Biology

Key Unit Competence

Explain the role of artificial and natural selection in the production of varieties of animals and plants with increased economic importance

Learning objectives

By the end of this unit, I should be able to:

–Explain that natural selection occurs as populations have the capacity to produce many offspring that compete for resources.–

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

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

–Explain how a change in allele frequency in a population can be used to measure evolution.

–Describe how selective breeding (artificial selection) has been use to improve the milk yield of dairy cattle.–Outline the following examples of crop improvement by selective breeding:

–The introduction of disease resistant varieties of wheat, tomatoes, Irish potatoes, and rice.

–Inbreeding and hybridization to produce vigorous, uniform varieties of maize

–Interpret graphs on how fur length affects the number of individuals at different temperatures.

–Use the Hardy-Weinberg principle to calculate allele, genotype and phenotype frequencies in populations.–Differentiate between natural and artificial selection.

–Appreciate that the environment has considerable influence on the expression of features that show continuous (or Quantitative) variation.

–Appreciate the importance of selective breeding (artificial selection) to improve features in ornamental plants, crop plants, domesticated animals and livestock

Introductory activity

Plants like Alula (Brighamia insignis) also known as cabbage on a stick, wild Ginkgo biloba, Angel’s trumpets; and animals like dinosaurs, ichthyosaur (reptile), ancestor finches, passenger pigeon, western African black rhinoceros, dark-mice population in desert, giraffe with short neck etc which existed in some years back are no longer exist today. Moreover, today, there are both animals like jersey, dogs, Japanese koi fish, snails, male bird of paradise, peacock, wood duck...; and plants such as wheat, cabbages, lemon-orange, tomatoes, avocado, peach and corn among others which have not existed before.

What do you think are the factors contributing to the species extinction on one hand and arrival of new species on the other hand?

16.1 Natural selection

Activity 16.1

Use available school resources such as internet, and library; search information about evolution or natural selection and answer the following:

1. Discuss how natural selection does occur.

2. Discuss the benefits of natural selection to a population.

3. Observe the graphs below, analyse and interpret them and then deduce different types of natural selection.

Key: Blue line indicates a given population after natural selection while Red line indicates a given population before natural selection

4. Construct and interpret graphs on how temperature affects fur length in a population of a particular mammal such as gorilla, camel, and rabbit.

5. Discuss about natural selection with specific examples such as antibiotic resistance in bacteria, pesticide resistance in insects and mammals and industrial melanism.

16.1.1 Natural selection as a means of evolution as well as capacity to survive and reproduce

Throughout the lives of the individuals, their genomes interact with their environments to cause variations in traits from genotypic to phenotypic variations among the individuals in a population because of differences in their genes.

Individuals with certain variants of the trait may survive and are capable to reproduce more than less successful individuals with unfavourable characters; therefore, the population evolves. Over time, this process can result in populations that specialise for particular ecological niches (microevolution) and may eventually result in speciation (the emergence of new species also known as macroevolution). In other words, natural selection is a key process to change organisms and make them suitable to different environment.

The variants that are best adapted to their natural environment such as abiotic conditions, predation, competition to food, space, light, water and resistance against diseases will be selected for survival and can reproduce. By reproduction, organisms transmit their physical traits contained within their genes or alleles to their next generation. The individuals that best suited or fitted to the stated before environmental conditions will have the best chance to survive and produce fertile offspring due to characteristic features or favourable characteristics that give them an advantage in the struggle for existence being intraspecific or interspecific competition. However, those with unfavourable characteristics are more likely to die due to lack of resources or not having access to resources. The high or birth rate gives a selective advantage whereas high mortality or death rate gives them a selective disadvantage.

As environmental conditions gradually change, certain characteristics within a population also gradually change; thus, randomly varying population are favoured, and natural selection occurs. This is known as the survival of the fittest. The fittest in evolution is defined as the ability of an organism to pass on its alleles to subsequent generations, compared with other individuals of the same species.

16.1.2 Types of natural selection

As it has been mentioned, environment is a responsible agent of natural selection. Thus, it selects and determines individuals in different ways according to different types of natural selections. Those natural selections are stabilizing selection, directional selection, and disruptive selection among other.

a. Stabilising selection

Stabilising selection is a type of natural selection in which a population mean stabilises on a particular non-extreme trait value as result of genetic diversity decreases as illustrated in the figure below.

