Genetic Diversity and Adaptation:

Gene: A length of DNA that codes for a polypeptide

Locus: the position of a gene on a chromosome or DNA molecule

Allele: One of the different forms of a particular gene

Homologous Chromosome: A pair of chromosomes, one maternal one paternal, that have the same gene loci

Mutations are any chnage to the quantity or the base sequence of DNA of an organism. If they occur during gamete formation they may be inherited which often produces a distinct, sudden change in an individual. Gene mutations are any change to one or more nucleotide bases, or a change in the sequence of bases in the DNA. 

Th sequence of triplets on DNA is transcribed into mRNA and is then translated into a sequence of amino acids that make a polypeptide. Any chnages to one or more bases in the DNA triplets could result in a change in the amino acid sequence. Gene mutations can arise spontaneously during DNA replication, these incluse buse sustitution and deletion.

Substitution: the mutation where a nucleotide in the DNA is replaced by another nucleotide that has a different base. For example, GTC codes for glutanine. if it is changed to GTG then this will instead code for histidine. The polypeptide produce will differ in one amino acid which could be significant because if this changes the primary structure it will cause a change in the tertiary structure of the protein. As a result the protein may not function properly. 

Deletion: when a nucleotide base is lost from the normal DNA sequence. This loss is considerable as it changes the sequence of the entire gene and therefore polypeptide. One deleted nucleotide causes all the triples in the sequence to be read differently as frame shift occurs, moving all the nucleotides up one space to the left, therefore changing all the amino acids that will be made. 

Chromosome Mutations: change in the structure or number of whole chromosomes

Chromosome mutations are spontaneos and occur for 2 reasons:

• Change in whole sets of chromosomes: occur when organisms have 3 or more sets of chromosomes rather than 2. Called polyploidy and occurs more often in plants
  
• Changes in the number of individual chromosomes: Sometimes individual homologous pairs of chromosomes fail to separate during mitosis. This is known as non-disjunction, usually results in a gamete have either one too many or one too few chromosomes. 

Cell division occurs in one of 2 ways:

• Mitosis: Produces 2 genetically identical daughter cells, has same number of chromosomes as parent cell
• Meiosis: Produces 4 daughter cell, each with half the number of chromosomes as the parent cell

Two gametes fuse in sexual reproduction to produce offspring. If each gamete had the full set of chromosomes (46) then the cell produced would have double the number (92). This doubling would continue in each generation. In order to maintain a constant number of chromosomes, the number of chromosomes must be halved, this occurs as a result of meiosis. In animals it occurs in the formation of gametes.

Every diploid cell has 2 complete sets of chromosomes, one set per parent. During meiosis homologous pairs of chromosomes separate so only one chromosome from each parent is in each daughter cell, this is the haploid number of chromosomes (23). Two haploid gameted fuse during fertilisation, and the diploid number is restored.

Involves 2 divisions that occur one after the other:

1) Meiosis I: Homologous chromosomes pair up and during crossing over, their chromatids wrap around and exchnage equivalent  portions of their chromosomes. At the end of this cell division the homologous pairs have separated,with one chromosome from each pair going into one of the two daughter cells. 

2) Meiosis II: The chromatids move apart, four cells have been formed, each contain 23 chromosomes

Meiosis produces geentic variation among offspring which could lead to adaptations that lead to better survival. Genetic variation is brought about in 2 ways:

• Independant segregation of homologous chromosomes
• New combinations of maternal and paternal alleles by crossing over 

During meiosis I, each chromosome lines up along side its homologous partner. This means that there will be 23 homologous pairs of chromosomes lying side by side, they arrange themselves at random. One of each pair will pass to each daughter cell using independent segregation which is the random allocation of chromosomes to a daughter cell during meiosis. The independant segregation depends on how the pairs line up in the parent cell. 

Each member of a homologus pair of chromosomes has exactly the same genes and therefore determines the same characteristics (e.g. blood group). The alleles of these genes may differ (e.g. blood group A or B). Independant assortment produces new genetic combinations. The stages of indepents segregation are:

• Stage 1: (E.g.) 2 Chromosomes are lined up down the middle of the cell in pairs of characteristics they code for. As there are 2 there are 2 ways possible arrangements of the cells at the start of meiosis. Both are equally probable, but each produces a different outcome in terms of characteristics passed on via the gametes. 
• Stage 2: At the end of meiosis 1 the homologous chromosomes have segregated into 2 separate cells
• Stage 3: At the end of meiosis 2, chromosomes have separated into chromatids producing 4 gametes for each arrangement. The actual gametes are different and they depend on the original arrangement of the chromosomes at stage 1. 

