5.2 Natural selection

Question 1

Diagram showing natural selection

Answer 1

Question 2

Explain how not all variation is visible

Answer 2

-The phenotype of a species is one way to observe certain characteristics, but many variations are invisible.

-The range of alleles present in a species accounts for the variation in the population and is a good measure of a healthy gene pool.

Question 3

Give three causes of variation

Answer 3

-Mutation

-Meiosis

-Sexual reproduction

Question 4

How is mutation a cause of variation?

Answer 4

-Any change to the DNA sequence is classified as a mutation.

-It can range from a single base change to removal of one segment of a chromosome.

Question 5

How is meiosis a cause of variation?

Answer 5

-It produces gametes with unique combinations of alleles, thus increasing the genetic variation of individuals within the species.

-In sexually reproducing organisms, meiosis can lead to variation in a species along with mutations and sexual reproduction.

-Meiosis causes variation by helping to create a new combination of genes in the organism’s gametes through crossing over and independent assortment of chromosomes.

Question 6

How are sexual reproduction and random fertilisation causes of variation?

Answer 6

The combination of gametes is random and results in a zygote that has genes from both of its parents.

Question 7

What organisms only reproduce asexually?

Answer 7

Most prokaryotes and some eukaryotes.

Question 8

What organisms can reproduce both sexually and asexually?

Answer 8

Some fungi can reproduce both sexually and asexually, as can certain plants and animals.

Question 9

What does an organism produce if it reproduces asexually?

Answer 9

It produces a clone that is an identical genetic copy of itself because it is not combining genes with any other organism.

Question 10

What is the only way that asexual species can increase variation?

Answer 10

-Through mutation

-However, mutations can also cause change in sexually reproducing organisms

Question 11

Diagram of asexual reproduction in a strawberry plant

Answer 11

Question 12

What are mutations and what do they lead to?

Answer 12

-Mutations are any change in the genome of an organism.

-These mutations in the genome may alter the coding for proteins, so that the alleles of the organism change.

-These changes are often harmful to the organism, but at times they can provide an advantage or be neutral, causing no change.

Question 13

What are the different types of mutations?

Answer 13

-Nonsense mutations

-Missense mutations

-Silent mutations

Question 14

What are nonsense mutations?

Answer 14

-Harmful mutations are those that cause a gene not to function properly or not to function at all.

-These are nonsense mutations.

Question 15

What are missense mutations?

Answer 15

Missense mutations may cause variation that benefits the organism by changing a codon, which in turn alters the protein.

Question 16

What are silent mutations?

Answer 16

-Those in which the codon still codes for the same amino acid, so no actual change in the protein occurs.

-These silent mutations do not have any effect on the variation of the species.

Question 17

Give an overview of how natural selection changes a gene pool

Answer 17

-According to natural selection, organisms better adapted to their environment tend to survive and produce more offspring.

-Successive cycles of selection of the ‘fittest’ or best adapted from the varying members of a population bring about evolution, as the number of individuals possessing that adaptation (hence the genes that code for it) increases in frequency, and characteristics (and their genes) that do not confer an advantage are gradually lost from the population.

-Thus the gene pool changes over time.

Question 18

Explain the significance of crossing over to natural selection

Answer 18

-Crossing over occurs on homologous chromosomes between non-sister chromatids during prophase 1.

-When the homologous chromosomes start to line up on the spindle fibers, they are in pairs.

-Chromosomes are not rigid, so the ends can flop around a little while they are moving.

-The centromeres, however, are attached to the spindle, so do not allow as much movement as the ends of the chromosome.

-When non-sister chromatids touch or cross over each other, genes can be exchanged from one to the other resulting in a new combination of genes on the chromosome.

-Then the homologous chromosomes line up during metaphase 1.

-The way the chromosomes line up is random, so this also leads to a new combination of genes in the gametes.

Question 19

When can natural selection only occur?

Answer 19

-If there is variation among members of the same species.

-Adaptations are characteristics that make an individual suited to its environment and way of life.

Question 20

Give examples of adaptations that have arisen through natural selection

Answer 20

-Development of special hair structures in the fur coat of polar bears that insulate them from the intense cold of the Arctic and provide camouflage with the snow and ice.

-Development of intricate and subtle forms in orchids to entice certain insects to pollinate them, as well as the hairs on the insects that help to collect pollen, which they spread, helping to pollinate other plants of the same species, providing more genetic variation for the orchid.