As illustrated in the above figure, in stabilizing selection, natural selection favours the individuals in the population with the intermediate phenotypes. These individuals have greater survival and reproductive success. Individuals with extreme phenotypes are less adaptive and are therefore eliminated. An example is the newly-born human babies who are under 2.27 Kg or over 4.54 kg are less likely to survive than babies weighing between 2.27 and 4.54 kg. Despite of this, with advances in medical science, the survival chances of newly-born underweight or overweight babies have now been improved.

b.Directional selection

Directional selection is a mode of natural selection in which a single or new fit phenotype is favoured when exposed to environmental changes, causing a population genetic variance or allele frequency to continuously shift in one direction or one end of the spectrum of existing variation.

A classical description of directional selection has been identified in eighteenth and nineteenth century in England as illustrated in the figure 16.1.b above. Prior to the industrial revolution, the moths were predominately light in colour, which allowed them to blend in with the light-coloured trees and lichens in their environment. As soot/black powder began spewing from factories, the trees darkened and the light-coloured moths became easier for predatory birds to spot.

Over time, the frequency of the melanic form of the moth increased because their darker coloration provided camouflage against the sooty tree; they had a higher survival rate in habitats affected by air pollution. The result of this type of selection is a shift in the population’s genetic variance towards the new and fit phenotype.These individuals with extreme phenotypes have greater survival and reproductive success.

c. Disruptive or diversifying selection

In disruptive selection, both the extreme phenotypes in the population are selected and become more prevalent. The individuals with extreme phenotypes or end- phenotypic spectrum have greater survival and reproductive success. The disruptive selection pressure increases the chances of the advantageous alleles to be passed on to the next generation. By disruptive selection, the intermediate phenotype is selected against and gradually decreases in number from generation to generation, and may become extinct.

From the above figure, disruptive selection many generations may cause the formation of two separate gene pools and the formation of new species.

Disruptive selection is mostly seen in many populations of animals that have multiple male mating strategies such as; rabbits, mice, and lobsters among others and is often the source of speciation or drives to speciation.

n rabbits as illustrated in the figure 16.1.c, a hypothetical population in which grey and Himalayan (grey and white) rabbits are better able to blend with a rocky environment than white rabbits. Large dominant alpha lobster males obtain mates by brute force, while small males can sneak in for furtive copulations with the females in an alpha male’s territory. In this case, both the alpha males and the sneaking males will be selected for, but medium-sized males, which cannot overtake the alpha males and are too big to sneak copulations, are selected against.

In scenario case of mice, those living at the beach where there is light-coloured sand interspersed with patches of tall grass. Light-coloured mice that blend in with the sand would be favoured, as well as dark-coloured mice that can hide in the grass. Medium-coloured mice, on the other hand, would not blend in with either the grass or the sand, thus, would more probably be eaten by predators. The result of this type of selection, is increased genetic variance as the population becomes more diverse.

The three types of natural selection are summarized in the figure 16.1.d above. It shows populations of species which are selected by the environment particularly the temperature on fur colour and those which decreases to extinction.

Self-assessment 16.1

1. Distinguish among the different of natural selection.

2. Describe what is meant by industrial melanism and how is beneficial to peppered moth

3. Discuss how natural selection is one way of evolution and allows individual can survive and reproduce

16.2 Artificial selection

Activity 16.2

From your daily experience and or carry out project work on plants (cabbage, banana, wheat, maize, tomatoes, irish potatoes, and rice) and animals (cattle and chicken) at your school or home. Do also research through internet and textbooks and then answer to the questions below:

1. Discuss what is meant by artificial selection

2. Distinguish between inbreeding and outbreeding selection

3. Discuss how selective breeding or artificial selection has been used to improve the yield or production of plant crops such as maize, wheat, tomatoes, and rice as well as milk and meat

Artificial selection is selective breeding that occurs when humans instead of environmental forces select and determine the desirable alleles of plants or animals to be passed on to successive generations. Artificial selection has been practiced by humans for several centuries. It has played an important role in the evolution of modern crop plants, farm animals and domestic pets from the wild ancestors. For example, farming took place about 7000 years ago. The first crops humans selected and domesticated include barley and wheat. By artificial selection, some scientists argue that artificial selection and biotechnology can combine characteristics within a short period of time that natural selection would require thousands or millions of years to carry out.

It exerts/influences a directional selection pressure which leads to changes in the frequencies of alleles and genotypes which have been selected by nature in the population.

16.2.1 Advantages of artificial selection

Some of the advantages of artificial selection are:

–It is the quickest and more certain method of producing offspring for a desirable characteristic.

