Flashcards in Mechanisms/evidence for evolution Deck (91)
What are 5 causes for variation?
1. Random assortment
2. Crossing over
4. Random fertilization
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
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
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
What is random fertilization?
Chance alone is responsible for which sperm meets which egg
What are mutations?
- Sudden and permanent changes in the DNA of a chromosome and may result in totally new characteristics in an individual
What is a species?
Organisms belonging to the same species who are capable of producing fertile offspring under natural conditions
What is a population?
A group of organisms of the same species living together in a particular place at a particular time
What is a gene pool?
- The sum of all the alleles in a given population
- Can change over time
- 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
What is evolution?
A gradual change in phenotype thought to be caused by a change in allele frequency
What happens when there is a change in allele frequency?
Changes in allele frequency → phenotypic changes → the gene pool changes
Changes to allele frequency can be brought around by:
2. Natural selection
3. Random genetic drift
5. Barriers to gene flow
6. Genetic diseases
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
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
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
- 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
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
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
Islander group polulations
- Have high IA allele frequency
- No IB alleles
- Mainlanders are the reverse
- Blood groupings do not provide a survival advantage
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
- 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
- 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
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
Types of barriers to gene flow
Geographical barriers → ocean, river, canyon
Socio-cultural barriers → government, religion, race, income level
- 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
- 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
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
- 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