Biology

Key Unit Competence 

Analyze the relevance of theories of evolution and explain the process of speciation.

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

 – State the general theory of evolution that organisms have changed over time. – Discuss the molecular evidence that reveals similarities between closely related organisms with reference to mitochondrial DNA and protein sequence data. 

– Explain the causes of present day evolution. 

 – Explain the role of pre-zygotic and post-zygotic isolating mechanisms in the evolution of new species. 

– Explain how speciation may occur as a result of geographical separation (allopatric speciation), and ecological and behavioural separation (sympatric speciation).

– Explain why organisms become extinct, with reference to climate change, competition, habitat loss and killing by humans. 

– Explain large-scale extinctions in earth’s history 

– Observe and interpret mitochondrial, DNA and protein sequence data and investigate the similarities of closely related organisms.  

– Relate diagrams of Darwin’s finches to the mechanism of evolution. 

– Research evidence for evolution. 

– Acknowledge that over the years the theories of evolution have undergone modifications as more evidence is collected.  

– Appreciate that over prolonged periods of time, some species have remained virtually unchanged, while others have changed significantly and many others have become extinct.

Introductory activity 

1. The coyote, jackal and dingo are closely related species of the dog family. Their distribution is shown on the map.

Suggest and explain how these three distinct species evolved from a common ancestor. 

2. Observe and analyse the pictures below. From your observation and analysis, do you think there is relationship between individuals? If yes, which ones? Is there any difference? If so, what does it cause or has caused it?

17.1 Theories of evolution 

Activity 17.1 

Use the school library and search additional information on the internet, read the information related to evolution 

1. Write a short note on the term evolution. 

2. Identify the importance of studying evolution

Evolution is the process by which new species are formed from pre-existing ones over a period of time. It is not the only explanation of the origins of the many species which exist on earth, but it is the one generally accepted by the scientific world at the present time.  Evolution is marked by emergence of new species from preexisting species and the disappearance of some species.  The species that disappear are said to become extinct. 

Studying evolution helps to understand the biological forces that cause organisms to develop from simple to more complex organisms to the extent of new species emerging. It also helps to know how different organisms relate to each other and one another. 

The evolution is explained through different theories namely; Lamarckism, Darwinism, Neo-Darwinism, and Special creation. 

1. Lamarckism/ Lamarckian inheritance theory

Lamarckism is briefly described as follows:

– An organism can pass on characteristics that it acquired to its offspring. 

– Organisms evolve overtime due to the environmental factors that act up on that organism. For example: A giraffe’s neck grows longer overtime because the giraffe’s desire for treetop leaves.

Figure 17.1:  Lamarck’s giraffe: A giraffe’s neck grows longer overtime because the giraffe’s desire for treetop leaves.

a. Assumptions of Lamarck’s theory 

– Organisms tend to increase in size as they become more complex to a predetermined limit. 

– When influenced by the environment, body changes can be induced in organisms.

– Organisms acquire new features because of need. 

– Development of an organ and its effectiveness is promoted by its use whereas its disuse brings about decline. 

– Acquired features are inherited by future generations.

b. Merits/Advantages 

– Lamarck was able to show that the environment influences the course of evolution. 

– He observed that features are passed down from parents to their offspring. 

– He was able to recognize that as organism increase in size, they become more complex to a predetermined limit. (Predetermine: to determine or decide in advance) 

c. Demerits /disadvantages

 – Acquired changes are not heritable as they are influenced by genes. 

– Somatic changes are not heritable as they are not passed through reproduction. 

– The process of gametogenesis is not related to occupation or their activity. 

– Use or disuse of somatic cells does not affect gamete formation.

2. Darwinism/Theory of natural selection

The term Darwinism has been applied to the evolutionary theories of Charles Darwin (1809-1882). Darwin’s theory of natural selection is important landmark in the evolutionary process and the origin of species. Darwin’s theory of evolution had a great impact because it was supported by a wealth of evidence. 

According to Darwin’s theory:

– Each species living today arose from a pre-existing species. 

