18.
Variation and Selection
Variation
-is the differences that exist between organisms of the same species and even different
species.
Variation occurs in:
-complexion; ability to roll the tongue; shape of earlobes; blood groups; sex; height;
intelligence, sex, height, body mass, etc
Two types of variation:
Continuous variation and Discontinuous variation
Continuous variation: - individuals show a range of phenotypes between two extremes.
e.g. in height - very short and very tall.
-occurs in height; weight; length of leaves; size of feet; number of seeds in a pod;
intelligence etc.
A histogram can be used to show the frequencies of heights in class.
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A normal distribution curve can be drawn as below
Discontinuous variation: - it is where individuals are grouped into limited
distinct phenotypes with no intermediates e.g. A, B, AB and O blood groups and
sex in most organisms
▪ it is influenced by genes alone.
▪ occurs in Sex; Blood groups; Ability to roll the tongue; Hitch hiker’s finger; Shape of
earlobe etc.
A bar chart is used to show the frequencies e.g. of blood groups.
Causes Of Variation
-the genes making the genotypes/alleles of an organism
-the environment which surrounds an organism
-genes brings about genetic variation
-environment brings about environmental variation
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What causes genetic variation?
*Mutations
*Meiosis
*Random mating
*Random fertilisation
A. Mutations
-a mutation is an unpredictable change in a gene or a chromosome.
-often harmful or bring uncalled for suffering in both people and animals.
-bring about new alleles & phenotypes in the population
-happen spontaneously
Causes:
1. ionizing radiation by X-rays; gamma, beta and alpha radiations;
-DNA base sequences are damaged
-such damage in gamete producing cells of the ovaries and testes and result in
mutations being passed to the next generations
2. Chemicals such as lead, mercury, and those in tobacco smoke
-chemicals causing mutations are called mutagens
-mutagens causing cancer are called carcinogens
Mutations bring about changes in DNA base sequences, hence changes in proteins
produced which can be harmful or not functional causing different conditions
Examples:
Albinism
Down’s syndrome
Sickle cell anaemia
Some mutations can be of benefit in some organisms
Adaptive Features
-an adaptive feature is that caused by an organism’s genes, making it to survive and
reproduce in its environment
-some adaptive features are obvious e.g. fish has gills for gas exchange
-others are amazing
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-Camouflaging –bodies having colour blending with the environment, it is against
predation
-leaf tailed gecko blends with dry leaves and can easily catch its pray
-seahorses blend sea fans to derail predators
-some insects look like leaves
-tuna fish is streamlined and has strong muscles for swimming very fast to catch its prey
-springboks have long thin strong legs to jump and flee from danger, brown and white fur
for camouflage
-giraffes – have long legs and necks to reach for leaves, skin furs designed to blend with
trees and bushes
- lions- strong legs and strong jaws and teeth to fight and to kill prey
-cheetah- light and swift body to catch prey
-organism best suited (fit) to its environment survive
-fitness is the probability of an organism to survive and pass on its genes to the next
generation in a given environment
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Xerophytes: - plants surviving in very dry/desert conditions.
Adaptation of xerophytes
➢ closing stomata
➢ waxy cuticle
➢ hairy leaves
➢ reducing surface area of the leaves
➢ deep or spreading roots
➢ sunken stomata on underside of leaves
Closing stomata
-deserts have very high afternoon temperatures
-stomata close to prevent excessive water loss by transpiration
Waxy cuticle
-leaves have a thick layer of waxy cuticle secreted by the epidermal cell
-the cuticle prevents loss of water by evaporation
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Hairy leaves
-hairs trap a layer of moist air around the leaf reducing loss of water vapor by transpiration
-a micro humid environment is created around the leaf
Reducing surface area of the leaves
-the smaller the surface the les water vapour will be lost by diffusion
-cacti has very tiny leaves, others reduced to thorn
-spines act as deterrents, animals will not eat them
-cactus us chloroplast in its stems for photosynthesis
deep or spreading roots
-deep roots to reach water deep down in the soil
-roots spread covering a wider area to capture as much water as possible soon after
flash floods
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Sunken stomata on underside of leaves
-xerophytes have very few stomata in their leaves
-stomata are in pits on the lower side of the leaves
-pits trap water vapour
-diffusion gradient is reduced and less water is lost to the environment
Though plants elsewhere also have stomata on the lower’s surfaces of leaves, have cuticle,
have deep or spreading roots, desert plants display more.
