Chapter 18- Populations and Evolution

Question 1

What is a population?

Answer 1

• Group of organisms of the same species that occupies a particular space at a particular time and can potentially interbreed.
• There is variation in the phenotypes of organisms in a population, due to genetic and environmental factors.

Question 2

What is a species?

Answer 2

• Group of similar organisms that can reproduce to give fertile offspring.
• Exist as one or more populations.

Question 3

What is a gene pool?

Answer 3

All the alleles of all the genes of all individuals in a particular population at a given time.

Question 4

What is allele frequency?

Answer 4

The number of times an allele occurs within the gene pool- usually given as a percentage or decimal of the total population.

Question 5

How does allele frequency vary?

Answer 5

• Allele frequency- affected by selection- due to environmental factors.
• Unless an allele leads to a phenotype with an advantage or disadvantage compared with other phenotypes- allele frequency in the population will stay the same from one generation to the next.

Question 6

How are recessive alleles maintained in a population.

Answer 6

Heterozygous individuals act as a store of recessive alleles in the population, despite not expressing the allele in their phenotype.

Question 7

Using the example of cystic fibrosis, explain how allele frequencies work.

Probably not going to come up.

Answer 7

• Dominant allele- F- normal mucus production.
• Recessive allele- f- production of thicker mucus.
• Individual humans have two alleles in every cell- one in each of the pair of homologous chromosomes the gene is in.
• Alleles- the same in every person- only count one pair of alleles per gene per individual when considering a gene pool.
• 10,000 people in a population- twice as many- 20,000 alleles in the gene pool of the gene.
• Three different combinations- homozygous dominant- FF, homozygous recessive- ff, heterozygous- Ff.
• Allele frequencies- need to realise heterozygous combination could be written as Ff or fF- conventional to put the dominant allele first.
• Population of 10,000 people- if all 10,000 had genotype FF:
  o Probability of anyone being FF would be 1.0 and probability of anyone being ff would be 0..
  o Frequency of the F allele- 100%, frequency of the f allele- 0%.
• Population was all heterozygous:
  o Probability of anyone being Ff would be 1.0.
  o Frequency of the F allele 50%.
  o Frequency of the f allele 50%.

Question 8

What does working out the allele frequency of populations with mixed genotypes require?

Answer 8

The Hardy Weinberg principle.

Question 9

What is the Hardy-Weinberg principle?

Answer 9

• Hardy-Weinberg principle- mathematical model to predict allele frequencies of a particular gene in a population, alongside genotype and phenotype frequencies.
• Predicts that allele frequencies will not change from generation to generation. Assumption that the proportion of dominant and recessive alleles of any gene in a population remains the same from one generation to the next.

Question 10

When does the Hardy-Weinberg principle apply?

Answer 10

• No mutations occur.
• The population is isolated- no flow of alleles in/out of the population- no immigration, or emigration.
• No natural selection- alleles are equally likely to be passed to the next generation.
• Population is large.
• Mating within the population is random- all possible genotypes can breed with each other.
• Unlikely the conditions are met but the Hardy-Weinberg principle still useful in studying gene frequencies.

Question 11

How can the Hardy-Weinberg equation be used?

Answer 11

• You may be asked about collecting data about the frequency of observable phenotypes within a single population.
• You could be asked to calculate allele, genotype and phenotype frequencies in a population from appropriate data using the Hardy–Weinberg equation.
• Use the equations to determine the probability of any allele in a population.
• Hardy-Weinberg equations can also be used to test whether the Hardy-Weinberg principle applies to alleles in a population- to test whether selection or other factors are influencing the allele frequencies.
• If frequencies change between generations in a large population- something is influencing the population.

Question 12

When are the Hardy-Weinberg equations in the spec used?

Answer 12

When a gene has two alleles.

Question 13

What are the values used in the Hardy-Weinberg equation?

Answer 13

• p and q can be randomly assigned to alleles, but it is usually assigned as follows:
• p= frequency of dominant allele A.
• q= frequency of recessive allele a.

Question 14

What is the allele frequency Hardy-Weinberg equation and how is it derived?

Answer 14

• There are only two alleles.
• The total frequency of all the possible alleles for a characteristic in a certain population is 1.0, so the combined frequencies of the two alleles must be 1.
• p+q= 1.