– All species have evolved from one ancestral type. 

– Natural selection provides the mechanism for one species to change into another. The main evidence for his first suggestion, which has been called descent with modification, comes from fossils. 

Essential features of Darwin’s theory of natural selection 

Charles Darwin conducted extensive research on plants and animals in order to study the process of evolution. The essential features of the theory Darwin put forward are:

 – Overproduction of offspring: All organisms produce large numbers of offspring which, if they survived, would lead to a geometric increase in the size of any population 

– Constancy of numbers: Despite the tendency to increase numbers due to overproduction of offspring, most populations actually maintain relatively constant numbers. 

– Struggle for existence: Darwin deduced on the basis of 1 and 2 that members of the species were constantly competing with each other in an effort to survive. In this struggle for existence only a few would live long enough to breed 

– Variation among offspring: The sexually produced offspring of any species show individual variations, so that generally no two offspring are identical. 

– Survival of the fittest by natural selection: Among the offspring there will be some better able to withstand the prevailing conditions. That is, some will be better adapted (fitter) to survive in the struggle for existence. These types are more likely to survive long enough to breed. 

– Like produces like: Those that survive to breed are likely to produce offspring similar to themselves. The advantageous characteristics that gave them the edge in the struggle for existence are likely to be passed on to the next generation. 

– Formation of new species: Over many generations, the individuals with favorable characteristics will breed, with consequent increase in their numbers. The development of a number of variations in a particular direction over many generations will gradually lead to the evolution of a new species.

Darwin’s theory was based on three main observations: 

– Within a population are organisms with varying characteristics, and these variations are inherited (at least in part) by their offspring. 

– Organisms produce more offspring than are required to replace their parents 

– On average, population numbers remain relatively constant and no population gets bigger indefinitely. 

From these observations, Darwin came to the conclusion that within a population many individuals do not survive, or fail to reproduce. In his study of birds, he found that after arriving at the islands, the Finches were dispersed in varied environmental conditions. In due course of time, the anatomy of birds was modified naturally as an adaptation to the prevailing conditions especially food regimes.

Figure 17.2: Five of Darwin’s finches 

• Assumptions of Darwinism 

– Most organisms have the potential to produce large number of offspring or progeny than the environment can support. This leads to still competition as the numbers of organisms are fairly stable. 

– All organisms, even of the same species vary in a few characteristics, 

– Only those organisms of a given species with variations that adapt them to the environment, survive the competition and live. There is survival for the fittest by natural selection. 

– The features favored/selected by nature survive and are inherited. Therefore, new species may develop by natural selection, which is one of the forces of evolution.

 • Merits of Darwin’s theory of natural selection 

– Species always change as the environment changes. 

– Species are compared with their ancestors due to presence of similarities in characteristics. 

– Enough data are / can be collected for explaining variation in a population that may result into formation of a new species.

• Demerits of Darwin’s theory of natural selection 

– Not all variations inherited, except for only genetic variations. 

– It provides inadequate explanation of existence of many vestigial structures in organisms.

– Explanation on deleterious mutations that are retained in a population is not adequate.

3.  Neo-Darwinism 

 The modern theory of evolution is called Neo-Darwinism (neo= new) because it incorporates new scientific evidence, particularly from genetics and molecular biology. For example, we now know that the variations that are so important in natural selection come about by random and spontaneous changes in genes, particularly from mutations in reproductive cells. According to neo-Darwinism, nature selects those individuals with beneficial mutations and allows them to be passed to their offspring through reproduction from generation to generation. The mutations are transmitted within the population and if selected by nature, they may form a new species. 

4. Special creation 

It is believed that a special being, God created the universe and all living organisms. In this theory, heavens and earth were first created. Light, day and night were created next and subsequently, all living things with human beings the last in the creation. It shows that there was direct creation of organism with no precursor to life. 

