Topic 4 - natural selection and genetic modification

describe the work of Alfred Russel Wallace

Alfred Russel Wallace developed the theory of speciation, and therefore evolution by natural selection

• on his travels, he had the idea that the individuals who did not have characteristics to help them survive a change in the enviroment would die out

• he published joint studies with Darwin

• the publication of ‘On the Origin of Species’ meant Darwin received the credit for the theory

• he continued to work across the world to collect evidence - one of his most important works was warning colouration in animals

• much more evidence over time has resulted in our current understanding

explain the steps of the process of speciation

1. variation exists within a population as a result of genetic mutations

2. alleles which provide a survival advantage are selected through natural selection

3. populations of a species can become isolated, for example through physical barriers such as rock fall preventing them from breeding together

4. different alleles may be advantageous in the new enviroment, leading to them being selected

5. over time the selection of different alleles will increase the genetic variation between the two populations

6. when they are no longer able to breed together to product fertile offspring, a new species has formed

Charles Darwin

• scientist and naturalist

• put forward the theory of evolution

• this was supported by experimentation and his knowledge of geology and fossils that he discovered on a round the world expedition

• published ‘On the Origin of Species’ in 1859

Theory of Evolution

• variation exists within species as a result of mutations in DNA

• organisms with characteristics most suited to the enviroment are more likely to survive to reproductive age and breed successfully - called survival of the fittest

• the beneficial characteristics are then passed on to the next generation

• over many generations

controversy surrounding his ideas

1. it contradicted the idea that God was the creator of all species on Earth

2. there was not enough evidence at the time as few students had been done on how organisms change over time

3. the mechanism of inheritance and variation were not known at the time

explain how the emergence of resistant organisms supports Charles Darwin's theory of evolution including antibiotic resistance in bacteria

bacteria are labelled resistant when they are not killed by antibiotics which previously were used as cures against them

• bacteria reproduce at a fast rate

• mutations during reproduction can result in new genes, such as the gene for antibiotic resistance. this is the creation of a new strain

• exposure to antibiotic resistance can reproduce and pass on the advantageous gene to their offspring. therefore, the presence of these new, resistant bacteria supports Darwin’s theory of natural selection (as the new bacteria have been selected by the enviroment to have a feature (resistance) advantageous to survival)

• this population of antibiotic resistant bacteria increses

• bacterial diseases spread rapidly because people are not immune to these new resistant bacteria and there is no treatment for it

an example is MRSA

• called a ‘superbug’ as it is resistant to many different types of antibiotics

• common in hospitals: spreads when doctors and nurses move to different patients

how do fossils give evidence for human evolution

fossil evidence shows how developments in organisms arose slowly. this is because we can use carbon dating and related techniques to estimate when a fossil was formed, giving us a more complete picture of how an organism or species developed over time.

describe the evidence for human evolution based on fossils, Ardi from 4.4 million years ago

Ardi - Ardipithecus ramidus, or Ardi is the oldest known human ancestor - estimated to have lived 4.4 million years ago

her fossilised skeleton contains many ‘humanoid’ features but also resembles an ape - thus, it is phenotypically somewhere between the two. the presence of this ‘intermediate’ organism is good evidence that natural selection, and eventually evolution, occurred gradually.

the bone structure in Ardi’s feet also gives a clue - it is different from that of chimpanzees, suggesting that the two evolved separately rather than together

describe the evidence for human evolution based on fossils, Lucy from 3.2 million years ago

Lucy - this fossilised skeleton dates from 3.2 million years ago. her bone structure suggests that she walked in an upright, human-like position. however, Lucy had a small, chimp-like skull and brain and therefore represents another intermediate between apes and early humans

describe the evidence for human evolution based on fossils, Richard Leakey’s discovery of fossils from 1.6 million years ago

fossils discovered by the archaeologists Louis and Mary Leakey in the 1950s helped support the theory of natural selection, especially an early fossil which contained remnants of stone tools (thought to be an early toolmaker), and Homo habilis, which is now considered to be one of the most important early human species

Early Stone Age tools

early stone age tools - homo habilis (1.5 million years ago)

• used basic pebble tools (‘Oldowan tools’) created by smashing rocks together

• these tools had simple uses, such as cracking nuts

Late Stone Age tools

late stone age tools - homo neanderthalensis (40,000 years ago) and modern homo sapiens

• these more advanced species used pointed arrowheads, spears and hooks

• this enabled more advanced tasks to be carried out, such as catching fish

methods to date those tools

1. radiometric carbon dating - by looking at the natural radioactive decay of an isotope of Carbon (carbon-14) we can estimate how long ago an organism lived. if any once-living material is found with a tool, such as a piece of wood or fur, we can date this to find the age of the rock