Question 15

What is the genotype frequency equation for the Hardy-Weinberg equation and how is it derived?

Answer 15

• Total frequency of all possible genotypes for one characteristic in a certain population is 1.0.
• There are only 4 possible genotypes for 2 alleles- the frequencies of individual genotypes must add to 1.0.
• AA+ Aa+ aA+ aa= 1
• p²+2pq+q²= 1
• p²= homozygous dominant genotype
• 2pq= heterozygous genotype (carriers).
• q² = homozygous recessive genotype.
• Remember to label which one is which to ensure marks.

Question 16

How do you work out genotype/ phenotype frequencies using the Hardy-Weinberg principle.

Answer 16

• Genotype- alleles an organism has.
• Phenotype- expression of the genotype in the environment.
• The genotype frequencies can be used to work out phenotype frequencies if you know how the genotype relates to the phenotype.

Question 17

Other than monohybrid crosses, what other situations can the Hardy-Weinberg equations work in.

Answer 17

Work if two alleles are codominant or if you don’t know which allele is recessive and which is dominant- in these cases p represents one allele and q represents another- random allocation but need to be consistent with the use of each letter.

Question 18

Look at notes for examples of using the Hardy-Weinberg equations in calculations.

Answer 18

Answer on revision card.

Question 19

How do you use the Hardy-Weinberg principle to show if external factors are affecting the allele frequency.

Answer 19

• Hardy- Weinberg principle- predicts frequencies of alleles in a population don’t change form one generation to the next as long as the population is larger, there is no immigration, emigration, mutations or natural selection and mating is totally random.
• Hardy- Weinberg equations- can show allele frequency changed from one generation to the next- principle doesn’t apply to the population so a factor must be affecting the allele frequency.

Question 20

What is variation?

Answer 20

• Variation- differences that exist between individuals.
• Intraspecific variation- variation within a species.
• Individuals within a population of a species may show a wide range of variation in phenotype- need to be able to explain why.

Question 21

What is variation caused by?

Answer 21

• Variation- mostly caused by a combination of genetic and environmental factors affecting the phenotypes of organisms in a population.
• Two forces affect genetic variation in populations: genetic drift and natural selection.

Question 22

What causes genetic variation?

Answer 22

• Individuals of the same species have the same genes but different alleles- which cause genetic variation.
• Differences in alleles occur in living individuals but change in each generation.
• Primary source of genetic variation- mutation- changes in the DNA base sequence lead to the formation of new alleles.
• Further genetic variation is caused by meiosis and random fertilisation of gametes during sexual reproduction.

Question 23

Describe the sources of genetic variation in detail.

Answer 23

• Mutation- changes to genes (through changes in the DNA base sequence leading to new alleles) and chromosomes passed onto the next generation- main source of variation.
• Meiosis- crossing over of chromatids and independent segregation of homologous chromosomes- produces new combinations of alleles before they are passed into the gametes which are all genetically different.
• Random fusion of gametes- sexual reproduction- new combinations of alleles and offspring different from parents- gametes fusing at fertilisation is random- adds variety of offspring parents produce.

Question 24

What happens if variation is largely caused by genetic factors.

Answer 24

• If variation is caused largely by genetic factors organisms fit into a few distinct forms but there are no intermediate types- characters are controlled by a single gene- discrete.
• E.g. ABO- A, B, AB, O= controlled by a single gene.
• Variation largely due to genetic factors- represented graphically as discrete- bar chart/ pie graph.
• Environmental factors have little influence on these types of factors e.g. blood types.

Question 25

Describe how the environment affects variation in the phenotype.

Answer 25

• Environment influences all organisms- affect ways genes are expressed and causes variation within a species.
• Genes set limits but the environment determines where within the limits an organism may lie.
• E.g. height- genetically determined but also depends on nutrition to enable proper growth.
• Environmental influences- include climate, soil conditions, pH, food availability and lifestyle.
• Characteristics of organism’s grade into one another- form continuum e.g. height and mass.
• Characters- display type of variation not controlled by a single gene but many genes- polygenes.
• Environmental factors- major factor determining where on the continuum an organism lies.
• E.g. individuals genetically predetermined to be the same height grow to different heights due to variations in environmental factors e.g. diet.