Self-assessment 17.1 

1. Give the biological meaning of evolution 

2. How does neo- Darwinism differ from Darwin’s original theory of evolution?

17.2. Evidence of evolution

Activity 17.2 

Use the school library and internet to search and read the information related to evidence of evolution with particular emphasis on molecular evidence.

Make a table showing that the molecular evidence reveals similarities between closely related organisms with reference to mitochondrial DNA and protein sequence data.

17.2.1 Palaeontology: the study of fossil 

A fossil is the remains of an organism that lived in the past, preserved by a natural process (for example, in rock, peat, or ice). Fossils include; bones, shells, footprints, and faeces. Most of fossils are found in sedimentary rocks formed by layers of silt. Rocks and their fossils can be dated approximately on the basis of how long it takes for sedimentary rocks to be laid down. However, these estimates are very rough. More accurate estimates come from measuring the radioactivity of crystals of igneous rock in the strata.

Figure 17.3: Fossil formation. Fish B becomes a fossil much later than fish A. The deeper the rock layer, the older the fossil.

The level of radioactivity is greatest when the crystals first form. As they age, the isotopes decay: uranium to lead, and potassium to argon. The older the rock, the less original radioactive material remains. Fossils can therefore be detailed by analyzing the amounts of uranium and lead, or potassium and argon, they contain. Potassiumargon dating is often used to date fossils because potassium is a common element found in many types of rock, and it decays to argon very slowly. This allows rocks up to 3000 million years old to be dated. Sometimes younger fossils can be dated by radioactive carbon dating.

17.2.2 Comparative biochemistry and cell biology 

The most persuasive evidence that all organisms have evolved from a common ancestor comes from studies comparing the cell biology and biochemistry of different organisms, which reveal that: 

– The genetic code contained within nucleic acids is almost universal 

– Physiological processes vital to all organisms, such as respiration, follow very similar metabolic pathways. 

– ATP is the universal energy currency 

The cellular and biochemical details of organisms are quite similar, but any differences can give an idea of how closely different species are related. Species that are closely related would be expected to differ only slightly from each other. Detailed comparisons of DNA, metabolic pathways, key proteins, and organelles such as ribosomes have been used to work out the evolutionary relationships of organisms. For example, ribosomes inside mitochondria and chloroplast are similar to those in bacteria, suggesting that these organelles may have evolved from bacteria. Mammalian blood proteins can be tested to see how similar they are to human blood proteins: blood serum from the mammal in question is added to rabbit serum containing anti-human antibodies

17.2.3 Comparative embryology

Observations have shown that species that are known to be closely related show a similar embryonic development. Therefore, species that show a similar embryonic development are assumed to be closely related, even if the adult stages are very different. For example, echinoderms (the phylum containing starfish and sea urchins) are believed to be related to chordates (the phylum including vertebrates) because of similarities in their early embryonic development.

17.2. 4  Comparative anatomy 

Comparative anatomy is the study of biological structures in different organisms. The scientists look at structures that are similar in different organisms or species. Example: limbs of vertebrates such as human beings, goats and wings of birds are used for different purposes but they have a basic design structure, this is known as homologous structure. The forelimbs of humans are for manipulation, fore limbs of birds (wings) are for flight and fore limbs of a goat are for walking; this shows that all these animals are from common ancestors. Analogous structures are the ones, which look different, but they perform similar functions e.g. insect, birds and bats all have wings used for flight but they have different structural organization.

 Another important line of evidence for evolution comes from DNA analysis. Any permanent change in form or function of an organism must be preceded by a change in its DNA. Organisms which have much of their DNA in common must be closely related, i.e. they have split from a common ancestor comparatively recently (in geological terms). For example, humans and chimpanzees have 99% of their DNA in common which suggests a close relationship and relatively ‘recent’ divergence from a common ancestor.

Self-assessment 17.2

1. By what process do:

a. Analogous structures evolve so that they look alike?

b. Two related but geographically separate groups evolve similar adaptations independently?

