Evolution over time can follow several different patterns. Factors such as environment and predation

pressures can have different effects on the ways in which species exposed to them evolve. The three main

types of evolution are: divergent, convergent, and parallel evolution.

A = divergent, B = convergent, and C = parallel.

Divergent evolution, is a phenomenon in which initially similar populations

accumulate differences over evolutionary time, and so become increasingly

distinct (i.e., they "diverge"). In the views of both Darwin and Wallace, and

thus of traditional evolutionary theory, divergence serves two purposes:

1. It allows a given type of organism to survive in modified form by utilizing

new niches;

2. This increase in diversity supposedly boosts a habitat's carrying capacity.

All three of these taxa look an awful lot alike, and if we were scoring them

based on just their morphology, we would likely conclude they share a common

ancestor. They all have flippers, dorsal fin, tail, streamlined body. In reality they

do not share a recent common ancestor, their last common ancestor was

MILLIONS of years ago. This is an example of CONVERGENT EVOLUTION. These

different lineages developed these character separately... they evolved toward

a similar morphology. This is because they are all water dwelling, fish eating

predators, that need to be able to move swiftly in water.

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An evolutionary tree, or phylogeny, represents the pattern of relationships between groups of organisms. It is

often used in discussing evolutionary concepts because it shows the evolutionary history of a species.

Biologists often represent time on phylogenies by drawing the branch lengths in proportion to the amount of

time that has passed since that lineage arose. If the tree of life were drawn in this way, it would have a very

long trunk indeed before it reached the first plant and animal branches.

Phylogenies are not ladders, so ‘the higher up the phylogeny” an organism is does not mean it is more evolved

or more advanced. For any speciation event on a phylogeny, the choice of which lineage goes to the right and

which goes to the left is arbitrary. The following phylogenies are equal.

Evolution encompasses changes of vastly different scales — from something as insignificant as an increase in

the frequency of the gene for dark wings in beetles from one generation to the next, to something as grand as

the evolution and adaptive radiation of the dinosaur lineage. These two extremes represent classic examples

of micro- and macroevolution. Microevolution happens on a small scale (within a single population), while

macroevolution happens on a scale that transcends the boundaries of a single species. Evolution at both of

these levels relies on the same mechanisms of evolutionary change and natural selection.

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–

Any slight change in the genotype of a species over a few generations is formed is known as microevolution.

These small changes in populations may lead to development of new species (macroevolution).

Microevolution is the changes in allele frequencies with in a species of a population that occur over a relatively

short period of time.

Such a change might come about because:

• Mutation

• migration/gene flow - the population received new immigrants carrying the gene

• Genetic drift- In each generation, some individuals may, just by chance, leave behind a few more

descendants than other individuals. The genes of the next generation will be the genes of the “lucky”

individuals, not necessarily the healthier or “better” individuals.

• natural selection favoring a gene

Evolution at this scale can be observed over short periods of time. Examples of how changes in populations

due to microevolution have happened:

• Peppered moths becoming more or less melanistic according to the levels of pollution in Britain –

Industrial melanism.

• Mosquitoes evolving resistance to DDT

• Whiteflies evolving resistance to pesticides

• Darwin’s finches developing larger or smaller beaks in response to seed size, which in turn changes due

to rainfall patterns

• Gonorrheal bacteria strains evolving resistance to penicillin

• HIV strains evolving resistance to antiviral medicines

• Drug resistant TB bacteria (XDR, MDR and TDR)

• Tibetan Sherpas evolving a circulatory system with a greater surface area to cope with the high

altitudes of the Himalayas

Macro evolution is evolution on a grand scale. It results in the formation of new species. This is what we see

when we look at the over-arching history of life: stability, change, lineages arising, and extinction (ie these are

the patterns of macroevolution).

The basic evolutionary

mechanisms of mutation,

migration, genetic drift, and

natural selection produce

major evolutionary change if

given enough time.

The tree of life is a metaphor describing the relationship of all life on Earth in an evolutionary context. There

are also patterns to macroevolution. All of the changes, diversifications, and extinctions that happened over

the course of life's history are the patterns of macroevolution affecting the tree of life.

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Many lineages on the tree of life exhibit stasis, which just means that they don't

change much for a long time, as shown in the figure to the right.

