Genetics, populations, evolution and ecosystems

Question 1

What is meant by the term genotype?

Answer 1

Genetic constitution of an organism, all of the alleles that an organism carries on its chromosomes.

Question 2

What is meant by the term phenotype?

Answer 2

the set of observable characteristics of an individual resulting from the interaction of its genotype with the environment

Question 3

What are alleles and how do they arise?

Answer 3

Variations of a particular gene (same locus) → arise by mutation (changes in DNA base sequence)

Question 4

How many alleles of a gene can be found in diploid organisms?

Answer 4

● 2 as diploid organisms have 2 sets of chromosomes (chromosomes are found in homologous pairs)
○ But there may be many (more than 2) alleles of a single gene in a population

Question 5

What is a dominant allele?

Answer 5

Always expressed (shown in the phenotype)

Question 6

What is a ressecsive allele?

Answer 6

Only expressed when 2 copies present (homozygous recessive)
/ NOT expressed when dominant allele present (heterozygous)

Question 7

what are codominant alleles?

Answer 7

Both alleles expressed / contribute to phenotype (if inherited together)

Question 8

What is meant by the terms homozygous?

Answer 8

Alleles at a specific locus (on each homologous chromosome) are the same

Question 9

What is meant by the term hetrozygous?

Answer 9

Alleles at a specific locus (on each homologous chromosome) are different

Question 10

What do monohybrid crosses show?

Answer 10

inheritance of one phenotypic characteristic coded for by a single gene

Question 11

What do dihybrid crosses show?

Answer 11

inheritance of two phenotypic characteristics coded for by two different genes

Question 12

What is a sex-linked gene?

Answer 12

A gene with a locus on a sex-chromosome (normally X)

Question 13

Explain why males are more likely to express a recessive X-linked allele

Answer 13

This assumes males are XY and females are XX, as in humans. In some organisms, it is swapped. In these cases, females (XY) would be more likely to express a recessive X-linked allele
females (XY) would be more likely to express a recessive X-linked allele.
● Females (XX) have 2 alleles → only express recessive allele if homozygous recessive / can be carriers
● Males (XY) have 1 allele (inherited from mother) → recessive allele always expressed

Question 14

Explain how autosomal linkage affects inheritance of alleles

Answer 14

● Two genes located on same autosome (non-sex chromosome)
● So alleles on same chromosome inherited together
○ Stay together during independent segregation of homologous chromosomes during meiosis
● But crossing over between homologous chromosomes can create new combinations of alleles
○ If the genes are closer together on an autosome, they are less likely to be split by crossing over

Question 15

If an organism carries two different alleles it is said to be……

Answer 15

Heterozygous at that locus

Question 16

If an organism carries two copies of the same allele it said to be ……..

Answer 16

Homozygous at the locus

Question 17

What is mono hybrid inheritance?

Answer 17

Inheritance of a characteristic controlled by a single gene

Question 18

Inheritance of a characteristic controlled by a single gene

Answer 18

The likelihood of the different alleles of that gene being inherited by the offspring of certain parents

Question 19

What phenotypic ratio would you expect to see if you did a dihybrid cross with 2 heterozygous Parents?

Answer 19

9 : 3 : 3 : 1 dominant both : dominant first, recessive second : recessive first, dominant second : recessive both

Question 20

What does sex linked mean?

Answer 20

The alleles that code for them are located on a sex chromosome (normally X)

Question 21

What is an autosome?

Answer 21

any chromosome that is not a sex chromosome

Question 22

What are genes on the same autosome said to be? Why?

Answer 22

Linked - they’ll stay together during the independent segregation of chromosomes in meiosis 1 and their alleles will be passed on to their offspring together

Question 23

What can cause autosomal genes not to stay together during independent segregation ?

Answer 23

If crossing over splits them up first

Question 24

What is independent segregation?

Answer 24

The random division of homologous chromosomes into separate daughter cells during meiosis

Question 25

What is epistasis?

Answer 25

A phenomenon when the genotype of one gene can mask the effects of a separate gene

Question 26

What happens if the epistatic allele is recessive?

Answer 26

Two copies of it will mask the expression of the other gene

Question 27

Describe when a chi-squared (X2) test can be used

Answer 27

● When determining if observed results are significantly different from expected results (frequencies) ○ Eg. comparing the goodness of fit of observed phenotypic ratios with expected ratios ● Data is categorical (can be divided into groups eg. phenotypes)

Question 28

Suggest why in genetic crosses, the observed phenotypic ratios obtained in the offspring are often not the same as the expected ratios

Answer 28

● Fusion / fertilisation of gametes is random ● Autosomal linkage / epistasis / sex-linkage ● Small sample size → not representative of whole population ● Some genotypes may be lethal (cause death)

Question 29

What is a population?

Answer 29

● A group of organisms of the same species in a particular space at a particular time ● That can (potentially) interbreed (to produce fertile offspring)

Question 30

What is a gene pool?

Answer 30

All the alleles of all the genes in a population at any one time

Question 31

What is allele frequency?

Answer 31

Proportion of an allele of a gene in a gene pool (decimal or percentage)

Question 32

What does the Hardy-Weinberg principle state and what are the conditions under which the principle applies?