Question 26

How may variation caused by environmental factors be represented graphically and give an example.

Answer 26

• Heights of a population plotted of individuals against heights- obtain normal distribution curve- bell-shaped curve.

Question 27

Why is it hard to draw conclusions on the causes of variation.

Answer 27

• Variation due to combined effects of genetic differences, usually polygenes, and environmental influences.
• Hard to distinguish between effects of many genetic and environmental influences that combine to produce differences between individuals- results in being difficult to draw conclusions about the causes of variation.
• Conclusions are tentative and should be treated with caution.

Question 28

What type of variation leads to evolution.

Answer 28

Only genetic variation.

Question 29

Which theory underpins modern biology.

Answer 29

The theory of evolution.

Question 30

What is evolution?

Answer 30

A change in the allele frequency of a population.

Question 31

How does evolution occur?

Answer 31

By natural selection or genetic drift.

Question 32

When does natural selection occur?

Answer 32

• When alleles that enhance the fitness of the individuals that carry them rise in frequency.
• Every organism- subjected to process of selection based on its suitability for survival under conditions that exist.

Question 33

What are selection pressures?

Answer 33

• Environmental factors- affect the chances of organism surviving and reproducing- limit the population of a species.
• Mainly predation, disease, competition for means of survival.
• Vary depending on time and place.

Question 34

Describe the process of natural selection.

Answer 34

• Selection pressures- create a struggle for survival and determine frequency of alleles in the gene pool.
• As individuals of the same species have variation due to different alleles produced by mutations, some individuals are better adapted to the selection pressures than others- differential levels of survival and reproductive success.
• Individuals with phenotypes that provide selective advantages that increases their chance of survival/reproduction are more likely to survive and produce more offspring and pass on their genes- including favourable alleles that determine their phenotype to the next generation
• This means a greater proportions of the next generation inherit the beneficial alleles.
• This generation is more likely to survive, reproduce and pass on their genes if they express the beneficial phenotype- differential reproductive success- the frequency of the beneficial alleles in the gene pool increases in each generation.

Question 35

What does evolution by natural selection depend on?

Answer 35

• Production of offspring- supported the available supply of food, light, space etc.
• Genetic variety within populations of all species.
• Variety of phenotypes selections operate against.

Question 36

How is reproduction rate altered to enable survival, and how are reproduction and natural selection linked.

Answer 36

• All species have the potential to increase their numbers exponentially if growth is unchecked by disease, competition, predation, and climate.
• In nature populations rarely increase at an exponential rate due to environmental factors.
• Death rate of slow-breeding species- high.
• High reproductive rates evolved in many species to ensure large enough population survives to breed and produce the next generation- compensates for high death rates from predation, competition, and natural disasters.
• Some species evolved lower death rates along with high parental care- lower death rates help to maintain population size.
• Over-production and natural selection are linked- where there are too many offspring for the available resources- competition amongst individuals- intraspecific competition for limited resources available.
• Greater numbers= greater competition= more individuals will die and struggle to survive.
• Deaths- not random- individuals in a population best suited to the conditions more likely to survive than those less well adapted.
• Individuals- more likely to breed and pass on their more favourable allele combinations to the next generation- different allele frequency from the previous one.

Question 37

Why is variation important for natural selection?

Answer 37

• Population- evolved a combination of alleles better adapted to conditions- selection process depends on individuals of a population being genetically different from each other.
• Organisms do not produce identical offspring if they have a selective advantage, even if it produces individuals that are less suited as conditions can change.
• Having a wide range of genetic differences in a population means some will have combinations of genes needed to survive in any new set of circumstances.
• Population- little genetic variation- vulnerable to new disease sand climate change.
• Important that a species is capable of adapting to changes resulting from the evolution of other species.
• Larger population= more genetically varied individuals and greater chance that one or more individuals will have a combination of alleles that lead to a phenotype which is advantageous for survival.
• Individuals- therefore more likely to breed and pass their allele combinations ot future generations.
• Variation- potential for a population to evolve and adapt to new circumstances.

Question 38

What is the importance of differential reproductive success?

Answer 38

• Differential reproductive success- has an effect on the allele frequencies within a gene pool of the population.
• Any sexually mature individual of a species in a population can breed with any other member of the same species- alleles combine.