Cells produced in meiosis are gametes and are genetically different due to the different combinations of maternal and paternal chromosomes and the alleles they contain. Each gamete has a different make up and their random fusion therefore produces variety in offspring

During meiosis 1 the chromosomes line up with their homologous partner and the following takes place:

• Chromatids of each pair break off and twist around one another
• Tension builds during twisting process, portions of the chromatids break off
• Recombination occurs when the broken portions rejoin with the chromatids of its homologous pair, equivalent portions of chromatids are exchanged
• This way new genetic combinations of maternal and paternal alleles are produced

Crossing over is when the chromatids cross over each other many times 

Random pairing of sperm and egg during fertilisation increases the possible chromosome combinations in offspring. The possible number of these combinations can be calculated using the formula:

(2n)2

n = the number of pairs homologous chromosomes

Genetic Diversity is the total number of different alleles in a gene pool. It is represented by the variation in genetic material of members of the population. Differences between members of a population can occur due to:

• Differences in the base sequence of the sections of DNA which form their genes 
• Differences in chromsomes due to:
  • crossing over and independant assortment of chromosomes during meiosis
  • The recombination of parental chromosomes in the zygote
• The base sequences of the non-coding DNA

Genetic diversity is increased with the greater number of alleles that all the species possess, and it decreases when a species has a reduced number of alleles

Population: A group of individuals of the same species that live in the same place and can interbread

Not all alleles of a population are equally likely to be passed onto the next generation. Only certain individuals are reproductively successful and so pass on their alleles

How Species Evolve:

• Because individuals of a population vary genetically, characteristics of individuals are slightly different from one another
• Resources (food, water, mates, space) enable mating species to populate are finite
• Organisms have the potential to over-reproduce, but on average numbers stay stable
• Individuals with varietns of gene, including advantagous mutations, that express these characteristics, better enable the individuals to survive than the individuals without these characteristics. Those with the advantagous characteristics are more likely to reproduce and their offspring will inherit the varient genes which control the favourable characteristics
• Natural selection is the process which results in individuals inheriting genes that control favourable characteristics
• Speciation is when populations become differnet species as a result of the accumulated difference between them. This happens as genes which control the favourable characteristics accumulate in the in the population from each generation through natural selection

Selection is the process by which organisms who are better adapted to their environment tend to survive and breed, while those who are less well adapted tend not to. Different environmental conditions favour different characteristics in the population.

Selection can occur for 2 reasons:

• Directional selection: Changes the characteristics of the population. It's the type of selection that favours individuals that vary in one direction from the mean population
• Stabalising Population: Preserves the characteristics of a population. This type of selection will favour average individuals

Polygenes are characteristics are influenced by more than one gene (occurs in most genes). These types of charactristics are more influenced by the environemt than ones determined by a single gene. Effects of the environment on polygenes produce individuals in a population tat vary about the mean, the variation can be seen on a normal distribution curve

In stable environments selection pressure is low, so it is good for an individual not to have any extreme charactristics and those who do are selected against. Those individuals with characteristics close to the average are selected and therefore are more likely to survive and produce offspring and pass on the 'average' genes to the next generation. If the trend is long term, the species changes very little and therefore decendants look similar to their ancestors. 

This is stabilising selection. Graphs will show a small range of expression of particular genes. 

Where the environmnt is changing rapidly, selection pressure is high and new species arise more quickly. Selection pressure promotes the adaptation of individuals to the altering circumstances. This means they are much more likely to survive and reproduce to pass on their advantagous characteristics. Eventually decendents will look very different to their offspring due to such drastic genetic adaptations.

On a graph this will be seen when the mean of the range of expression of a particular characteristic will have significantly shifted in one direction away from the original mean

There are 3 types of environmental adaptation that occur due to natural selection:

• Anatomical: changes to the physicsl look of an organism, e.g. short ears of polar bears
• Physiological: changes to the internal processes, e.g. oxidising of fat rather than carbohydrate in kangeroo rats produces additional water in the desert
• Behavioural: changes to the way an organism behaves in a given environment, e.g. hibertation of animals over winter