Topic 17: Selection and evolution

​

17.1 Variation

 

​Students should be able to:

1) explain, with examples, that phenotypic variation is due to genetic factors or environmental factors or a combination of genetic and environmental factors

2) explain what is meant by discontinuous variation and continuous variation

3) explain the genetic basis of discontinuous variation and continuous variation

4) use the t-test to compare the means of two different samples

​

1. Explain, with examples, that phenotypic variation is due to genetic factors or environmental factors or a combination of genetic and environmental factors

Phenotypic variation refers to the differences in physical traits or characteristics that exist between individuals within a population. These differences arise because of variation in the genetic information that individuals inherit, differences in the environments they experience, or a combination of both.

• Genetic factors: Variation caused by differences in DNA sequences inherited from parents. For example, blood groups in humans are determined solely by genes inherited from the parents and are largely unaffected by the environment. Another example is eye color, which is controlled by different alleles.
   

• Environmental factors: Variation caused by non-genetic influences in an individual’s surroundings. For instance, identical plants grown in different soil types or light conditions may vary in height or leaf color due to environmental factors alone. Likewise, identical twins may have slight differences in weight or muscle tone due to their different lifestyles or nutrition.
   

• Combination of genetic and environmental factors: Many traits result from the interaction of genes and the environment. For example, human height is influenced by inherited genes but can also be affected by nutrition, health, and living conditions during growth. Similarly, skin color can be genetically determined but also influenced by sun exposure.
   

Thus, phenotypic variation arises from complex interactions between an organism’s genotype and the environment it develops in.

​

 

2. Explain what is meant by discontinuous variation and continuous variation

Variation within populations can be categorized into two main types: discontinuous variation and continuous variation.

• Discontinuous variation refers to traits that fall into distinct, separate categories with no intermediate forms. Individuals show one phenotype or another with no blending. For example, human blood groups (A, B, AB, O) are discrete categories controlled by specific alleles. Another example is the ability to roll one’s tongue—people can either do it or not, with no in-between.
   

• Continuous variation describes traits that show a range of phenotypes that gradually blend from one extreme to another, often producing a normal distribution curve. These traits can take many values within a range, such as human height, skin color, or weight. Continuous variation is usually influenced by multiple genes (polygenic inheritance) and environmental factors, leading to a wide diversity of phenotypes.
   

Understanding these types helps explain how traits are inherited and how populations can evolve.

​

 

3. Explain the genetic basis of discontinuous variation and continuous variation

The underlying genetics of discontinuous and continuous variation differ fundamentally:

• Discontinuous variation is typically controlled by a single gene locus with different alleles producing distinct phenotypes. These traits follow simple Mendelian inheritance patterns, where dominant and recessive alleles determine the phenotype. Since only a few alleles and genes are involved, the phenotype is distinct and categorical.
   

• Continuous variation arises from the combined effect of many genes, often called polygenes or quantitative trait loci (QTLs), each contributing a small additive effect to the trait. Because so many genes influence the phenotype, there is a wide range of possible trait values, resulting in continuous variation. Environmental influences also play a role in modifying these traits, adding to the range of variation seen.
   

This polygenic inheritance combined with environmental effects produces the smooth gradient of phenotypes observed in continuous traits.

​

 

4. Use the t-test to compare the means of two different samples (formula provided)

The t-test is a statistical method used to determine if there is a significant difference between the means of two independent samples, such as the heights of plants grown under two different treatments. It accounts for sample sizes, means, and variability.

• The formula uses the means, standard deviations, and sample sizes of both groups to calculate a t-value.
   

• This t-value is then compared to a critical value from the t-distribution table (based on degrees of freedom and significance level).
   

• If the calculated t-value exceeds the critical value, it suggests that the difference between the two sample means is unlikely due to random chance alone, indicating a statistically significant difference.
   

The t-test helps biologists validate hypotheses by quantitatively assessing experimental results.