Question 39

How do/ don’t environmental factors affect alleles?

Answer 39

• Environmental changes- affect probability of allele being passed on in a population and number of times it occurs within the gene pool.
• Environmental factors—don’t affect the probability of a mutant allele arising- affect the frequency of a mutant allele already present in the gene pool.
• Environmental factors- create variation within a population.
• Environmental factors- agent for constancy or change according to the selection pressure they exert.

Question 40

Give an overview of each type of selection.

Answer 40

• Stabilising selection- preserves the average phenotype- phenotypes around the mean of a population by favouring average individuals- selection against extreme phenotypes.
• Directional selection- changes in the phenotypes of a population by favouring phenotypes that vary in one direction from the mean of the population- selection for one extreme phenotype.
• Disruptive selection- favours individuals with extreme phenotypes rather than phenotypes around the mean of the population.

Question 41

What is importnat to mention in selection questions?

Answer 41

• Remember to mention the type of selection in questions.
• Remember to mention what the selection pressure/ selective advantage is- name nutrients e.g. glucose/ competitors/ predators.

Question 42

What is stabilising selection and draw the graph?

Answer 42

• Eliminates extremes of the phenotype range within a population, reducing it and therefore the capacity for evolutionary change.
• Individuals with alleles for characteristics towards the middle of the range area morel likely to survive and reproduce.
• Occurs where the environmental conditions are constant over long periods of time.
• Mean remains the same, but fewer individuals at extreme.
• E.g. body mass of children at birth.
• Graph on revision card.

Question 43

Describe how fur length relates to stabilising selection.

Answer 43

• May be beneficial to have longer/ shorter hair depending on the temperature.
• If the environment fluctuates- both extremes survive as there are years/ months where one type of individual has a selective advantage but the other does not- not enough time for evolution to occur and more beneficial to have variation.
• If environment is constant, individuals at extremes of the range have a lower chance of survival as its harder to maintain the right body temperature- very short/ long hair- selective disadvantage- selected against.
• Average fur length- selective advantage- more likely to survive, reproduce, and pass on their alleles- selected.
• Alleles for average fur length increase in frequency.
• Over time the proportion of the population with average fur length increases and the range of fur length decreases.
• Offspring graph- range of fur lengths decreased- narrower graph.
• Proportion with average length fur increases- resulting in a taller graph in the average fur length region.

Question 44

Describe directional selection.

Answer 44

• Individuals with alleles for a single extreme phenotype are more likely to survive and reproduce, usually in response to an environmental change.
• Within a population- range of genetically different individuals with respect to a phenotype- continuous variation forms a normal distribution curve.
• Normal curve- mean represents the optimum value for the phenotypic character under existing conditions.
• If environmental conditions change- optimum value for survival changes. Mutations produce alleles that help organisms to survive in new conditions e.g. antibiotic resistance.
• Some individuals- left/ right of mean – have the combination of alleles with the new optimum for the phenotypic character- selection pressure favouring the combination of alleles resulting in the mean moving left/ right of original position.
• Individuals to the left/right of the mean with the advantageous allele are more likely to survive and reproduce leading to an increase in the allele frequency of the advantageous allele in the population.
• One extreme of a range of variation being selected against in favour of other extreme.

Question 45

In directional selection, which alleles frequency increases faster.

Answer 45

The allele frequency of dominant alleles increases faster as they are always expressed when present in the homologous pair of chromosomes- only one needs to be present.

Question 46

What is directional selection dependent on?

Answer 46

Area- different types of environment in different populations.

Question 47

Describe how directional selection works with cheetah speed.

Answer 47

• Individuals with alleles for increased speed- more likely to catch prey than slower individuals- more likely to survive, reproduce and pass on their alleles.
• Frequency of alleles for high speed increases and the population becomes faster.
• Average speed has moved towards the extreme faster end.

Question 48

Describe disruptive selection.

Answer 48

• Opposite of stabilising selection.
• Individuals with alleles for extreme phenotypes are more likely to survive and reproduce.
• Characteristics towards the middle of the range are lost.
• Least common but most important in bringing evolutionary change.
• Environmental factor e.g. temperature- takes two/more distinct forms and favours more than one phenotype.
• Temperature alternated between hot and cold- two separate species- one active in the winter with long fur, and one active in the summer with short fur.

