10.3 Gene pools and speciation

Question 1

What is a species?

Answer 1

A group of organisms that can interbreed to produce fertile offspring (this definition can only be used with sexually reproducing species).

Question 2

What is the ability of species to evolve limited by?

Answer 2

The genetic variations that exist.

Question 3

What is a population?

Answer 3

This refers to individuals of the same species living in the same place and time.

Question 4

What is a gene pool?

Answer 4

• The set of all the variations of all genes in any population.
• A gene pool is similar to the genome of an individual, but includes the alleles of all individuals in the population.

Question 5

Alleles in gene pools

Answer 5

• Some alleles will be common in the gene pool, and others will be rare.
• All members of a population share a gene pool.
• When they reproduce, their alleles may combine with any of the alleles found in the gene pool depending on the choice of partner.

Question 6

Define gene pool

Answer 6

A gene pool consists of all the genes and their different alleles present in an interbreeding population.

Question 7

What is allele frequency?

Answer 7

The allele frequency, usually expressed as a percentage or proportion, measures how common an allele is in a population.

Question 8

What is the depth, or richness, of a gene pool measured by?

Answer 8

By the number of alleles and their relative frequencies.

Question 9

Diagram showing a simplified version of the gene pool of dogs

Answer 9

Question 10

Characteristic alleles in dog breeds

Answer 10

• Dog breeds have particular characteristic alleles that confer their distinctive body shape and size, their fur color and length, and their personality traits.
• In similar breeds, such as different types of collies or terriers, many of the same alleles are common.
• In breeds that are very different, there will be numerous alleles that are common in one breed but rare in the other.
• However, dogs are a single species.
• Through sexual reproduction between breeds, every possible combination of alleles in the gene pool could exist in individual dogs.

Question 11

Whenever genetic variation exists, so does ___

Answer 11

The potential for genetic change.

Question 12

What is genetic drift?

Answer 12

Random fluctuations in allele frequency

Question 13

In what situations is there little genetic change in a population?

Answer 13

If the population has allele frequencies that are already well adapted to the environment, there is often little genetic change because natural selection acts to maintain the status quo.

Question 14

What is a prerequisite for evolution?

Answer 14

Genetic variation

Question 15

What is the original source of all genetic variations?

Answer 15

Mutation in DNA

Question 16

What is the significance of mutations in DNA

Answer 16

• The original source of all genetic variations is mutation in DNA.
• These mutations are often harmful, sometimes neutral, and occasionally beneficial.
• Though each mutation is a random event, mutations do occur in all living things.
• Mutations form alleles, and evolution occurs whenever there is a change in allele frequencies.

Question 17

Why does natural selection act on the entire collection of alleles that constitute the individual?

Question 18

How do new allele combinations in individuals occur?

Answer 18

• Through crossing over and random orientation of homologous chromosomes in meiosis and the fusion of genetic material from two parents.
• Thus, given enough time and a large enough population, alleles that are helpful become more common in the population, even though every individual contains a combination of beneficial and harmful alleles.

Question 19

Evolution requires that allele frequencies ___

Answer 19

Change with time in populations.

Question 20

Overview of natural selection and selective pressure

Answer 20

• In a population, individuals with different combinations of alleles may show differential survival and reproductive success because of natural selection.
• Selective pressure leads to the evolution of the population.

Question 21

What happens when populations are isolated from each other?

Answer 21

• Genetic drift and different selective pressures can cause the populations to evolve differently.
• Over time, when enough differences accumulate, the populations may speciate.

Question 22

What is selective pressure?

Answer 22

• Environmental factors can affect the rates of survival and reproduction of certain phenotypes (which are determined by alleles).
• This influence of natural selection is called selective pressure.

Question 23

What is selective pressure caused by?

Answer 23

By both biotic and abiotic factors that change the rate of survival and reproduction of a segment of a population.

Question 24

Selective pressure can be ___ or ___.

Answer 24

Weak

Strong

Question 25

Explain how selective pressure can be weak or strong

Answer 25

• For example, an allele that changes fur color to make a predator stand out in its environment might be strongly selected against if it can’t approach prey without being detected.
• If an allele offers slightly better protection against a rare parasite, it might experience a weak, positive selective pressure.
• Over time, the selective pressures of natural selection change allele frequencies and drive evolution.

Question 26

How many types of effects on phenotype, and therefore allele frequencies, does selective pressure have?