–It selects and breeds animals and plants that can adapt and tolerate to live in certain habitats or different environmental conditions such as heat, cold, day length, and salinity or pH changes in the soil.

–It produces organisms that are resistant to pests, diseases or herbicides.

–It selects and breeds crop plant such as wheat, barley, rice, and maize plants for high productivity or yield per unity area.

–It selects and breeds farm animals for better quality and quantity of milk, meat production and wool quality.

–It leads to plants of fast germination seeds capacity, higher growth rate, early maturation, better absorption of water, mineral salts or fertilizers. This allows the planting of the same type of crop two or three times in one season and therefore increases their production.

–Animals for sports or hobbies such as horses for racing and transport; pigeons for flight capacity and plumage type; dogs as guardians or for hunting, racing and appearance; orchids, roses and other flowers to produce more colourful bloom; koi (a beautiful ornamental fish of striking colours-reds, golds, blues, yellows, metallic silvers and even greens) fish for appearance from coloured mutants of common food carp are produced.

16.2.2 Types of artificial selection

Inbreeding and outbreeding are the two distinguishable types of artificial selection.

a. Inbreeding

Inbreeding is the selective crossing between individuals that have a similar genotype or are more closely related than if they had been chosen at random from the entire population. Examples of inbreeding include; selfing in plants, mating between offspring with one of the parents, among siblings or closely related individuals.

It has noticed that after several generations, the force of selection of inbreeding increases the frequency of homozygous genotypes. Thus, the organism is probably purebred, or homozygous for the selected characteristics. By inbreeding, organism tends to maintain the desirable characteristics such as increase the quantity and quality of milk by jersey cows (high cream content), produce maize plants and others of uniform height to facilitate mechanical harvesting, increase oil content of linseed oil to reduce cost of production and extraction, increase yields from plant crop and ivestock, use less land for farming or raising livestock but increase, breading of horses for racing, and produce varieties of dogs for competition or as security guard for example.

Even though, inbreeding is advantageous as described in above; it also presents disadvantages that include:

–After several generations of excessive inbreeding, it results into inbreeding depression. The inbreed progeny have decreased/loss vigor resulting from excessive selective inbreeding between closely related organisms which increases homozygosity (production of individuals with harmful or undesirable phenotypic characteristics), poor growth and yield and decline in fertility than non-inbred individuals.

–There is an increased risk of lowered diseases resistance as genetic variation is reduced. Thus, inbreeding is not encouraged by animal breeders.

b.Outbreeding

Outbreeding is the controlled mating or crossing between distantly related individuals (plants and animals) with desired characteristics e.g. the cross between Elaeis guineensis (African oil palm or macaw-fat) variety dura with Elaeis guineensis variety pisifera to produce the hybrid oil palm Elaeis guneesis variety tenera, with fruits of high oil content and do not drop off easily. They may come from two breeds of the same species or may come from different species. Outbreeding is more advantageous than inbreeding because:

–The progeny also known as hybrids usually show more variation than progeny produced by inbreeding. The hybrids usually have new and superior phenotypes and have greater potential to adapt to environmental changes for example wheat, tomatoes and rice produced by outbreeding are capable to resist to diseases.

–Increases heterozygosity and new opportunities for gene interaction. Harmful recessive alleles are masked by dominant alleles.

However, in some cases outbreeding results in hybrid vigour; healthier; or larger offspring. And the hybrid produced between genetically different species are often sterile. They do not have sets of homologous chromosomes and meiosis cannot proceed to produce fertile gametes.

Self-assessment 16.2

1. Explain how artificial selection is beneficial to man.

2. Distinguish between inbreeding from outbreeding.

16.3 Allele frequency and its causes

Activity 16.3

Use available school resources such as internet, library, and teachers; search information about allele frequency, selection, the founder effect and genetic drift and or use pictures (a) and (b) given in question of this activity or use bean seeds of different colour and play a game as instructed:

a. Take 15 bean seeds and then put all in one plastic bottle such as the one of mineral water or power soap

b. Take other three empty bottlesc. Shake the bottle containing bean seeds and randomly distribute seeds into the three bottles. Record and discuss the observations

d. Repeat events in step c) at least three times.

e. Draw the conclusion by linking the discussion to what they have read on allele frequency, founder effect, and genetic drift

Then, do the following:

1. Discuss what is meant by allele frequency

2. Discuss how forces of mutation and natural selection affect the allele frequencies

3. Analyse the figures below and then describe how the founder effect and genetic drift affect the allele frequencies in populations

4. Discuss how a change in allele frequency in a population can be used to measure evolution

16.3.1 Allele frequency in a population as determinant of evolution

Genetic variation which confirms evolution is determined by; mutation, natural selection, the founder effect, and genetic drift among others.

a. Mutation and natural selection

In a particular period, why do some organisms survive while others die? These surviving organisms generally possess traits or characteristics that bestow / give them traits or benefits of great value benefits that help them survive (e.g. better camouflage, mating, faster swimming or running, or digesting food more efficiently) as discussed before. Each of these characteristics is the result of a mutation or a change in the genetic code.