​

​

17.2 Natural and artificial selection

 

​Students should be able to:

1) explain that natural selection occurs because populations have the capacity to produce many offspring that compete for resources; in the ‘struggle for existence’, individuals that are best adapted are most likely to survive to reproduce and pass on their alleles to the next generation

2) explain how environmental factors can act as stabilising, disruptive and directional forces of natural selection

3) explain how selection, the founder effect and genetic drift, including the bottleneck effect, may affect allele frequencies in populations

4) outline how bacteria become resistant to antibiotics as an example of natural selection

5) use the Hardy–Weinberg principle to calculate allele and genotype frequencies in populations and state the conditions when this principle can be applied (the two equations for the Hardy–Weinberg principle will be provided, as shown in the Mathematical requirements)

6) describe the principles of selective breeding (artificial selection)

7) outline the following examples of selective breeding:

• the introduction of disease resistance to varieties of wheat and rice

• inbreeding and hybridisation to produce vigorous, uniform varieties of maize

• improving the milk yield of dairy cattle

​

1. Explain that natural selection occurs because populations have the capacity to produce many offspring that compete for resources; in the ‘struggle for existence’, individuals that are best adapted are most likely to survive to reproduce and pass on their alleles to the next generation

Natural selection is a fundamental process that drives evolution. It occurs because organisms produce more offspring than the environment can support, leading to competition for limited resources such as food, space, or mates. This competition results in a struggle for existence where not all individuals survive.

Those individuals whose inherited characteristics give them an advantage in their particular environment are more likely to survive, reproduce, and pass on their beneficial alleles to their offspring. This process is often summarized as “survival of the fittest,” where “fitness” refers to an organism’s reproductive success.

Over many generations, advantageous traits become more common in the population, while less favorable traits are eliminated. Natural selection shapes populations by favoring traits that increase survival and reproductive success under current environmental conditions.

 

​

2. Explain how environmental factors can act as stabilising, disruptive and directional forces of natural selection

Natural selection can take different forms depending on how the environment influences the fitness of individuals with different phenotypes:

• Stabilising selection favors individuals with intermediate phenotypes and selects against extreme variations. This results in the maintenance of the status quo for a trait. For example, human birth weight is under stabilising selection because both very low and very high birth weights have higher mortality, so babies with average weights survive best.
   

• Directional selection favors individuals at one extreme of a phenotypic range, causing a shift in the population’s trait distribution over time. For instance, if the environment changes such that larger beak size helps birds access food better, the average beak size will increase over generations.
   

• Disruptive selection favors individuals at both extremes of a trait distribution but selects against the average phenotype. This can lead to two distinct subpopulations and potentially to speciation. An example is a habitat with two distinct types of food sources favoring birds with either very small or very large beaks, but not medium-sized.
   

Understanding these modes explains how populations adapt to their environments or diverge into new species.

​

 

3. Explain how selection, the founder effect and genetic drift, including the bottleneck effect, may affect allele frequencies in populations

Allele frequencies within populations—the proportion of different genetic variants—can change over time by several mechanisms:

• Natural selection changes allele frequencies by increasing the frequency of alleles that enhance survival or reproduction.
   

• The founder effect occurs when a small group of individuals separates from a larger population to establish a new population. This small group’s genetic makeup may not represent the original population’s diversity, causing certain alleles to become more common or rare purely by chance.
   

• Genetic drift refers to random fluctuations in allele frequencies, especially in small populations, due to chance events rather than natural selection. It can lead to loss or fixation of alleles over time.
   

• The bottleneck effect happens when a population undergoes a drastic reduction in size due to events like natural disasters. The survivors’ gene pool may be unrepresentative of the original population, reducing genetic diversity and altering allele frequencies.
   

These processes show that evolution can be influenced by chance as well as by adaptive changes.

​

 

4. Outline how bacteria become resistant to antibiotics as an example of natural selection

Bacteria evolve antibiotic resistance through natural selection:

• Within a bacterial population, some bacteria may have mutations that make them less susceptible or resistant to an antibiotic.
   

• When exposed to the antibiotic, susceptible bacteria die, while resistant bacteria survive and multiply.
   

• These resistant bacteria pass their resistance genes to offspring, increasing the frequency of resistance in the population.
   

• Over time, this leads to populations of bacteria that are much harder to kill with antibiotics, posing a major challenge to medicine.
   

This example illustrates how natural selection acts rapidly in populations with short generation times.

