Chapter 16 - Evolutionary Mechanisms

Question 1

Define Species.

Answer 1

A species is a group of individuals that share many characteristics and are able to interbreed under natural conditions to produce fertile offspring.

Question 2

Define population.

Answer 2

A population is a group of organisms of the same species living together in a particular place at a particular time.

Question 3

When studying populations, what do geneticists (scientists who specialise in the study of inheritance) prefer to do?

Answer 3

They prefer to consider the characteristics of the population as a whole and not those of the individuals that make up the population.

Question 4

What is Gene pool?

Answer 4

• Geneticists find it convenient to pool the genotypes of all the individuals capable of reproducing and refer to this as the gene pool.
• Thus the gene pool is the sum of all the alleles in a given population.

Question 5

Define Allele frequencies.

Answer 5

• In studying a population, geneticists are interested in how often each allele of a gene occurs in the gene pool for that population.
• Knowing how often a characteristic occurs in a population, geneticists are able to work out the frequency of each of the alleles for a particular gene.

Question 6

How does the gene pool in different populations differ?

Answer 6

• Populations that differ in the characteristics they posses are likely to be different in the frequencies of the various alleles of a gene in their respective gene pools.
• Thus, any two populations having differing characteristics are likely to have different gene pools.

Question 7

What are some factors that may cause the frequency of particular alleles in a population to change over time?

Answer 7

• chance events
  eg. mutation that alters the expression of a gene or more natural means, where changes in the environment may result in variations to the alleles frequencies.

Question 8

What are mutations?

Answer 8

Characteristics which occur when new variations in offspring, which show no resemblance to either parent, may occur quite suddenly and purely by chance.

Question 9

What is a mutant?

Answer 9

It is an organism with a characteristic resulting from a mutation.

Question 10

What are the two main types of mutation?

Answer 10

1. Gene Mutations - changes in the DNA of a single gene so that the traits normally produced by that gene are changed or destroyed.
2. Chromosomal Mutations - all or part of a chromosome is affected.

Question 11

Explain Somatic Mutation?

Answer 11

• Mutations can occur in the body cells or the reproductive cells of a person.
• A mutation in the body cells, also called somatic cells, is called a somatic mutation.
• Only the characteristics of the individual with the somatic mutation are affected.
• Reproductive cells are not affect, so when the individual dies, the mutation is lost.
• Somatic mutations are often the cause of many cancerous growth.

Question 12

Explain Germline (or germinal) mutation.

Answer 12

• Mutations that affect the reproductive cells can result in the gametes containing the changed genetic information that can then be passed on to the next and subsequent generations.
• In such cases the person in whom the mutation occurs is not usually affected, but that individual produces gametes with changed DNA.
• A germline mutation may, by chance, result in characteristics that improve the survival chances of the individual.
• Such a characteristic is likely to be passed on to future gnerations resulting in a change to the allele frequences of the gene pool.

Question 13

Define Natural Selection.

Answer 13

The process whereby organisms better adapted to their environment tend to survive and produce more offspring.

Question 14

When does natural selection take place ?

Answer 14

• When such a situation occurs, nature is favouring one set of alleles at the expense of others.
• Natural selection is major cause of changes to allele frequencies.

Question 15

Is Natural Selection random?

Answer 15

• Nope; Alleles are not passed on randomly to the next generation.
• The alleles passed on tend to be the ones that enhance survival and reproduction of the individual and therefore of the species.

Question 16

What is a random genetic drift?

Answer 16

• A phenomenon which occurs in small populations, where there is often a random, non-directional variation in allele frequencies.
• AKA Sewall-Wright efft, after the man who first recognised its significance in causing changes to allele frequencies.
• It is a purely chance occurrence, a change in allele frequencies that may occur where a population is very small.

Question 17

What is another phenomenon similar to genetic drift?

Answer 17

• Founder Effect
• This effect occurs when a small group moves away from its homeland to a totally new area and establishes a community, which later expands.
• The migrant group, being such a small sample of the original population, is usually not genetically representative of them.
• This new community, therefore, generally shows features that are not typical of the original homeland population.

Question 18

What is Achromatopsia?

Answer 18

It is an inherited form of total colour blindness.

Question 19

How can migration cause changes in allele frequencies in a gene pool?

Answer 19

Geneticists describe migration as gene flow from one population to another.

-Therefore, if immigrants to a certain country bring alleles that are not already in the population, the frequencies for the alleles of that gene will be altered.

Question 20

Why are populations often kept apart by barriers that inhibit the amount of interbreeding between them?

Answer 20

• As no two environments are exactly the same, the environmental pressures on one population will be different from the pressures on the other.
• Over time, the allele frequencies of each gene pool will change, depending on which characteristics are favoured for survival.

Question 21

Which is the most common barriers to interbreeding in the early human populations?

Answer 21

Geographical barriers.

Question 22

What are Geographical barriers?

Answer 22

-Include oceans, mountain ranges, large lake systems, deserts and expensive ice sheets.

