UNIT 17 EVOLUTION AND SPECIATION
UNIT 17: EVOLUTION AND SPECIATION
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 othershave become extinct.
Introductory activity
1. The coyote, jackal and dingo are closely related species of the dogfamily. 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 hascaused 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 andone 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 becausethe 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 putforward 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 anadaptation 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 notadequate.
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. Itshows that there was direct creation of organism with no precursor to life.
Application 17.1
1. Give the biological meaning of evolution
2. How does neo- Darwinism differ from Darwin’s original theory ofevolution?
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 proteinsequence 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 ofigneous rock in the strata.
Living fish, A dies enclosed in sediment hard parts fossilised
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 byradioactive 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) becauseof similarities in their early embryonic development.
Figure 17.4: Comparison of embryos from different vertebrates: Although the adults are quite different,
the early embryonic stages are similar
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 allhave wings used for flight but they have different structural organization.
Figure 17.5: The forelimbs of the following vertebrates show the basic pattern of limb bones with modifications
which are adapted to their methods of locomotion.
17.2.5 DNA evidence
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’ divergencefrom a common ancestor.
Application 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 acommon 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 writeshort 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.
Figure 17.6: The white winter coat of a mountain hare provides excellent camouflage from predators
when viewed against snow.
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, becomingbetter 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 thefollowing 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 bekilled.
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 “crossover”:
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 look
a 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 traitsthat get passed down to future generations of another species (dogs).
Application 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.
Figure 17.10: A hypothetical example of allopatric speciation
Sympatric speciation
Sympatric literally means. (‘Same country’.) Sympatric speciation occurs when
organisms inhabiting the same area become reproductively isolated into two groupsfor 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 callof a mate or particular actions of a partner.
Application 17.4Distinguish 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 naturalselection in speciation.
The role of natural selection in evolution
Natural 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 callreproductive 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 selectioninclude:
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 individualswho do not possess the characteristic.
Application 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 ondifferent 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 witha 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 browncoloration moves from one population to another.
Figure 17.15: Illustration of migration
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 speciesinto different species to fill different niches is called adaptive radiation
17.8 Convergent evolution
Unrelated species independently evolve similarities when adapting to similarenvironments
Figure 17.16: Convergent evolution
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 extinctiontoday include:
– Exotic species introduced by humans into new habitats. They may carry
disease, prey on native species, and disrupt food webs. Often, they can outcompete
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 allthe 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. Bythat time, Homo sapiens had evolved.
Application 17.6
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 somespecies
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 andlargest 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 example.
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.
9. What do you understand by Lamarckism? How does it differ from Darwinism?
10. How can you convince that evolution is still in progress?
11. A Darwin and Lamarck contribution to science is unparalleled. Discuss.
12. Explain the importance of modern genetics to the theory of origin ofspecies by natural selection
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Electronic links_
http://www.fao.org/docrep/006/Y4955E/y4955e06.htm
https://www.google.rw/search?q=Animal+cloning&rlz=1C1GGRV_en-
RW752RW752&oq=Animal+cloning&aqs=chrome..69i57.10995j0j7&-
sourceid=chrome&ie=UTF-8
http://vle.du.ac.in/mod/book/print.php?id=11914&chapterid=23696
https://en.wikipedia.org/wiki/Dolly_(sheep)
http://sphweb.bumc.bu.edu/otlt/MPH- Modules/PH/GMOs/GMOs3.html
https://www.google.rw/search?q=t-dna&rlz=1C1GGRV_enRW752RW752&oq=t-DN
A&aqs=chrome.0.0l6.14641j0j7&sourceid=chrome&ie=UTF-8
https://www.omicsonline.org/open-access/genetic-transformation-of-cells-using-
physical-methods-2157-7412-5-237.php?aid=29908
https://www.rxlist.com/antibiotic_resistance/drugs-condition.htm
https://www.lakeforest.edu/live/news/4396-the-role-of-horizontal-gene-transferin-
antibiotic
https://www.ncbi.nlm.nih.gov/pubmed/24201341
https://www.britannica.com/list/extinct-in-the-wild-but-still-around-5-plants-andanimals-
kept-alive-by-humans
https://sites.google.com/a/hvrsd.org/charles-darwin-webquest/post-darwin-populationwww.dictionary.com/browse/natural-selection.