Answer 26

Three

Question 27

Diagram showing natural selection: stabilizing, directional and disruptive pressures

Answer 27

Question 28

Explain stabilizing selection

Answer 28

• Stabilising selection is widespread.
• It occurs when the existing variations that are beneficial are already common. Stabilising selection acts against extremes of a trait, for example, the colour of a sand crab.
• Seagulls prey on the crabs.
• The more the colour of the crab deviates from the background colour, the less likely it is to survive.
• New colours may be reintroduced by mutation occasionally, but the stabilising selection exerted by seagulls will maintain the match between the colour of crabs and the sand.

Question 29

Explain directional selection

Answer 29

• Directional selection occurs when one extreme of a trait offers a survival or reproductive advantage.
• The sand crabs living on the beach would experience directional selection if the background sand colour changed, for example, if flooding covered the beach with darker-coloured silt.
• Then darker-coloured crabs would have an advantage in avoiding predators and would survive longer and have more successful offspring.
• Dark colour alleles would become more common and light colour alleles less common.
• Directional selection increases allele frequencies at one phenotypic extreme and reduces them at the other.
• A classic example of directional selection is increasing neck length in giraffes over many generations.

Question 30

Explain disruptive selection

Answer 30

• In disruptive selection, the most frequent phenotype becomes a disadvantage, and individuals at both extremes have better rates of survival and reproduction.
• In the sand crab example, perhaps the female crabs inherit a genetic preference for males that stand out against the sandy background.
• This type of preference is fairly common in animal species.
• Choosing a mate that can survive even when it stands out to predators suggests a very strong assortment of other alleles.
• In this case, crabs lighter and darker than the background would have the most success in passing their alleles to the next generation.

Question 31

Similarities and differences between stabilizing, directional, and disruptive selection

Answer 31

• In stabilizing and directional selection, the population remains a single group.
• In disruptive selection, the population may break into two groups.
• Individuals with extreme phenotypes may also benefit from a genetic preference to mate with similar individuals because their offspring would have less chance of inheriting the disadvantageous intermediate phenotype.
• Thus, disruptive selection may cause one population to split into two, which may eventually become two species.

Question 32

Natural selection changes allele frequencies in a population because some ___ survive and reproduce more successfully than others.

Answer 32

Individuals

Question 33

Most blackbirds produce four to five eggs in each clutch.

Blackbirds producing fewer or more eggs than this have reduced fitness.

What type of selection is this?

Answer 33

Stabilizing selection

Natural selection acts against genetic tendencies to have fewer eggs or more eggs.

Question 34

How is allele frequency usually expressed and what does it measure?

Answer 34

It is usually expressed as a percentage or proportion, and measures how frequently an allele from a specific gene locus occurs in a population

Question 35

Map showing the frequency of two alleles influencing eye color in a number of human populations.

The ancestral allele codes for darker iris color, while the derived (mutant) allele codes for lighter iris color.

The pie charts show the proportion of the two alleles.

Answer 35

Question 36

For any gene, the sum of all its allele frequencies must be ___, while individual alleles may be ___

Answer 36

100%

Any fraction of the whole.

Question 37

Analyzing maps showing the frequency of alleles for the exam

Answer 37

• You are expected to be able to analyze images similar to Figure 1.
• For example, you should be able to identify regions where the ancestral allele is the only one present (Africa in this case), in how many populations the derived allele is the most common (four in this case), and the population with the greatest percentage of the derived allele, and so on.
• Similar images may show more than two alleles, for example, the ABO blood group, but the principles of analysis are the same.

Question 38

What allele frequency is it helpful to examine when studying evolution and natural selection?

Answer 38

• The allele frequencies between populations, or in one population over time.
• However, for eukaryotes, alleles occur in pairs in the genotypes of individuals.
• Genotype frequency is different than allele frequency.

Question 39

Table showing the number of individuals with different genotypes for height in a pea plant population

Answer 39

Question 40

What is a genotype frequency?

Answer 40

The number of individuals with a given genotype as proportion of the entire population.