For eg. The original inhabitants of Australia were isolated for thousands of years by ocean barriers that formed as sea levels rose.

Question 23

What caused Sociocultural barriers?

Answer 23

As human populations developed and became culturally more complex, other types of barriers to interbreeding such as religious groups began to form.

Question 24

What are some examples of Sociocultural barriers today?

Answer 24

Economic status, educational background and social position are barriers to interbreeding.

Question 25

Genetic disease results from…

Answer 25

Changes to allele frequencies in a gene pool.

Question 26

What happens when there is an allele causing an inherited, fatal disease ?

Answer 26

It would be expected to be gradually eliminated from a population because people with the allele would die and would not pass it on to the next generation.

Question 27

What is Tay-Sachs disease?

Answer 27

TSD is a hereditary disorder of lipid metabolism that occurs most frequently in individuals of Jewish descent from Eastern Europe (the so-called Ashkenazi Jewish population).

Question 28

What causes Tay-Sachs disease?

Answer 28

• Tay-Sachs is caused by a missing enzyme that results in the accumulation of a fatty substance in the nervous system.
• Death usually occurs by the age of 4 or 5.

Question 29

Why is the fatal disease (Tay-Sachs) much higher amongst the Ashkenazi Jews?

Answer 29

• Genetic Drift; Jewish populations have tended to be small and isolated, factors that increase the chances of genetic drift.
• Observations on individuals with 1 Tay-Sachs allele, who are heterozygous would be able to resist the disease Tuberculosis (TB).

> But individuals with 2 normal alleles are more susceptible to TB, thus they are more likely to die from it.

> Individuals with 2 Tay-Sachs allele, who are obviously homozygous would die early in life.

> Therefore, heterozygotes would have a survival advantage and would be more likely to reproduce and pass their alleles on to the next generation.

Question 30

What is Sickle-Cell Anaemia?

Answer 30

• The disease occurs when a person inherits the allele from both parents.
• It results in the red blood cells being crescent (or sickle) shape.
• This disease is usually fatal, as the sickle-shaped cells do not carry as much oxygen as normal red blood cells and they also stick together and block small blood vessels.

Question 31

Facts about Sickle-Cell Anaemia.

Answer 31

• It occurs mainly in black Africans, or in people of black African ancestry.
• In the tropical zone of Africa, up to 40% of some populations carry the allele for sickle-cell anaemia.

Question 32

What is Sickle-cell trait?

Answer 32

• Individuals with only one allele for sickle-shaped cells show no ill effects unless oxygen is in short supply.
• When this occurs their red blood cells show mild sickling.

Question 33

How does Sickle-cell trait give certain advantages to those who have it?

Answer 33

• It provides a degree of immunity to malaria, a disease prevalent in parts of the world where the sickle-cell gene is found.
• For this reason, the allele is maintained in areas where malaria is present.

Question 34

What is Special Creation?

Answer 34

It is the belief that God individually created each species.

Question 35

What is Evolution?

Answer 35

It is the gradual change in the characteristics of a species.

• The theory of evolution through natural selection was put forward independently by Charles Darwin and Alfred Russel Wallace in 1858.
• However, it is Darwin’s name that is usually associated with this theory because of the massive amount of supporting evidence he collected.

Question 36

What is the Bionomial system?

Answer 36

Carolus Linnaeus established the basis of this system (our very present system) of classification and the binomial system of naming organisms using the generic (genus) and specific (species) names.

Question 37

What is the general hypothesis put forwards by Lyell who wrote the book The Principles of Geology?

Answer 37

• That the natural forces existing in the past were much the same as those existing in his own time.
• This hypothesis implied that the Earth’s surface has been gradually moulded over a very long period of time, by such simple forces as changes in temperature, running water and earth movements.
• Lyell’s ideas provided Darwin with a concept of constant change against which he could view his own work.

Question 38

What idea did Thomas Malthus, a British clergyman and political economist, provided?

Answer 38

• Malthus, in an essay on The Principle of Population, pointed out that the human population was increasing at a rate far exceeding the rate of food production.
• Drawing examples from natural populations of plants and animals, he demonstrated that natural reproduction rates exceeded the available resources; that is, more plants and animals are produced than can possibly survive.
• Darwin realised that under these circumstances a struggle for existence would occur, with the favourable variations being preserved and the unfavourable ones being gradually lost from the population.

Question 39

What 3 observations were Darwin’s theory of natural selection based on?

Answer 39

1. Variation: Darwin noted that all members of a species vary. He made no attempt to explain the source of this variation. However, he did point out that these variations were passed on from one generation to the next, characteristics displayed by the parents being passed on to their offspring.
2. Birth Rate: inspired by Malthus, Darwin realised that all living organisms reproduce at a rate far greater than at which their available food supply and other resource increase. This would normally result in overcrowding.
3. Nature’s Balance: Darwin observed that, although the birth rate of organisms was very high, each species tended to maintain its numbers at a relatively constant level.