​

 

5. Use the Hardy–Weinberg principle to calculate allele and genotype frequencies in populations and state the conditions when this principle can be applied (formulas provided)

The Hardy-Weinberg principle provides a mathematical model to describe the genetic equilibrium of a population where allele and genotype frequencies remain constant from generation to generation, given certain conditions.

The formulas:

• p+q=1p + q = 1p+q=1 (where ppp and qqq are the frequencies of two alleles)
   

• p2+2pq+q2=1p^2 + 2pq + q^2 = 1p2+2pq+q2=1 (where p2p^2p2, 2pq2pq2pq, and q2q^2q2 represent the genotype frequencies)
   

Conditions for Hardy-Weinberg equilibrium include:

• No mutation
   

• No migration (gene flow)
   

• Large population size (to prevent genetic drift)
   

• Random mating
   

• No natural selection
   

Deviation from these conditions means allele frequencies can change, indicating evolutionary forces at work.

​

 

6. Describe the principles of selective breeding (artificial selection)

Selective breeding is a process where humans intentionally breed organisms with desirable traits to produce offspring that express those traits more strongly.

• The breeder selects parent organisms with favorable phenotypes.
   

• These parents are bred together to combine and amplify the desired traits in the next generation.
   

• Over many generations, selective breeding can produce plants or animals with improved yield, disease resistance, or specific physical characteristics.
   

Artificial selection is a powerful tool in agriculture and animal husbandry for improving crop varieties and livestock breeds.

​

 

7. Outline examples of selective breeding

• Disease resistance in wheat and rice: Breeders select strains that naturally resist fungal infections or pests and crossbreed them to combine resistance genes, improving crop survival and yield.
   

• Inbreeding and hybridisation in maize: Inbreeding produces genetically uniform lines, while crossing these lines (hybridisation) produces vigorous, high-yield hybrids due to heterosis or hybrid vigor.
   

• Improving milk yield in dairy cattle: Cattle with high milk production are selectively bred to enhance this trait in future generations, increasing efficiency in dairy farming.
   

​

17.3 Evolution

 

​Students should be able to:

1) outline the theory of evolution as a process leading to the formation of new species from pre-existing species over time, as a result of changes to gene pools from generation to generation

2) discuss how DNA sequence data can show evolutionary relationships between species

3) explain how speciation may occur as a result of genetic isolation by:

• geographical separation (allopatric speciation)

• ecological and behavioural separation (sympatric speciation)

​

1. Outline the theory of evolution as a process leading to the formation of new species from pre-existing species over time, as a result of changes to gene pools from generation to generation

Evolution is the gradual change in the genetic composition of populations over successive generations. It leads to the formation of new species from existing ones through accumulated genetic changes.

• Changes in allele frequencies in a population’s gene pool occur due to mutation, natural selection, genetic drift, and gene flow.
   

• Over long periods, these changes can produce reproductive isolation between populations.
   

• Once populations can no longer interbreed successfully, new species arise.
   

• This process explains the diversity of life on Earth and how organisms adapt to their environments

​

​​

 

2. Discuss how DNA sequence data can show evolutionary relationships between species

DNA sequencing allows scientists to compare the genetic material of different species:

• Closely related species have more similar DNA sequences, indicating a recent common ancestor.
   

• More distantly related species show greater differences.
   

• By analyzing these genetic similarities and differences, scientists can construct evolutionary trees (phylogenies) that map relationships and divergence times.
   

• DNA evidence often corroborates or refines evolutionary relationships inferred from morphology or fossils.
   

This molecular approach has revolutionized our understanding of the tree of life.

​

 

3. Explain how speciation may occur as a result of genetic isolation by:

​

• Geographical separation (allopatric speciation)

• Populations become physically separated by barriers such as mountains, rivers, or oceans.
   

• This isolation prevents gene flow between groups.
   

• Over time, different mutations, natural selection, and genetic drift act independently on each population.
   

• The gene pools diverge sufficiently so that even if the barrier is removed, the populations can no longer interbreed, forming new species.
   

• Ecological and behavioural separation (sympatric speciation)

• Speciation occurs without physical separation.
   

• Populations occupy different ecological niches or habitats within the same area, reducing interbreeding.
   

• Behavioral differences, such as mating calls or breeding times, also prevent gene flow.
   

Genetic differences accumulate until reproductive isolation occurs, creating new species despite shared geography.