Biology Unit Two

Biology unit two- aqa

HideShow resource information
  • Created by: Leah
  • Created on: 07-06-10 17:08

Causes of variation


Interspecific Variation- the variation that exists between different species. Eg. Horses differ from ducks which vary from humans etc.

Intraspecific Variation- The variation that occurs within a species. e.g. the length of a giraffe's neck.

Intraspecific variation is caused by both genetic and environmental factors.

Although individuals of the same species may appear similar, no two individuals are ever exactly alike. Variation can be caused by genetic and environmental factors.

1 of 19


* All members if a species have the same genes-thats what makes them the same species.

* But all individuals within a species have different versions of those genes - alleles.

* The alleles of an organism make up it's genotype.

* Different genotypes result in variation in phenotype - the characteristics displayed by an organism.

* Examples of genetic variation in humans include eye colour or blood type.

* you inherit your genes from your parents. This means that variation is


2 of 19


the appearance (phenotype) of an individual is also affected by the environment.

Identical twins are genetically identical. they have the same alleles, so any differences between them will be environmental. For example, if they grew up in different areas they may speak differently, or if one suffered from an illness as a child their development may have been effected.

Variation can often be a combination of genetic and environmental factors,

*for example a person may have the genes to potentially grow to 6ft tall. Whether they do grow to this height will depend on environmental factors such as their diet.

* another example is that the amount of melanin in a persons skin is partially controlled by genes, but a persons skin colour is also influenced by the amount of sunlight they're exposed to.

3 of 19

Investigating Variation

To study variation you have to sample a population. You cant look at the whole population as it's be impossible to catch them all and it's be too time consuming. Samples are used as models of the whole population.

The sample has to be random as it will be used to draw conclusions about the whole population. Therefore its important that it represents the whole population and isn't biased.

4 of 19


DNA contains genes which are instructions for proteins.

*Genes are sections of DNA found on chromosomes.

* Genes code for proteins

* Proteins are made from amino acids

* Different protreins have a different order and number of amino acids

* its the order of nucleotide bases in a gene that determines the order of amino acids in a protein.

* each amino acid is coded for by a sequence of three bases in a gene (a triplet)

* Different sequences of bases code for different amino acids.

5 of 19

Introns- An intron is a section of DNA that doesnt code for amino acids.

Exons- all the bits that do code for amino acids.

The nature and development of organisms is determined by genes.

6 of 19

Meiosis and genetic variation.

Meiosis is a type of cell division. Cells that divide by meiosis are diploid to start of with but the cells that are formed as a result of it are haploid_the chromosome number halves. Without meiosis you'd get double the number of chromosomes when gametes (sperm and egg) fused. not good.

Chromatids cross over in meiosis.

- during meiosis, homologous pairs of chromosomes come together and pair up. The chromatids twist around each other and bits of them swap over. The chromatids still contain the same genes but now have a different combination of alleles.

7 of 19

Genetic Diversity

Variation in DNA can lead to genetic diversity.

Genetic diversity within a species or population of a species is caused by differences in alleles. All humans have a gene for blood type but different alleles of blood type may come and go. The more alleles in a population the more genetically diverse it is.

Genetic Diversity within a population is increased by-

1. Mutations in DNA- forming new alleles.

2. Different alleles being introduced into a population when individuals from a different population migrate into them and reproduce. This is known as gene flow.

8 of 19

A genetic bottleneck is an event that causes a big reduction in the population, eg. when a large number of organisms within a population die before reproducing. This reduces the number of different alleles in the gene pool and therefore reduces genetic diversity.

Selective breeding includes choosing which organisms reproduce

Changes in genetic diversity arent just bought about by natural events like bottlenecks or migration. Selective breeding of plants or animals by humans has resulted in reduced genetic diversity in some populations.

Selective breeding can cause problems for the organisms involved.

Arguments for- produces high yielding animals. can make animals more resistant to disease.

Arguments against- it can cause health problems (eg. dairy cows have a short life expectancy) reduces genetic diversity

9 of 19

Cell division- mitosis.

Mitosis is cell division that produces genetically identical cells.

