P2B - Variation and evolution 2

Variation and main causes

Variation and main causes

Variation: all the differences in the characteristics of individuals in a population

• genetic causes:

  • alleles that people inherit like red hair, brown eyes

• environmental causes:

  • colour of some flowers depends on pH of the soil

  • language in humans

• combination of genes and environment causes

  • height - certain alleles make people likely to grow taller, but must also have enough calcium for bones to fully develop

Causes of genetic variation

• species have a huge amount due to mutations (random changes to DNA)

• happen all the time and most have no effect on the phenotype

• but some mutations can influence the phenotype

  • alleles for tallness may lead to tallness if diet is suitable

• a small number of mutations actually determine the phenotype

Beneficial phenotypes and rabbit e.g

• it’s very rare for a mutation to lead to a new phenotype

• sometimes the new one can be beneficial for environmental changes

• this leads to a rapid change in species e.g

  • 50s - 99% of UK wild rabbits died from virus

  • mutations of the 1% = rabbits were resistant to virus and survived

  • rabbits had a new phenotype that is now widespread in the UK

v1 - variation

Evolution by natural selection

• Life developed 3+ billion years ago

• All species have evolved from simple life forms (single cells)

Processes of natural selection with rabbit e.g

• Environment changes - gets colder

• Rabbits that have thick fur alleles = more likely to survive than with thinner fur

• They can survive the cold and are more likely to reproduce

• Their offspring can inherit the thicker fur alleles = they are also more likely to survive and reproduce

• Over many generation, the alleles for thicker fur will be more common so the overall population will have thicker fur than before

• Population has evolved

Processes of natural selection with fox and rabbit e.g

• A predator moves into the rabbits area (fox)

• Rabbits with better eyesight/hearing alleles have an advantage = more likely to detect the fox than other rabbits

• Advantageous allele rabbits are more likely to survive and reproduce

• Beneficial alleles can be passed onto their offspring

• Over many generations the advantageous alleles will be widespread in the rabbit population

• Population has evolved

Evolution def

The change in the inherited characteristics of a population over time through a process of natural selection

How species are made

• when 2 populations of one species become so different in phenotype that they can no longer interbreed to produce fertile offspring

• these 2 populations are now separate species

• pygmy rabbits are too small to breed with normal rabbits

v2 - evolution by natural selection

4 examples of selective breeding seen today

• dogs = gentle nature

  • coming from wild wolves, they are very useful by alerting threats, protecting from attacks, warmth, company = selectively bred over 1000+ years = dogs

• food crops = disease resistance

• cows = more meat/milk

• plant = large flowers

Selective breeding process

Breeding large cows for meat:

1. From a mixed population of cows, choose largest male and female

2. Breed

3. Sexual reproduction produces variation in offspring = offspring will be a mix of larger and smaller animals

4. Select largest male and female offspring and breed

5. Repeat over many generations till all offspring are large

Selective breeding problem and dog e.g

• if we breed together closely related animals/plants, we can get inbreeding

• inbreeding causes some breeds to be prone to disease or inherited defects

• e.g: dog breeds were selectively bred for hundreds of gens = many breeds develop inherited disorders like joint problems, heart disease or epilepsy

v3 - Selective breeding

Genetic engineering

• genes from one organism (human) are cut out and transferred to cells of a different organism (bacteria)

• the bacteria genome is modified and now includes a human gene

Genetic engineering with insulin

• insulin is a hormone involved in blood glucose regulation in humans

• type 1 diabetes = can’t make own insulin = must inject insulin regularly

• bacteria have been genetically modified to contain human insulin gene

• they produce insulin that can be purified and used for t1 diabetes

Genetic engineering with plants

• can produce genetically modified (GM) crops

• they usually produce a greater yield than normal crops

• can be resistant to disease or insect attack OR produce bigger/better fruits

• some are resistant to herbicides = farmers can spray to kill weeds without harming crops

Worries of GM crops

• safe - could they harm insects or wild flowers?

