Chapter 15 - Evidence for relationships between organisms

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> Comparison of DNA base sequences

comparing DNA and proteins of different species helps scientists to determine the evolutionary relationships between them. When one species gives rise to another species during evolution, the DNA of the new species will initially be very similar to that of the species that gave rise to it. Due to mutations, the sequences of nucleotide bases in the DNA of the new species will change. Consequently over time, the new species will accumulate more and more differences in its DNA. As a result, we would expect species that are more closely related to show more similarity in their DNA base sequences than species that are more distantly related. As there are millions of base sequences in very organism, DNA contains a vast amount of information about the evolutionary history of all organisms.

> DNA hybridisation

- depends upon a particular property of the DNA double helix. When the DNA is heated, its double strand separates into its two complementary single strands. When cooled, the complementary bases on each strand recombine with each other to reform the original double strand. Given sufficient time, all strands in a mixture of DNA will pair up with their partners.

DNA from 2 species is extracted, purified and cut into short pieces.

The DNA from one of the species is labelled by attaching a radioactive or fluorescent marker to it. It is then mixed with unlabelled DNA from the other species

The mixture of both sets of DNA is heated to separate their strands

The mixture is cooled to allow the strands to combine with other strands that have a complementary sequence of bases

Some of the double strands that reform will be made up of one strand from each species - hybridization. The new strands are called hybrid strands, these can be identified because they are 50% labelled.

These hybrid strands are separated dcfdand the temperature is increased in stages

At each temperature stage the degree to which the two strands are still linked together is measured.

If the two species are closely related they will share many complementary nucleotide bases and so there will be more hydrogen bonds linking them together in the hybrid strand. The stronger the hybrid strand, the higher the temperature needed to separate it into its two single strands. The higher the temperature at which the hybrid strand splits, the more closely the two species are related, the lower the temperature at which it splits, the more distantly the species are related.

> comparison of amino acid sequences in proteins - the sequence of amino acids in proteins is determined by DNA, the degree of similarity in the amino acid sequence of the same protein in two species will therefore reflect how closely related the two species are. Once the amino acid sequence for a chosen protein has been determined for two species, the two sequences are compared. This can be done by counting either the number of similarities or the number of differences in each sequence.

> Immunological comparisons of proteins - antibodies of one species will respond to specific antigens on proteins in the blood serum of another.

- Serum albumin from species A is injected into species B

- Species B produces antibodies specific to all the antigen sites on the albumin from species A

- Serum is extracted from species B, this serum contains antibodies specific to the antigens on the albumin from species A

- Serum from species B is mixed with serum from the blood of a third species C

- The antibodies respond to their corresponding antigens on the albumin in the serum of species C

- The response is the formation of a precipitate

- The greater the number of similar antigens, the more precipitate is formed and the more closely the species are related

- The fewer the number of similar antigens, the less precipitate is formed and the more distantly the species are related

> Why is courtship behaviour necessary - reproduction is important to all organisms as it is the means y which a species can survive over time. Females of most species only produce eggs at specific times it is therefore important to ensure that mating is successful and that the offspring ahve the maximum chance of survival. Courtship behaviour helps to do this by enabling individuals to :

- recognise members of their own species - to ensure that mating only takes place between members of the same species because only members of the same species can produce fertile offspring

- identify a mate that is capable of breeding - both partners need to be sexually mature, fertile and receptive to matin

- form a pair bond that will lead to successful mating and raising of offspring

- synchronise mating - so that it takes place when there is the maximum probability of the sperm and egg meeting

Courtship behaviour is used by males to determine whether the female is at the receptive stage, if she responds with the appropriate behavioural response, courtship continues and is likely to result in the production of offspring.

Typically a male carries out some action, this action acts as a stimulus to the female, who responds with a specific action o her own. Her response acts as a stimulus to the female who responds with a specific action of her own. Her response acts as a stimulus to the male to carry out a further action, the ritual proceeds in this way in that is called a stimulus-response chain. The chain of the actions is the same for all members of a species but differs for members of different species. In this way both individuals recognise that their partner is of the same species and they they may be prepared to mate, the longer the courtship sequence, the more likely it is that mating will result, if at any point one of the pair fails to respond then the sequence fails and ends.