Question 41

Genotype frequency in pea plant population table

Answer 41

• In this population, there are two alleles (T and t) for the gene coding for height.
• There are 4,900 homozygous dominant (TT) individuals in the population.
• The frequency of the TT genotype in this population is 0.49, or 49%, as shown below:

f(TT)=4,900/10,000=0.49

• Since there are 10,000 individuals in the population and each individual carries 2 alleles for the height gene, there are 20 000 alleles for the height gene in the population.
• The allele frequencies can be calculated by counting the number of T alleles (or t alleles) and dividing by the total number of alleles.
• Each homozygous tall individual has two copies of the ‘T’ allele.
• In this population, the 4,900 TT individuals have a total of 9,800 copies of the T allele, or 4,900×2.
• Each of the 4,200 heterozygous individuals has only one copy of the T allele (and one copy of the t allele). Thus, the allelic frequency for the T allele will be:

f(T)=((2×4 900)+4 200)/20 000=0.7

-The frequency for the t allele will be:

f(t)=((2×900)+4 200)/20,000=0.3

• By convention, the frequency of one of the alleles (usually the dominant allele) is represented as p and the other as q.
• In this case p= the frequency of the T allele, or f(T).
• Hence, in the pea plant population p=0.7 and q=0.3.
• If there are only two alleles, as in this case, the sum of p and q must be 1.0.
• Every allele is either T or t, so the sum must be 100% of the alleles for height in the population.

Question 42

How can researchers study selective pressure?

Answer 42

By comparing trends in allele frequencies between geographically isolated populations, or within a population over time.

Question 43

Hardy-Weinburg principle

Answer 43

This topic relates strongly to the Hardy–Weinberg principle, which is useful in understanding allele frequencies and evolution but is not directly required for this course. The Hardy–Weinberg principle includes two equations:

Allele frequency: p+q=1

Genotype frequency: p2+2pq+q2=1

The genotype frequencies assume that the chance of inheriting an allele is the same as its frequency. Therefore, the chance of being homozygous dominant is p×p (or p2). Heterozygotes may inherit their alleles in two ways: dominant allele from the mother and recessive from the father or vice versa. Thus the chance of being heterozygous is 2×p×q (or 2pq) .

Hardy–Weinberg states that allele frequencies will remain the same from generation to generation unless there are evolutionary influences, such as genetic drift, natural selection, or non-random mating.

Question 44

Calculations to do with allele frequencies for exam

Answer 44

You should be able to:

• Carry out simple calculations to determine allele frequencies
• Compare the allele frequencies of geographically isolated populations.

Question 45

The table below shows the allele frequencies for the two co-dominant alleles, C and A, for the lipoprotein lipase (LPL) gene in various human populations.

The frequency of the allele C is highest in the ___

Answer 45

African population

Question 46

The table below shows the allele frequencies for the two co-dominant alleles, C and A, for the lipoprotein lipase (LPL) gene in various human populations.

Assuming that the size of the Chinese population is 1.2 billion (1.2×10^9), calculate the number of Chinese people who have the genotype AA.

Give your answer to three significant figures.

Answer 46

42,800,000

The probability of a Chinese individual having a copy of the A allele is 0.189. The probability of them having a second A allele is 0.189 × 0.189 = 3.57 × 10-2 = 3.57%.

Therefore the number of individuals in the Chinese population who have the genotype AA = 3.57/100×1.2×10^9 = 42840000 = 4.28 × 107 (answer to 3 significant figures)

Question 47

The table below shows the allele frequencies for the two co-dominant alleles, C and A, for the lipoprotein lipase (LPL) gene in various human populations.

In which population is the frequency of allele A the highest?

Answer 47

Chinese population

Question 48

In rabbits, fur texture is controlled by a gene locus with a dominant allele (R) for rough fur and a recessive allele (r) for smooth fur.

In a population of 200 rabbits, 8 are homozygous for rough fur, 64 are heterozygous, and 128 are homozygous for smooth fur.

What is the allele frequency of the r (smooth fur) allele?

Answer 48

80%

To find the allele frequency for r, divide the number of r alleles by the total number of alleles. There are 200 diploid rabbits, and therefore 400 alleles total for the fur texture gene in the population. There are 128 rabbits with the genotype rr, so 256 r alleles in the homozygotes. There is one r allele in each heterozygote (Rr), so 64 copies. The total number of r alleles is 256+64=320. 320/400=0.8 or 80%.

Question 49

What is speciation?

Answer 49

• A species may gradually change with time to the point where ancestors are so different from their descendants that the two groups would be considered different species.
• This is called speciation.

Question 50

What is required to turn one species into another?

Answer 50

Genetic variation and time (usually with the help of some type of selective pressure).