3. Give two pieces of evidence from comparative biochemistry that support the theory that all species living today are descended from a common ancestor

17.3 Causes of evolution

Activity 17.3

Use the school library and internet to search and read the information related to the causes of evolution. Make a list of different causes of evolution and write short summary in your own words on the meaning of each cause.

17.3.1 Competition changes in the environment

Imagine that we are plunged into a new Ice Age. The climate becomes much colder, so that snow covers the ground for almost all of the year. Assuming that rabbits can cope with these conditions, white rabbits now have a selective advantage during seasons when snow lies on the ground, as they are better camouflaged (like the hare in figure 17.6). Rabbits with white fur are more likely to survive and reproduce, passing on their alleles for white fur to their offspring. The frequency of the allele for white coat increases at the expense of the allele for agouti. Over many generations, almost all rabbits will come to have white coats rather than agouti.

17.3. 2 Mutations

Because they are random events, most mutations that occur produce features that are harmful. That is, they produce organisms that are less well adapted to their environment than ‘normal’ organisms. Other mutations may be neutral, conferring neither an advantage nor a disadvantage on the organisms within which they occur.

Occasionally, mutations may produce useful features. Imagine that a mutation occurs in the coat colour gene of a rabbit, producing a new allele which gives a better camouflaged coat colour than agouti. Rabbits possessing this new allele will have a selective advantage. They will be more likely to survive and reproduce than agouti rabbits, so the new allele will become more common in the population. Over many generations, almost all rabbits will come to have the new allele. Such changes in allele frequency in a population are the basis of evolution. Evolution occurs because natural selection gives some alleles a better chance of survival than others. Over many generations, populations may gradually change, becoming better adapted to their environments.

17.3.3 Effect of drugs or chemical resistance

Antibiotic resistance is a severe problem throughout the world. For example, some strains of the common bacterium Staphylococcus aureus are resistant to antibiotics such as penicillin and methicillin. Penicillin resistance has probably evolved in the following way:

–By chance, a mutation produces an individual bacterium with an allele that allows it to produce an enzyme, penicillinase, which deactivates penicillin

–This bacterium is immediately resistant to penicillin. (As bacteria have only one strand of DNA and one copy of each gene, the mutant allele is expressed immediately and is not masked by a dominant allele.)

–If the population to which the mutant belongs is exposed to penicillin, the mutant will survive and reproduce whereas those without the mutant will be killed.

17.3. 4 Industrialization

Many species of organisms, especially insect species, have two or more adult body forms that are genetically distinct from one another, but which are contained within the same interbreeding population. This condition is known as polymorphism (another type of natural selection). The peppered moth (Biston betularia), for example, has two main forms with different wing colours. One form has pale wings with dark markings; the other form is called melanic because the wings contain large amounts of melanin (a black pigment), so they are almost black.

17.3. 5 Gene recombination

Despite these efforts there are still some copying errors and accidental damage, permanent changes, or mutations. These may be responsible for thousands of inherited diseases, and mutations that appear in cells throughout the lifetime of an individual. These may lead to many types of cancer. DNA repair thus becomes important to prevent mutations and inherited diseases.

17.3.6 DNA Recombination

DNA sequences in cells thus are maintained from generation to generation with very little change. While this is true, there is evidence that the DNA sequence in chromosomes does change with time and the DNA gets rearranged over time. The combination of the genes on the genome may change due to such DNA rearrangements. In a population, this sort of genetic variation is important to allow organisms to evolve in response to a changing environment. These DNA rearrangements are caused by a class of mechanisms called genetic recombination.

a. Homologous DNA recombination

The most important form of genetic recombination is homologous recombination. The process involves the basic facts such as two double double-stranded DNA molecules that have regions of very similar (homologous) DNA sequence come together so that their homologous sequences are in tandem. Then they can “cross-over”: in a complex reaction, both strands of each double helix are broken and the broken ends are re-joined to the ends of the opposite DNA molecule to re-form two intact double helices, each made up of parts of the two different DNA molecules.

b.Non homologous DNA recombination

In homologous recombination, DNA rearrangements occur between DNA segments that are very similar in sequence. A second, more specialized type of recombination, called site-specific recombination, allows DNA exchanges to occur between DNA double helices that are dissimilar in nucleotide sequence.

