The Development of Understanding of Genetics & Evolution

What theory did Charles Darwin propose?

Evolution by natural selection

What does Darwin’s theory of natural selection state?

• individuals in a species show a wide range of variation caused by differences in genes

• individuals with characteristics most suited to the environment have a higher chance of survival and more chances to reproduce

• alleles that code for advantageous characteristics are therefore passed to offspring at a higher rate than those that code for characteristics less suited to survival

• over many generations these beneficial characteristics become more common in the population and the species changes, or evolves

What did this theory of natural selection become known as?

Survival of the Fittest

What are the 4 key principles to remember?

• variation exists in a population due to mutation

• some individuals have advantageous alleles that increase their chance of survival

• surviving individuals are more likely to reproduce and pass on their alleles

• over time the advantageous alleles become more common in the population

Why was the theory of evolution only gradually accepted?

• it challenged the idea that god made all the animals and plants that live on earth

• there was insufficient evidence at the time 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

  • the theory of evolution by natural selection developed over time and from information gathered by many scientists

What was Lamarck’s theory of evolution?

changes in an organism during its lifetime can be inherited

What two main ideas did it involve?

• a characteristic that is used frequently by an organism becomes better and stronger, whereas a characteristic that isn't used gradually disappears

• the beneficial characteristics that are used frequently (and are improved as a result) are passed to offspring

give an example of what lamarck suggested with giraffes

• Giraffes had a short-necked ancestor that would frequently stretch its neck to reach the high branches so it could feed on the leaves

• This repeated stretching could very slowly elongate the giraffe's neck and that this would be passed to the giraffe’s offspring

• Over time and many generations, the giraffe would evolve to have the very long neck it has today

What do we now know about lamarck’s ideas of inheritance?

This type of inheritance cannot occur and that Lamarck’s ideas were incorrect

Who was Alfred Russel Wallace and what did he do?

A scientist who after conducting own travels around world, independently developed his own theory of evolution based on the process of natural selection

Who did he publish scientific papers on this theory with in 1858?

Charles Darwin

What is Wallace best known for? (2 things)

-His work studying the warning colouration of species (particularly butterflies) and how this must be an example of a beneficial characteristic that had evolved by natural selection, as the warning colouration helps to deter predators

• Developing the theory of speciation

Speciation is a process that results in what?

the formation of a new species

At what point has speciation occured?

When populations of the same species become so different that they are unable to interbreed and produce fertile offspring

how can speciation occur?

describe this in more detail

a result of a combination of isolation and natural selection

• Populations of the same species can become isolated from one another due to the formation of a physical barrier (eg. a new river or mountain range) – this is known as geographic isolation

• The environment will be different on either side of this physical barrier (eg. different climates or different food available)

• The environmental differences on either side will provide different selection pressures and natural selection will cause a different set of characteristics to become more common in the two isolated populations

• Over many generations, individuals from the two populations will have become so distinct (genetically, behaviorally, physically) that they will no longer be able to interbreed and produce fertile offspring

• The two populations are now separate species

Who was Gregor Mendel and what did he do?

Give an example

• an austrian monk

• in 19th century carried out breeding experiments of plants

• he studied how characteristics were passed on between generations of plants

• e.g. he conducted studies with pea plants and looked at how the height characteristic was inherited

What was one of his observations?

The inheritance of each characteristic is determined by ‘units’ that are passed on to descendants unchanged

Using the example of pea plants, what did Mendel show?

That height in pea plants was the result of separately inherited ‘hereditary units’ passed down from each parent plant to the offspring plants - this particular experiment showed that the ‘unit’ for tall plants (t) was dominant over the unit for short plants (t)

What did his work eventually provide?

The foundation for modern genetics

Why was the importance of Mendel’s discovery not recognised until after his death?

• his studies were totally new to science in the 19th century

• there was no knowledge of the mechanisms behind his findings (DNA, genes and chromosomes had not been discovered yet)

When was the behaviour of chromosomes during cell division observed?

What did scientists realise the chromosomes’ behaviour was similar to?

Where did scientists believe Mendel’s ‘units’ must be located and what do we now know these units to be?