In fact, some lineages have changed so little for such a long time that they are

often called living fossils. For example: Crocodiles and Coelocanths. Coelacanths

comprise a fish lineage that branched off of the tree near the base of the

vertebrate clade. Until 1938, scientists thought that coelacanths went extinct 80

million years ago. But in 1938, a South African scientist, JBL Smith, discovered a

living coelacanth from a population in the Indian Ocean that looked very similar

to its fossil ancestors. Hence, the coelacanth lineage exhibits about 80 million

years' worth of morphological stasis.

Patterns of lineage-splitting can be identified by constructing and examining a phylogeny. The phylogeny

might show cladogenesis or anagenesis.

is an evolutionary

splitting event where a parent species

splits into two distinct species, forming a

clade. A clade is a life-form group

consisting of a common ancestor and all

its descendants – representing a single

branch on the tree of life.

is when the ancestral

species gradually accumulates changes

and eventually, when enough is

accumulated, the species is sufficiently

distinct and different enough from its

original starting form that it can be

labelled as a new species.

Paleontologists see the role of

cladogenesis as more important in

evolution than that of anagenesis. Anagenetic trends are often micro-evolutionary where as cladogenetic

trends are macro-evolutionary.

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Adaptive radiation is the burst of

divergence from a single lineage to

give rise to many new species from a

single ancestor. This patten of

macroevolution, which is a form of

cladogenesis, occurs to fill up new

ecological niches in the case of the

finches on the Galapagos islands.

Over time, the organisms that have

evolved from the common ancestor

adapt and change even further.

Extinction is extremely important in the history of life. It can be a frequent or

rare event within a lineage, or it can occur simultaneously across many

lineages (mass extinction).

Over 99% of the species that have ever lived on Earth have gone extinct.

In this diagram, a mass extinction cuts short the lifetimes of many species,

and only three survive.

–

states that evolution

generally occurs uniformly and by the steady and

gradual transformation of whole lineages (called

). In this view, evolution is seen as

generally smooth and continuous.

Is a theory in

evolutionary biology which proposes that most

species will exhibit little net evolutionary change for

most of their geological history, remaining in an

extended state of stasis. When significant

evolutionary change occurs (generally restricted to

rare and rapid events), cladogenesis occurs.

is the process by which a species

splits into two distinct species, rather than one

species gradually transforming into another. Often

this form of evolution would explain the absence of

transitional fossils.

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is a process that produces random changes in the frequency of characteristics in a

population. Genetic drift results from the role that chance plays in whether a given trait will be passed on to

the next generation. It is important in small populations because chance plays a greater role than natural

selection in determining which individuals reproduce and pass on their genes.

is the process whereby the organism with the best adaptations for an environment

are able to survive, reproduce and pass on their genes to their off spring. The process is not random as

organisms are selected for survival based on the traits which will lead to better adaptation to the

environment.

Natural selection was based on Darwin’s idea of evolution made from FOUR observations he made.

1. More offspring are produced than are required

• He found that populations usually produced more offspring than the environment could

support, yet populations generally remained stable in size over time.

2. Natural variation

• No two individuals are exactly the same,

this variation is a result of a combination of

genes which are randomly shared from the

parents.

3. A change in environment leads to differential

reproduction

• Since populations remained stable it was

reasoned that better adapted individuals

would be likely to reproduce while the less

adapted would die or fail to reproduce.

This is known as differential reproduction

or “survival of the fittest”.

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4. Characteristics were inheritable

Natural selection is one of the basic mechanisms of evolution, along with mutation, migration, and genetic

drift. Natural selection is a simple mechanism that causes populations of living things to change over time.

Natural selection only operates on variation in inherited characteristics.

It can be broken down into five basic steps, abbreviated here as V.I.S.T.A.:

There is in traits.

For example, some beetles are pale and some are dark(phenotype) due to

genetic variation (genotype).

These characteristics are by the offspring

There is - dark beetles are easier to spot and get eaten by birds.

The pale ones survive

There is (and thus inheritance).

The surviving pale beetles reproduce and have pale baby beetles because

this trait is coded for by the genes.

End result over :

The more advantageous trait (the to the environment), pale

coloration, which allows the beetles to survive and reproduce, and to have

offspring, becomes more common in the population. If this process

continues, eventually, all individuals in the population will be pale.

Variation – genotypic and phenotypic

Inheritance,

Selection (and Reproduction)

Time

Adaptation.