• more research on health effects of eating GM crops?

Gene therapy and issues

• genetic modification explored as a way to treated inherited disorder in humans

• long term effects aren’t known - if we modify a gene we don’t know potential effects on other genes

Steps of genetic engineering

1. Identify the gene wanted for transfer (human, animal, plant) and use enzymes to isolate it

2. Transfer gene into a small circle of DNA called a plasmid

   • plasmids are from bacteria - very useful for transferring DNA from one organism to another

   • can also use a virus instead

   • since both transfer DNA from one organism to another they are called VECTORS

3. Desired gene is transferred into the target organisms cells (animal, plant or microorganism)

   • Always transfer gene at an early stage in the organisms development - if transferring to animal, do it at early embryo stage

   • this makes sure all cells receive the transferred gene = organism develops with characteristics we want

v4 - genetic engineering

Advantage of cloning plants

• since the clone = genetically identical to the original, we know what its exact characteristics will be (e.g flower colour)

How seeds are produced

• by sexual reproduction = all offspring will be different

Cloning plant methods

• cuttings

• tissue-culture

Cuttings process and pros

1. small piece of plant is removed

2. end is dipped in rooting powder - contains plant hormones that encourage plant to develop roots

3. cuttings = produce genetically identical clone of starter plant

   • works really well for just a few clones, not for hundreds

Tissue-culture process, conditions and pros

1. divide plant into hundreds of tiny pieces - each piece has small number of cells

2. incubated with plant hormones - stimulate growth and development into fully grown clones

   • conditions must be sterile - don’t want microorganisms introduced

   Pros:

   • very useful in commercial plant nurseries - thousands of clones cheaply and quickly

   • gardeners can be certain they will get the wanted characteristics

   • preserves rare species of plants

v5 - Cloning plants

Cloning by embryo transplants

E.g Horses (can be done with any mammal):

1. sperm and egg from animal with wanted characteristics

2. fertilisation produces a fertilised egg

3. allow egg to develop into early stage embryo

   • cells in this must NOT have started to specialise

4. use glass rod to split embryo into 2

5. transplant the 2 embryos into host mother - grow and develop into clones

Problem with embryo transplant and how to overcome

• since we start with a sperm and egg, we can’t be certain that offspring will have the characteristics we want

• overcome by using adult-cell cloning

Adult-cell cloning stages and key benefit

Sheep:

1. remove cell from wanted animal (skin cell)

2. remove nucleus from cell - nucleus has genetic info from the animal

3. take unfertilised egg cell from same species and remove its nucleus and discard

4. insert nucleus from original adult body cell into empty egg cell = egg only has DNA from the animal we are cloning

5. electric shock the egg to make it divide and form and embryo with the same DNA as the adult skin cell we started with

6. once embryo develops into a ball of cells, we insert it into the womb of an adult female and continue its development

7. host mother gives birth to clone

   • clone looks nothing like host mother because it has none of her DNA

Benefit: since we are cloning from an adult, we know the characteristics that the clone will have

v6 - cloning animals

How theory of evolution by natural selection was developed

1. 1800s - Charles Darwin on 5 year expedition visiting 4 continents on HMS Beagle, collecting vast number of diff plants and animals. Observed many organisms on a 5 week visit to Galapagos Islands.

2. Fascinated by living organisms variety - studied geology and fossils for many years, which showed that many species of animals and plants alive today are similar to extinct species

3. After many years of experimentation and discussion he developed the theory of evolution by natural selection

Darwin’s proposals with giraffe e.g

• in a species we can see a wide range of genetic variation for any characteristic

  • some giraffes have longer necks that others

• individuals with characteristics most suited to environment are more likely to survive and breed successfully

  • long neck giraffes can reach the most nutritious top leaves = survive and breed

• the characteristics enabling these individuals to survive are passed on to the next generation

  • offspring of giraffes could inherit a longer neck

Darwin’s controversial book and why

1859 theory was published in “On the Origin of Species” book:

• Controversial and only gradually accepted because:

  • people strongly believed that God made all living animals and plants - his theory challenged that idea

  • scientists felt there wasn’t enough evidence

  • people didn’t understand how characteristics are inherited - genetics weren’t understood until 50 years after his theory was published

Jean-Baptiste Lamarck’s theory and giraffe e.g

• suggests when a characteristic is regularly used, it becomes more developed

• this strengthened characteristic is then passed onto the offspring

  • every time a giraffe stretches to reach tree top, it’s neck will grow slightly longer

  • offspring will inherit this longer neck

  • over many generations, giraffes will have the current neck length we see today

Problem with Jean-Baptiste Lamarck’s theory and how this helped Darwin

• we now know that in most cases, changes that occur in an organism’s lifetime can’t be passed onto offspring = his theory is incorrect

• scientists now accept Darwin on how species change

v7 - darwin and natural selection

Story of Alfred Russel Wallace’s theory and speciation

Theory of evolution by natural selection:

• Travelled all over world looking at different animals/plants - interested in warning colouration in animals and wanted to know how these evolved

  • bright red colour of frog = warns predators that it’s poisonous

• Studies resulted him in independently proposing the theory of evolution by natural selection

Speciation:

• He was interested in speciation - how new species form

• Noticed that closely related species were often separated by geographical barriers like wide rivers

• since then more evidence has led to our current understanding of speciation

Wallace and Darwin working together

• Darwin and Wallace realised they both developed the same theory = in 1858 they jointly published their findings

• Following year, Darwin published “On the Origins of Species” which presented natural selection to a wider audience

Speciation in snails (can apply to any organism)

Island with 1 species of snails = they can all interbreed so any beneficial mutation spreads through the whole population:

1. Environment change - river changes course, separating the population into 2 groups. River is a geographical barrier.

   • Separation = no interbreeding between the 2 groups

2. Over time, natural selection will favour different alleles on the 2 sides of the island

   • e.g food sources on one side may be slightly different to other

3. No interbreeding = any mutations that occurs can’t spread between the 2 populations

   • Over many generations, the 2 populations of snails will begin to change

4. River changes course and snails now mix

   • phenotypes of the 2 group are so different that they can’t reproduce to make fertile offspring = 2 different species

Speciation key facts

• It’s the formation of 2 or more species from the original species

• In order for it to take place, we need a geographical barrier to separate a population and prevent interbreeding between the populations

Speciation process by isolation

• two populations of a species can become geographically separated because of the environment

• isolation can prevent interbreeding and the combination of genes within a species

• different mutations can take place in the isolated groups and create different phenotypes within a particular location

• over time species may evolve to be different to each other, and they will not be able to interbreed

v8 - Speciation

Gregor Mendels studies

• Middle of 1800s, carried out thousands of breeding experiments on pea plants

  • at the time scientists didn’t understand how inheritance worked - many believed characteristics were blended when inherited

• He looked at pod shape and flower colour of pea plants

• He realised from this that characteristics are not blended during inheritance

  • e.g pea pod shape has no effect on flower colour

Gregor Mendels discovery of genetics

• Said that characteristics are determined by inherited units that don’t change when passed onto descendants

  • these units are now called genes

• He also showed that some characteristics may be masked and reappear in later generations

  • these are now called recessive alleles

Mendel vs other scientists and how genetic discovery developed from 18-1900s

• He published it in a scientific paper but scientists still thought of blending characteristics to be true

• Didn’t understand that Mendel made a major discovery. Soon work was forgotten.

• Late 1800s, scientists looked at chromosome behaviour during cell division. At same time, they rediscovered Mendels work

• Early 1900s, realised his units behaved in a similar way to chromosomes. Units were renamed genes, and scientists realised they must be located on chromosomes.