Question 21

Why does variation in a population occur?

I. Meiosis
II. Natural selection
III. Artificial selection
IV. Sexual reproduction
V. Mutation

Answer 21

I, IV and V

II and III are not correct because selection, whether natural or artificial leads to a decrease in variation because the characteristic that is not helpful to the organism will die out.

Question 22

Overpopulation in nature is ___

Answer 22

Rare

Question 23

Why is overpopulation rare in nature?

Answer 23

-In a healthy ecosystem, the number of individuals in a particular population remains fairly stable.

-This is despite the fact that some species produce millions of spores, fertilised eggs or seeds.

-The more offspring they produce, the higher chance there is that some will survive.

Question 24

Species tend to produce ___ offspring than their environment can support.

Answer 24

More

Question 25

What is a consequence of the overproduction of offspring?

Answer 25

-That not all of the offspring will survive.

-This ensures a power struggle within the population and indirectly ensures that those individuals best adapted to the environment will survive.

-These types of struggles could include finding food, hiding from predators, finding mates, or surviving disease.

-Those individuals that survive long enough to reproduce will contribute to the next generation.

Question 26

What does survival to an age where an organism can reproduce mean?

Answer 26

-That the individual was well adapted. Its offspring will inherit the genes for these characteristics.

-In this way, better adapted individuals reproduce to pass on characteristics to their offspring and hence contribute to the survival of the species.

Question 27

What will happen to organisms that are not well adapted to their environment?

Answer 27

-Those organisms that are not as well adapted are more likely to die, for example due to predation before they reach reproductive age, and that way their genes, or specific alleles, will be eliminated from the population.

-Alternatively, they may survive but produce fewer offspring.

Question 28

How does a gene pool eventually only contain characteristics that make the population successful?

Answer 28

With a large number of offspring, statistically, only the best adapted will survive long enough to pass on their genes, resulting in a population with a gene pool containing characteristics to make that population successful.

Question 29

Explain the different forms of adaptations (physical, behavioral, etc.)

Answer 29

-Some adaptations are physical and may be seen in the appearance of the organism while other physical adaptations may be more difficult to see such as their internal structure, biochemistry, or development.

-Behavioural adaptations may also be difficult to see without close observation.

-For example, bird mating dances and calls.

Question 30

Acquired characteristics vs. natural selection in chimpanzees (reword)

Answer 30

-Natural selection means that only the best adapted will pass on their genes to the next generation.

-This has nothing to do with acquired characteristics, such as the ability to use a tool (certain chimpanzees) or the loss of a horn in a rhinoceros.

-Both of these are not passed on genetically, however, certain characteristics giving the organism the ability to acquire that characteristic may be inherited, such as the opposable thumb needed by the chimpanzee to use the tool.

Question 31

A population has substantial genetic variation. Under the influence of natural selection, what is the correct sequence of the following events?

1. Well-adapted individuals leave more offspring than do poorly adapted individuals.
2. A change occurs in the environment.
3. Gene pool of the population changes.
4. Poorly adapted individuals have a decreased chance of survivorship.

Answer 31

2 → 4 → 1 → 3

Question 32

Foxes- needed?

Question 33

What is genetic drift?

Answer 33

Random fluctuations in allele frequency.

Question 34

How do changes occur within species to make them better adapted to the environment?

Answer 34

-Characteristics are favoured that make a species better adapted to its environment, and if that environment changes, those with genes that confer characteristics that are well adapted to the new environment will survive and pass on these genes to their offspring.

-This means that, with time, natural selection increases the frequency of characteristics that make individuals better adapted and decreases the frequency of other characteristics.

Question 35

What effect can environmental pressures have on genetic variation?

Answer 35

Environmental pressures may cause natural selection, which will increase the allele frequency of the favoured characteristic, decreasing variation.

Question 36

Diagram showing the change in allele frequency of the gene pool due to industrial melanism

Answer 36

Question 37

Explain the speed at which evolution happpens

Answer 37

-Evolution is slow. It takes a lot of time to see changes appearing, but evolution depends to a large extent on the generation time.

-For mice, with a generation time of only 3–6 months, changes become visible after several hundreds of years.

-But for a species such as the African elephant, with a slower generation time of about 60–70 years, it would take many thousands of years before any change became apparent.

-Genetic drift will also appear faster in smaller populations with a smaller gene pool.

-As these changes come about, some unfavorable characteristics may be completely lost.