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MECHANISMS OF VARIATION FOR EVOLUTION (SPECIFRICALLY NATURAL SELECTION)

• Cause changes to the nucleotide sequences in the DNA of genes

• Allows microevolution to take place

• Three types of mutations:

A -Neutral mutations: The change of sequence in the DNA may or may not change the sequence of

the amino acids in the protein but does not change the function of the protein therefore no

change in the species

B - Lethal mutations: The DNA sequence changes, the amino acid sequence changes and the

function of the protein changes – leads to unfavourable change in the phenotype – the effect is

serious and may even cause death.

C - Fixed mutations: The DNA sequence changes, amino acid sequence changes, function of the

protein changes – result is a favourable change in phenotype. NB Gives an advantage to the

individuals in the population with this change.

• Mutations lead to phenotypic variation:

a. Caused by genetic differences ie a change in the allele of a gene.

b. May be discontinuous – due to a single pair of alleles such as tongue rolling

c. May be continuous – polygenic – resulting in a range of phenotypes such as height.

d. May be influenced by environment – height will depend on diet in a child

After mutation and /or the reshuffling of genes during sexual reproduction and /or gene flow there are

genotypic variations in a population. As a result the physical characteristics of an individual will change

and phenotypic variations will be found in the population.

If the phenotypic variations are favourable individuals will:

• be better adapted

• to survive in the environment

• and thus breed – the mutated genes are now part of the individual’s new genotype and will be

passed on to succeeding generations.

• Crossing over during gametogenesis when haploid gametes are formed due to meiosis

• Independent assortment during Metaphase I and II of meiosis

• Fertilization, randomness of which sperm fertilizes the ovum

• Randomness of which ovum is released during ovulation

Gene flow is the movement of genes between populations. This may happen through migration of the

organisms or the movement of gametes (so pollen). It is especially important in the sea, where larvae are

widely dispersed.

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The peppered moth has been studied in a lot of detail over the past 200

years as it has a very interesting evolution over a short time period.

Originally, most of the peppered moths were a light, speckled colour, as

you can see in the top moth in the image. This colouring allowed the

moths to be camouflaged when they rested upon the light-coloured trees

and lichens in their habitat. However, not all the peppered moths were

this light colour. There was some variation and there were a few which

were a much darker, grey colour. They could not camouflage themselves

as well as the light coloured one. The darker coloured moth is shown

below in the following image.

During the Industrial Revolution in England, there was a huge increase in

the number of factories. These factories mainly burnt coal as an energy

source, which increased the amount of pollution and soot in the air. The pollution caused the lichen on the

trees to die off. The soot coated the trees in the peppered moths' habitat. These trees now did not have any

lichen and they were a dark grey colour because of the soot covering them.

The light coloured moths were therefore not camouflaged anymore and could be seen easily by predators

when they rested on the trees. As a result, more of the light-coloured moths were eaten by birds and didn't

have a chance to mate and lay eggs. Therefore the number of light-coloured moths decreased. In comparison,

the few moths that were a dark grey colour were now at an advantage as they were now the same colour as

the soot covered trees and could hide. These darker-coloured moths could therefore go on to have more

offspring. Over time, this resulted in more and more of the moths being dark-coloured.

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–

Background information:

One of the best-known examples of natural selection in action is that of the peppered moth – Biston betularia.

This moth spends its days resting on the bark of trees. Its main predator is birds. The moth has been intensively

studied for approximately 200 years.

The colour of the moth is genetically determined. Two forms of this moth occur:

The light-coloured form or typica. It has speckled

greyish wings.

The typica form of Biston betularia

The dark-coloured form or carbonaria. Its wings

are dark in colour.

The carbonaria form of Biston betularia

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Prior to the industrial revolution (in the early 1800s) the bark of trees in England was predominantly light grey

and was often covered in lichen. During the industrial revolution of the early 1800s, sulphur dioxide emissions

from the coal burning factories killed the delicate lichens on the trees, and soon, together with the soot emitted

from the factory chimneys, the bark of the trees was predominantly dark. Since the advent of the Clean Air laws

in the late 1900s, sulphur dioxide pollution and the sootiness of the air have both decreased, the bark of the

trees has gradually lightened again.

lichen

FORMATION OF A NEW SPECIES

: Species is a group of organisms that closely resemble each other and are able to breed

among themselves and produce fertile offspring

: A population of is a group of individuals of the same species occupying a particular

habitat.

: Is the formation of a new species from an old one.