2. stratifying rock layers - looking at the layer of sediment in which a rock was found is a useful tool for archaeologists. each layer of sediment, and everything within it, must have been formed at the same time. therefore, we can date once-living fossils in this layer and use this to estimate when the tools were formed.

how this proves evolution

looking at how anatomical features developed over time using fossils also provides important evidence for evolution.

a pentadactyl limb is a limb with five digits. this can be seen in a number of organisms implying that they all come from a common ancestor - and that each ‘branches off’ at some stage of evolution. this could have been due to different selection pressures within different enviroments

the human hand has five digits (four fingers and a thumb), but bats, cats, horses and birds also have this pattern with their limbs. however, that does not mean that we evolved directly from these animals but humans are distantly related to them via a common ancestor

five kingdoms vs three domains, five kingdoms system

five kingdoms system

the five kingdoms classification splits all organisms into one of 5 groups:

• animals

• plants

• fungi

• prokaryotes (eg. bacteria)

• protists (eg. algae, amoebas and other single-celled eukaryotic organisms)

each kingdom is then subdivided into a phylum, class, genus and species. these are different for each organism. for example, a human (Homo sapiens) would be in the animal kingdom, its phylum is Chordata, class is Mammalla and order is Primate

five kingdoms vs three domains, three domain system

three domain system

• developments in science such as the improvement of the microscope and increased knowledge of biochemistry (eg. RNA sequence analysis) found that some species were more distantly related than first thought

• Carl Woese added three large groups called domains above kingdoms

  • archae: primitive bacteria which live in extreme environments such as hot springs

  • bacteria: true bacteria (despite having similar features to archae)

  • eukaryota: organisms who have a nucleus enclosed in membranes, includes the kingdoms protists, fungi, plants and animals

what is the binomial naming system

the binomial naming system is based on the genus and species: for example, Homo sapiens is of the genus Homo, which also contains Homo habilis and Homo erectus etc

what is selective breeding

selective breeding is when humans choose which organisms to breed in order to produce offspring with a certain desirable characteristic (eg. animals with more meat, plants with disease resistance or big flowers.

this has been happening for many years since animals were domesticated and plants were grown for food

• parents with desired characteristics are chosen

• they are bred together

• from the offspring those with desired characteristics are bred together

• the process is repeated many times until all the offspring have the desired characteristic

problem with selective breeding

the problem is that it can lead to inbreeding

• breeding those with similar desirable characteristics means it is likely you are breeding closely related individuals

• this results in the reduction of the gene pool, as the number of different alleles reduce (as they mostly have the same alleles)

• this means if the enviroment changes or there is a new disease, the species could become extinct as they all have the same genetic make up (so the chance of a few organisms having survival advantage and not dying is reduced). this is particularly relevant in selective breeding of plants, as one disease could spread rapidly and destroy the entire population of crops. this could cause severe economic problems, especially for the farmers who rely on income from their crops

• another problem is that the small gene pool leads to a greater chance of genetic defects being present in offspring, as recessive characteristics are more likely to be present. this is particularly relevant in domesticated animals, which have a much higher frequency of genetic conditions than normal

what is tissue culture

tissue culture is a method of culturing living tissue, ie. making it grow outside the organism, within a growth medium. this is especially useful for plants - we can produce an entire field of identical cloned crops using just a small cutting. tissue culture can also be used to culture animal and human tissues outside of the body

how is this performed in plants

1. take the plant that you would like to clone - for example, a plant with desirable characteristics

2. using tweezers, remove a piece of tissue from a fast growing region of the plant, eg the root or shoot tip

3. using aseptic technique (maintaining sterile conditions), place the tissue on a special growth medium (containing hormones and nutrients)

4. once the tissue has developed enough (eg. produced shoots and roots), it can be transferred to compost for further growth

benefits of tissue culture

• produces lots of offspring with a specific desirable feature

• increasing the number of crops resistant to bad weather, for example, can increase crop yields

• can help extremely endangered species, or even bring back species that have become extinct

risks of tissue culture

• the gene pool is reduced through producing clones, meaning it is less likely that the population will survive if a disease arises with low diversity in the population

• clones have a low survival rate, and tend to have some genetic problems

• it may lead to human cloning

what is genetic engineering

modifying the genome of an organism by introducing a gene from another organism to give a desired characteristic

• plant cells have been engineered for disease resistance or to have larger fruits

• bacterial cells have been engineered to produce substances useful to humans, such as human insulin to treat diabetes

stages of genetic engineering

1. genes from chromosomes are ‘cut out’ using restriction enzymes

2. the same restriction enzymes are used to cut the vector (such as a virus or bacterial plasmid) into which the genes will be placed