17.3.7 Artificial selection

Over the years, humans have used artificial selection to create crazy specific dog breeds Over the past 150 years or so, humans have been specifically mating dogs that looka certain way to create the animals we now keep as pests via a process known as breeding. This is artificial selection, where one species (humans) directs the traits that get passed down to future generations of another species (dogs).

Self-assessment 17.3

Write short summary on industrialization and gene recombination as causes of evolution

17.4 Speciation

Activity 17. 4

Use the school library and internet and read the information related to speciation.1. How does speciation occur?

2. How does one species evolve into two or more new species?

Evolution occurs whenever the inherited characteristics of a population or of a species change over a period of time. When these changes lead to the formation of one or more new species, speciation has taken place. A species can be defined as a group of organisms with similar features which can interbreed to produce fertile offspring, and which are reproductively isolated from other species.The central part of this and most other definitions of species is that members of the same species can interbreed to produce fertile offspring. Thus, although donkeys can interbreed with horses to produce offspring called mules, donkeys and horses are regarded as separate species because mules are infertile.

Organisms which do not interbreed to produce fertile offspring under normal circumstances are regarded as reproductively isolated, and they belong to separate species. Mechanisms that prevent breeding between populations and which can eventually lead to speciation are called isolating mechanisms. Mechanisms that prevent the formation of hybrids are called prezygotic isolating mechanisms, Prezygotic (before a zygote is formed) isolating. Mechanisms include:

–Individuals not recognising one another as potential mates or not responding to mating behaviour

–Animals being physically unable to mate

–Incompatibility of pollen and stigma in plants

–Inability of a male gamete to fuse with a female gamete.

The mechanisms that affect the ability of hybrids to produce fertile offspring are called postzygotic isolating mechanisms. Postzygotic isolating mechanisms include:

–Failure of cell division in the zygote

–Non-viable offspring (offspring that soon die)

–Viable, but sterile offspring.

The most important isolating mechanism is thought to be geographical isolation, in which two populations originally of the same species are separated from each other by a physical barrier such as a mountain, river, or ocean.

Allopatric speciation

When geographical isolation leads to new species being formed, allopatric speciation is said to have occurred. (Allopatric means literally ‘different countries’. Any physical barrier that prevents members of different populations from meeting must inevitably prevent them from interbreeding. Note that although geographical isolation is the original cause of allopatric speciation, the two isolated populations diverge so much from each other that when reunited they are unable to interbreed. Other isolating mechanisms now keep the two species from breeding together.

Sympatric speciation

Sympatric literally means. (‘Same country’.) Sympatric speciation occurs when organisms inhabiting the same area become reproductively isolated into two groups for reasons other than geographical barriers. Such reasons might include:

1. The genitalia of two groups may be incompatible (mechanical isolation): It may be physically impossible for the penis of a male mammal to enter the female’s vagina

2. The gametes may be prevented from meeting: In animals, the sperm may not survive in the female’s reproductive tract or, in plants; the pollen tube may fail to grow.

3. Fusion of the gametes may not take place: Despite the sperm reaching the ovum, or the pollen tube entering the micropyle, the gametes may be incompatible and so will not fuse.

4. Development of the embryo may not occur (hybrid inevitability): Despite fertilization taking place, further development may not occur, or fatal abnormalities may arise during early growth

5. Polyploidy (hybrid sterility): When individuals of different species breed, the sets of chromosomes from each parent are obviously different. These sets are unable to pair up during meiosis and so the offspring cannot produce

6. Behavioral isolation: Before copulation can take place, many animals undergo elaborate courtship behavior. This behavior is often stimulated by the colour and markings on the members of the opposite sex, the call of a mate or particular actions of a partner.

Self-assessment 17.4

Distinguish between allopatric and sympatric speciation.