Question 51

Explain how sometimes an ancestral species gives rise to two or more new species, increasing the total number of species

Answer 51

• This requires more than genetic variation.
• In order to have one portion of Species A become Species B, while another part does not – perhaps it remains Species A or becomes Species C – it is necessary that there be some sort of barrier, or isolation, between groups of the starting species.

Question 52

Diagram showing how sometimes an ancestral species gives rise to two or more new species, increasing the total number of species

Answer 52

Question 53

What happens when there is no isolation between populations of a species?

Answer 53

• They have a single gene pool.
• Any allele can be inherited together with any other allele in new individuals because of sexual reproduction.

Question 54

Why did sexual reproduction evolve?

Answer 54

• To increase genetic diversity.
• It mixes alleles into new combinations through crossing over in prophase I, independent assortment of homologues in metaphase I, and most importantly, combination of DNA from two different parents.

Question 55

What is there little chance for without reproductive isolation?

Answer 55

Without isolation, there is little chance for differences to accumulate and turn only a portion of an interbreeding population into a new species.

Question 56

What does a gene pool include?

Answer 56

All the alleles in an interbreeding population.

Question 57

Explain what reproductive isolation leads to

Answer 57

• Reproductive isolation of populations is equivalent to dividing the gene pool in two.
• By reducing or eliminating sexual reproduction that leads to fertile offspring, reproductive isolation reduces or eliminates the flow of alleles between gene pools.
• Changes, such as the introduction of new alleles through mutation, remain in that population’s gene pool only.
• Over time, genetic differences accumulate that can eventually prevent reproduction between populations and establish permanently separate species.

Question 58

Define speciation

Answer 58

• The formation of one or more new species from an ancestral species.
• Production of two or more new species occurs due to reproductive isolation between populations.

Question 59

Define reproductive isolation

Answer 59

The failure of individuals from two populations to mate and produce fertile offspring, resulting in the reduction or elimination of gene flow between the populations.

Question 60

Explain how reproductive isolation can be established

Answer 60

• Reproductive isolation can be established by many types of barriers.
• Some barriers may involve physical separation of populations, but an isolating barrier can be anything that reduces the chance of fertile offspring between two groups.
• There are several main ways a population can become reproductively isolated, three of which are discussed here: geographic isolation, behavioural isolation and temporal isolation.

Question 61

What are the three main ways a population can be reproductively isolated that are discussed here?

Answer 61

Geographic isolation, behavioural isolation and temporal isolation

Question 62

Give examples of how geographic isolation can happen

Answer 62

• The most obvious type of isolation between populations is a physical barrier that keeps them in two separate places.
• Depending on the nature of the species, geographic barriers could be entire oceans between continents or a small strip of land between neighbouring ponds.
• Sometimes a small population becomes isolated after a storm, and sometimes an established population is divided by a natural phenomenon like a landslide or a river changing course.

Question 63

What is allopatric speciation?

Answer 63

When a new species develops as a result of part of a population becoming geographically isolated from other populations.

Question 64

Diagram showing basic examples of geographic barriers

Answer 64

Question 65

Define geographic isolation

Answer 65

The separation of populations by a physical barrier that reduces or prevents gene flow.

Question 66

Explain how the Ensatina salamanders are an example of geographic isolation

Answer 66

• Occasionally a species has a large enough range and slow enough gene flow that populations at the extremes of the geographic range cannot interbreed.
• As Ensatina salamanders from the ancestral population in the north expanded their range southward, salamanders on the coast experienced different selective pressures than inland salamanders, and salamanders in the south experienced different pressures than more northern salamanders.
• Today, even though most neighboring species show some degree of hybridisation (and are therefore not fully separate species), the southernmost species cannot interbreed with the ancestral population.

Question 67

Diagram showing how geographic isolation can lead to speciation (salamanders)

Answer 67

Question 68

When does behavioral isolation occur?

Answer 68

When genetically-influenced differences in behaviour have the effect of reducing or preventing mating between different portions of the population.

Question 69

Give an example of behavioral isolation

Answer 69

• For example, a variation in the production of the mating song in male crickets may be mirrored by a preference for different variations in female crickets.
• Although mating would produce fertile offspring between any pairing in the entire population, individuals mate primarily with their own song group.

