Flashcards in Mechanisms/evidence for evolution Deck (91)
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1
What are 5 causes for variation?
1. Random assortment
2. Crossing over
3. Non-dis-junction
4. Random fertilization
5. Mutations
2
What is random assortment?
- Chromosomes are sorted into daughter cells randomly during meiosis, so there are many possible combinations of chromosomes that can come from the mother and father
3
What is crossing over?
- Process where during meiosis, pieces of chromatids may be broken off and attached to a different chromatids
- This results in a changed sequence, or recombination of the alleles along the resulting chromosome
4
What is non-dis-junction?
- One or more members of a chromosome pair fail to separate during meiosis
- This results in gametes that have more or less than the correct number of chromosomes
- If such gametes are involved in fertilisation, the resulting embryo will have the incorrect number of chromosomes
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What is random fertilization?
Chance alone is responsible for which sperm meets which egg
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What are mutations?
- Sudden and permanent changes in the DNA of a chromosome and may result in totally new characteristics in an individual
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What is a species?
Organisms belonging to the same species who are capable of producing fertile offspring under natural conditions
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What is a population?
A group of organisms of the same species living together in a particular place at a particular time
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What is a gene pool?
- The sum of all the alleles in a given population
- Can change over time
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Allele frequencies
- Can increase or decrease
- Different populations have different allele frequencies
- EG. Scandinavians have a high allele frequency for blue eyes and blond hair
- EG. Chinese have a high allele frequency for straight dark hair
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What is evolution?
A gradual change in phenotype thought to be caused by a change in allele frequency
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What happens when there is a change in allele frequency?
Changes in allele frequency → phenotypic changes → the gene pool changes
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Changes to allele frequency can be brought around by:
1. Mutations
2. Natural selection
3. Random genetic drift
4. Migration
5. Barriers to gene flow
6. Genetic diseases
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What is a mutation?
- A sudden and permanent change brought about by a change in the sequence of bases in a strand of DNA
- Gene or chromosomal mutations
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What are somatic mutations?
- Body cells experience a mutation
- Body cells arising from the mutant cell inherits the mutation
- Subsequent offspring do not inherit the mutation
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What are germinal mutations?
- If a mutation occurs in a gamete, then any offspring resulting from this gamete will inherit the mutation
- This mutation can then be inherited by following generations
- This will change the allele frequency in the long term
- Mutations may or may not affect the survival chances of an offspring
- Mutations change allele frequencies
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Natural selection
- There is competition between individuals
- Selection pressures make some genetic traits more favorable for survival
- Those with the traits survive and reproduce
- Favorable traits are passed onto offspring
- The allele frequency of favorable traits increase
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Random genetic drift
- Only usually occurs in small populations
- By chance (not because it is advantageous) the allele frequency in a population changes
- Some random event (not associated with an increased chance of survival eg. An earthquake) change the allele frequency
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An example of random genetic drift
EG. The Dunkers in Germany
- Small religious group who only intermarry within the population
- Their allele frequencies for blood groupings, mid-digital hair, ear lobes and handedness are markedly different from the general population
- These features have no survival advantage
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Islander group polulations
- Have high IA allele frequency
- No IB alleles
- Mainlanders are the reverse
- Blood groupings do not provide a survival advantage
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The founder effect
- A sub-group of random genetic drift
- A small group moves away from the original population to begin a new population
- The allele frequency of the emigrating group just happens to be different from the frequencies of the original population
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Achromatopsia
- Inherited total colour blindness
- An example of random genetic drift
- After a typhoon, only 20 people survived on a Micronesian Island
- One of these was heterozygous for Achromatopsia
- The current population now has a high frequency for this allele
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Migration
- A gene flow from one population to another
- As individuals join a population, they change the allele frequencies
- Large migrations have a considerable impact on allele frequencies
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Barriers to gene flow
- Can stop the interbreeding between populations
- Isolated populations may be subject to different environments with different selection pressures
- Results in different gene pools
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Types of barriers to gene flow
Geographical barriers → ocean, river, canyon
Socio-cultural barriers → government, religion, race, income level
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Genetic diseases
- Can changes the allele frequencies in a population
- It is expected that the frequency of a fatal allele will decrease in a population overtime
- This is not always the case
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Tay-sachs disease
- A recessive autosomal trait
- Homozygotes lack an enzyme that results in the accumulation of a fatty substance in the nervous system
- Most frequently occurs in individuals of Jewish decent form Eastern Europe (1 in every 2500 births)
- Frequency worldwide occurs 1 in every 500 000 births
- Death usually occurs by the age of four or five
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Reasons for this change in allele frequency (Tay-sachs)
- Jewish populations have tended to be small and isolated, increasing the chances of genetic drift
- Those who are heterozygous for Tay-sachs, have increased resistance to tuberculosis (TB)
- If this is the case, heterozygotes have an advantage in situations where TB is prevalent
- Individuals with two normal alleles would be more susceptible to TB, and would possibly die, while individuals with two Tay-sachs alleles would die in early life
- Heterozygotes would have a survival advantage and would be more likely to reproduce and pass on their alleles to the next generation
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Sickle-cell anaemia
- Causes the flattening (sickling) of erythrocytes preventing them from carrying oxygen
- Fatal in homozygotes
- The allele frequency is unexpectedly high in some African populations
- It was discovered that heterozygotes have a resistance to malaria
- Homozygotes for sickle-cell anaemia would die from the disease
- Homozygotes for healthy RBCs die from malaria
- Heterozygotes have the greatest survival
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