17.5 Roles natural selection in speciation

Activity 17.5

Use the school library and internet, read the information related to the roles of natural selection in speciation.

In your own words, write a short summary on the roles of each type of natural selection in speciation.

The role of natural selection in evolutionNatural

selection leads to evolutionary change when individuals with certain characteristics have a greater survival or reproductive rate than other individuals in a population and pass on these inheritable genetic characteristics to their offspring. Simply put, natural selection is a consistent difference in survival and reproduction between different genotypes, or even different genes, in what we could call reproductive success.

The reason that natural selection is important is that it›s the central idea, stemming from Charles Darwin and Alfred Russel Wallace that explains design in nature. It is the one process that is responsible for the evolution of adaptations of organisms to their environment. Three essential components of evolution via natural selection include:

1. Genetic Diversity – Populations of individuals are genetically diverse. Even members of the same species have characteristics that vary from one individual to the next.

2. Fitness – In any given environment, some individuals have characteristics that put them at an advantage over individuals who do not possess those same characteristics.

3. Population Shift – In any given environment, those individuals who have advantageous characteristics will generally be healthier, live longer, and leave more offspring than individuals who do not possess those characteristics. The population will, over time, contain more and more individuals with the advantageous characteristic, and fewer individuals who do not possess the characteristic.

Self-assessment 17.5

1. Some individuals of the Rwandan swallowtail butterfly (Papillio machaon) 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. Explain why the intermediate colour forms would be at a selective disadvantage.

2. What is the role of natural selection in evolution?

17.6 Mechanism of speciation

Activity 17.6

Use the school library and search additional information on the internet, read the information related to mechanism of speciation. Write a short report on different mechanisms of speciation

a. Continental drift

The continents which now exist have not always appeared as they do today. At one time, the earth had a single large land mass called Pangaea. This is thought to have broken up into two parts, a northern Laurasia and a southern Gondwanaland. Over millions of years, the two great land masses split up and moved by a process called continental drift to form our present continents. The theory that these land masses were once joined is supported by the discovery in Australia, South Africa, South America, and Antarctica of fossils belonging to the same extinct species. Fossils in North and South America show differences between the species, suggesting that these two continents have only joined together relatively recently. Before this, their fauna (animals) and flora (plants) were geographically isolated and evolved independently.

Australia shows many excellent examples of species that evolved independently following its geographical isolation. It is thought that Australia became isolated

about 120 million years ago, when marsupials (mammals without a placenta but with a pouch in which the young develop) and eutherian mammals (mammals with a true placenta) diverged from a common ancestor

b.Migration

Migration also called gene flow is any movement of individuals, and/or the genetic material they carry, from one population to another. Gene flow includes lots of different kinds of events, such as pollen being blown to a new destination or people moving to new cities or countries. If gene versions are carried to a population where those gene versions previously did not exist, gene flow can be a very important source of genetic variation. In the graphic below, the gene version for brown coloration moves from one population to another.

17.7 Divergent evolution

A single species evolves into several new species that live in different ways.

The five of Darwin’s finches are a good example. There are separate species of finch in the group, all of which probably evolved from individuals belonging to one mainland species. The islands have few other bird species. In the absence of competition, the finches became adapted to fill all the available niches. In particular, they evolved a wide range of beak sizes and shapes so that they could take advantage of the food sources on the different islands. The evolution of an ancestral species into different species to fill different niches is called adaptive radiation

17.8 Convergent evolution

Unrelated species independently evolve similarities when adapting to similar environments

17.1: Table isolating mechanisms

17.9 Extinctions

Extinct means that a species that has died out

17.9.1 Causes of Extinction

The single biggest cause of extinction today is habitat loss. Other causes of extinction today include:

–Exotic species introduced by humans into new habitats. They may carry disease, prey on native species, and disrupt food webs. Often, they can out-compete native species because they lack local predators.

–Over-harvesting of fish, trees, and other organisms. This threatens their survival and the survival of species that depend on them.