Question 70

Explain behavioral isolation

Answer 70

• New alleles that occur in one group are unlikely to spread to the other due to a lack of interbreeding.
• Differences accumulate between the song groups, making it increasingly unlikely and eventually impossible for successful interbreeding to occur.
• This is a form of sympatric speciation, the divergence of a population into two new species while in the same geographic area.

Question 71

Diagram of behavioral isolation in the form of mating song preferences

Answer 71

Question 72

What factors (besides variations in mating song) can lead to behavioral isolation?

Answer 72

• Besides variations in mating song, behavioral isolation may include other courtship practices such as bird dance, mating sites, and firefly lighting patterns.
• Behavioral isolation may also include practices that are not directly related to reproduction but cause non-random mating.
• For example, preference for a particular sub-niche or sub-habitat, such as soil pH in flowers or beetles, may mean that some individuals are more likely to encounter and reproduce with individuals that share their behavioural variations.

Question 73

Define behavioral isolation

Answer 73

Any behaviour that acts to reduce or eliminate gene flow between portions of a population, such as variations in courtship song or location.

Question 74

Define sympatric speciation

Answer 74

• When a portion of a population develops into a new species while still living in the same geographic area as the ancestral population.
• Sympatric speciation can occur due to behavioral, temporal, or other forms of speciation.

Question 75

When does temporal speciation occur?

Answer 75

• When portions of a population are reproductively active at different times.
• Portions of the population may mate or flower at different times of day or during different seasons.

Question 76

Give an example of temporal speciation

Answer 76

• For example, Anaxyrus americanus toads and Anaxyrus fowleri toads have some sympatric populations. In laboratory settings, they easily hybridise and produce mostly fertile offspring.
• However, in nature, they rarely interbreed because A. americanus mate only during a few days in early summer, while sympatric A. fowleri mate a few weeks later.

Question 77

Pictures of American toad (on the left) and Fowler’s toad (on the right) show temporal isolation.

Answer 77

Question 78

Explain how temporal isolation can be seen during a single day

Answer 78

• This can happen in populations that are active primarily at dawn versus dusk.
• Another example is migratory birds that select mates and nesting sites when they arrive at their mating grounds.
• Early arrivers tend to mate with other early arrivers, and, because arrival time is likely to have a genetic influence, over time early arrivers may accumulate differences from the late-arriving portion of the population.

Question 79

What can temporal isolation eventually lead to?

Answer 79

Sympatric speciation

Question 80

Define temporal isolation

Answer 80

Any shift in the timing of a behaviour that acts to reduce or eliminate gene flow between portions of a population.

Question 81

Give examples of other types of isolation (not geographic, behavioral, or temporal)

Answer 81

• Other types of isolation exist, for example, mechanical isolation, where an anatomical barrier exists that prevents mating, and hybrid infertility, where mating between species occurs and produces healthy but sterile offspring.
• Whatever the type of isolation, genetic differences can accumulate and lead to speciation.

Question 82

Why can the natural world make it difficult to determine whether two groups are populations or separate species?

Answer 82

• Some populations can acquire many visible differences while still producing fertile offspring, while in other cases two species may look indistinguishable although they are not able to mate.
• Populations may experience any degree of isolation from complete to none, and acquire any degree of genetic difference.
• Sometimes populations become very different until a barrier is removed, but then reintegrate into a single group.

Question 83

When does incipient speciation occur?

Answer 83

When populations that are genetically distinct can still interbreed.

Question 84

Can speciation be undone?

Answer 84

Once speciation is complete it can never be undone, but until that time any genetic differences accumulated during isolation could be reversed.

Question 85

Two populations of worm species live in the moist, grassy soil on either side of a stretch of bare and sandy soil.

The sandy soil forms a barrier that is usually too dry for worms to cross.

What type of isolation is this an example of?

Answer 85

The populations are geographically isolated.

Question 86

What is evolution?

Answer 86

The cumulative change in heritable (genetic) characteristics of a species, which requires changing allele frequencies

Question 87

What happens to alleles during evolution

Answer 87

Over time, new alleles are formed by mutation, and some alleles disappear completely.

Question 88

What happens during evolution if there is sufficient genetic change?

Answer 88

New species can emerge

Question 89

Describe the speed of speciation due to the divergence of isolated populations

Answer 89

• This can occur very gradually (a model known as gradualism), abruptly (even instantaneously in the case of polyploidy), and there can be long periods of little change interspersed with short periods of rapid evolution (known as punctuated equilibrium.)
• However, ‘short’ periods in an evolutionary timescale may be a hundred thousand years or more.