GO TO THE FOLLOWING WEB SITE TO EXPLORE HOW A SPECIES IS FORMED DUE TO GEOGRAPHIC ISOLATION

http://evolution.berkeley.edu/evolibrary/article/0_0_0/evo_42

Genotypic variation leading to phenotypic variation .Eg: Adaptive radiation of Galapagos finches. Through

mutations and thus genetic variation , as well as some measure of geographic isolation (even in the small area

of the Western Cape) South Africa’s King Protea may have reached a stage in evolutionary divergence where

we may soon have five different national flowers.

can be referred to as the mating of closely related individuals of the same species. This can

happen due to geographic isolation (Eg lemurs on Madagascar) or a genetic bottle neck due to a catastrophic

environmental event (Toba catastrophe theory suggests that a bottleneck of the human population occurred

c. 70,000 years ago, proposing that the human population was reduced to perhaps 10,000 individuals when

the Toba super volcano in Indonesia erupted and triggered a major environmental change), genocide

(Holocaust extermination of Jews) and immigration (a few Dutch settlers in South Africa formed the

Afrikaaners).

There are many religious and sociocultural reasons for inbreeding in humans. Closely related individuals are

more likely to have the same alleles therefore inbreeding reduces the gene pool. It results in less variation and

thus evolution will not occur. If inbreeding continued over generations, it can result in inbreeding depression

which leads to many defects and a decline in offspring vitality, less resistant to disease, smaller babies, higher

mortality rate, physical abnormalities, have recessive genetic diseases, show a decrease in heterozygous

genes, and fail to reproduce and the line dies out.

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EXAMPLES OF INBREEDING IN POPULATIONS:

1. White lions of Timbavati

2. Lions of Umfolozi

3. The Vadoma tribe of Zimbabwe show inbreeding

depression due to the their isolation, they have

developed and maintained ectrodactyly, and due to

the comparatively small gene pool, the condition is

much more frequent than elsewhere

4. More severe illnesses exist among certain Jewish

groups. Ashkenazi Jews, for example, have a

particularly high chance of suffering from Tay-Sachs disease, a fatal condition in young children

5. Some royal societies have also been known to practice inbreeding to protect royal blood lines. For

example, the English Royal family has had many hemophiliac members due to inbreeding.

CASE HISTORY 1. INBREEDING IN THE CHEETAH

About 10 000 years ago climate change caused all but one group of

cheetahs to become extinct . (see “bottle neck”) The result was that

close relatives were forced to breed and become genetically inbred.

In most species, related individuals share about 80% of the same

genes, in cheetah it is about 99%. This has led to:

• Fewer cubs in a litter

• low survivorship

• poor sperm quality

• greater susceptibility to disease

A lack of genetic diversity means a species is less able to have the variation that helps to adapt to change in

environment. A virus that infects one cheetah is likely to infect all cheetahs in the population and may kill

them all, leading to extinction. Currently, the threatening virus is feline infectious peritonitis, which has a

disease rate in domestic cats from 1%–5%; in the cheetah population it is ranging between 50% to 60%.

CASE HISTORY 2: INBREEDING IN THE AMISH

The Amish population of Lancaster County, Pennsylvania, is an example of inbreeding within a close knit

religious community. As a result, the Amish suffer from a variety of genetic disorders including Crigler-Najjar

syndrome and Ellis - van Creveld (EVC) syndrome, a disease caused by inheritance of two mutated copies of

the EVC gene. Symptoms of the disease include short-limbed dwarfism with polydactyly (additional fingers or

toes), bone malformations in the wrist, heart defects, and prenatal eruption of the teeth.

CASE HISTORY 3: WHITE LION

White lion are not albino lions. The white coat colour is homozygous

recessive. White Lions are endemic to one place only on earth: the

Greater Timbavati region in South Africa. This region is characterised

by white sandy riverbeds and in the winter the long grass in this area

is scorched pale. In this habitat they are very well camouflaged. In

their natural habitat, the White Lions are "apex predators" - i.e. they

have been recorded as hunting successfully during the day and at

night, killing prey as large as giraffe.

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These lions are better adapted to their natural habitat than tawny lions, so this characteristic spread to create

a new subspecies of lion. Sadly, they are often bred in camps in South Africa for use as trophies to be killed

during canned hunts.

Outbreeding is the mating of unrelated individuals of a species. This ensures increases the gene pool and in

return results in genetic variation (Heterozygous) hybrid vigour. Outbreeding in plants tends to mean more

productive, more fertile and have a greater chance of survival and greater diversity. Outbreeding, also called

outcrossing, is the transfer of gametes from one individual to another, genetically different individual.

