Mechanisms of evolution

Question 1

Population

Answer 1

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

Question 2

Gene pool

Answer 2

Sum of all alleles in a given population, changes over time with allele frequencies increasing and decreasing

Question 3

Causes of variation (5)

Answer 3

Random assortment, crossing over, non-disjunction, random fertilisation, mutations

Question 4

Random assortment

Answer 4

Multiple combinations of chromosomes that can come from the mother and father as chromosomes are sorted randomly in daughter cells

Question 5

Crossing over

Answer 5

when homologous chromosomes pair up with each other and exchange different segments of their genetic material forming recombinant chromosomes

Question 6

Non-disjunction

Answer 6

when one or more chromosomes pairs fail to separate resulting in gametes with more or less chromosomes

Question 7

Random fertilisation

Answer 7

chance decides which sperm meets which egg

Question 8

Mutations

Answer 8

permanent change in the DNA of a chromosome that may result in a totally new characteristic in an individual, Random mutations case changes in allele frequency thus changing the gene pool, E.g- blue eyes, being able to drink milk

Question 9

Allele frequencies

Answer 9

Measured in percentages, 95% of the population don’t carry the cystic fibrosis allele meaning the frequency of the cystic fibrosis allele is 5%, Chinese have a high allele frequency for straight dark hair

Question 10

Evolution

Answer 10

Gradual change in phenotype thought to be caused by a change in allele frequency, Changes in allele frequency result in phenotypic changes
Gene pool changes

Question 11

Causes of changes to allele frequencies

Answer 11

Mutations, Natural selection, Random genetic drift, Migration, Barriers to gene flow, Genetic diseases

Question 12

Natural selection criteria (6)

Answer 12

Variation in the population, Competition between individuals, Selection pressures make some genetic traits more favourable for survival, Those with favourable traits survive, reproduce and pass it onto offspring, Those without the favourable trait die, Allele frequency of favourable trait increases

Question 13

Natural selection

Answer 13

Directional- works towards the advantageous trait, Occurs in large and small populations

Question 14

Random genetic drift

Answer 14

By chance, the allele frequency in a population changes (not because it is advantageous), Some random event (famine, natural disaster) that is not associated with an increased chance of survival changes the allele frequency, Non-directional, No selection pressures/not related to the environment, More likely in small populations, Chance occurrence

Question 15

Examples of random genetic drift

Answer 15

Dunkers in Germany- small religious group who only intermarry within the population, allele frequencies of blood groupings, mid-digital hair, ear loves and handedness are different from the general population, Some Islander group populations have a high allele for A blood group and no B blood group whilst mainlanders are reverse

Question 16

Bottleneck effect

Answer 16

results in genetic drift, When there is a disaster of some sort that reduces a population to a small handful that rarely represents the actual genetic makeup of the initial population

Question 17

Founder effect

Answer 17

Often leads to genetic drift, Small group moves away from the original population to begin a new population, Allele frequency of the emigrating group just happens to be different form the frequencies of the original population, Eg Amish populations- descended from a few dozen people who migrated from Germany in the 1700’s, over the last 40 years 61 babies from 21 families were born with microcephaly (small cranium), all families descended from an Amish couple from nine generations before who had the genes for microcephaly

Question 18

Migration

Answer 18

Genre flow from one population to another, As individuals join a population, they change allele frequencies, Large migrations have considerable impacts on allele frequencies

Question 19

Barriers to gene flow

Answer 19

Stop interbreeding between populations, Isolated populations may be subjected to different environments with different pressures resulting in different gene pools

Question 20

Geographical barriers

Answer 20

Ocean, mountain range, dessert

Question 21

Sociocultural barriers

Answer 21

Race, religion, money, status

Question 22

Genetic diseases

Answer 22

Can change allele frequencies in a population, Generally, it is expected that the frequency of a fatal allele will decrease in a population over time but this is not always the case (shown in Tay-Sachs disease and sickle-cell anaemia)

Question 23

Tay-sachs disease

Answer 23

Recessive autosomal (non-sex chromosome) trait, Homozygotes recessive lack and enzyme and accumulate lipids in the nervous system, Causes fatty substances to build-up in the nervous system meaning people usually die at about 5 years old, Results in mental and physical disabilities such as blindness, deafness, inability to swallow, atrophied muscles, Some Jewish populations (religion barrier) form Eastern Europe have high frequencies for this allele, Being heterozygous for the trait gives you resistance to Tuberculosis (caused by bacteria) meaning in crowded ghettos, heterozygotes had a higher change of survival against Tuberculosis meaning they survive and reproduce, Frequency of the allele increased because of environmental pressures

Question 24

Sickle-cell anaemia

Answer 24

Recessive trait, Causes flattening of erythrocytes (red blood cells) due to a mutation in a gene, preventing them from carrying oxygen, Fatal in homozygotes recessive, Allele frequency in unexpectedly high in some Black African populations, Symptoms- fatigue, shortness of breath, anaemia, swelling and inflammation of hands or feet, lung and heat injury/leg ulcers, Heterozygotes have resistance to malaria meaning they have greatest survival, Homozygotes for sickle-cell anaemia die from the disease, Homozygotes for health erythrocytes die from malaria

Question 25

Speciation steps (5)

Answer 25

Variation- wide variety of genes present, Isolation- isolation of gene pool/barriers to gene flow, Struggle- Individuals that possess the suitable genes are more likely to survive and produce offspring meaning more offspring are produced that can survive, Selection- suitable genes are passed onto offspring whilst individuals without the suitable gene die off, Speciation- no longer to reproduce with others/new species, evolution brought about by an increase in the frequency of advantageous alleles and a decrease in the frequency of disadvantageous alleles