Question 49

Describe disruptive selection with regards to bird beak sizes.

Answer 49

• Birds with large beaks- specialised to eat large seeds.
• Birds with small beaks- specialised to eat small seeds.
• Majority of seeds are large/ small and few are medium sized, bird with medium-sized beaks have a reduced chance of survival as they are unable to eat either large or small seeds.
• Birds with large or small beaks are more likely than birds with medium—sized beaks to survive, reproduce and pass on their alleles.
• Over time, alleles for a large beak and a small beak increase in frequency but alleles for a medium-sized beak decrease in frequency.
• Proportion of the population having a small or large beak increases.

Question 50

Describe polymorphism.

Answer 50

Some species- have two or more distinct forms- genetically distinct but exist within the same interbreeding population.

Question 51

Describe polymorphism in peppered moths.

Answer 51

• Light until c.19th as dark coloured moths due to mutation but they were too visible against het light background of the lichen-covered trees and rocks- more subject to predation than the better camouflaged light forms.
• Black mutants- became more common after c.19th as buildings blackened by soot and pollutants killed lichens- better camouflaged and the light form predated on more frequently.
• Selective predation- favours individuals that lie at one extreme or the other.

Question 52

How does evolution alone differ to speciation.

Answer 52

• Evolution- change in allelic frequency in populations so that they become better adapted to their environment, but as the two populations can interbreed they are still one species.
• Speciation- to become 2 different species- 2 populations need to become reproductively separated.

Question 53

How does genetic drift differ to natural selection.

Answer 53

• Evolution by natural selection- selection pressures change the allele frequencies of a population over time.
• Genetic drift- instead of environmental factors affecting which individuals survive, breed and pass on their alleles, the alleles that are passed on are dictated by chance- random.

Question 54

How do genetic drift and natural selection influence species and each other.

Answer 54

• Can lead to differences in allele frequency between two isolated populations- if enough differences develop over time this could lead to reproductive isolation and speciation.
• Natural selection and genetic drift- work alongside each other to drive evolution.
• One process may drive evolution more than the other depending on population size.

Question 55

What populations does genetic drift affect.

Answer 55

Genetic drift is only important in causing changes in allele frequency in small populations.

Question 56

Describe the process of genetic drift.

Answer 56

• Individuals within a population show variation int their genotypes.
• The allele for one genotype by chance is passed onto more offspring than others- number of individuals with the allele increases.
• If by chance the same allele is passed on more often, this can lead to evolution as the allele becomes more common in the population.

Question 57

Describe why genetic drift only works in small populations.

Answer 57

• Small populations- few members of a small population have a smaller variety of alleles than the members of a large population- genetic diversity is less.
• Individuals breed- genetic diversity of the population is restricted to the few alleles in the original population.
• Only a small number of different alleles- not an equal chance of each being passed on.
• Those alleles passed on will quickly affect the whole population as their frequency is high.
• Mutations to one of these allele that is selectively favoured will more quickly affect the whole population- frequency high.
• Effects of genetic drift- greater and the population changes rapidly- more likely to develop into a separate species.

Question 58

Why does genetic drift not affect larger populations.

Answer 58

• Large populations- effect of a mutants allele- diluted- frequency less in the larger gene pool- effects of genetic drift less and development into a new species slower.
• Larger populations- chance factors even out across the population.

Question 59

How could you investigate genetic drift.

Answer 59

• You may be asked to apply knowledge of sampling to the concept of genetic drift.
• You may be asked to devise an investigation to mimic the effects of random sampling on allele frequencies in a population.
• You may be asked to use computer programs to model the effects of natural selection and of genetic drift.

Question 60

In what case may selection not occur even if a genetic condition is lethal.

Answer 60

Selection may not occur if organisms have already reproduced because symptoms of a disease develop at a later date e.g. Huntingdons.

Question 61

Why is it important to investigate selection in a natural environment.

Answer 61

• The organism may not behave in the same way.
• Courtship behaviour may not be the same.
• Organisms may have been used in other previous experiments affecting results.
• Timings may be too short for e.g. mating.