Outbreeding in animals, often results in a “new species” referred to as which often are sterile and

cannot reproduce.

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CASE HISTORY OF OUTBREEDING IN DOMESTIC DOGS:

Hip dysplasia in large dogs such as German shepherds is inherited through a recessive gene. Due to the limited

numbers of German Shepherds in South Africa, the disease became widespread in pedigreed dogs. It was only

when German shepherds were imported from other countries to mate with the local German Shepherds

(increasing the gene pool) that the incidence of hip dysplasia decreased.

The ‘founder effect’ in genetics refers to an event when a small number of individuals carrying a fraction of

their population's genetic variation are the founders of a new society elsewhere. As a result, the new

population may be distinctively different in their genes from the parent population. It may also lead to a

population bottleneck. Eg: in the figure below the incidence of genetic cancer increases in a population due to

the founder effect.

In extreme cases the founder effect may lead to speciation and subsequent evolution of new species.

MIGRATION AND FOUNDER EFFECT:

Due to various migrations throughout human history, founder effects are

quite common. This has led to:

• Reduced genetic variation from the original population (a bottle

neck).

• Non-random sample of the genes in the original population.

• Clusters of genetic diseases

This effect is easy to recognize in genetic diseases, but of course, the frequencies of all sorts of genes are

affected by founder events. For example in South Africa, research projects aimed at learning more about the

role of BRCA1 (breast cancer susceptibility gene one) and BRCA2 (breast cancer susceptibility gene two) within

the Caucasian Afrikaner led to the identification of three founder mutations.

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CASE HISTORY 1:

The Afrikaner population of Dutch settlers in South Africa is descended mainly from a few colonists. Today, the

Afrikaner population has an unusually high frequency of the gene that causes Huntington’s disease because

those few original Dutch colonists just happened to carry that gene with unusually high frequency.

CASE HISTORY 2:

In 1688 Ariaantje and Gerrit Jansz emigrated from Holland and got married in South Africa, one of them

bringing along an allele for the metabolic disease porphyria. Today more than 30 000 South Africans carry this

allele and, in every case examined, can trace it back to this couple — a remarkable example of the founder

effect.

CASE HISTORY 3:

Familial hypercholesterolaemia is a common autosomal domininant genetic disorder which results in patients

having significantly increased levels cholesterol and low density lipoprotein (LDL) cholesterol. The high

cholesterol levels are caused by a deficiency or a defect in the LDL receptor which results in severe and

premature coronary artery disease. The mutations in the gene encoding the LDL receptor are found on

chromosome 19.

Familial hypercholesterolaemia has a prevalence of 1/500 in America and Europe. Within South Africa, the

prevalence is very high in particular population groups. In the Afrikaner population a prevalence rate of 1/72

has been reported, whilst in the Ashkenazi Jewish population the prevalence is 1/67.

A genetic bottle neck is caused for

example by a catastrophic event

like an earthquake or a flood or

genocide that kills entire sections

of a population. Bottlenecks can

cause a founder effect even

though it isn't strictly a new

population but a small group of

survivors of the old. In this way a

population bottleneck would

reduce genetic variation further.

This may be further exacerbated by the resulting inbreeding. Migration such as seen in the few Dutch colonists

who were the ancestors of today’s Afrikaaners will also lead to a bottle neck.

The Toba catastrophe theory suggests that a bottleneck of the human population occurred about 70,000 years

when the human population was reduced to less than 15,000 individuals when the Toba super volcano in

Indonesia erupted and triggered a major environmental change.

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If the population isn’t fraught with lethal genetic disorders, it may rebound to a substantial size — but that

doesn’t mean it’s safe. It takes millennia for genetic diversity to develop (through the slow accumulation of

changes to the DNA sequence), so even a large population may still bear the low-diversity signature of

bottlenecks past.

That’s a major reason that cheetahs, for example, are

hovering on the brink of extinction. Of course, cheetahs

face the same human threats (habitat loss, poaching, etc.)

that most African wildlife does. But while other species are

recovering slowly under watchful conservation eyes, the

cheetah isn’t sprinting back. The secret is written all over its

DNA.

Where most mammals share about 80 percent of their genes with other members of their species, cheetahs

share 99 percent — more than you or I have in common with even our closest relatives (save for identical

twins). So the miraculous genetic reshuffling of sexual reproduction — which evolved to produce varied

offspring to meet a variable world — can’t help cheetahs claw back into synchrony with a changing

environment.

