Genetic diversity and adaptation

Any change to the quantity/ base sequence of the DNA of an organism = mutation Mutations occurring during the formation of gametes may be inherited, often producing sudden and distinct differences between individuals Any change to one or more nucleotide bases (A,T,C, G) or a change in the sequence of the bases (read in triplets) in DNA= gene mutation Any changes to one or more bases in DNA triplets could result in a change in the amino acid sequence of the polypeptide Gene mutations arise spontaneously, e.g. during DNA replication and include base substitution and base deletion

Substitution = the type of gene mutation in which a nucleotide in a DNA molecule is replaced by another nucleotide that has a different base (e.g. C replaced with T) E.g. DNA triplet of bases GTC that codes for the amino acid glutamine. If the final base C is replaced by G, then GTC becomes GTG which is the DNA triplet that codes for the amino acid histidine which replaces the amino acid glutamine. The polypeptide produced will differ in a single amino acid. The significance of this difference will depend upon the precise role of the original amino acid. If it is important in forming bonds (e.g. hydrogen bonds) that determine the tertiary structure of the final protein, then the replacement amino acid may not form the same bonds. The protein may then be a different shape and therefore not function properly. If the protein is an enzyme, its active site may no longer fit the substrate and it will no longer catalyse the reaction The effect of the mutation is different if the new triplet of bases still codes for the same amino acid as before. This is due to the degenerative nature of the genetic code, in which most amino acids have more than one triplet. – there may be no change in the polypeptide produced and so the mutation will have no effect.

A gene mutation by deletion arises when a nucleotide is lost from the normal DNA sequence. The loss of a single nucleotide from the thousands in the gene can have considerable consequences Usually the amino acid sequence of the polypeptide is entirely different and so the polypeptide is unlikely to function correctly. This is because the sequence of bases in DNA is read in triplets. One deleted nucleotide causes all the triplets in the sequence to be read differently because each has been shifted to the left by one base. This means all of the triplets after where a nucleotide has been deleted will be different and result in many different amino acids being produced & a non functional polypeptide

Chromosome mutations = changes in the structure or number of whole chromosomes Changes of whole sets of chromosomes – occur when organisms have three or more sets of chromosomes rather than the usual two. This is called polyploidy and mostly occurs in plants

Changes in the number of individual chromosomes – sometimes individual homologous pairs of chromosomes fail to separate during meiosis. This is called non-disjunction and usually results in a gamete having either one or more fewer chromosome. On fertilisation with a gamete, offspring have more or fewer chromosomes than normal in all of their body cells. E.g. down’s syndrome, where individuals have an additional chromosome 21

Meiosis – produces four daughter cells, each with half the number of chromosomes as the parent cell. (haploid cells) In sexual reproduction, two gametes fuse to give offspring. If each gamete had a full set of chromosomes (diploid number) then the cell they produce would have double the number. In humans, the diploid number of chromosomes is 46 which means that the cell would have 92 chromosomes. The doubling of the number of chromosomes would continue at each generation. In order to maintain a constant number of chromosomes in the adult of a species, the number of chromosomes must be halved at some stage in the life cycle. – the halving is a result of meiosis. Meiosis occurs in the formation of gametes. Every diploid cell of an organism has two complete sets of chromosomes, one provided by each parent. During meiosis, homologous pairs of chromosomes separate, so that only one chromosome from each pair enters a daughter cell. This is known as the haploid number of chromosomes which in humans, is 23. when two haploid gametes fuse at fertilisation, the diploid number of chromosomes is restored.

Involves two nuclear divisions so that sex cells can be haploid Crossing over = homologous chromosomes pair up and their chromatids wrap around each other. Equivalent portions of these chromatids may be exchanged in crossing over. Meiosis also produces genetic variation among the offspring which may lead to adaptations that improve survival chances. Meiosis brings about genetic variation by: independent segregation of homologous chromosomes and new combinations of maternal and paternal alleles by crossing over.

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 gene Homologous chromosomes – a pair of chromosomes, one material and one paternal that have the same gene loci

During meiosis 1, each chromosome lines up alongside its homologous partner – 23 homologous pairs of chromosomes lying side by side. When these homologous pairs arrange themselves in this line, they do so at random. One of each pair will pass to each daughter cell. Which one of the pair goes into the daughter cell and with which one of any of the other pairs depends on how the pairs are lined up in the parent cell. Since the pairs are lined up at random, the combination of chromosomes of maternal and paternal origin that go into the daughter cell at meiosis 1 is a matter of chance = independent segregation.