Question 62

What should you mention when explaining the selective advantage of a certain feature?

Answer 62

Think about predation, courtship behaviour, nutrition, competition etc.- think about why the organism would want to evolve to have the feature.

Question 63

What should you remember to mention with regards to timing on selection.

Answer 63

• Remember that selection has a time lag as initially there will only be a few organisms with the favourable mutation, individuals with the allele have to have offspring- takes many generations for the allele to become the most common.
• Always mention the impact on the offspring- the offspring are what selection affects.

Question 64

What should you mention if a question is about mutations

Answer 64

• Mutations are spontaneous/ random.
• Only the rate of mutation can be affected by the environment.
• Different species do not interbreed/ produce fertile offspring- mutation cannot be passed from one species to another.

Question 65

Describe the significance of common ancestry.

Answer 65

• All new species arise from an existing species- results in different species sharing a common ancestry, as represented in phylogenetic classification.
• Common ancestry explains the similarities between all living organisms, such as common chemistry (e.g. all proteins made from the same 20 or so amino acids), physiological pathways (e.g. anaerobic respiration), cell structure, DNA as the genetic material and a ‘universal’ genetic code.

Question 66

How does common ancestry affect small populations.

Answer 66

Small populations are isolated so inbreeding/ small gene pool leads to a high frequency of a certain allele- allele may be inherited from a common ancestor.

Question 67

What is a species?

Answer 67

Group of individuals that have common ancestry- share same genes but different alleles and are capable of breeding with each other to produce fertile offspring.

Question 68

What is speciation.

Answer 68

Development of a new species from an existing one.

Question 69

What causes speciation..

Answer 69

• Natural selection and isolation result in change in the allele and phenotype frequency and lead to the formation of a new species.
• Speciation occurs when populations of the same species become reproductively isolated from each other.
• Reproductive separation of two populations can result in the accumulation of difference in their gene pools as evolution by natural selection- changes allelic frequencies in a population- causes change in phenotypes.
• New species arise when these genetic differences lead to an inability of members of the populations to interbreed and produce fertile offspring- changes in alleles, genotypes and phenotypes prevent individuals with these changes from breeding with individuals without them successfully.
• • Changes often take hundreds of generations.
• Changes lead to reproductive separation and formation of separate species.

Question 70

Describe the process of speciation.

Answer 70

• Species have one or more populations.
• Individuals- breed only with others in the same population as they are isolated from the other populations but are capable of breeding with individuals in other populations.
• If a population becomes isolated from other populations of the same species, there will be no gene flow between the isolated population and the others.
• If a population becomes separated from another population- populations- experience different selection pressures due to different environments.
• Natural selection- leads to changes in allele frequencies of each population based on the selection pressures as each population evolves- remember to name the selection pressure/ characteristic being altered.
• The populations have different mutations and selection pressures- this may lead to the accumulation of genetic differences in the isolated population, compared with the other populations- becomes genetically different with different combinations of alleles.
• The adapted organisms breed with each other and pass on their adaptions to their offspring but their allele combinations do not get passed onto the offspring of other populations.
• Different combinations of alleles due to selection pressures produces different phenotypes.
• Adaptive radiation- evolution of organisms due to selection pressures into a wide variety of phenotypes adapted to its local environment.
• These genetic and physical differences may ultimately lead to organisms in the isolated population becoming unable to breed and produce fertile offspring with organisms from the other populations, even if they were no longer physically separated.
• This reproductive isolation means that a new species with its’ own separate gene pool has evolved and no interbreeding can occur.

Question 71

Name the different isolating mechanisms.

Answer 71

• Geographical- isolated by physical barriers- oceans, mountain ranges, rivers.
• Ecological- different habitats within the same area- individuals rarely meet.
• Temporal- breeding seasons of each population don’t meet so they don’t interbreed as they aren’t reproductively active at the same time e.g. flowering or mating seasons at different times of the year.
• Behavioural- mating stimulated by courtship behaviours- mutations cause variations in behaviour that aren’t attractive to the rest of the species- prevents mating.
• Mechanical- anatomical differences prevent mating occurring- changes in shape or function of genitalia- prevent individuals from mating successfully and breeding.
• Gametic- gametes prevented from meeting due to genetic or biochemical incompatibility- pollen grains fail to germinate/grow when land on stigma of different genetic makeup, sperm are destroyed by chemicals in the reproductive tract of females.
• Hybrid sterility- fusion of gametes from different species- cannot produce viable gametes e.g. mules- horse and donkeys- 63 chromosomes (horse 2n=64, donkey 2n=64), impossible for chromosomes to pair during meiosis- gametes not viable- mule sterile.