• In the mid 1900s, scientists determined the DNA structure and how genes function

v9 - Mendel and genetics

Fossils

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

• provide very good evidence for evolution

How fossil form

• when parts of organisms have not decayed

  • happens when conditions needed for decay are absent - e.g temp is too cold, not enough oxygen or water

• forms even if an organism has decayed, if parts of the organism are slowly replaced by minerals during the decay process

• fossils can be the preserved traces of organisms

  • animals can leave footprints/burrows

  • plants can leave preserved spaces where roots were

Problem with fossils

• Many of the earliest forms of life were soft-bodied organisms = no shell/skeleton

• They very rarely form fossils

• Many that did form have been destroyed by changes to rocks in the Earths crust

• Few early fossils = scientists can’t be certain how life began on Earth

What fossils show

• Show that a huge number of species have become extinct

• Extinct = no remaining individuals alive

Reasons for species extinction

• Catastrophic event

  • e.g asteroid colliding with Earth leading to dinosaur extinction

• Environment change

  • changing weather patterns

• New disease or predator killing entire species

• A new more successful species evolves and competes with it - e.g for scarce food/water

v10 - Evidence for evolution: fossils

Evolution of bacteria and antibiotic resistance

• Reproduce at a fast rate = Bacteria evolve quickly

• Bacterial diseases were deathly but in the 40s, doctors began to treat these with antibiotics (penicillin), which kill bacteria.

• In the last few years, certain strains of bacteria are no longer killed by antibiotics = they have evolved to be antibiotic-resistant

  • e.g MRSA bacteria

• Evolution of bacteria is an example of natural selection

How antibiotic resistance happens

Genetic variation is caused due to a mutation in a population:

1. Mutation may make a bacterium antibiotic-resistant

2. If human uses an antibiotic, all are killed except the mutation

3. The strains survives and reproduces without any competition from other bacteria

4. Over time, population of resistant strain rises

5. Strain spreads since people aren’t immune to it and there is no effective treatment

How to reduce development of antibiotic-resistant bacteria

1. Doctors shouldn’t prescribe antibiotics inappropriately

   • no point in using antibiotic to treat a virus

2. Patients must complete their course of antibiotics so all bacteria is killed

   • none can survive to mutate and be resistant

3. Restrict use of antibiotics in farming

Issues of developing new antibiotics

• long time

• very expensive

• can’t keep up as new antibiotic-resistant bacteria emerge

v11 - evidence for evolution: resistant bacteria

Carl Linnaeus studies

• 1700s he classified species into different categories based on structure and characteristics

• Divided all living organisms into 2 kingdoms - animal and plant

• Divided each kingdom into a number of smaller categories

Carl Linnaeus categories and acronym

• Kingdom - King

• Phylum - Philip

• Class - Came

• Order - Over

• Family - For

• Genus - Good

• Species - Soup

Fill in the missing classification

Class

Binomial system

• Binomial means 2 names - every organism is named from their genus and species

• It’s based on characteristics we can see

  • shape of body or number of toes

• If asked for an animals binomial name, you write (genus) (species)

  • E.g Polar bear’s binomial name is Ursus Maritimus

• Biology has major advances since this system was developed

Alternatives to basing the binomial system on characteristics we can see

Biology has major advances since this system was developed:

• use microscopes for internal structures

• analyse organism’s biochemistry (DNA) and look for similarities with other species

Three domain system

• Developed by Carl Woese who compared the biochemistry of different organisms

• Nowadays we use this system

In this system:

• archae - primitive bacteria

  • often found in extreme conditions like hot springs

• true bacteria

  • kind that live in human digestive system

• eukaryota - animals, plants, fungi and protists (amoeba)

Evolutionary tree

• shows how closely related organisms are to each other

• made by using classification data on living organisms e.g DNA, but on extinct organisms we must use fossils

  • problem: many species fossil records are incomplete

Describe the evolutionary tree diagram

• species 1&2 are closely related to each other

• species 3 is more distantly related

• species 1,2&3 all share a common ancestor

v12 - classification