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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

5

What is random fertilization?

Chance alone is responsible for which sperm meets which egg

6

What are mutations?

- Sudden and permanent changes in the DNA of a chromosome and may result in totally new characteristics in an individual

7

What is a species?

Organisms belonging to the same species who are capable of producing fertile offspring under natural conditions

8

What is a population?

A group of organisms of the same species living together in a particular place at a particular time

9

What is a gene pool?

- The sum of all the alleles in a given population
- Can change over time

10

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

11

What is evolution?

A gradual change in phenotype thought to be caused by a change in allele frequency

12

What happens when there is a change in allele frequency?

Changes in allele frequency → phenotypic changes → the gene pool changes

13

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

14

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

15

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

16

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

17

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

18

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

19

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

20

Islander group polulations

- Have high IA allele frequency
- No IB alleles
- Mainlanders are the reverse
- Blood groupings do not provide a survival advantage

21

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

22

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

23

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

24

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

25

Types of barriers to gene flow

Geographical barriers → ocean, river, canyon
Socio-cultural barriers → government, religion, race, income level

26

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

27

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

28

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

29

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

30

Thalassemia

- A recessive disease in which anaemia results from defects in the formation of haemoglobin
- A reduction in the amount and shape of red blood cells
- Homozygotes recessive have two defective genes, which can be fatal
- Patients require regular blood transfusions and special drugs to remove excess iron
- Homozygotes dominant do not have the disease