The original cheetah bottleneck probably happened about 10,000 years ago, but other bottlenecks have been

much more recent. The European bison, or wisent, population dwindled to 12 in the 1920s; California’s sea

otters trace their ancestry to only 50 individuals alive in 1938. We may yet see the legacy of those bottlenecks

in conservation efforts.

You should be able to

draw this graph and

explain it

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Speciation requires that the two species be unable to produce viable offspring together or that they avoid

mating with members of the other group. This is achieved in many ways other than geographic isolation.

BREEDING/FLOWERING AT DIFFERENT TIMES OF YEAR:

In the mist belt forests of KZN there are three species of yellow wood tree. It is highly likely that they remain

separate species because they form cones and pollen at different times of the year.

SPECIES SPECIFIC COURTSHIP:

Genetically-based changes to different mating location, mating time, or mating rituals help to complete the

process of reproductive isolation and speciation.

In every group, the ability of an animal to recognize potential

mates depends on the presence of signals, called the "mate

recognition system". In some animals this depends on specialized

anatomical features, such as the horns of a kudu or the special hair

pattern on the face of Vervet monkeys. Some animals need to

perform special behaviors for mate recognition to occur, for

example the dances that cranes do.

Like any other phenotypic characteristic, these features may evolve over time. Two populations that lose

genetic contact will be very unlikely to evolve in the same way. If separated for any period of time, members

of one population will not recognize the members of the other as potential mates – for example Vervet

monkeys and Samango monkeys do not recognise each other as mates even when they live in the same area.

The Samango seldom leaves the trees and their vocalization is totally different to a Vervet monkey.

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ADAPTATION TO DIFFERENT POLLINATORS:

(See video “Sexual encounters of the floral kind” in plant reproduction)

Flowers have coevolved with their pollinators and the different pollinators may have driven speciation.

Reproductive isolation is achieved with pre-pollination barriers that limit the transfer of pollen from

individuals of one species to the stigmas of another species.

Proteas are pollinated by birds, rodents, insects and wind. This co

evolution with different pollinators may account for the many varieties of

Proteas in such a small geographic area as the Western Cape, often

growing right next to each other.

For example there are only 10 wind-pollinated Proteas in southern Africa:

These are characterized by not secreting nectar - most do not even have

nectaries - and being odourless. They may grow next to a Protea such as

the one in the photograph.

This plant is rodent pollinated and is adapted for this by having sweet

nectar and flowers that hang close to the ground. Floral isolation can

work through floral morphology (shape) - allowing only certain pollinators

access to rewards e.g. through long floral spurs or tubes or placement of

pollen on different body parts of a pollinator.

TIMING OF FLOWERING:

O breviceps and O clavaeformis are two types of evening primrose that

grow in the desert. The two species live side by side, flower at the same

time of the year and are pollinated by the same insects - solitary bees.

The flowers of O breviceps open before sunrise and are pollinated by

insects active early in the day whereas O clavaeformis flowers late

afternoon and is thus pollinated by bees that are active at this time of the

day. In this way an exchange of pollen between the two species is

prevented and the species remain separate (there is little hybridization).

DIFFERENT SHAPED SEX ORGANS:

Hard for us to imagine, but damsel flies’ penises show that the different sex organs ensure no sex between

different species!

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Each population contains genotypic variations. These variations are important as they increase a species

chance of adapting and thus surviving under different conditions. Variation can eventually lead to speciation.

There are two types of speciation:

• Geographic (allopatric) speciation, which is due to part of the population becoming isolated

• Sympatric speciation, which occurs in a population that occupy the same geographical area.

Allopatric speciation is just a fancy name for species being formed by geographic isolation.

Something in the environment prevents two or more groups from mating with each other regularly, eventually

causing that lineage to form two separate species. Isolation might occur because of great distance or a

physical barrier that develops due to continental drift, earthquake, erosion, formation of a desert, diversion of

a river.

Ancestral species

Species A

Species B

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Sympatric speciation does not require large-scale geographic distance to reduce gene flow between parts of a

population. Different species form because different genetic differences develop in the offspring due to a

variety of reasons. Different little populations may vary according to behavior, feeding, colour patterns, and

stay in different parts of the lake; breeding within their own small group, until eventually they are unable to

reproduce with other members of the species.

