4.6.2 Variation and evolution

4.6.2.1 Variation

Genotypes

• A genotype is the collection of genes an organism has.
  • This is inherited from the parents.

Phenotypes

• A phenotype is the physical characteristics of an organism.
  • All organisms all have different appearances and characteristics – i.e. different phenotypes.
  • The phenotype is determined by environmental and genetic factors.

Variation

• Variation is the differences in phenotypes in a population.
• There can be variation in
  • Physical appearance (e.g. different weight)
  • Social factors (e.g. personality)
  • Academic factors (e.g. natural talents)
  • Many other things

Reasons for Variation

• There are three main reasons for variation:
  • Genes
  • Environment
  • A combination of Genes and Environment

Genes

• The set of genes that have been inherited from the mother and father can control many characteristics.
  • For example, blood type is controlled by genes only, there is no environmental influence.
  • Other examples of genetic phenotypes are natural eye colour and the ability to roll your tongue.

Environment

• The environment in which an organism grows and develops will play a large role in phenotype.
  • For example, an accent is controlled solely by the environment.

Genes and Environment

• The majority of factors are not solely controlled by either variable.
  • Weight is an example of a combination of both.
  • For example, overweight parents with a low metabolism can have a light child.
  • This is because weight is controlled not only by genes but also by environmental factors such as diet and exercise.

Variation

• Variation can occur:
  • Within Species
  • Between Species

Variation

• Variation can occur:
• Within species – individuals of the same species tend to have more similarities and have less variation than individuals of different species.
  • They tend to produce similar proteins and have a similar body anatomy, but genetic variation is almost always present.

• Variation can occur:
• Between species – individuals in different species tend to be very different and have lots of variation.
  • For example, a lion has a tail whereas a human does not.

Mutation

• Variation occurs within species due to mutations,
  • Mutations are a spontaneous, random change in genetic material due to a mistake in replication
• When the genetic material is duplicated during reproduction, a mutation can occur leading to a change in genotype.
  • However, this does not always lead to a change in the phenotype.
• Sometimes the mutation is silent, which means that it doesn’t affect the final phenotype.
• However, other mutations can affect phenotype and lead to the new organism having different features.
• If the mutation is beneficial, it can give the new organism an advantage and can therefore be passed on to then next generation.

4.6.2.2 Evolution

Evolution

• Evolution refers to a change in the alleles in a population of a species.
• It leads to changes in the phenotypes of species over time, in order to make them more suitable to their environment.
• This has occurred through a process called natural selection.

Natural Selection

• Natural selection is a process that leads organisms to be more and more adapted to their surroundings in order to survive for longer and reproduce more.
• It was coined and studied by the famous biologist, Charles Darwin.

Survival of the Fittest

• The theory of ‘survival of the fittest’ nicely encompasses natural selection.
  • Survival of fittest basically means that the more adapted the organism is to the surroundings, the more likely it is to survive and reproduce.

Survival of the Fittest: An Example

• An example of this are the deer mice.
• These animals migrated to the sand hills in Nebraska and changed colour from dark brown to light brown.
• This protected them from predators.
• The lighter mice survived for longer and reproduced more until the allele for this phenotype became common throughout the species.

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On the Origin of Species

• Charles Darwin wrote a book called ‘On the Origin of Species’ in the 1850s, in which he attempted to describe evolution and how it works.
• One of the most important principles coming from this is that all species of living things evolved from very simple life forms.
  • Eventually organisms became more and more complex through evolution.
  • This process began more than three billion years ago.

Stages of Natural Selection

1. Variation exists
2. Selection occurs
3. Breeding of Beneficial Alleles

1. Variation exists

2. Selection occurs

3. Breeding of Beneficial Alleles

Understanding Speciation

• Speciation is the formation of a new species.
  • Species form when the phenotypes of two organisms in the same species gets distinctly different.
• There is a key way of working out whether a new species is formed.
  • If two organisms can no longer freely interbreed to produce fertile offspring, they belong to different species.