Each member of a homologous pair of chromosomes has exactly the same genes and therefore determines the same characteristics. However, the alleles of these genes may differ. The independent assortment of these chromosomes therefore produces new genetic combinations. At the end of meiosis 1, the homologous chromosomes have segregated into two separate cells At the end of meiosis 2, the chromosomes have segregated into chromatids producing four gametes. The actual gametes are different. Gametes will be genetically different as a result of the different combinations of the maternal and paternal chromosomes/alleles that they contain. These haploid gametes fuse randomly at fertilisation. The haploid gametes produced by meiosis fuse to restore the diploid state. Each gamete has a different make up and their random fusion therefore produces variety in the offspring. Two different genetic make ups are combined and even more variety results.

During meiosis 1, each chromosome lines up alongside is homologous partner and then: The chromatids of each pair become twisted around one another During this twisting process, tensions are created and portions of the chromatids break off Usually it is the equivalent portions of homologous chromosomes that are exchanged In this way, new genetic combinations of maternal and paternal alleles are produced The chromatids cross over one another many times so it is called crossing over. The broken off portions of chromatid recombine with another chromatid so this is called recombination.

Crossing over therefore increases genetic variety even further.

Homologous pairs of chromosomes line up at the equator of a cell during meiosis 1. Either one of a pair can pass into each daughter cell (independent segregation) and so there are a large number of possible combinations of chromosomes in any daughter cell. •2^n where n = the number of pairs of homologous chromosomes in a human haploid cell = 2^23

All members of the same species have the same genes but different alleles Genetic diversity = the total number of different alleles in a population

Population = a group of individuals of the same species that live in the same place and can interbreed

A species consists of one or more populations The greater the number of different alleles that all members of a species possess, the greater the genetic diversity of that species Genetic diversity is reduced when a species has fewer different alleles The greater the genetic diversity, the more likely that some individuals in a population will survive an environmental change This is because a wide range of alleles and characteristics. Therefore this gives greater probability that some individual will possess characteristics that suits it to the new environmental conditions. Genetic diversity is a factor that enables natural selection to occur

Differences between the reproductive success of individuals affects allele frequency in populations. Within any population of a species there will be a gene pool containing a wide variety of alleles Random mutation of alleles within this gene pool may results in a new allele of a gene which in most cases will be harmful However in certain environments, the new allele of a gene pool might give its possessor an advantage over other individuals in the population

These individuals will be better adapted and therefore more likely to survive in their competition with others

These individuals are more likely to obtain the available resources and so grow more rapidly and live longer. As a result, they will have a better chance of breeding successfully and producing more offspring, passing on their advantageous alleles

Only those individuals that reproduce successfully will pass on their alleles to the next generation As these new individuals also have the new ‘advantageous’ allele, they in turn are more likely to survive and so reproduce successfully Over many generations, the number of individuals with the new ‘advantageous’ allele will increase at the expense of the individuals with the ‘less advantageous’ alleles over time, the frequency of the new ‘advantageous’ allele in the population increases while that of the ‘non-advantageous’ ones decrease What is advantageous depends upon the environmental conditions at any one time

Selection is the process by which organisms that are better adapted to their environment tend to survive and breed, while those that are less well adapted tend not to. Every organism is subject to the process of selection, based on its suitability for surviving the conditions that exist at the time Selection may favour individuals that vary in one direction from the mean of the population. This is called directional selection and changes the characteristics of the population Selection may favour average individuals. This is called stabilising selection and preserves the characteristics of a population Most characteristics are influenced by more than one gene. These types of characteristics are more influenced by the environment than ones determined by a single gene. The effect of the environment on polygenes produces individuals in a population that vary about the mean. When you plot this variation on a graph you get a normal distribution curve

If the environmental conditions change, the phenotypes that are best suited to the new conditions are most likely to survive. Some individuals that fall either left or right of the mean will posses a phenotype more suited to the new conditions. These individuals will be more likely to survive and breed. They will therefore contribute more offspring to the next generation than other individuals. Over time, the mean will then move in the direction of the individuals. E.g. the effectiveness of some antibiotics at killing bacteria was reduced so directional selection took place, favouring those bacteria with resistance to antibiotics- they survived and reproduced, whereas the bacteria without this advantageous gene were selected against Directional selection results in phenotypes at one extreme of the population being selected for and those at the other extreme being selected against.

If environmental conditions remain stable, it is the individuals with phenotypes closest to the mean that are favoured. These individuals are more likely to pass their alleles onto the next generation. Those individuals with phenotypes at the extremes are less likely to pass on their alleles. Stabilising selection therefore tends to eliminate the phenotypes at the extremes.

E.g. human birth weights . Body mass of babies at birth is within a relatively narrow range- due to infant mortality. If baby's birth weight is either too low or too high, it is less likely to survive (be selected against) than an average birth weight which is selected for

Natural selection result in species that are better adapted to the environment that they live in. These adaptions may be: Anatomical = such as shorter ears and thicker fur in artic foxes compared to foxes in warmer climates Physiological = for example oxidising of fat rather than carbohydrate in kangaroo rats to produce additional water in a dry desert environment Behavioural = such as the autumn migration of swallows from the UK to Africa to avoid food shortages in the UK winter