Sympatric speciation can occur as a result of the formation of:

1. Hybrid species

• Hybridisation = when two genetically different parents mate – form hybrid organism. (In animals,

offspring may be sterile.)

• More common in plants.

• The new species is isolated from the ancestral species in terms of reproduction.

2. Polyploid plants

• Errors during cell division produce plants with extra sets of chromosomes.

• New plants are reproductively isolated from original species because crossbreeding between the

original and new species cannot occur.

EXAMPLE 1: FINCHES ON GALAPAGOS

ISLANDS

There are now at least 13 species of

finches on the Galapagos Islands, each

filling a different niche on different

islands. All of them evolved from one

ancestral species, which colonized the

islands only a few million years ago. This

process, whereby species evolve rapidly

to exploit empty ecospace, is known as

adaptive radiation.

When Charles Darwin stepped ashore on

the Galapagos Islands in September 1835,

it was the start of five weeks that would

change the world of science, although he

did not know it at the time. Among other

finds, he observed and collected the

variety of small birds that inhabited the

islands, but he did not realize their

significance, and failed to keep good

records of his specimens and where they

were collected. It was not until he was

back in London, puzzling over the birds,

that the realization that they were all

different, but closely related, species of

finch led him toward formulating the

principle of natural selection.

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In his memoir, The Voyage of the Beagle, Darwin noted, almost as if in awe, "One might really fancy that, from

an original paucity of birds in this archipelago, one species had been taken and modified for different ends."

Indeed, the Galapagos have been called a living laboratory where speciation can be seen at work. A few

million years ago, one species of finch migrated to the rocky Galapagos from the mainland of Central or South

America. From this one migrant species would come many -- at least 13 species of finch evolving from the

single ancestor.

This process in which one species gives rise to multiple species that exploit different niches is called adaptive

radiation. The ecological niches exert the selection pressures that push the populations in various directions.

On various islands, finch species have become adapted for different diets: seeds, insects, flowers, the blood of

seabirds, and leaves.

The ancestral finch was a ground-dwelling, seed-eating finch. After the burst of speciation in the Galapagos, a

total of 14 species would exist: three species of ground-dwelling seed-eaters; three others living on cactuses

and eating seeds; one living in trees and eating seeds; and 7 species of tree-dwelling insect-eaters.

Scientists long after Darwin spent years trying to understand the process that had created so many types of

finches that differed mainly in the size and shape of their beaks.

Most recently, Peter and Rosemary Grant have spent many years in the Galapagos, seeing changing climatic

conditions from year to year dramatically altering the food supply. As a result, certain of the finches have lived

or died depending on which species' beak structure was best adapted for the most abundant food -- just as

Darwin would have predicted.

EXAMPLE 2: CICHLID FISH IN MALAWI LAKE

The great lakes of Africa: Lake Malawi, Lake Tanganyika and Lake Victoria are all swarming with hundreds

of species of endemic Cichlid fish species viz. 700, 250 and 450 respectively,. They all arise from a

common unspecialized ancestral organism which was likely to be feeding on insects and possibly other types

of food each of which occupies and is adapted to a distinct ecological niche.

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It has been noted that closely related species may differ in colour. Mates are selected according to colour

e.g. in one population, the females selected red males and in another blue males. The fish may also be

plankton feeds, algae feeders, predators, scavengers, parasite cleaners and egg stealers. It also seems that

basic design of the fish where they have a second pair of jaws (as shown in the diagram below) allows cichlids

to diversify and feed in many different ways .

Within the same lake, the cichlids adapted to the following selection pressures:

Specific habitats, specific foods and specific sexual strategies.

1. Habitat-specific selection

• There are two main habitats in the lake – sandy (bottom of lake) and rocky (sides of the

lake).

• The fish separated from each other to occupy these two different niches and eventually

they adapted to these niches.

• The fish in each niche developed separately and survived to reproduce and pass on their

favourable traits.

2. Food-specific selection

• As the population of fish grew in the lake there was more Intraspecific competition.

• The selection pressure became a lack of food. Since the lake had many empty feeding

niches, the fish began to occupy these.

• The fish in the different niches developed separately and evolved into different species of

fish.

3. Sexual-specific selection

• Bright colours and patterns in the males were a reproductive strategy to attract females for

mating.

• Random mutations gave rise to these brightly coloured, patterned fish.

• Females responded well to these new colours and these males were able to reproduce

successfully, creating a new species.