Question 72

How does speciation result in increased diversity.

Answer 72

• Evolutionary change over a long period of time has resulted in a great diversity of species, increasing biodiversity.
• Diversity of life is due to speciation and evolutionary change over millions of years.
• One population of organisms divide into new populations and evolve into separate species.
• New species divided again and new populations evolve into separate species.
• Process repeated to create millions of new species.

Question 73

Name the two types of speciation.

Answer 73

Allopatric and sympatric speciation.

Question 74

What is allopatric speciation?

Answer 74

• Allopatric- speciation where two populations become geographically separated.
• Reproductive isolation- occurs when a physical barrier e.g. flood or earthquake- divides a population of a species- some individuals separated from the main population- geographical isolation.
• No gene flow occurs between the two populations.

Question 75

What causes geographical seperation in allopatric speciation.

Answer 75

Barriers are different for different species e.g. oceans are a barrier for mice but not birds/fish.

Question 76

Describe the process of allopatric speciation.

Answer 76

• Environmental conditions on either side of the barrier vary e.g. climate- different selection pressures.
• Natural selection influences the populations differently- different changes to allele frequencies as different alleles will be more advantages in the different populations.
• Directional selection often occurs if environmental conditions differ.
• Allele frequencies also change as mutations occur independently in each population.
• Genetic drift may also affect the allele frequencies in one or both of the populations.
• Each species will evolve adaptions to local conditions.
• Changes in allele frequency lead to differences accumulating in the gene pools of the separated populations- changing phenotype frequencies.
• Individuals from the different populations change so much that they can’t breed with each other to produce fertile offspring- reproductively isolated- two groups become separate species.

Question 77

Describe speciation of Galapagos finches.

Answer 77

• Allopatric speciation.
• Single species colonised one of the Galapagos islands- population increased and established in other habitats on the same and different islands.
• Population- evolved adaptions to suit new environment including food resources- different size of beak to deal with different seed types.
• Geographic separated from population- changes led to populations becoming different- no longer interbreed- form separate species.

Question 78

Describe the process of sympatric speciation.

Answer 78

• Sympatric speciation- occurs when a population becomes reproductively isolated without any physical separation- populations are in the same area- no geographical isolation- occurs in the same habitat/ environment.
• The populations gene pools are kept separate- no gene flow.
• Occurs when random mutations within the population prevent individuals that carry the mutation from breeding with other members of the population that don’t carry the mutation.
• Disruptive natural selection often occurs- results in changes in allele frequencies and different alleles are passed on/ selected.
• The individuals eventually cannot breed to produce fertile offspring.

Question 79

Why is sympatric speciation rare?

Answer 79

Difficult for a section of a population to become reproductively isolated from the rest of the population without being geographically isolated.

Question 80

Describe how polyploidy may lead to sympatric speciation.

Answer 80

• Most eukaryotic organisms- diploid- 2 sets of homologous chromosomes.
• Polyploidy- increases the number of chromosomes.
• Different number of chromosomes- can’t reproduce sexually to give fertile offspring.
• Polyploid organism in diploid population- reproductively isolated from diploid organisms.
• Polyploid organism- reproduces asexually- new species develops.
• Polyploidy- only leads to speciation if it isn’t fatal and more polyploid organisms can be produced asexually.
• More common in plants than animals.

Question 81

Describe the speciation of the apple maggot fly.

Answer 81

• Sympatric speciation.
• Initially only laid its eggs inside hawthorns.
• Apple trees introduced- fly started to lay eggs in apples.
• Females- lay eggs on the type of fruit in which they developed.
• Males- look for mates in the type of fruit in which they developed.
• Hawthorn flies- mate with each other and apples mate with each other.
• Mutations in each population- led to evolution of genetic differences- over time results in being incapable of successfully breeding with one another and therefore being separate species.