4.1 Evolution 1

Question 1

Define natural selection

Answer 1

Natural selection is a process in which individuals that have certain inherited traits tend to survive and reproduce at higher rates than other individuals

Question 2

Define evolution (broad definition)

Answer 2

evolution is defined as descent with modification from a common ancestor

Question 3

Define allele frequency

Answer 3

Allele frequency is a measure of chances of finding a particular allele of a gene within the population

Question 4

Define and differentiate microevolution and macroevolution

Answer 4

Microevolution:
The change in allele frequency within a population or species over short periods of time. It does not give rise to a new species
- Small scale
- Few generations
- Within a population/ species
- No new species is formed
- Descendant is of the same type as ancestor

Macroevolution
Gradual changes in allele frequencies usually over longer periods of time and may eventually give rise to a new species
- Large scale
- Thousands/ millions of generations
- At/ above the level of species
- New species is formed
- Descendants are different from ancestors as new species have emerged

Question 5

What are the 5 agents of evolutionary change?

Answer 5

1. Natural selection
2. Disruption of gene flow
3. Genetic drift
4. Nonrandom mating
5. Mutation

Question 6

Agents of evolutionary change:
1. Natural selection
Explain why variation (as a result of mutation, meiosis and sexual reproduction) is important in natural selection
Explain the role of natural selection in evolution
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Answer 6

Overproduction of offspring

Variation within a population

Struggle for survival

Survival of the fittest by natural selection

Constancy of numbers

Formation of a new species over a long period of time

Like produces like

Question 7

Define variation and explain how variation arises within a population

Answer 7

Variation is differences between individuals of the same species due to the presence of different alleles
It includes morphological, physiological, biochemical and genetic differences
Variations arise spontaneously and are not dictated by the need of organisms to survive better in an environment
Variations help ensure perpetuation of species and hence safeguard species from extinction when the environment changes
Variation arises due to meiosis and sexual reproduction with the random fusion of male and female gametes

Question 8

Does meiosis and random fusion of gametes introduce new alleles to the population?

Answer 8

No; they merely reshuffle existing alleles among individuals resulting in differences in phenotypes
Mutations however introduce new alleles resulting in new phenotypes and hence increased variation

Question 9

Describe survival of the fittest by natural selection

Answer 9

When the environment changes, variation allows inidivuals with a selective advantage (i.e. those with favourable alleles that confer an advantageous characteristic to the individuals) survive and reproduce more successfully than others; these survivors have characteristics that are selected for/ favoured by the environment; survivors get a chance to produce viable and fertile offspring

Question 10

What is selection pressure?

Answer 10

Any cause that reduces reproductive success in a proportion of a population

Question 11

Explain the formation of a new species over a long period of time

Answer 11

The unequal ability of individuals to survive and reproduce will lead to a gradual change in a population
With each succeeding generation, the proportion of individuals with advantageous characteristics increases while those lacking the characteristics decreases
Favourable characteristics and hence favourable genotypes accumulate over time, changing allele frequency
Over hundreds and thousands of generations, a new species may form

Question 12

Describe the 3 types of natural selection

Answer 12

1. Directional selection
   Phenotype at one extreme is selected for
   Favours what are initially relatively rare individuals
   Once the new mean phenotype coincides with new optimum environmental conditions, stabilising selection takes over
2. Disruptive selection
   Intermediate phenotypes are selected against
   Favours individuals on both extremes of a phenotypic range
   Possible to result in polymorphism where two or more forms are found in one species
3. Stabilising selection
   Extreme phenotypes are selected against
   Favours more common intermediate variants in a population
   Does not promote evolutionary change but maintains phenotypic stability

Question 13

Define gene flow

Answer 13

Gene flow is the transfer of alleles from one population to another through the movement of fertile individuals or their gametes

Question 14

What is the effect of gene flow and its disruption?

Answer 14

Gene flow tends to reduce differences in allele frequencies between neighbouring populations
On the other hand, disruption to gene flow can result in differences in allele frequencies in different populations over time

Question 15

Describe genetic drift and its effects

How is it different from natural selection?

Answer 15

Genetic drift is a change in allele frequency due to chance events (which allele ends up lost from the original gene pool is an indiscriminate random event)
Genetic drift tends to reduce genetic variation in populations through the losses of alleles; the smaller the original population, the greater the impact of genetic drift
Genetic drift is different from natural selection in that alleles are lost from random individuals and not individuals with disadvantageous phenotypes

Question 16

Describe two events of genetic drift and how they affect allele frequency

Answer 16

1. Founder effect
   A few, random individuals from a larger population became pioneers of a newly isolated population and are not likely to carry all the alleles present in the original population
   The new population is small and reproductively isolated. If there is continuous breeding within this pioneer population, rare alleles may become more common
2. Bottleneck effect
   A catastrophic event leading to a drastic reduction in population, resulting in reduction of allele frequencies and even elimination of alleles in a random nature
   The few, random suriviving individuals constitute a random genetic sample of the original population
   Certain alleles may be over-represented or under-represented among the survivors

Question 17

What is a population?