Speciation Example

• Natural selection can lead to formation of a new species if there is a split in the population.
• For example, geographical isolation could separate two populations of monkeys over two islands.

This mutation spreads via natural selection to the other monkeys on the same island. There is a reproductive barrier (the river) between both populations, so the mutation doesn’t spread to the other monkeys.

Over time, the two populations become so different that they can no longer bree together – two different species are formed.

4.6.2.3 Selective breeding

Selective Breeding

• Selective breeding involves choosing plants and animals with the best traits (e.g. most food producing) and breeding them more.
  • In this way, we are breeding for particular genetic characteristics.
  • These characteristics can help the organism in many ways.

• Humans have been doing this for thousands of year.
• Farmers often find wild crops or wild animals with good traits, and then breed them to produce lots of their own plants and animals with the desired characteristics.

Selective Breeding in Plants

• All sorts of crops are selectively bred. This includes corn, peaches and bananas.
• They are bred selectively for many beneficial characteristics, including:
  • Large fruits
  • Resistance against disease
  • Greater aesthetic beauty, such as larger flowers or more unusual flowers

Selective Breeding in Animals

• Animals are bred selectively to increase the changes of certain characteristics.
• These include:
  • Increased meat yield
  • Increased milk yield
  • Large eggs from chickens
  • Desirable characteristics in domestic dogs, such as a gentle nature

Selective Breeding Step-By-Step

1. Choose parents with the desired characteristics.

If you are trying to breed cows with a high yield of milk, choose a cow that has a high milk yield and a bull, who’s mother had a high milk yield. They must be from a mixed population

2. Breed them together.

Breed the selected parents together. They will both have the desired genes and hence the offspring will also gain the desired genes.

Selective Breeding Step-By-Step

1. Choose parents with the desired characteristics.

2. Breed them together.
3. Breed the offspring with the desired characteristics

4. Continue this process over generations.

Selective Breeding Positives

1. Economic benefit

If a farmer can grow the crops with the largest fruits and the animals with the greatest yield of meat, they will get the most money.

2. Domesticating animals

You can select for more docile household pets or animals that are safer to farm with.

3. Prevent disease

You can prevent spread of disease amongst crops by using selective breeding to develop crops resistant to disease spread by pests.

Selective Breeding Negatives

1. Selecting for rare diseases

Some of the more selected for organisms have unexpected disadvantages. Through

selective breeding, these disadvantages may accidentally be selected for.

2. Reduced genetic variation

By selecting for specific alleles, the genetic variation in a population decreases. This can

reduce defences of the animals and plants against disease and insect attacks.

4.6.2.4 Genetic engineering

Genetic Engineering Process

• Genetic engineering involves introducing a gene from one organism into the genom of another organism.
  • Genetic engineering is also known as genetic modification.

Genetic Engineering Step-By-Step

• During genetic engineering, we follow certain steps. Genes can be cut out from the chromosomes of humans and stuck into other organisms.
• For example, we can genetically engineer bacteria to make them produce insulin, so that we can mass produce insulin in a fermenter:

1. Select a desired phenotype

2. Find the gene which causes this phenotype

3. Insert the gene into the recipient

4. Culture the recipient

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Genetic Engineering in Plants

• Genetic engineering of crops can produce genetically modified (GM) crops.
• Farmers can use genetic engineering to produce better crops.
  • This will give them more money and help the consumer.
  • They produce crops that are resistant to disease or have large fruits.
  • Crops can also be made resistant to herbicides, which means that herbicide can be used to kill the weeds, but the crop survives, aiding the yield.

Genetic Engineering in Plants

• Crops can be genetically engineered to aid humanitarian situations.
  • Golden Rice was produced to be given in regions that suffered from Vitamin A deficiencies.
  • It contains a substance called Beta-Carotene that provides its golden colour.

