Evolution

• (a) define the term variation; • (b) discuss the fact that variation occurs within as well as between species; • (c) describe the differences between continuous and discontinuous variation, using

examples of a range of characteristics found in plants, animals and microorganisms; • (d) explain both genetic and environmental causes of variation; • (e) outline the behavioural, physiological and anatomical (structural) adaptations of

organisms to their environments; • (f) explain the consequences of the four observations made by Darwin in proposing

his theory of natural selection; (HSW1) • (g) define the term speciation; • (h) discuss the evidence supporting the theory of evolution, with reference to fossil,

DNA and molecular evidence (HSW1, 4, 7a, 7b); • (i) outline how variation, adaptation and selection are major components of evolution; • (j) discuss why the evolution of pesticide resistance in insects and drug resistance in

microorganisms has implications for humans (HSW6a, 7c). •

Variation

• Variation is the differences between individuals- no two individuals are exactly alike even if they look similar

• Identical twins start as one cell that divides and then separates into two cells, although they started with the same original DNA, the subsequent divisions have introduced changes, also environmental changes may have taken place in the womb or after birth.

Variation within species

• Members of the same species can show variation

• Examples are: eye, hair and skin colour, nose shape, eye shape etc

Variation between species

• Variation between species is more obvious- it is what we use to separate one species and another

Continuous Variation• There are usually 2

extremes and a full range of intermediate values between the extremes- most individuals are close to the mean value and the number at the extremes is low

• Examples are: human height, length of leaves on an oak tree, length of stalk of a toadstool

Discontinuous Variation• There are two or more

distinct categories, with no intermediate values

• There may be more than one type than the other or they may be distributed evenly

• Examples are: sex- male or female in plants and animals

• Some bacteria have flagella, some do not

• Human blood groups- A, B, AB or O

Inherited and Genetic Variation

• Genes define our characteristics

• The combination of alleles that we inherit is not the same as any other living thing

• Human cells have approx 25000 genes, many with different alleles, so the chance of having the same ones is extremely remote

Environmental Variation• Characteristics affected by the

environment

• Examples: skin tone can become darker in the sun, hawthorn trees may grow sideways in areas with strong gales, affecting the direction of growth, lack of water can affect the height of growth

• Environmental and genetic variation can be linked e.g. humans now have a better diet, and so can grow taller, however your genes also limit final height

Adaptations

• Adaptations can be behavioural, physiological, or anatomical

• Adaptations help the organism to cope with environmental stresses and obtain the things they need e.g. to find water, gather nutrients, respond to changes, defend from predators

Behavioural Adaptations

• An aspect of the behaviour of an organism that helps it to survive the conditions it lives in

• Example: an earthworm has no eyes, so cannot tell if a bird is about to eat it, however on touch, earthworms rapidly contract and recoil into their burrows- this adaptation allows them to avoid being eaten

Physiological Adaptations

• A physiological or biochemical adaptation ensures the correct functioning of cell processes

• Example: yeast (Saccharomyces) can respire sugar either aerobically or anaerobically depending on oxygen availability- this also involves producing the correct enzymes for the type of respiration required

Anatomical Adaptations

• Anatomical means structural

• Any structure that enhances the survival of an organism is an adaptation

• Example: bacteria like Legionella have flagella that help them move independently

Case Study: Plants• Xerophytes grow in areas with a low

water supply e.g. cacti• Behavioural adaptations: open and

close stomata, open stomata at night only, fold and roll leaves when lack of water, open stomata when short of water to wilt leaves and reduce surface area

• Physiological adaptations: stem with accordion-fold structure that folds more in dry periods, when water is plentiful the cells expand and the stem enlarges and becomes more rounded

• Anatomical: shallow roots spread out over a large area, long roots e.g. camel thorn tree (40m long), fleshy stem and leaves, waxy leaves, curled and folded leaves with sunken stomata

Charles Darwin• The theory of evolution was

not his idea• He proposed a mechanism

for this process• He called it natural selection• Darwin was interested in the

variation between members of the same species and this underpinned his work

• He studied in the Galapagos Islands and worked on a ship called the HMS Beagle

The Theory of Natural Selection

Darwin made 4 important observations:

1. Offspring generally appear similar to their parents

2. No two individuals are identical

3. Organisms have the ability to produce large numbers of offspring

4. Populations in nature tend to remain fairly stable in size

Developing the Theory• Darwin read an essay by Thomas

Malthus who said that the human population would outstrip its food supply- Darwin disagreed and said that competition for food and resources would keep the population in check

• Darwin saw that too many young are produced and that this leads to competition for food and resources

• As all offspring are different, the better adapted ones will obtain all the food and survive long enough to reproduce and pass this onto their offspring

• The less well adapted ones are likely to die before they reproduce, so the population does not grow indefinitely

Link to Evolution

• Over a long period of time, small variations arise and if beneficial, will be passed on to the next generation

• Over many generations a species has many variations and may even change into another species that has so many changes it could never reproduce with its ancestors as the DNA has changed so much

Darwin’s Conclusions

• There is a struggle to survive

• Better adapted individuals survive and pass on their characteristics

• Over time, a number of random changes may give rise to a new species

Selection Pressures

If you can survive after undergoing these selection pressures, you are more likely to pass on your random adaptations

• Availability of suitable food; adapted to eat the available food

• Predators; adaptations to avoid being seen and eaten, or to escape

• Diseases; survival of diseases• Physical and chemical factors; living in cold

places, shade, hot summers

Speciation

• Speciation means the formation of a new species from a pre-existing one

• Does not occur suddenly• Long, slow accumulation

of changes meaning the species cannot interbreed freely to produce fertile (viable) offspring

• Takes many generations• Faster in bacteria as

reproduction happens every 20 minutes

How does Speciation Occur?

• Some reproductive barrier occurs

• Some organisms unable to breed in the group

• Variations providing a benefit will be passed on

• A collection of small changes means the individuals become so different they cannot breed with the rest of the group

Reproductive barriers

• Geographical separation: different members of the same species living on different islands will not breed freely, speciation is likely to occur e.g. galapagos islands- this is known as allopatric speciation

• Biochemical change preventing fertilisation e.g. behavioural dance not recognised or sexual organs change. This is known as sympatric speciation