Section 7 - Genetics, populations, evolution and ecosystems: 18 Populations and evolution

Question 1

What is a Population

Answer 1

A group of organisms of the same species that occupy a particular space at a particular time (Can potentially interbreed)

Question 2

What is a ‘Gene Pool’

Answer 2

All of the alleles of all the genes of all the individuals in a population at a given time
- Alleles are the same in all cells of an individual, so only one set of alleles is counted per gene, per interval, in the gene pool

Question 3

What is Allele frequency

Answer 3

The relative frequency of an allele within a population

Question 4

What is the Hardy-Weinberg Principle

Answer 4

Principle stating that the allele and genotype frequencies within a population remain constant across generations (In the absence of mutations and selection pressures)
- ∴ Allele frequencies can be calculated within a population

Question 5

What is the Hardy-Weinberg Equation used to determine the allele frequency of a single gene (2 alleles)

Answer 5

P+q = 1

P: Probability of an individual having the dominant allele for a particular gene
q: Probability of an individual having the recessive allele for a particular gene

• Can be used to determine the allele frequency for a particular gene, within a population

Question 6

What is the Hardy-Weinberg Equation used to determine the allele frequency of a particular gene based on the genotypes of individuals

Answer 6

P^2 + 2Pq + q^2 = 1

P^2: Probability of an individual being Homozygous dominant
2Pq: Probability of an individual being Heterozygous
q^2: Probability of an individual being Homozygous recessive

• Can be used to determine the allele frequency for a particular gene, based on genotypes and observable phenotypes
  
  • P^2 + 2Pq: Probability of an individual expressing dominant phenotype
  • q^2: Probability of an individual expressing recessive phenotype
• Derived by considering the the principle for allele probabilities, when two alleles are present in the genotype
  
  • (P+q)^2 = P^2 + 2Pq + q^2 = 1

Question 7

What assumptions are made for the Hardy-Weinberg principle to be true

Answer 7

• No mutations arise
• Population is isolated, so there is no flow of alleles in/out
• No selection, so all alleles are equally likely to be inherited
• Population is large
• Mating in the population is random

Although most of these conditions are never fully met in naturally populations, the Hardy-Weinberg is still used for determining allele frequencies

Question 8

How do genetic factors cause variations in phenotype

Answer 8

• Mutations
  
  • Sudden changes to gene and chromosome that are potentially inherited by future generations
  • eg. Deletion, substitution, etc
  • Mutations are the main source of variation
• Meiosis
  
  • Nuclear division resulting in new combinations of alleles that pass into gametes
  • Independent assortment of chromosomes results in genetically unique gametes
• Random fertilisation
  
  • Which gamete fuses with which at fertilisation in sexual reproduction is random, so increases variation in the off-spring
  • New allele combinations cause the off-spring to be different to their parents, resulting in variation between generations

Question 9

How do environmental factors cause variations in phenotype

Answer 9

• Environmental factors affect the way genes are expressed
• The genes set the limits for a particular trait, but the environment influences where within these limits the phenotype will be
• eg. Plant may have genotype to grow tall, but poor environmental conditions such as low soil nitrates, will limit plant growth

Question 10

What is Natural selection

Answer 10

Process by which individual organisms more suited and better adapt for environmental conditions survive and pass their advantageous genes on to the next generation.
(Leads to Evolution)

Question 11

What is a selection pressure and how does it lead to Natural selection

Answer 11

Environmental factor that limits the population size of a population
- eg. Predation, Disease, competition, etc.
- Effects allele frequency, as individuals with certain alleles will have advantageous phenotypes under the selection pressure, so are more likely to survive and breed (Natural selection)

Question 12

What conditions must be met by a population to allow natural selection to occur

Answer 12

• Organisms produce more off-spring than can be supported by the available resources/environment (leads to competition)
• Genetic variation within the population
• Variety of different phenotypes within the population

Question 13

Why is the ‘Over-production of off-spring’ important for natural selection to occur

Answer 13

• Over production of off-spring leads to there being too many individuals for the available resources
• Results in intraspecific competition between individuals in a population
• ∴ The individuals that survive are the ones more suited to the environment
• ∴ Advantageous alleles are passed on to the next generation (natural selection)

Question 14

Why is variation within a population important for natural selection to occur

Answer 14

A wide range of different alleles means that when environmental conditions change (selection pressure), it is more likely that at least some individuals will have advantageous traits
- ∴ Less genetic diversity makes the population more vulnerable to extreme selection pressures (eg. diseases)
- Variations in phenotype allows for intraspecific competition to lead to the survival of advantageous individuals (natural selection)
- Variation allows for a ‘continuum’ of traits, giving a normal distribution to allow natural selection to occur