Question 90

When does gradualism occur?

Answer 90

When major changes are the cumulative product of slow but continuous minor changes.

Question 91

Explain gradualism

Answer 91

• In this model, mutation or immigration occasionally introduces new alleles to the gene pool, which are then acted on by natural selection, becoming more common or disappearing.
• In each generation, allelic shifts are subtle but tend to increase the frequency of any advantageous alleles.
• Individuals with the helpful trait survive and reproduce to pass on these alleles to the next generation.
• Over long periods of time, perhaps millions of years, the cumulative changes in alleles become increasingly visible in the form of phenotypic changes in the population.
• Evolution by gradualism is a smooth and continuous process.

Question 92

Give an example of gradualism

Answer 92

• For example, two populations of the same species may be in two different environments.
• Perhaps only one area contains a particular predator or food source. Each population will have a range of alleles, to begin with, and will experience different random mutations.
• From the alleles available, any that help to avoid the predator will become common in the population where the predator is present due to selective pressure.
• At some point, the gene pools of the two populations will be so different that, even if reunited, members of the two populations would be unable to produce fertile offspring.

Question 93

The theory of gradualism explains ___

Answer 93

Some, but not all, evolutionary transitions documented in the fossil record.

Question 94

Speed of evolutionary transitions

Answer 94

• Some evolutionary transitions appear to have occurred rapidly.
• The fossil record shows long periods of stability with little phenotypic change.
• At other times, major phenotypic shifts appear rapidly (although the definitions of ‘major’ and ‘rapid’ may vary.)

Question 95

Explain punctuated equilibrium

Answer 95

• In examples of punctuated equilibrium, species are generally stable during long periods while stabilizing selection works to maintain the existing phenotype of the species.
• However, the periods of stability, or equilibria, are ‘punctuated’ by rapid bursts of phenotypic change.
• The rapid phenotypic changes are often the result of major upheavals such as long-term climate shifts or the arrival, through immigration or evolution, of new species with a strong ecological impact.
• During these times of change, natural selection may favor phenotypes that were previously at a disadvantage.
• Thus, stabilizing selection would shift to directional or disruptive selection.

Question 96

Punctuated equilibrium implies ___

Answer 96

Long periods without appreciable change and short periods of rapid evolution.

Question 97

Diagram comparing gradualism versus punctuated equilibrium

Answer 97

Question 98

Diagram showing gradualism versus punctuated equilibrium in butterfly species

Answer 98

Question 99

At what speeds can speciation occur?

Answer 99

• Speciation due to the divergence of isolated populations can be gradual.
• Speciation can also occur abruptly after long periods of stability.

Question 100

Compare and contrast gradualism and punctuated equilibrium (separate)

Answer 100

• Both gradualism and punctuated equilibrium are models of evolution with supporting evidence.
• `It is likely that even during periods of relative stability, a gradual change occurs and eventually produces new species.
• It is equally likely that during times of upheaval, selective pressures change rapidly leading to the abrupt development of new species.
• The key understanding is that the rate of evolution can vary.
• Even in gradualism, it is not suggested that the pace of evolution is completely consistent, only that the accumulation of change occurs a bit at a time.
• The details of the rate of evolution remain an area of study.

Question 101

In the early 1970s, Stephen J. Gould, Niles Eldredge, and a few other leading researchers found that there is sufficient fossil evidence to show that some species remained essentially the same for millions of years and then underwent short periods of very rapid, major change leading to new species.

This supports the hypothesis that ___

Answer 101

Speciation must have occurred via punctuated equilibrium.

Question 102

In the punctuated equilibrium models, the ‘sudden’ appearance of a new species in the fossil record implies that ___

Answer 102

Speciation occurred rapidly in geologic time.

Question 103

More similarities between gradualism and punctuated equilibrium

Answer 103

• They both deal with speciation events on the scale of thousands or millions of years.
• In both cases, speciation is driven by the accumulation of allelic differences at different loci and can apply to all organisms.

Question 104

Explain how speciation can occur by polyploidy

Answer 104

• Speciation can also occur by polyploidy, the possession of more than two complete sets of chromosomes.
• Polyploidy can cause instantaneous speciation, involves increasing the number of chromosome sets, and usually occurs in plants, which have a much higher tolerance for alternate chromosome numbers than animals.

