Natural Selection

Noadswood Science, 2011

Natural Selection

To understand how natural selection can lead to evolution

Wednesday, April 19, 2023

Natural Selection

Mach each statement to its title…

1. All living things in a species are not the same

2. There is not enough food or space for all of them

3. Some individuals have features which help them survive

4. They are more likely to have offspring – more of the next generation have the useful feature

A. Competition

B. Survival

C. Variation

D. Reproduction

Natural Selection

1. Variation – all living things in a species are not the same

2. Competition – there is not enough food or space for all of them

3. Survival – some individuals have features which help them survive

4. Reproduction – they are more likely to have offspring – more of the next generation have the useful feature

Variation

Each species has individuals showing variation (they have slight differences) if reproduction is sexual

Competition

Between and within a species there is competition (e.g. for food; water; space; mates; shelter etc…)

Get off my land

Adaptation

The better adapted individuals are more likely to survive (better at living within their environment) – survival of the fittest

I’m hungry

Yum yum

Reproductively Successful

These survivors are more likely to pass on their genes, so their offspring are more likely to express the beneficial traits

You’ll all be nice and tall

Evolution

The basic idea behind the theory of evolution is that all the different species have evolved from simple life forms

These simple life forms first developed more than three billion years ago - the Earth is about 4.5 billion years old

Natural Selection & Evolution

The theory of evolution states that evolution happens by natural selection: - Individuals in a species show a wide range of variation

(due to differences in genes)

Individuals with characteristics most suited to the environment are more likely to survive and reproduce

The genes that allowed the individuals to be successful are passed to the offspring in the next generation

Natural Selection & Evolution

Individuals that are poorly adapted to their environment are less likely to survive and reproduce, meaning that their genes are less likely to be passed to the next generation

Given enough time, a species will gradually evolve…

Mutations

Mutations occur when an organism develops a new characteristic that no other member of the species has had before – e.g. someone being born with blue hair

Some mutations are beneficial, some are neutral, but most are non-advantageous

A mutation is a change in a gene, DNA or chromosomes in a cell which leads to genetic variation (mutations usually occur due to errors during DNA replication)

Mutations

DNA exists as two helical strands held together through the base pairs by hydrogen bonding: -

DNA mutations: substitution; deletion; insertion; and inversion

Mutations

Each gene is the instruction for making one protein

Sometimes a mistake is made when the gene’s DNA is copied

The gene may code for a different protein

Mutations do happen naturally, but they can also be caused by some chemicals, and ionizing radiation

Useful Mutations

Most mutations do not help the organism – the different protein that is made cannot do its job well

However a very small number may help the organism survive in some environments, e.g. bacteria with mutations that make them resistant to certain antibiotics such as MRSA

Sickle-cell anaemia is a serious blood disease: people with two copies of the disease allele can be very ill, but people who carry just one copy of the allele have protection from malaria increasing their chances of survival in countries where malaria is common

Evolutionary Evidence

Evolution is difficult to observe because it usually takes many years to happen

This is one reason why the theory of evolution is still a theory, not a law – since no-one was around millions of years ago to make observations, take notes and carry out experiments, evolution cannot be proved in the usual scientific sense

However, rapid changes in species have been observed that support the theory…

Antibiotic-Resistant Bacteria

Microorganisms such as bacteria and viruses reproduce very rapidly and can evolve in a relatively short time

The bacterium E. coli can have its DNA damaged or changed during replication, and most of the time this has no effect, or causes the death of the cell

Occasionally, the mutation is beneficial - for the bacteria, e.g. allowing resistance to an antibiotic

When that antibiotic is present, the resistant bacteria have an advantage over the bacteria that are not resistant, so these reproduce more proficiently passing on the resistance to future generations

Peppered Moths

Before the industrial revolution in Britain, most peppered moths were of the pale variety (they were camouflaged against the pale birch trees they rest on)

Moths with a mutant black colouring were easily spotted and eaten by birds, giving the white variety an advantage, and they were more likely to survive to reproduce

Airborne pollution in industrial areas blackened the birch tree bark with soot meaning that the mutant black moths were now camouflaged, while the white variety became more vulnerable to predators

This gave the black variety an advantage, and they were more likely to survive and reproduce – over time, the black peppered moths became far more numerous in urban areas than the pale variety – natural selection directly influenced by man

Peppered Moths

The pale peppered moths camouflage

well against the pale birch tree

The darker peppered moths camouflage

well against the blackened birch tree

Peppered Moths Experiment

1. Cut 10 small pieces of white and 10 small pieces of black paper

2. Place them on a large piece of white paper

3. Cover your eyes with a scarf or jumper so you can just see through (this is more realistic as the peppered moth’s main predators don’t have eyesight as good as us)

4. Pick off 10 pieces of paper (the ones easiest to find)

5. Record how many are white / black

6. Now repeat this using black paper

What have you found and what conclusions could your draw from this?

Natural Selection – Rats

Warfarin is used as a rat poison – put the statements in order to show how natural selection has allowed some rat populations to become resistant to this original poison…

Natural Selection – Rats

A mutation in a rat’s sex cells make its offspring resistant to

warfarin

The mutated gene is passed on and is common in the rat

population

The number of resistant rats increases with each generation

Warfarin kills most rats

Mutations may happen when DNA is copied in the cell

People use warfarin to kill the rats

The resistant rats have a better chance of surviving and

reproducing

The resistant rats breed and pass their features on

Mutations are a change in the genetic code

Mutations can be caused by some chemicals or ionizing radiation

Natural Selection – Rats

A mutation in a rat’s sex cells make its offspring resistant to

warfarin

The mutated gene is passed on and is common in the rat

population

The number of resistant rats increases with each generation

Warfarin kills most rats

Mutations may happen when DNA is copied in the cell

People use warfarin to kill the rats

The resistant rats have a better chance of surviving and

reproducing

The resistant rats breed and pass their features on

Mutations are a change in the genetic code

Mutations can be caused by some chemicals or ionizing radiation