Answer 17

A population is a group of interbreeding individuals belonging to a species and sharing a common geographic area

Question 18

Explain why the population is the smallest unit that can evolve

Answer 18

Evolution is a measure of changes in allele frequencies in a population over successive generations
Long-term effects of natural selection are at the level if the gene and population rather than the individual
The survivors interbreed, thereby passing on favourable alleles to the next generation. Over time, these individuals contribute to allele frequencies changes that can be measured in a population.
Time is needed to measure these allele frequency changes
Allele frequency cannot be measured in one individual but can be measured in a population.
Whilst natural selection occurs through interactions between individual organisms and their environment, it. is the collective genetic response of the population that determines the survival of the species and the formation of new species.
Hence, a population is the smallest unit that can evolve.

Question 19

Define and explain the advantages and limitations of the biological species concept

Answer 19

A species is a group of organisms capable of interbreeding to form fertile, viable offspring and are reproductively isolated from other such groups

Advantage;
Organisms can be studied to see if they can interbreed and produce fertile, viable offspring

Limitations:
The definition cannot be applied to:
Asexually reproducing organisms and extinct species
and some plants that are capable of polyploidy

Question 20

Define and explain the advantages and limitations of the genetic species concept (explain genetic isolation)

Answer 20

A species is a group of genetically compatible interbreeding organisms in a natural population that is genetically isolated from other such groups. Organisms in a species have sufficient similiarity in their DNA sequences and share the same number of chromosomes

Genetic isolation results as 2 genomes diverge to the point that they are genetically distinct and evolve independently of each other
Genetic isolation maintains integrity of gene pools in natural populations. However, if the 2 sister species are mated on rare occasions, fertile viable offpsirng may still be possible (genetically but not reproductively isolated)

Advantages:
Genetic data from mitochondrial and nuclear DNA to identify species can be unambiguous in deducing evolutionary relationships

Limitation:
Technology required to study DNA sequences is relatively expensive

Question 21

Define and explain the advantages and limitations of the ecological species concept

Answer 21

A species is a group of organisms sharing the same ecological niche

Advantages:
Every organism has a niche hence the identity of a species is unambiguous

Limitations:
It cannot be applied to unrelated species that occupy similar niches
Many animals are elusive and determining niches can be time-consuming and difficult

Question 22

Define niche

Answer 22

Both the place where an organism lives and its interactions with the environment

Question 23

Define and explain the advantages and limitations of the morphological species concept

Answer 23

A species is a group of organisms sharing similar body shape and other structural features

Advantage:
Every organism has morphological features that can be easily studied

Limitations:
It is difficult to determine the degree of difference that is required to indicate separate species as well as what structural features should be used to distinguish the differences
Some organisms may be structurally similar but have different evolutionary origins

Question 24

Define and explain the advantages and limitations of the phylogenetic species concept

Answer 24

A species is the smallest group of organisms that share a most recent common ancestor and can be distinguished from other such groups. Phylogentic history of a species can be obtained by comparing homologous morphological structures and/or homologous molecular sequences with those of other organisms

Advantages:
Avoids mistakenly classifying organisms based on superficial morphological similarities as the characteristics that are compared are based on common ancestry/ homology
To determine phylogeny, genetic, DNA sequences, amino acid sequences, morphological, fossil evidence, hybridisation studies are conducted so phylogenetic concept potentially. presents most accurate representation of a species
Provides accurate historical information about speciation history that biological concept of a species cannot

Limitation:
Accuracy of phylogenetic tree is dependent on availability, diversity and accuracy of source data. If for instance, DNA evidence was not used, the phylogenetic tree may not be that reliable

Question 25

Define speciation

Answer 25

Speciation is a process by which one or more new species arises from a previously existing species

Question 26

What is required for speciation to occur?

Answer 26

Disruption of gene flow

Question 27

What are the two types of speciation and what is the difference?

Answer 27

1. Allopatric speciation
   Members are geographically isolated due to a physical barrier resulting in gene flow disruption and evolutionary changes to occur independently within each sub-population i.e. different genetic changes from accumulation of mutations and changes in allele frequencies through genetic drift and natural selection within each sub-population; over time, a new species forms
2. Sympatric speciation
   Members are not geographically isolated. However a reproductive barrier emerges resulting in gene flow disruption and evolutionary changes to occur independently within each sub-population i.e. different genetic changes from accumulation of mutations and changes in allele frequencies through genetic drift and natural selection within each sub-population; over time, a new species forms

Question 28

Describe three ways in which sympatric speciation may arise

Answer 28

Physiological isolation where mating between individuals of different sub-populations existing in the same geographical area is not possible due to unique physiology

Behavioural isolation where mating between certain individuals of a population existing in the same geographical area does not occur, due to unique mating rituals and preferences

Reproductive isolation through the emergence of a rreproductive barrier

• prezygotic mechanism where mating/ fertilisation is prevented
• postzygotic mechanism that causes zygote fatality or prevents hybrids from developing into fertile adults