Hydrophytes: - plant that survive in very wet places
-no water problems
-no need for adaptations to conserve water
e.g. the water hyacinth and water lily.
-no deep roots- roots just fix the plant into the soil under water
-stalk and leaves have hallow spaces-filled with air.
-air spaces for floating and expose their leaves to light for photosynthesis.
-stomata are mostly on the upper surfaces
-thin cuticle on leaf surfaces
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-below is a photograph of water lily
Selection
Natural Selection
-natural selection is when nature decides which organism survives and pass on its genes to
next generation.
-organisms best suited to the environment, thus organisms with advantageous adaptive
features survive and pass on their genes/alleles to the next generation.
-natural selection is happening all the time
Summary of natural selection
1. Variation: - some mutations bring some selective advantage which causes other
organisms to survive and breed.
- camouflaged moths survived in Britain in 1918 than their white counterparts
-gene mutations, meiosis and fertilisation give rise to new individuals with better
adaptations
2. Overproduction: - production of large numbers of eggs by Sockeye salmon fish,
ensures that enough offspring survive and breed.
-Overproduction is to cater for those which are consumed by predators.
-zebra foals are in large numbers in spring, only a few reach adult age, due to predation
3. Survival of the fittest -plants compete for light, water, nutrients and space, animals
compete for food, water, space and mates.
*Organisms that are able to fight in this struggle for existence will survive and breed.
*Competition is fiercest among organisms of the same species
*They have the same structures and ways to get those resources.
4. Passing on of alleles for useful adaptations
-best adapted individuals survive and reproduce.
-best alleles increase in the population and others decrease
Stabilising selection
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-individuals at the extremes do not survive, they are not well adapted e.g. female sparrows
with very long or very short wings are often killed in stormy weathers.
-natural selection does not always bring change.
-changes come when there is a change in the environment or if there is a mutation.
-natural selection keeps populations the same.
Mutation
-a new allele coffering an advantage to an individual -giving it new adaptive features
will make the organism to survive and pass on the allele
Change in the environment
-probably a feature that used to give an organism an advantage over the others no longer
save the purpose. In normal African environments grass and the environment in general
have a greyish-brownish look. Rabbits with grey or agouti colour are well camouflaged.
The allele for grey or Agouti favour and selected for, unlike the allele for white fur.
Suppose the climate changes and Africa starts to experience snow (a new ice age), white
fur allele to enjoy a selective advantage. White rabbits start to multiply as they get
camouflaged by the ice.
Examples of natural selection
*Distribution of the dark and pale peppered moth in Britain
*Antibiotic resistance
Distribution of the dark and pale peppered moth in Britain
-before the industrial revolution of the 19th century Oak trees where of a normal bark
of greyish colour with lichens contributing to the colour.
-the peppered moth is a nocturnal fly. Day times in perches on tree trunks and branches
-the moth has two phenotypes- dark and pale colours
-the dark colour is controlled by a dominant allele, but is due to a mutation
-the normal pale colour is controlled by a recessive allele
The pale peppered moths were very common before the industrial revolution
-they were more camouflaged by the Oak tree background. Predation by birds was
very low, they were not very visible.
-the mutant dark moths were scarce due to predation-they were darker against a clear
background
-natural selection was in favour of the recessive allele.
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A lot of soot from burning coal, darkened the Oak tree trunks during the 19th century as
industry boomed
-the dark peppered moth started to enjoy a selective advantage and their numbers increased
compared to those of the pale peppered moth. The dark moth became camouflaged.
-this is an example of a change in the environment
An investigation to measure the survival of the dark and pale moths in polluted and
unpolluted environments
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Antibiotic Resistance In Bacteria
-the process by which adaptive features develop gradually to make an organism fit to
survive in its environment is called adaptation
-bacteria can also develop adaptive features to survive in a changing environment, though
to the disadvantage of men
-drugs are chemicals used to ease symptoms or cure a disease, they affect metabolic
reactions in the body
-medicinal drugs, e.g. antibiotics are drugs that keep us healthy
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-Antibiotics are chemicals used to kill bacteria or inhibit their growth, without damaging
our cells
-Discovered in 1940s
-antibiotics are not used against viral infections—viruses do not have metabolism of their
own(like protein synthesis); they do not have cell walls.