Question 105

Explain polyploidy in terms of chromosomes

Answer 105

• Polyploidy refers to the presence of more than two complete sets of chromosomes in a cell.
• Common ploidy levels are haploid (n), and diploid (2n), while common polyploid levels are triploid (3n) and tetraploid (4n) but can rarely be as high as hexadecaploid (16n) or more.

Question 106

How is polyploidy different from aneuploidy?

Answer 106

Polyploidy is different from aneuploidy, which refers to non-standard numbers of a single chromosome. For example, in humans Down’s syndrome is caused by aneuploidy; specifically trisomy 21 (three copies of chromosome 21). The chromosome number would be written 2n+1 .

Question 107

What can polyploidy be caused by?

Answer 107

• By the total non-disjunction of chromosomes during mitosis or, most commonly, meiosis I.
• The entire meiotic spindle fails and all sister chromatids are inherited together

Question 108

Diagram showing an example of ploidy levels in a species where n=3

Answer 108

Question 109

Describe polyploid organisms and their reproduction

Answer 109

• When a polyploid organism forms, it may be instantly isolated from its parent species.
• Meiosis requires that each chromosome form a tetrad with its homologous partner during prophase I.
• Both diploid and tetraploid individuals will always have even numbers of homologous chromosomes and can therefore produce gametes for sexual reproduction.
• Assuming they develop normally, tetraploid individuals face no reproductive barriers with other tetraploids.

Question 110

If a tetraploid individual reproduces with a diploid, their offspring will be ___

Answer 110

Triploid

Question 111

Triploid organisms are usually ___

Answer 111

Sterile

Question 112

Why are triploid organisms usually sterile?

Answer 112

This is because not all chromosomes have a homologous partner.

Question 113

Why may diploid and tetraploid matings be considered separate species immediately?

Answer 113

Since diploid and tetraploid matings do not produce fertile offspring.

Question 114

What industry is ployploidy useful in?

Answer 114

Crops

Question 115

Why is triploidy useful in crops?

Answer 115

• Triploidy is very useful when seedless fruit is desired because, since sexual reproduction fails, the embryo-containing seeds do not develop.
• For example, commercial bananas and seedless watermelon are triploid varieties that are cultivated asexually through cloning.

Question 116

Polyploid species can have a large variety of ___ (usefulness of polyploidy in crops).

Answer 116

Alleles at the same locus

Question 117

Explain how polyploid species can have a large variety of alleles at the same locus

Answer 117

• For example, a tetraploid individual can carry four different alleles for each gene because it has four copies of each chromosome.
• This allows the species to develop new uses for some genes without losing their original function.
• Many new and useful varieties can be developed in this way.

Question 118

What state does polyploidy often produce?

Answer 118

Hybrid vigour, a state where the hybrid exhibits traits that are more desirable than either parent.

Question 119

Example of polyploidy in the genus Allium (check)

Answer 119

-In the genus Allium (plants such as garlic, onion, and leeks), asexual reproduction is common.

-The failure of chromosomes to separate during mitosis in asexual reproduction has led, in many instances, to a doubling of the number of chromosomes.

-Figure 2 shows Allium canadense (2n=14), the closely related Allium canadense lavendulae (4n=28), and Allium validum (8n=56). The typical haploid number for Allium species is n=7.

-A. canadense lavendulae is tetraploid while A. validum is octoploid.

Question 120

Is polyploidy more frequent in plants or animals?

Answer 120

It is much more frequent in plants

Question 121

Polyploidy in animals

Answer 121

• Although polyploidy is much more frequent in plants, it may also occur in animals.
• For example, the Xenopus frog genus contains different species with a variety of ploidy levels ranging from two sets of chromosomes to twelve (dodecaploid, 12n).

Question 122

Describe the chromosomes in polyploid individuals

Answer 122

• Polyploid individuals will have chromosome numbers that are some multiple of the haploid number.
• For example, if n=5, polyploid individuals could have chromosome numbers of 15, 20, 25, and so on, but not 6, 9, or 16.

Question 123

Allium ampeloprasum possesses 32 chromosomes and is a tetraploid organism.

It is likely that it evolved through polyploidy from ___

Answer 123

Allium cepa with a chromosome number of 16.

Allium ampeloprasum has 32 chromosomes and is tetraploid (4n=32). It probably evolved from an A. cepa with 2n=16 through a polyploid event that led to a doubling of chromosome number.