Answer 32

● Allele frequencies will not change from generation to generation, given: ○ Population is large ○ No immigration / emigration (to introduce / remove alleles) ○ No mutations (to create new alleles) ○ No selection for / against particular alleles ○ Mating is random

Question 33

What is the Hardy-Weinberg equation?

Answer 33

p^2 + 2pq + q^2 = 1 p + q = 1 ● p = frequency of one (usually dominant) allele of the gene ● q = frequency of the other (usually recessive) allele of the gene ● p^2 = frequency of homozygous (usually dominant) genotype ● 2pq = frequency of heterozygous genotype ● q^2 = frequency of homozygous (usually recessive) genotype

Question 34

Explain why individuals within a population of a species may show a wide range of variation in phenotype

Answer 34

● Genetic factors ○ Mutations = primary source of genetic variation ○ Crossing over between homologous chromosomes during meiosis ○ Independent segregation of homologous chromosomes during meiosis ○ Random fertilisation of gametes during sexual reproduction ● Environmental factors (depends on context - eg. food availability, light intensity)

Question 35

What is evolution?

Answer 35

● Change in allele frequency over time / many generations in a population ● Occurring through the process of natural selection

Question 36

Describe factors that may drive natural selection

Answer 36

● Predation, disease and competition for the means of survival ● These result in differential survival and reproduction, ie. natural selection

Question 37

Explain the principles of natural selection in the evolution of populations

Answer 37

1. Mutations- Random gene mutations can result in [named] new alleles of a gene 2. Advantage- Due to [named] selection pressure, the new allele might benefit its possessor [explain why] → organism has a selective advantage 3. Reproduction- Possessors are more likely to survive and have increased reproductive success 4. Inheritance -Advantageous allele is inherited by members of the next generation (offspring) 5. Allele frequency- Over many generations, [named] allele increases in frequency in the gene pool

Question 38

Explain the effects of stabilising selection

Answer 38

● Organisms with alleles coding for average /modal variations of a trait have a selective advantage (eg. babies with an average weight) ● So frequency of alleles coding for average variations of a trait increase and those coding for extreme variations of a trait decrease ● So range / standard deviation is reduced

Question 39

Explain the effects of directional selection

Answer 39

● Organisms with alleles coding for one extreme variation of a trait have a selective advantage (eg. bacteria with high resistance to an antibiotic) ● So frequency of alleles coding for this extreme variation of the trait increase and those coding for the other extreme variation of the trait decrease

Question 40

Explain the effects of disruptive selection

Answer 40

● Organisms with alleles coding for either extreme variation of a trait have a selective advantage ● So frequency of alleles coding for both extreme variations of the trait increase and those coding for the average variation of the trait decrease ● This can lead to speciation

Question 41

Describe speciation (how new species arise from existing species)

Answer 41

1. Reproductive separation of two populations (of the same species) 2. This can result in accumulation of differences in their gene pools 3. New species arise when these genetic differences lead to an inability of members of the populations to interbreed and produce fertile offspring

Question 42

Describe allopatric speciation

Answer 42

1. Population is split due to geographical isolation (eg. new river formed) 2. This leads to reproductive isolation, separating gene pools by preventing interbreeding / gene flow between populations 3. Random mutations cause genetic variation within each population 4. Different selection pressures / environments act on each population 5. So different advantageous alleles are selected for / passed on in each population 6. So allele frequencies within each gene pool change over many generations 7. Eventually different populations cannot interbreed to produce fertile offspring

Question 43

Describe sympatric speciation

Answer 43

1. Population is not geographically isolated 2. Mutations lead to reproductive isolation, separating gene pools by preventing interbreeding / gene flow within one population, eg. ○ Gamete incompatibility ○ Different breeding seasons (eg. different flowering times) ○ Different courtship behaviour preventing mating ○ Body shape / size changes preventing mating 3. Different selection pressures act on each population 4. So different advantageous alleles are selected for / passed on in each population 5. So allele frequencies within each gene pool change over many generations 6. Eventually different populations cannot interbreed to produce fertile offspring

Question 44

Explain genetic drift and its importance in small populations

Answer 44

● Genetic drift = a mechanism of evolution in which allele frequencies in a population change over generations due to chance (NOT natural selection) ● Some alleles are passed onto offspring more / less often by chance ○ Regardless of selection pressures and whether alleles give a selective advantage ● So strongest effects in small populations with no interbreeding with other populations (no gene flow), as gene pool is small and chance has a greater influence ○ Eg. when a population is sharply reduced in size (bottleneck effect) ○ Eg. when a small, new colony forms from a main population (founder effect) ● This can reduce genetic diversity - some alleles have much higher frequencies, others are lost

Question 45

What is a community?

Answer 45

All the populations of different species living in the same place (habitat) at the same time

Question 46

What is an ecosystem?

Answer 46

A community and the non-living (abiotic) components of its environment

Question 47

What is a niche?