Mutations occur spontaneously, but not all mutations are heritable; they are passed down to offspring only if the mutations in the gametes. These heritable mutations are responsible for the rise of new traits in a population. Populations or gene pools evolve as gene frequencies change otherwise individual organisms cannot evolve. Variation in populations is determined by the genes present in the population’s gene pool as illustrated in figure below, which may be directly altered by mutation.

In natural selection, those individuals with superior traits will be able to compete and get more resources as there are more organisms than resources and produce more offspring. The more offspring an organism can produce, the higher its fitness. As novel traits and behaviours arise from mutation, natural selection preserves the traits that confer a benefit.

As mutations create variation, natural selection gradually affects the frequency of that advantageous trait in a population.

b.The founder effect

The founder effect occurs when part of a population becomes isolated and establishes a separate gene pool with its own allele frequencies. When a small number of individuals become the basis of a new population, this new population can be very different genetically from the original population if the founders are not representative of the original. Therefore, many different populations, with very different and uniform gene pools, can all originate from the same, larger population. Together, the forces of natural selection, genetic drift, and founder effect can lead to significant changes in the gene pool of a population.

c. Genetic drift

Genetic drift is an overall shift of allele distribution in an isolated population, due to random fluctuations in the frequencies of individual alleles of the genes. When selective forces are absent or relatively weak, gene frequencies tend to drift or change due to random events. This drift halts when the variation of the gene becomes “fixed” by either disappearing from the population or replacing the other variations completely. Even in the absence of selective forces, genetic drift can cause two separate populations that began with the same genetic structure to drift apart into the two divergent populations.

In the above simulation, there is fixation in the blue gene variation within five generations. As the surviving population changes over time, some traits (red) may be completely eliminated from the population, leaving only the beetles with other traits (blue).

16.3.2 Allele frequency

Natural selection affects a gene pool by increasing the frequency of alleles that give an advantage, and reducing the frequency of alleles that give a disadvantage. The allele frequency (or gene frequency) is the rate at which a specific allele appears within a population. In population genetics, the term evolution is defined as a change in the frequency of an allele in a population. Frequencies range from 0, present in no individuals, to 1, present in all individuals. The gene pool is the sum of all the alleles at all genes in an interbreeding population.

A gene for a particular characteristic may have several variations called alleles. These variations code for different traits associated with that characteristic. For example, in the ABO blood type system in humans, three alleles (IA, IB, or i) determine the particular blood-type protein on the surface of red blood cells. A human with a type IA allele will display A-type proteins (antigens) on the surface of their red blood cells. Individuals with the phenotype of type A blood have the genotype IAIA or IAi, type B have IBIB or IBi, type AB have IAIB, and type O have ii.

A diploid organism can only carry two alleles for a particular gene. In human blood type, the combinations are composed of two alleles such as IAIA or IAIB. Although each organism can only carry two alleles, more than those two alleles may be present in the larger population. For example, in a population of fifty people where all the blood types are represented, there may be IA alleles than i alleles. Population genetics is the study of how selective forces change a population through changes in alleles and genotypic frequencies.

Using the ABO blood type system as an example, the frequency of one of the alleles, for example IA, is the number of copies of that allele divided by all the copies of the ABO gene in the population, i.e. all the alleles. Allele frequencies can be expressed as a decimal or as a percent and always add up to 1, or 100 percent, of the total population. For example, in a sample population of humans, the frequency of the IA allele might be 0.26, which would mean that 26% of the chromosomes in that population carry the IA allele. If we also know that the frequency of the IB allele in this population is 0.14, then the frequency of the i allele is 0.6, which we obtain by subtracting all the known allele frequencies from 1(thus: 1-0.26-0.14=0.6). A change in any of these allele frequencies over time would constitute evolution in the population.

