environment.
If there was no variation, it is unlikely the population would have survived all the environmental changes.
If there was no variation, it is unlikely the population would have survived all the environmental changes.
It's important to understand that dominant alleles aren't always the most common in a population.
For example, Huntington's disease occurs when someone holds at least one allele for it (the person is heterozygous, or homozygous dominant) - you don't hear much about people with Huntington's because it's not very common.
More information about Huntington's can be found here.
For example, Huntington's disease occurs when someone holds at least one allele for it (the person is heterozygous, or homozygous dominant) - you don't hear much about people with Huntington's because it's not very common.
More information about Huntington's can be found here.
The number of dominant or recessive alleles present in a population completely depends on whether or not that allele is beneficial in that environment. Let's use guinea pigs as our example again. Hypothetically, in New Zealand, guinea pigs will benefit from being hairy as it makes it easier to stay warm in our climate. However, if there were hairy guinea pigs found in the top part of Australia, or somewhere in Africa, where the climate is a lot warmer, having no hair may be more beneficial as it allows them to keep cool. Hairless alleles are recessive, however if a population of hairy and hairless guinea pigs was found in the top of Queensland, it might be that the hairy ones die of heat exhaustion and the hairless ones survive, reproduce and pass on the alleles for hairlessness. This would make the hairless allele more common in that population.
How genetic variation can be increased:
Sexual reproduction
Sexual reproduction involves two parents who produce gametes via MEIOSIS which fuse together to make a unique offspring. Asexual reproduction requires one parent and no gametes, and is used to produce offspring that are genetically identical to itself.
Advantages of sexual reproduction:
Disadvantages of sexual reproduction:
The advantages must outweigh the disadvantages for the process of sexual reproduction to be so common in everyday life. How genetic variation can be decreased:
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Mutations
During meiosis, DNA is replicated (this is the first step). During this process, it is possible for mistakes to be made. A mistake will change the sequence of bases and can result in a different amino acid being used, thus a different protein may be produced instead of the desired one. This changes the phenotype! If this occurs during meiosis, the gamete(s) will carry the mutation and, if they are involved in fertilization (remember not every gamete gets fertilized), then the offspring may display a new trait! This means more differences in the population which increases variation!
Who says that the mutation is bad? This new protein could allow an animal to become more camouflaged in its environment (allowing it to survive, reproduce and pass on the mutation) or it could cause the animal to become more obvious in its environment (causing its predator to easily spot and kill it, no reproduction or passing down of mutation). This is just one example. Mutations increase variation of a population!
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Selective breeding
- Selecting the individuals you want to breed from, and leaving the ones you don't.
- By only allowing the organisms with the most desirable traits (traits the you/the breeder want to become more common) to breed, unwanted alleles can be 'weeded out' of a population, leaving only the alleles desired. As a result, you are left with pure-breeding pedigrees (always the same alleles passed on). Humans do this all the time with plants and animals.
Problems with selective breeding
- The number of alleles in the population gets steadily less and less. This is because only the desirable alleles are being passed on, and the undesirable ones are not - over time they will be left out of the population. This can lead to problems of inbreeding,as all the animals or plants that are left are closely related to each other.
- Less variation (a result from selective breeding) means a population is less likely to survive an environmental change.
- Also, once an allele has been lost from a population, it is gone forever. Lets say a disease from the old (more varied) population made its way into the new population, there may not be a trait present anymore to allow any individuals to survive and reproduce. These problems are even more exaggerated with cloning and genetic modification! The only way new alleles can be introduced (or re-introduced) into a population is if a foreign member of the same species enters the population, carrying new/different alleles and mates with a member of your population. Mutations also produce new alleles.