Biology 12

Palaeontology

• This involves the collection of evidence of past life or fossils

The fossil record shows:• similarities between prehistoric organisms and organisms

living today eg mammoths & elephants• changes in some species such as horses, whales, pigs• some organisms have changed little throughout their history.

This may be due to little change in the selection pressures

These organisms are called living fossils eg coelacanth, crocodile, cockroach

Some problems with using fossils includeincomplete records eg human recordsproblems with dating eg changing CO2 levels affect carbon dating

Comparative Embryology

• Similar organisms pass through similar stages of development. The closer the similarities in the appearance of the embryo/foetus at each stage, the greater the degree of relationship inferred.

• Some scientists believe the development from egg to complete organism may reflect the evolutionary history of that organism, as many embryos possess structures which are not found in the adult, but appear in organisms in the ancestral tree eg gill slits and tails in all mammals (including humans)

Comparative anatomyThis refers to • Homologous structures are structures that are

similar due to common ancestors on the evolutionary tree

Examples include forelimbs of vertebrates• Vestigial structures are reduced structures or

remnants of structures once possessedExamples include pelvis of whales, human canine &

appendixThese structures indicate changes in structure &

ancestral link to those possessing full structuresOne problem with comparative anatomy is the presence

of analogous structures, which are structures that are similar to similar selection pressures, not common ancestry

Examples include wings of birds and beetles, body shape of dolphins and sharks

• Phylogenetic trees are diagrams showing evolutionary relationships

Biogeography

• This refers to studies of the distribution of species, which suggest dispersal patterns and evolutionary history

• Examples include finches and other animals of Galopagos and other isolated islands, differences between species either side of the Wallace line (Australasia and Asia)

Biochemistry and Molecular Biology

• This involves the study of proteins and DNA in closely related organisms. The fact that closely related species have similar chemistry is seen as further evidence for evolution.

• Studies of proteins involve looking at amino acid sequences in common proteins shared by different species

• As amino acids are linked in sequences determined by the DNA, differences in amino acid sequences indicate the genetic composition of the organisms. The greater the similarities, the greater the degree of relationship presumed

DNA• DNA studies involve the analysis of the

nucleotide sequences (gene sequences)

• Genetic linkage groups involves looking at groups of genes close together on the same chromosomes

• DNA sequencing involves breaking up DNA into chunks to identify the actual sequence of bases in the DNA

• DNA hybridisation involves splitting DNA and merging strands from different species to look for differences in the chains

• Sources of DNA include nuclear DNA, mitochondrial DNA & chloroplast DNA (plants)

Immunology

A new technique that has immerged uses immunology to research the degree of relationship between species. This technique involves looking at antigens in different species.

The closer the relationship, the more antigens will be had in common

Artificial breeding

• This refers to human selection by breeding for particular characteristics (artificial selection)

• Examples include domestic animals (eg dogs, cats, cows, sheep, pigs, pigeons, etc) and plants (eg wheat, rice, fruit & vegetables) - Brussel sprouts, cauliflower, broccoli and cabbage have all been developed from the Kale plant.

Lamarck v Darwin

Evolutionary theoryWhat contributions have the following played?• Mendel: provided the concept of the gene (package

for inheritance) and the rules of genetics (how things are inherited)

• Lyell: provided the idea of constant geological change and the extremely long time span available in the life of the planet

• Malthus: provided the idea that populations increase faster than the food supply

• Wallace: discovered the Wallace line – differences between species in Australasia and Asia & came up with the same ideas of natural selection as Darwin

• Cricks, Watson and Wilson: discovered the molecular structure of DNA

Extended answer 2006 36( c) Contrast the effects of sexual and asexual reproduction on heritable variation

in species and relate these effects to survival in both changing and stable environments.

Extended answer 2006 36( c) Contrast the effects of sexual and asexual reproduction on heritable variation in species and

relate these effects to survival in both changing and stable environments. General points – up to 2 marks• Natural selection works by giving advantage or disadvantage to specific heritable variations• The greater the variability in a population, the greater the raw material for selection to act onSexual reproduction – up to 5 marks• Offspring inherit half their genes from each parent• Gene pool is shuffled by random mating• Meiosis gives opportunities for recombination, increasing genetic variation• Opportunity for mate selection (sexual selection/competition for mates)• In changing environment, variability increases chance that some forms will be suited to the

changes• Sexes must meet/2 parents/takes time to find mate• This may be difficult when population density is low & range is large• May require elaborate courtshipAsexual reproduction – up to 4 marks• No need for sexes to meet• Efficient/fast when population density is low• Offspring identical to parent, so heritable variation is reduced• No meiosis or recombination• In stable environment, there is less need for variability to cope with changes• Successful parents will have successful offspring

Extended answer 2006 37(c) Myxomatosis is a viral disease of rabbits that is transmitted by the bites of

mosquitoes or rabbit fleas. It was introduced into Australia in 1950 to control introduced rabbits and within two years reduced the rabbit population from 600 million to 100 million. However, genetic resistance in some rabbits was noticed soon afterwards and there is evidence that the virus also became less deadly compared to the original strain released. From your understanding of natural selection, explain: (i) How did the rabbit population become resistant? (ii) How did the virus become less deadly?

Extended answer 2006 37(c) Myxomatosis is a viral disease of rabbits that is transmitted by the bites of

mosquitoes or rabbit fleas. It was introduced into Australia in 1950 to control introduced rabbits and within two years reduced the rabbit population from 600 million to 100 million. However, genetic resistance in some rabbits was noticed soon afterwards and there is evidence that the virus also became less deadly compared to the original strain released. From your understanding of natural selection, explain: (i) How did the rabbit population become resistant? Up to 6 marks

• Rabbits varied in their susceptibility• While many rabbits were killed, some resistant rabbits survived/had

selective advantage• Offspring of these resistant rabbits passed on the resistant gene/allele• Over time, the rabbit population became more resistant• Resistance could be – by improved ability to kill virus when infected• Resistance could be – by reduced likelihood of being bitten by flea or

mosquito (less attractive to vector)• Resistance could be – by selection for rabbits whose eyesight was less

affected, so they were not predated on as early & lived longer

Extended answer 2006 37(c) Myxomatosis is a viral disease of rabbits that is transmitted by the bites of

mosquitoes or rabbit fleas. It was introduced into Australia in 1950 to control introduced rabbits and within two years reduced the rabbit population from 600 million to 100 million. However, genetic resistance in some rabbits was noticed soon afterwards and there is evidence that the virus also became less deadly compared to the original strain released. From your understanding of natural selection, explain:

(ii) How did the virus become less deadly? Up to 5 marks• Virus released had initial variability in virulence• Highly virulent viruses killed rabbits very fast/vectors do not feed on

dead rabbits• Less virulent strains killed rabbits slower or not at all• Less virulent strains were more likely to be transmitted, as there was

more time for rabbits to be bitten by vector• This way there was selection for less virulent strains of the virus & this

strain increased in the virus pool• Vectors may transmit virus strains of lower virulence with greater

efficiency