Answer 47

● The specific role of a species within its habitat, eg. what it eats, where and when it feeds ● Governed by its adaptation to both abiotic (non-living) and biotic (living) conditions

Question 48

Explain the advantage of species occupying different niches

Answer 48

● Less competition for food / resources ● If two species tried to occupy the same niche, one would outcompete the other

Question 49

What is carrying capacity?

Answer 49

The maximum (stable) population size of a species that an ecosystem can support

Question 50

List the factors that influence carrying capacity

Answer 50

Abiotic factors Eg. light intensity, temperature, soil pH & mineral content, humidity Interactions between organisms; a. Interspecific competition - between organisms of different species b. Intraspecific competition - between organisms of the same species c. Predation (predators kill and eat other animals, called prey)

Question 51

Explain how abiotic factors may affect population size / carrying capacity

Answer 51

● If conditions favourable, organisms more likely to survive & reproduce → increasing carrying capacity ● Eg. increasing light intensity increases rate of photosynthesis, increasing nitrates increases protein production and increasing phosphates increases phospholipid production in plants ○ This increases carrying capacity of a variety of plant species ○ So increases the number and variety of habitats, niches and food sources for animals ○ So increasing carrying capacity of a variety of animal species

Question 52

Explain how interspecific competition may affect population size

Answer 52

● Reduces [named resource] available to both species, limiting their chances of survival & reproduction ○ So reduces population size of both species ● If one species is better adapted, it will outcompete the other ○ So population size of less well adapted species declines, potentially leading to extinction

Question 53

Explain how intraspecific competition may affect population size

Answer 53

1. As population size increases, resource availability per organism decreases, so competition increases ○ So chances of survival & reproduction decrease → population size decreases 2. As population size decreases, resource availability per organism increases, so competition decreases ○ So chances of survival & reproduction increase → population size increases

Question 54

Explain the changes which occur in populations of predators & prey

Answer 54

Populations fluctuate in cycles, the predator population peaking after the prey (lag time): 1. Prey population increases so predators have more food ○ So more predators survive & reproduce 2. Predator population increases so more prey killed & eaten ○ So less prey survive & reproduce 3. Prey population decreases so predators have less food ○ So less predators survive & reproduce 4. Predator population decreases so less prey killed & eaten ○ So more prey survive & reproduce (cycle repeats)

Question 55

Describe how the size of a population of slow-moving or non-motile organisms can be estimated

Answer 55

1. Divide area into a grid / squares eg. place 2 tape measures at right angles 2. Generate a pair of coordinates using a random number generator (eg. on a calculator) 3. Place a quadrat here and count number / frequency of [named species] 4. Repeat a large number of times (10 or more) and calculate a mean per quadrat 5. Population size = (total area of habitat / quadrat area) x mean per quadrat

Question 56

Describe how the mark-release-recapture method can be used to estimate the size of a population of motile organisms

Answer 56

● Capture sample of species, mark and release ● Ensure marking is not harmful / does not affect survival ● Allow time for organisms to randomly distribute before collecting second sample ● Population = (number in sample 1 x number in sample 2) / number marked in sample 2

Question 57

Explain how the mark release recapture equation can be derived

Answer 57

population size= (number in sample 1 x number in sample 2 / number marked in sample 2)

Question 58

What assumptions does the mark-release-recapture method make?

Answer 58

1. Sufficient time for marked individuals to mix / distribute evenly within the population 2. Marking not removed so marked organisms are recaptured OR marking is not toxic so doesn’t affect chances of survival OR marking not visible (to predators) so doesn't affect predation 3. Limited / no immigration / emigration 4. No / few births / deaths / breeding / change in population size (or birth & death rate are equal)

Question 59

Suggest why the mark-release-recapture method can produce unreliable results in very large areas

Answer 59

● Unlikely that organisms will distribute randomly / evenly ● Less chance of recapturing organisms (that were marked initially)

Question 60

Describe and explain how primary succession occurs

Answer 60

1. Colonisation by pioneer species (first to colonise) 2. Pioneer species (and other species at each stage in succession) change abiotic conditions ○ Eg. they die and decompose, forming soil which retains water (humus / organic matter) 3. So environment becomes less hostile / more suitable for other species with different adaptations AND less suitable for previous species, so better adapted species outcompete previous species 4. As succession goes on, biodiversity increases 5. Climax community reached - final stable community (no further succession)

Question 61

Describe features of a climax community

Answer 61

● Same species present / stable community over a long time ● Abiotic factors (fairly) constant over time ● Populations (fairly) stable (around carrying capacity)

Question 62

Explain how conservation of habitats involves management of succession

Answer 62

● Further succession can be prevented to stop a climax community forming ○ By removing or preventing growth of species associated with later stages eg. by allowing grazing ● This preserves an ecosystem at a certain point / in its current stage of succession (plagioclimax) ● So early species are not outcompeted by later species and habitats / niches are not lost

Question 63

Describe the conflict between human needs and conservation as well as the importance of managing this

Answer 63

● Human demand for natural resources (eg. timber) is leading to habitat destruction / biodiversity loss ● Conservation is needed to protect habitats / niches / species / biodiversity ● Management of this conflict maintains the sustainability of natural resources ○ Meeting current needs without compromising the ability of future generations to meet theirs