Question 15

What are the 3 types of selection that can occur as a result of a selection pressure

Answer 15

• Stabilising selection
• Directional selection
• Disruptive selection

Question 16

What is stabilising selection

Answer 16

Selection that preserves the mean phenotype in a population by favouring ‘average’ individuals and selecting against extremes

Question 17

What are some examples of stabilising selection

Answer 17

• Mammals with medium length fur survive in an environment with a constant temperature (Long/short hair selected against)
• Babies with a body mass greater or less than average have a higher risk of dying soon after birth, ∴ average weight is advantageous and extremes are selected against

Question 18

What is Directional selection

Answer 18

Selection favouring individuals with phenotypes in one direction from the mean (selection of one extreme)

Question 19

What are some examples of directional selection

Answer 19

• Mammals with extremely long fur, in regions with a decreasing temperature, will be at an advantage, so will survive and pass on their genes to the next generation
• Peppered moths: Tree bark was darker due to soot in the industrial revolution, so darker coloured moths were advantageous, ∴ survived and bred

Question 20

What is Disruptive selection

Answer 20

Selection favouring individuals with both extreme phenotypes (selection against the mean)

Question 21

What are some examples of disruptive selection

Answer 21

• In regions where the temperature varies throughout the year (summer/winter), mammals with both long and short hair will be advantageous at some point, while the mean is selected against
• Breeding fish: Small fish can ‘sneak’ up to females and breed, and large males win mates through competition, so medium size males (average) are selected against

Question 22

How do selection pressures affect allele frequency within a population

Answer 22

Selection pressure affect the probability of certain alleles being passed on, so alter the allele frequency in the next generation
- Selection pressures mean that some individuals will be advantageous due to the genetic variation within the population
- These advantageous alleles will be more likely to be passed on (favourable in intraspecific competition) so will be more frequent in the next generation

Question 23

What is a species

Answer 23

Group of organisms with a common ancestry, that are capable of breeding to produce fertile offspring

Question 24

What is speciation

Answer 24

The formation of a new species from an existing one, due to isolation and separate processes of natural selection

Question 25

What 2 conditions must be met for speciation to occur

Answer 25

• 2 populations must become reproductively isolated ]
• The populations must experience different selection pressures

Question 26

What is Adaptive radiation

Answer 26

Process by which multiple species arise from a single ancestral species, due to the groups experiencing different environmental factors (leads to speciation)

Question 27

What is Genetic drift

Answer 27

Random changes in the gene pool of a population due to chance mutations

Question 28

What are the two types of speciation that can occur

Answer 28

• Allopatric speciation
• Sympatric speciation

Question 29

What is Allopatric speciation

Answer 29

Form of speciation that occurs when two populations are geographically isolated
- Physical barrier separates two groups of organisms
- eg. Ocean, river, mountain, etc.
- Selection pressure in each area cause different changes in allele frequency
- ∴ Natural selection/evolution occurs differently, so changes in allele frequency of the two groups are independent of each other
- Groups eventually become so different that they can no longer interbreed to produce fertile off-spring (now separate species)

Question 30

What is Sympatric speciation

Answer 30

Form of speciation that occurs when two populations are reproductively isolated (not physically separated)
- Random mutation leads to two groups (living in the same area) becoming reproductively isolated
- eg. Different courtship behaviour, anatomically incapable of breeding, etc.
- Changes in allele frequency of the two groups, due to mutations, are independent of each other
- ∴ Natural selection/evolution occurs differently
- ∴ Groups eventually become so different that they can no longer interbreed to produce fertile off-spring (now separate species)

Question 31

What are the possible isolation mechanisms for speciation to occur

Answer 31

• Allopatric speciation
  
  • Geographical: Populations isolated by physical barrier
  • Ecological: Populations inhabit different habitats in the same area, so rarely meet
• Sympatric speciation
  
  • Temporal: The breeding seasons of the populations don’t align, so they don’t interbreed
  • Behavioural: Random mutations may alter courtship behaviour of one group
  • Mechanical: Anatomical differences between the groups may make breeding physically impossible
  • Gametic: Gametes of the two groups may be unable to fuse due to genetic/biochemical incompatibility (eg. sperm destroyed by chemicals in female reproductive tract)

Question 32

What is Hybrid Sterility

Answer 32

When off-spring are formed from the fusion of gametes from two different species, they are often sterile and can’t produce viable gametes themselves

eg. Cross between a horse (2n = 64) and donkey (2n = 62) forms a mule with 63 chromosomes
∴ Chromosomes can’t line up in meiosis to form gametes, so a mule is sterile