Genetic Engineering in Animals

• Bacteria can be genetically modified to produce insulin.
  • Before, we used to use insulin from animals (e.g. porcine and bovine insulin), but this was inefficient.
  • Therefore bacteria have been engineered to produce insulin.

• Insulin is used to treat diabetes, a condition that leads to an inability to control glucose levels in the body.
• Insulin produced by bacteria can be injected to treat these patients.

• Medical research uses genetic engineering.
  • After the advent of using bacteria to produce insulin, there are many more avenues that are being considered through genetic modification.
  • They are looking into ways to treat HIV, sickle cell anaemia, Huntington’s and Cystic Fibrosis.

Genetic Engineering Cons

• Exam questions can often ask about the positives and negatives of genetic engineering.
• For the positives, you can use the examples from above.
• Here are some negatives:
  • Health risks
  • Ethical hazards
  • Unknowns

Health Risks

• Some genetically modified foods can lead to health risks.
  • For example they could contain chemicals that cause allergy or toxins.

Ethical Hazards

• Some do not believe that it is ethically fair to modify organisms and make newer life forms.

Unknowns

• There are many worries about the unknowns involved with genetically modified crops.
  • People worry that they could lead to a huge effect on the ecology of organisms.
  • They could lead to negative effects on the populations of wild flowers and organisms.

• Moreover, we do not know all of the effects of eating GM crops.
• There could be unforeseen consequences that we will only find out in the future.

Enzymes and Genetic Engineering

• For the higher tier, it is necessary to describe the steps of genetic engineering.
  • To do this, think about the basic steps we learnt above.
  • Genetic engineering involves removing DNA from the genome of one organism and placing it into the genome of another.

Enzymes and Genetic Engineering Step-By-Step

1. The required gene is isolated by restriction endonuclease.

2. The required gene is placed in a vector.

3. The vector is then introduced into the organism.

4.6.2.5 Cloning (biology only)

Tissue Culture

• Tissue culture is a method of cloning plants, which involves growing new, genetically identical plants from parts of a parent plant.
  • These parts are called explants.
• There are many steps in tissue culture:

Cuttings

• Using cuttings is an older, easier method of producing plants from a parent plant.
• It is often done by gardeners and does not require a sterile agar medium.

Embryo Transplantation

• Embryo transplantation produces genetically identical offspring and is done via transplanting embryos to numerous hosts.
  • It is quite a common process.

Embryo Transplantation

1. Fertilisation

Sperm is taken from one animal, an egg is taken from another

2. Artificial Insemination

An animal is then artificially inseminated

3. Development of Embryos

The zygotes are allowed to develop into embryos.

4. Removal of Embryos

Embryos are then removed from the uterus of the inseminated animal

5. Splitting of Embryos

These embryos are split apart to form smaller cells. This process must take place before

specialisation (when cells develop into different types of cells)

6. Transplant into Host Mothers

You then transplant all of these embryos into host mothers. These embryos will all be genetically identical to one another and so will be clones.

Adult Cell Cloning

• In adult cell cloning, an adult cell’s nucleus is used to replace the nucleus in an egg cell.
• Adult cell cloning has been around for around 20 years now.
• The first ever mammal was cloned in Edinburgh in 1996, and this was Dolly the Sheep.
• The process for adult cell cloning to produce Dolly is shown on the next slide

Advantages

• Cloning can produce animals that are primed to produce required proteins for the body.
  • As clones are produced, they will have the exact genetic information as the parent cell, so the required characteristics can easily be chosen.
• Endangered species can be cloned, in order to increase the population and then can breed to continue growing animals.

Disadvantages

• Adult cell cloning is a difficult process and requires lots of intense effort.
• As genetically identical organisms are produced, there is an increased risk of reducing the genetic variation.
  • This will reduce the size of the gene pool and lead to an increase in the incidence of genetic diseases.

• There are ethical queries.
  • Are humans playing God by cloning?
  • Will the cloning of animals finally transition into the cloning of humans?