Penicillin is an example of an antibiotic
-it stops cell wall formation in bacteria, the bacteria absorb water and burst.
-among the huge population of bacteria, one or two may have an allele giving them
resistance to penicillin-they will have a selective advantage
-they multiply/reproduce passing on the allele
-others die
-the new generation of bacteria will be penicillin resistant
Applying an antibiotic, can kill most of the bacteria, but a few may undergo a mutation
which give them a resistance to the chemical.
They may produce an enzyme that break down the antibiotic
These bacteria will then multiply successfully in the absence of their competitors
They reproduce passing on the gene for resistance
New strains of resistant bacteria survive today
e.g. Staphylococcus aureus, normally lives on our skins, can cause infections resulting in
boils.
-Drugs like penicillin, oxacillin, Amoxylin and methicillin were used successfully to kill
Staphylococcus aureus.
-the new strain is called MRSA (Methicillin Resistant Staphylococcus Aureus)
-the bacterium goes through random mutation and natural selection
-more antibiotics used, more resistance occur.
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How to conquer bacterial resistance?
-restrict use of antibiotics (no antibiotics for flu and cold, even for minor bacterial
infections)
-Patients should complete prescribed courses
-Doctors should stop unnecessary prescription of antibiotics
-New antibiotics should be made
-Use combinations of antibiotics
Artificial selection
Selective Breeding
-this is when man choses/decides which type of animal to breed or plant to grow basing on
their favourable/desirable features.
-humans are the selective agents not nature as in natural selection
-Man is after the best out of the crops and animals he keeps
-Man considers the following:
*High meat; milk and wool yielding
*Early maturity
*Rapid/fast growth
*Disease resistance
*Drought resistance
Animals and plants showing these features are bred over a period of time
Those which show an improvement in the desired features are kept for breeding.
Example:
A farmer has a herd of cattle. He always wants to improve one or two features of the herd
e.g. milk production
1. he chooses cows producing the most milk
2. chooses a bull whose mother and sisters produce more milk
3. breed the chosen cows with the chosen bull
4. the resulting calves are allowed to grow and those yielding more milk are selected and
bred with the best bull
5. he repeats for generations
He can also combine two or more desirable features like high milk yielding capacity
and docility.
-the act of choosing which animals or plants to breed is called artificial selection or
selective breeding
Dangers of selective breeding:
Inbreeding causes harmful recessive alleles to spread in the population.
Reduced variation
Individual animals cannot survive in the wild
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Comparison of Natural Selection and Selective Breeding
Natural selection Selective Breeding
For both: certain individuals survive and breed
1. environments chooses/selects Humans choose
2. random mating Only selected animals and plants
cross
3. survive due adaptive features Survive due to features desired by
breeder. The features often reduce
survival of the animal in the wild
4. any animal or plant has a chance to Only a plant or animal with best
survive even with the least features survive
adaptive features
5. greater variation Reduced/limited variation
6. slow change in population Rapid change in population feature
over a short period of time
19.Organisms And Their Environment
Ecology: - the study of living organisms and their environment.
Habitat: -a place where an organism lives and it has adaptive features to survive in that
area. Tadpoles live in a pond
Population: - a group of organisms of the same species living in the same habitat at the
same time. Many tadpoles make a population in the pond
Community: -populations of different species in an area.
Ecosystem: -is a community of organisms interacting among themselves and their physical
environment
Niche: - the way an organism lives its life and the role it plays in an ecosystem. Tadpoles
eat algae and weeds, excrete ammonia, they breath oxygen and breath out Carbon dioxide
in the water, they disturb pebbles and mud at the bottom of the lake
Physical factors in the environment:
Sunlight
Wind
Water
PH
Temperature
Ecosystems provide habitats for different organisms.
Examples of habitats are:
pond
lake
dam
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