3. ligase enzyme is used to attach the sticky ends of the gene and the vector together, to produce a recombinant gene product. the vector is placed in another organism at an early stage in development so the desired gene moves into its cells and cause the organism to grow with the desired characteristics. in plants the vector is put into meristematic cells (unspecialised cells) which can then produce identical copies of the modified plant

genetically modified crops

• they are engineered to be resistant to insects and to herbicides

• this will result in increased yields as less crops will die

genetic modification in medicine

• it may be possible to use genetic engineering to cure inherited disorders

• it is called gene therapy and involves transferring normal genes (not faulty) into patients so the correct proteins are produced

perceived benefits of genetic engineering

• it can be very useful in medicine to mass produce certain hormones in microorganisms (bacteria and fungi)

• in agriculture it can be used to improve yields by:

  • improving growth rates

  • introducing modification that allow the crops to grow in different conditions, eg. hotter or drier climates

  • introducing modification that allow plants to make their own pesticide or herbicide

• crops with extra vitamins can be produced in areas where they are difficult to obtain

• greater yields can help solve world hunger, which is becoming an increasing bigger issue due to population growth

perceived risks of genetic engineering

• GM crops might have an effect on wild flowers and therefore insects

  • GM crops are infertile and these genes could spread into wild plants, leading to infertility in other species, which affects the entire enviroment

  • growing with herbicides and pesticides can kill insects and other plants, which would reduce biodiversity

• people are worried that we do not completely understand the effects of GM crops on human health

• genetic engineering in agriculture could lead to genetic engineering in humans. this may result in people using the technology to have designer babies

• they pose a selection pressure, which could lead to increased resistance in other species, creating super weeds and pests

what are Bt crops and why/how are they used

bacillus thuringiensis is the name of the bacteria that produces toxins that kill insect larvae

this is a useful function for crops, so we use genes from the bacteria in crops to increase their insect resistance

genes are cut out from the bacteria using restriction enzymes, and re-inserted into the crop using ligase, as described above. the crop will then produce the toxic - any insects that eat the crop will die

as a reuslt, less of crop gets eaten by insects, increasing the crop yield and profits

however, there are concerns over this method - we don’t know if the toxin has any effect on human health, for example. killing insects also results in a loss of biodiversity, which can affect the entire ecosystem

what are the two most useful methods to cope with the agricultural demands of a growing human population

fertilisers and biological control

what do fertilisers do

fertilisers - fertilisers provide useful nutrients (nitrates and phosphates) to plants, making them more resistant to enviromental conditions and able to grow faster and larger - resulting in increased crop yields. however, excess fertiliser can often run off into rivers, killing fish and other wildlife and affecting biodiversity

what does biological control

biological control - biological control is the use of certain species to control population of other species. for example, Aphelinus abdominalis, the aphid killer wasp, has been used successfully to control aphid populations - which feed on frit crops. however, this reduces biodiversity, and again, has a knock-on effect across the whole ecosystem

perceived benefits of genetic engineering

• it can be very useful in medicine to mass produce certain hormones in microorganisms (bacteria and fungi)

• in agriculture it can be used to improve yields by:

  • improving growth rates

  • introducing modification that allow the crops to grow in different conditions, eg. hotter or drier climates

  • introducing modifications that allow plants to make their own pesticide or herbicide

• crops with extra vitamins can be produced in areas where they are difficult to obtain

• greater yields can help solve world hunger, which is becoming an increasingly bigger issue due to population growth

• it is possible to greatly increase the yield of a particular crop by selectively breeding only individuals that produce higher quality or a larger mass of food

• individual plants or animals can be bred to be resistant to a particular disease, which could increase crop yield

perceived risks of genetic engineering

• GM crops might have an effect on wild flowers and therefore insects

  • GM crops are infertile and these genes could spread into wild plants, leading to infertility in other species, which affects the entire enviroment

  • growing with herbicides and pesticides can kill insects and other plants, which would reduce biodiversity

• people are worried that we do not completly understand the effects of GM crops on human health

• genetic engineering in agriculture could lead to genetic engineering in humans. this may result in people using the technology to have designer babies

• they pose a selection pressure, which could lead to increased resistance in other species, creating super weeds and pests

• selecting for advantageous characteristics can sometimes cause severe health problems in the offspring - eg. chickens that have been bred to have more meat (muscle) are sometimes too large to be able to walk

• lack of genetic variation - despite the bred population being able to have resistance to a particular disease (or multiple diseases), if one of them has susceptibility to a different disease then they all do - and the entire population could be wiped out as a result

• there are ethical issues associated with selective breeding - many people consider it unethical to selectively breed for characteristics wanted by humans if it means that the offspring will suffer, or have a reduced quality of life as a result