In mitosis a parent cell divides to produce two genetically identical daughter cells (they contain an exact copy of the dna of the parent cell)

Mitosis is needed for growth of the multicellular organisms and for repairing damaged tissues. This is how we grow from child to adult, our cells divide and grow.

10 of 19

Interphase- The cell carries out all its normal functions but also prepares to divide. The dna is unravelled and replicated to double its genetic content. The organelles are also replicated so it has spare ones.





11 of 19

Cells of Multicellular organisms can differenciate

1. multicellular organisms are made up of many different cell types, eg. nerve cells, blood cells, skin cells

2. All these cell types are specialised- theyr designed to carry out a specific function

3. The structure of each cell is adapted to suit its particular job.

4. The process of becoming specialised is called differentiation.

Differentiated cells are adapted for specific functions

Similar cells are organised into tissues

tissues are organised into organs

organs are organised into systems.

12 of 19

Gas exchange surfaces have two major adaptations

1. they all have large surface areas

2. theyr thin and therefore provide a short diffusion pathway across the gas exchange surface.

The organism also maintains a steep concentration gradient of gases across the exchange surfaces.

Look at gas exchange page 71

13 of 19

Classification is all about grouping together similar organisms

Taxonomy involves naming organisms and organising them into groups based on their similarities and differences. This makes it easier for scientists to identify and study them.

1. there are seven levels of groups (taxonomic groups) used to classify organisms.

2. Organisms can only belong to one group at each level.

3. Similar organisms are first organised into large groups called kingdoms.

4. similar organisms from that kingdom are organised into a Phylum. similar organisms from that Phylum are then organised into a Class. And so on down the levels of the hierarchy. The hierarchy ends with species- this group contains only one type of organism (e.g. humans, dogs etc.)

A species is a group of similar organisms able to reproduce to provide fertile offspring.

14 of 19








15 of 19

Species can be classified by their DNA or Proteins

Dna can be compared directly or by using hybridisation

DNA hybridisation is used to see how similar DNA is without sequencing it.

1. DNA from two species are seperated into single strands and mixed together.

2. Where the base sequence is the same on both strands hydrogen bonds will form. The more hydrogen bonds that form-the more alike the DNA is.

3. The DNA is then heated to seperate the strands again. Similar DNA will have more bonds holding the strands together and wlil therefore need a higher temperature to break them apart.

Dna can be directly compared by looking at the sequencing of the bases . Closely related species will have a higher percentage of similarity in their DNA base order. E.g. humans and chimps have about 94% the same and humans and mice have around 85% the same.

16 of 19

Courtship behaviour-

Courtship behaviour is carried out by organisms to attract a mate of the rigth species.

-it can be fairly simple eg.releasing chemicals orr it can be more complicated like a series of displays.

- courtship behaviour is species specific - only members of the same species will respond to it.

- this can be used to classify organisms

- The more closely related species are, the more similar their courtship behaviour.

Some examples are

Fireflies give off pulses of light, the light sequences are specific to each species.

Male butterflies use chemicals to attract females.

17 of 19

Antibiotic action and resistance

Antibiotics are used to treat bacterial diseases.

1.Antibiotics are chemicals that either kill or inhibit the growth of bacteria.

2. Different types of antibiotics kill or inhibit the growth of bacteria in different ways.

3. Some prevent growing bacterial cells from forming the bacterial cell wall which usually gives the cell structure and support.

Mutations in bacterial dna can cause antibiotic resistance

1. the genetic material in bacteria is DNA

3. mutations are changes in the base sequences of an organisms DNA

4. If a mutation occurs in the DNA it could cause the protein to cause a different characteristic.

18 of 19

Species diversity is the number of different species and the abundance of each species within a community.

The higher the species diversity of plants and trees in an area, the higher the species diversity of birds, insects and animals as they have more habitats and food sources.

Diversity can be measured to help us monitor ecosystems and identify areas where it has been dramatically reduced.

Species diversity is measured using the index of diversity.

(pg 86)

19 of 19



thank you so much!!!!! these are great notes.....


thanks thanks tripple thanks :)

Similar Biology resources:

See all Biology resources »See all resources »