• late 19th to early 20th century

• mendel’s ‘hereditary units’

• on chromosomes

  • genes

When was the structure of DNA determined?

1953

What is one very clear piece of evidence for evolution?

Antibiotic resistance in bacteria

What is an antibiotic?

A chemical that can kill or inhibit the growth and reproduction of bacteria

Why are antibiotics extremely useful for humans?

Some bacteria are pathogenic and can cause life threatening disease

How often do bacteria reproduce and what does this mean as for the time span of evolution?

• 20 minutes

• evolution occurs in a much shorter time span

What might a chance mutation cause in bacteria?

What happens when the population is treated with this antibiotic?

What does this then mean?

• Some bacteria to become resistant to antibiotic (e.g. penicillin)

• some resistant bacteria do not die

• they continue to reproduce with less competition from non-resistant bacteria, which are now dead, meaning the genes for antibiotic resistance are passed on with a much greater frequency to the next generation

• over time the whole population of bacteria becomes antibiotic-resistant because the bacteria are better suited to their environment

Why is evidence for Darwin’s theory now available?

It has been shown that characteristics are passed on to offspring in genes

What has our understanding of genetics made clear?

The mechanism by which natural selection can occur

Where is there further evidence of evolution?

In the fossil record and our knowledge of how resistance to antibiotics evolves in bacteria

What are fossils?

The remains of organisms from millions of years ago, which are found in rocks

What are 3 ways fossils are formed?

• from parts of organisms that have not yet decayed because one or more conditions needed for decay are absent

• when parts of the organism are replaced by minerals as they decay

• as preserved traces of organisms such as footprints, burrows and rootlet traces

Many early forms of life were soft-bodied, what does this mean?

What traces there were have been mainly destroyed by what?

• they have left few traces behind - soft tissues often decay fully, leaving no trace in the fossil record

• geological activity (e.g. tectonic plate movements may have crushed fossils that had already formed)

• this is why scientists cannot be certain about how life began on earth

Where can evidence for early life forms on earth be found?

in the fossil record

What can we learn from fossils?

how much or how little different organisms have changed as life developed on earth

What are evolutionary trees?

Diagrams that show the relationship between species over evolutionary time

What does a new branch in the evolutionary tree show?

Where speciation has occurred (when a new species has evolved)

When do extinctions occur?

When there re no remaining individuals of a species still alive

species that are poorly adapted to their environment are less likely to what compared to species that are well adapted to their environment?

survive and reproduce

What happens if they are unable to survive and reproduce sufficiently to maintain their population numbers?

they will eventually go extinct

Name 5 factors that can lead to extinction

• new diseases

• new predators

• new, netter adapted and more successful competitors

• changes to the environment over geological time

• single catastrophic events

Extinction is occurring all the time. Although it is happening naturally for many of the reasons in the table above, … … have also massively increased extinction rates in recent history. We are very successful predators and have hunted many species to extinction (eg. the dodo). We also compete very effectively against other species for resources such as space, food and water.

human activities

what is an example of natural selection that humans have helped to develop due to overuse of antibiotics in situations where they were not really necessary?

bacteria that are antibiotic resistant

What should happen to slow the development of antibiotic resistant bacteria? (3 things)

• Doctors should avoid unnecessary prescriptions, such as for viral or mild infections

• Patients must complete their antibiotic course to ensure all bacteria are eliminated, preventing resistant mutations

• Antibiotic use in agriculture should be limited to reduce resistance spread

Developing new antibiotics is … and …, making it difficult to keep pace with emerging resistant strains

expensive and slow

What is MRSA and what is it an example of?

• a very dangerous bacterial strain that is resistant to most antibiotics, and so if someone gets infected with MRSA they cannot be treated easily

• this is an example of where increases in the population of antibiotic-resistant bacteria cause infections and diseases which are harder to control as it is difficult to find antibiotics that certain strains of bacteria are not resistant to

What are some scientists worried about in terms of antibiotics development for resistant bacteria?

Worried we may not be able to keep up with the demand for new antibiotics, as more and more antibiotic resistant strains evolve