[evolution] theories of evolution

[ BBC Bitesize 'Inheritance, variation and evolution ⇢ Evolution' page 1-9 ]

Evolution

A change in the inherited characteristics of a population over time through a process of natural selection which may result in the formation of a new species.

Natural Selection

The natural process whereby the best-adapted individuals survive longer, have more offspring and thereby spread their characteristics. Sometimes referred to as 'survival of the fittest'.

1st Stage of Natural Selection

A population shows variation in their features and this variation is because of differences in their genes.

2nd Stage of Natural Selection

Some organisms have features and adaptations which are better suited to their environment than other organisms, allowing them to survive and reproduce. Their genes are more likely to be passed on to the next generation, which results in these specific genes becoming more common.

3rd Stage of Natural Selection

Organisms without these features are less likely to reproduce and survive. Their genes are less likely to be passed on to the next generation.

4th Stage of Natural Selection

Over a period of time, the poorly-adapted organisms will die out and the advantageous alleles from the well-adapted organisms will be passed down to their offspring until, eventually, the whole population displays these features.

Charles Darwin

An English naturalist (1809-1882) who published ‘On the Origin of Species’, which outlined his theory of evolution.

Jean-Baptiste Lamarck

19th century, French scientist who developed an alternative theory of evolution before Charles Darwin.

Lamarck's theory involved two ideas:

• a characteristic which is used more and more by an organism becomes bigger and stronger, and one that is not used eventually disappears

• any feature of an organism that is improved through use is passed to its offspring

The theory of evolution through the process of natural selection was only gradually accepted because:

• the theory challenged the idea that God made all animals and plants that live on Earth (creationism)

• there was insufficient evidence when the theory was published to convince many scientists

• the mechanism of inheritance and variation was not known until 50 years after the theory was published

Alfred Russel Wallace

Naturalist who independently proposed the theory of evolution by natural selection. He is best known for studying warning colouration in animals and his theory of speciation.

Speciation

The formation of new and distinct species in the course of evolution due to populations becoming so different from each other, they are unable to interbreed and produce fertile offspring.

Species

A group of organisms which are able to interbreed and produce fertile offspring.

New species can arise as a result of:

• genetic variation - each population has a wide range of alleles that can control their characteristics

• natural selection - the alleles which help an organism to survive are selected in each population

• speciation - the populations become extremely varied and successful interbreeding cannot happen anymore

1st Stage of Speciation

Two populations of a species can become geographically separated because of the environment. This isolation can prevent interbreeding and the combination of genes within a species.

2nd Stage of Speciation

Different mutations can take place in the isolated groups and create different phenotypes between the populations.

3rd Stage of Speciation

Over many generations, natural selection will occur between the 2, separate populations. As a result, the populations will become too different to reproduce fertile offspring. This leads to a new species being formed.

Evidence for Evolution:

• rock fossils

• ice

• peat

• resistant bacteria

Fossils

The preserved remains of dead organisms from millions of years ago that can be found in rocks.

Main Ways of Forming Fossils in Rocks:

• absence of the conditions needed for decay eg. dead animals and plants can be preserved in amber, peat bogs, tar pits, or in ice

• preserved traces of organisms, such as footprints, burrows and rootlet traces → these become covered by layers of sediment, which eventually become rock

• hard body parts, such as bones and shells, which do not decay easily or are replaced by minerals as they decay

There are gaps in the fossil record due to early life forms because…

… their bodies were mostly soft and lacked skeletons, meaning they would not fossilise and because of geological activity destroying fossils.

Evolutionary Trees

Used to represent the relationships between organisms. Branches show places where speciation has occurred, and a new species has evolved.

Amber

A substance that is formed from hardened tree sap or resin. This allows the whole organism to be seen clearly.

Peat

A type of soil made of a large amount of dead plant material that cannot decay because of the acidic and anaerobic conditions. It forms over thousands of years in peat bogs and contain lots of carbon.

The main steps in the development of resistance in bacteria are:

1. random mutations occur in the genes of individual bacterial cells

2. some mutations protect the bacterial cell from the effects of the antibiotic

3. bacteria without the mutation die or cannot reproduce when the antibiotic is present

4. resistant bacteria can reproduce with less competition from normal bacterial strains

The evolution of the bacteria is an example of natural selection.

To reduce antibiotic resistance development:

• doctors should not prescribe antibiotics inappropriately, such as treating non-serious or viral infections

• patients should complete their course of antibiotics so all bacteria are killed and none survive to mutate and form resistant strains

• • the agricultural use of antibiotics should be restricted

Challenges Scientists Face when Developing New Antibiotics:

• slow development

• expensive

MRSA

Bacteria which is resistant to the antibiotic Methicillin.

Extinction

When there are no remaining individuals of a species alive. Animals that have not adapted well to their environment are less likely to survive and reproduce than those that are well adapted, resulting in possible extinction.

Factors causing Extinction:

• new diseases

• new predators

• new, more successful competitors

• changes to the environment over geological time, such as climate change

• a single catastrophic event, such as a massive volcanic eruption or a collision between an asteroid and the Earth

• speciation