Self-assessment 16.3

1. What is allele frequency?

2. Explain how mutation and natural selection are important in gene frequency?

3. In a situation where a trait is determined by two allele forms. What is the frequency of each allele form?

4. Using illustrations, explain genetic drift and founder effect.

16.4 Study of population genetic variation by Hardy-Weinberg principle

Activity 16.4

Use available school resources such as internet, library, search information about Hardy-Weinberg principle, allele, genotype and phenotype as well as allele frequency and then do the following:

1. What is Hardy-Weinberg principle

2. If the frequency of a recessive allele is 0.2. What is the frequency of a dominant allele?

3. Cross one homozygous dominant individual of yellow colour with one homozygous recessive pea plant of green colour. Calculate both genotype, phenotype and allele frequencies by using Hardy-Weinberg principle if the recessive allele is equal to 0.4.

The Hardy-Weinberg principle is a mathematical baseline way used to estimate the frequency of alleles, genotypes and phenotypes in a population. The principle assumes that in a given population, the population is large and is not experiencing mutation, migration, natural selection, or sexual selection.

The Hardy- Weinberg principle states that the frequency of alleles in a population can be represented by P + Q = 1, with P equal to the frequency of the dominant allele and Q equal to the frequency of the recessive allele.

The principle also states that the frequency of genotypes in a population is represented by

p2 + 2pq + q2 = 1, with p2 equal to the frequency of homozygous dominant genotype, pq equal to the frequency of the heterozygous genotype, and q2 equal to the frequency of the Homozygous recessive genotype.

The frequency of alleles can be estimated by calculating the frequency of the

recessive genotype, then calculating the square root of that frequency in order to determine the frequency of the recessive allele.

By referring to the above chart, by applying the expression of Hardy-Weinberg principle, if the dominant allele is illustrated below as Y is equal to 0.7 while the recessive allele noticed as y is equal to 0.3; then by using the Hardy-Weinberg principle p2+2pq+q2=1, if the number of individuals is given as 500 and number of alleles in a gene pool is 1000; genotypic and allelic frequencies are calculated as illustrated follow by Y2 + 2Yy + y2 = 1 and p + q = 1 respectively:

16.4.1 Hardy-Weinberg analysis

The Hardy-Weinberg principle states that a population’s allele and genotype frequencies will remain constant in the absence of evolutionary mechanisms. Ultimately, the Hardy-Weinberg principle models a population without evolution under the conditions such as; no mutations, no immigration/emigration, no natural selection, no sexual selection and a large population. Although there is no real-world population can satisfy all of these conditions, the principle stiff offers a useful model for population analysis.

16.4.1 Hardy-Weinberg equations and analysis

According to the Hardy-Weinberg principle, the variable p often represents the frequency of a particular allele, usually a dominant one. For example, assume that p represents the frequency of the dominant allele, Y, for yellow pea pods. The variable q represents the frequency of the recessive allele, y, for green pea pods. If p and q are the only two possible alleles of this characteristic, then the sum of the frequencies must add up to 1, or 100 percent. This can also be written as p+q=1, if the frequency of the Y allele in the population is 0.6, then we know that the frequency of the y allele is 0.4.

From the Hardy-Weinberg principle and the known allele frequencies, we can also infer the frequencies of the genotypes. Since each individual carries two alleles per gene (Y or y), we can predict the frequencies of these genotypes with chi square. If two alleles are drawn at random from the gene pool, we can determine the possibility of each genotype.

Self-assessment 16.4

1. Calculate the allelic, genotypic and phenotypic frequencies:

a. When a tall plant is crossed with a short one

b. When a heterozygous is crossed with another heterozygous

c. When heterozygous is crossed with a dominant homozygous. Note that 0.2 is given as a value of recessive allele

2. Calculate the phenotype, genotype and allele frequencies of populations/ hybrids obtained when the crossing is done between YY and Yy individuals. Note that the dominant allele is assumed to be 0.7.

End of unit assessment 16

1. Differentiate between natural selection from artificial selection

2. Some individuals of the swallowtail butterfly scientifically known as Papilio machaon of the family papilionidae pupate on brown stems or leaves; others pupate on green stems or leaves. Two distinct colour forms of the pupae are found, namely brown and green, with very few intermediates.

a. What type of natural selection does this example show?

b. Explain why the intermediate colour formed would be at selective disadvantage.

3. Why are heavy-metal tolerant plants rare in unpolluted regions?

4. What effect did the industrial pollution have on the frequency of the C (melanic) allele within a population of peppered moths.

5. Explain what is meant by heterozygous advantage, using the sickle-cell allele as an example.