–Global climate change, largely due to the burning of fossil fuels. This is raising Earth’s air and ocean temperatures. It is also rising sea levels. These changes threaten many species.

–Pollution, which adds chemicals, heat, and noise to the environment beyond its capacity to absorb them. This causes widespread harm to organisms.

–Human overpopulation, which is crowding out other species. It also makes all the other causes of extinction worse.

17.9.2 Large-scale extinctions in earth’s history

–During the late Precambrian, continents drifted, carbon dioxide levels fluctuated, and climates changed. Many organisms could not survive the changes and died out. Others evolved important new adaptations. These include sexual reproduction, cell specialization, and multi cellularity. The Precambrian ended with a mass extinction. It paved the way for the Cambrian explosion.

–The Paleozoic Era began with the Cambrian explosion. It ended with the Permian extinction. During the era, invertebrate animals diversified in the oceans. Plants, amphibians, and reptiles also moved to the land.

–The Mesozoic Era is the age of dinosaurs. They evolved from earlier reptiles to fill niches on land, in the water, and in the air. Mammals also evolved but were small in size. Flowering plants appeared for the first time. Dinosaurs went extinct at the end of the Mesozoic.

–The Cenozoic Era is the age of mammals. They evolved to fill virtually all the niches vacated by dinosaurs. The ice ages of the Quaternary Period of the Cenozoic led to many extinctions. The last ice age ended 12,000 years ago. By that time, Homo sapiens had evolved.

self-assessment

1. Describe the mechanism of continental drift.

2. Briefly explain why two types of organism may be regarded as separate species even though they can interbreed to produce fertile offspring

3. Describe the Rwanda policies to overcome the extinction of some species

End unit assessment 17

Multiple choice questions

1.A species of finch living on an isolated island shows variation in beak size. Birds with larger beaks can eat larger seeds. After a period of drought on the island, large seeds were more plentiful than small seeds and the average size of the finches’ beaks increased. What explains this increase in size of beak? a. Artificial selection acting against finches with small beaks

b. Directional selection acting against finches with small beaks

c. Increased rate of mutation resulting in finches with larger beaks

d. Stabilizing selection acting against finches with the smallest and largest beaks

2. Which effect of natural selection is likely to lead to speciation?

a. Differences between populations are increased.

b. The range of genetic variation is reduced.

c. The range of phenotypic variation is reduced.

d. Favourable alleles are maintained in the population.

Questions with short answers

3. Name two examples of adaptive radiation.

4. What effect did industrial pollution have on:

a. The frequency of the C (melanic) allele within a population of peppered moths.

b. The rate of mutation of the c allele to the C allele

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

le. 6. Answer the following questions:

a. Distinguish between homologous structures and analogous structures with specific examples.

b. Name the type of evolution exhibited by comparing:

(i) Flipper of whale and forelimb of desert rat.

(ii) Wing of a bat and wing of butterfly

(iii) Wing of a flamingo and wing of an insect

Essay questions

7. Explain the various evidences of organic evolution.

8. Explain Darwin’s theory of natural selection. The environment or nature selects the individual with variations that are favored by the environment. These compete with the others and able to reach sexual maturity, reproduce and pass over the favorable characteristics to their offspring.

9. What do you understand by Lamarckism? How does it differ from Darwinism?

10. How can you convince that evolution progress?

11. A Darwin and Lamarck contribution to science is unparalleled. Discuss.

12. Explain the importance of modern genetics to the theory of origin of species by natural selection

13. Answer the following questions:

a. Describe, with the use of examples, the genetic basis of resistance

b. Discuss the development of resistance in a named organism

14. Spartina anglicana is a species of grass which has originated as a result of the formation of a hybrid between two related species, S.maritima and S.alterniflora, as shown in the diagram below. The diploid numbers of chromosomes for S.maritima and S.alterniflora are given in the boxes.

a. Give the expected diploid number (2n) of chromosomes for the sterile hybrid

b. Explain why this hybrid is sterile

c. Suggest how doubling of chromosomes may have occurred to produce S.anglica