6.3 The development of understanding of genetics and evolution

6.3.1 Theory of evolution

FSL: GCSE Biology Revision "Evolution by Natural Selection"

- Lamarck observed was that giraffes for the longest necks could survive, reproduce, etc.
- What Lamarck got wrong was that he thought the necks developed over a lifetime and passed onto offspring if it was useful.
- Experiments were conducted where the tails were chopped off mice but offspring were born with tails, disproving Lamarck's theory.
- Charles Darwin, as a result of observations on a round the world expedition, backed by years of experimentation and discussion and linked to developing knowledge of geology and fossils, proposed the theory of evolution by natural selection.
- His ideas were not quickly accepted because his theory challenged the idea that God made all living organisms on Earth.
- There was little evidence at the time to support his theory.
- The mechanism of inheritance and variation was not known until 50 years after the theory was published.

6.3.2 Speciation

FSL: GCSE Biology Revision "Speciation"

Alfred Russell Wallace:

• Same ideas as Darwin
• Darwin's ideas were backed up by research, unlike Wallace's
• Darwin is more famous
• Darwin published the "origin of species" first
• Darwin and Wallace worked closely together after

- Speciation is when individuals in a population are separated, and evolve separately into different species.

1. Two populations of the same species are in an area
2. Physical barriers (e.g. new mountain ranges) separate the populations
3. Populations adapt to new environments due to different selection pressures (conditions have to be different on either side)
4. Development of a new species as different alleles are beneficial

6.3.3 The Understanding of Genetics

- In the mid-19th century Gregor Mendel carried out breeding experiments on plants.
- One of his observations was that the inheritance of each characteristic is determined by 'units' that are passed on to descendants unchanged.
- In the late 19th century behaviour of chromosomes during cell division was observed.
- In the early 20th century it was observed that chromosomes and Mendel’s 'units' behaved in similar ways.
- This led to the idea that the 'units', now called genes, were located on chromosomes.
- In the mid-20th century the structure of DNA was determined and the mechanism of gene function worked out.

6.3.4 Evidence for Evolution

Evidence for Darwin’s theory is now available as it has been shown that characteristics are passed on to offspring in genes. There is further evidence in the fossil record and the knowledge of how resistance to antibiotics evolves in bacteria.

6.3.5 Fossils

FSL: GCSE Biology Revision "Evidence for Evolution: Fossils"

- It is difficult to determine how and when life began because no one was there.
- Life™ is estimated to have begun around 3.5 billion years ago, but the earliest fossils are only around 2.5 billion years old because many early forms of life were soft-bodied, which means that they have left few traces behind. What traces there were have been mainly destroyed by geological activity.
- A fossil is the preserved remains of an organism that lived millions of years ago.
- Fossils can be formed in several ways, including:

• hard parts of the organism that do not decay easily (bones, shells, teeth)
• from parts of organisms that have not decayed because one or more of the conditions needed for decay are absent (e.g. encased in ice or amber)
• when parts of the organism are replaced by minerals as they decay (mineralisation):
  1. dead body buried by layers of silt
  2. decay leaves bones and teeth behind
  3. layers of silt become rock
  4. the bone is dissolved by water and in the space left behind, minerals from the water enter and form a rock-like substance shaped like the original bone
  5. the rocks become pushed up and as the softer rocks erode the harder fossil is exposed
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• as preserved traces of organisms, such as footprints, burrows and rootlet traces

6.3.6 Extinction

• changes in the environment that are too fast (e.g. climate change, habitat destruction)
• the extinction of a species' food source
• the introduction of a new predator or competitor
• new diseases
• invasive species that outcompete
• overpopulation
• over-hunting / over-fishing
• lack of genetic diversity
• natural disasters

6.3.7 Resistant Bacteria

FSL: GCSE Biology Revision "Evidence for Evolution: Resistant Bacteria"

- Antibiotic resistance is the ability of bacteria to survive exposure to antibiotics through random mutations.

1. A random mutation occurs in the DNA of an individual bacterium
2. The mutation protects the bacterial cell from the effects of the antibiotic - it becomes antibiotic-resistant
3. Bacteria without the mutation die when the antibiotic is present
4. Antibiotic-resistant bacteria survive and can reproduce with less competition from non-resistant bacterial strains
5. The genes for antibiotic resistance are passed on to offspring
6. Over time the whole population of bacteria becomes antibiotic-resistant

MRSA

- MRSA (methicillin-resistant Staphylococcus aureus) is a type of bacteria that has become resistant to many antibiotics, including methicillin.
- MRSA infections can be difficult to treat and can lead to serious complications, especially in people with weakened immune systems.
- MRSA is often spread in healthcare settings, such as hospitals and nursing homes, where people are more vulnerable to infections.
- To prevent the spread of MRSA, people should be isolated.

Preventing Spread

- To prevent the spread of antibiotic resistance, it is important that:

• Antibiotics should only be used when they are really needed
• People should take their whole prescribed dose
• Specific bacteria should be treated with specific antibiotics
• Medical staff should wash their hands with soap and water or use alcohol gel between patient visits and wear disposable clothing or clothing that is regularly sterilised
• Hospitals should have high standards of hygiene so that they are really clean
• Patients who become infected with antibiotic resistant bacteria should be looked after in isolation from other patients
• Visitors to hospitals and care homes should wash their hands as they enter and leave