Question 38

Diagram showing a genetic drift of the allele for dark fur colour in the fox population

Answer 38

Question 39

What is adaptive radiation?

Answer 39

• The rapid evolutionary diversification of a single ancestral line
• It occurs when members of a single species occupy a variety of distinct niches with different environmental conditions
• Consequently, members evolve different morphological features (adaptations) in response to the different selection pressures

Question 40

Example of adaptive radiation

Answer 40

• An example of adaptive radiation can be seen in the variety of beak types seen in the finches of the Galapagos Islands
• These finches have specialized beak shapes depending on their primary source of nutrition (e.g. seeds, insects, nuts, nectar)

Question 41

Diagram showing adaptive radiation

Answer 41

Question 42

What is Daphne Major?

Answer 42

• A volcanic island that forms part of the archipelago that is collectively referred to as the Galapagos Islands
• It is the native habitat of a variety of bird species known as Darwin’s finches (subfamily: Geospizinae)

Question 43

Darwin’s finches

Answer 43

• Darwin’s finches demonstrate adaptive radiation and show marked variation in beak size and shape according to diet
• Finches that feed on seeds possess small, powerful beaks – with larger beaks better equipped to crack larger seed cases
• In 1977, an extended drought changed the frequency of larger beak sizes within the population by natural selection
• Dry conditions result in plants producing larger seeds with tougher seed casings
• Between 1976 and 1978 there was a change in average beak depth within the finch population
• Finches with larger beaks were better equipped to feed on the seeds and thus produced more offspring with larger beaks

Question 44

Graph showing natural selection on Daphne Major

Answer 44

Question 45

Why is evolution only a theory?

Answer 45

-For a scientific theory to be accepted it must be able to explain and predict natural phenomena.

-The increase in the beak length and decrease in the beak width of these finches supports the theory of evolution by natural selection.

-Natural selection pressures caused adaptive radiation to occur where there were many small changes occurring in the species.

-This occurred until the species diverged and speciation arose.

Question 46

Fish called pikes are predators of algae-eating fish such as carp.

If a population of carp experiences predation pressure from pikes, which of the following is most likely to be observed in that carp population over the course of many generations?

Answer 46

Selection for larger female carp, bearing broods composed of more, and larger, young.

The selective pressure the pikes are exerting on the carp will result initially in the survival of larger carp. The carp population would have been reduced in number and more algae would be available for the carp population. After many generations, this could result in carp producing more eggs and larger broods comprising larger young. That is an evolutionary scenario with the highest probability

Question 47

Diagram showing the rise in the number of beta-lactamase enzymes isolated from the wild. The increased number of these enzymes indicates a decrease in the ability of antibiotics to treat bacterial infections.

Answer 47

Question 48

Antibiotic-resistant bacteria cannot be controlled or killed by ____.

Answer 48

Antibiotics

Question 49

Diagram showing how antibiotic resistance develops and spreads

Answer 49

Question 50

Diagram showing the reproduction of bacteria and increase of antibiotic resistance in a population of bacteria

Answer 50

Question 51

Example of antibiotic resistance

Answer 51

• An example of antibiotic resistance in bacteria can be seen in the evolution of Staphylococcus aureus (Golden staph)
• Golden staph can cause infections to the skin (lesions and boils) as well as more serious infections (pneumonia, meningitis)
• Historically, these infections were treated using the antibiotic methicillin
• Bacterial strains developed that were resistant to this antibiotic (methicillin-resistant Staphylococcus aureus – or MRSA)
• These strains proliferated while susceptible strains died out (methicillin-sensitive Staphylococcus aureus – or MSSA)
• MRSA infections are now especially present in hospitals and nursing homes, where the use of methicillin was most common
• Medical practitioners now prescribe alternate antibiotic agents to treat infections caused by Staphylococcus aureus

Question 52

Explain how antibiotic resistance develops

Answer 52

• In a bacterial colony, over many generations, a small proportion of bacteria may develop antibiotic resistance via gene mutation
• When treated with antibiotics, the resistant bacteria will survive and reproduce by binary fission (asexual reproduction)
• The antibiotic resistant bacteria will flourish in the absence of competition from other strains of bacteria (killed by antibiotic)
• Antibiotic resistant bacteria may also confer resistance to susceptible strains by transferring plasmids via bacterial conjugation
• The introduction of antibiotic (selection pressure) has caused the antibiotic resistance gene to become more frequent (evolution)