Question 40

What are the Interpretations that Darwin made on his observations?

Answer 40

1. Because of the excessive birth rate and limited resources, there must be a struggle for existence.
2. Because there was a range of variations in any species, those with characteristics best suited to their environment were the ones that were more likely to survive. [Survival of the Fittest]

Question 41

Define Survival of the Fittest.

Answer 41

• More organisms with favourable characteristics survived, while many of those with unfavourable characteristics died before they had an opportunity to reproduce and pass on the unfavourable characteristics.
• This is possible because there is a variation within any species.

Question 42

What is variation?

Answer 42

-It is when the members of a species differ from one another in their physical characteristics, body functioning and behaviour.

Question 43

What causes Variation?

Answer 43

• We now know that much of the variation that can be seen in a population is due to the effects of meiosis and fertilisation, and the simple principles first proposed by Mendel.
• Mendel studied characteristics that were determined by one pair of alleles.
• Many characteristics of organisms are controlled by more than on pair of alleles and this leads to a huge range of variation.

Question 44

What are mutations?

Answer 44

New variations, showing no resemblance to either parent. may occur quite suddenly and purely by chance.

Question 45

Summaries the principles of evolution through natural selection.

Answer 45

1. There is variation of characteristics within a species.
2. More offspring of a species are produced than can possible survive to maturity.
3. Due to excessive birth rate, and limited resources, there is a struggle for existence - competition for survival.
4. The individuals with characteristics best suited to the environment have more chance of surviving and reproducing - survival of fittest.
5. Favourable characteristics ( those with survival value) are passed on to the next generation.
6. In the gene pool, the proportion of alleles that produce favourable characteristics gradually increases.

Question 46

Why did the environment of early humans must have had a profound effect upon the characteristics that were selected as the most suitable for survival in the region where they lived?

Answer 46

Because as result, there are features of human body that appear to correlate well with the environments in which they occur.

For eg. human body shape or stature, can be correlated with resistance to the cold.

Question 47

Why would individuals with long bodies and short limbs have a survival advantage in very cold environments?

Answer 47

This is because they have a smaller surface area in relation to body volume than those with short bodies and long limbs.

Question 48

Why would many individuals with less favourable characteristics would have died before reproductive age?

Answer 48

• As fewer of the short-bodied, long-limbed individuals would survive in the extreme cold, fewer of the alleles for these characteristics would have been passed on.
• So the frequency of unfavourable alleles in the gene pool would gradually decrease.

Question 49

What environment suits Anopheles mosquitoes?

Answer 49

• It transmits the malarial parasite.
• It is not normally an inhabitant of tropical forests.
• It needs quiet, stagnant pools of water to raise its young.
• This habitat is more often found in open areas.

Question 50

How did the incidence of Malaria increased?

Answer 50

• As humans began to clear the forests of Africa for agriculture, they changed the environment in a manner that created additional breeding areas for Anopheles mosquitoes.
• Thus the incidence of malaria increased.

Question 51

How is Sickle cells formed?

Answer 51

• Because of the Mutation of the gene responsible for the production of normal haemoglobin.
• The mutant allele responsible for the sickle shape of the affected red blood cells causes the substitution of amino acid (valine) for another (glutamic acid) during the formation of the haemoglobin protein.
• The mutation affects only one of the 287 amino acids in the haemoglobin molecule, but this change is enough to affect the functioning of the red blood cell.
• The affected haemoglobin is often referred to as the haemoglobin S, and cells that contain it collapse into sickle shapes at low oxygen concentration.
• Individuals homozygous for this mutant allele suffer from sickle-cell anaemia, a disease that is usually fatal.

Question 52

What happens when a person with sickle-cell anaemia dies before reproducing?

Answer 52

• The allele that causes the disease would not be passed on to the next generation
• Therefore, you would expect that over many generations the frequency of the sickle-cell allele would gradually decrease until it was eliminated from the population altogether.

Question 53

What is Speciation?

Answer 53

It is the process of producing two species through the interbreeding of members from 2 different populations whom have been isolated for a very long period of time, and the environmental influences on each are different enough, major changes in the allele frequencies within each population could occur.

Question 54

Explain the Diagrammatic representation of variation, isolation, selection and speciation.

Answer 54

1. Variation- a population exists on an island.
   - > A range of variations exists within the population, which shares a common gene pool.
2. Isolation - the species is divided into two populations.
   - > A barrier has formed dividing the population into two. No interbreeding occurs between the two populations. Each population has a separate gene pool.
3. Selection - two subspecies begin to form.
   - > Different selection pressures act on each of the two populations over a number of generations. This brings about a change in the gene frequencies of each gene pool. Such changes lead to the evolution of separate subspecies.
4. Speciation - two species now exist.
   - > Over a long period of time the changes in the gene frequencies may be great enough to prevent the production of fertile offspring by interbreeding between the two populations from ever occurring again. When this happens, two species exist.