Diversity and Evolution

1. Explain how homology (anatomical, embryological and molecular) supports Darwin's theory of natural selection. [8]

Anatomical homology
1. anatomical resemblances representing variations on a structural theme present in their common ancestor
2. ref. examples: forelimbs of all mammals, including humans, cats, whales and bats show the same arrangement of bones from the shoulder to the tips of the digits/ vestigial legs in snakes
3. even though these appendages can have very different functions (lifting, walking, swimming and flying)

Embryology homology
4. comparison of early stages of animal development reveals additional anatomical homologies not visible in adult organisms
5. ref. examples: embryos of different types of vertebrates have a tail posterior to the anus/ pharyngeal (throat) pouches.
6. these embryonic structures develop into homologous structures with very different functions, such as gill slits in fishes and parts of ears and throat in humans.

Molecular homology
7. all forms of life use the same genetic machinery of DNA and RNA, and the genetic code is essentially universal.
8. DNA differences are accumulated as descendents evolve / closely related species share a greater portion of their DNA
9. ref. examples: organisms as dissimilar as humans and bacteria share many genes inherited from a distant common ancestor
*comparison of amino acid sequence of human haemoglobin with other vertebrates reveals the same pattern of evolutionary relationships as nonmolecular methods

In relation to Darwin’s theory
10. ref. similarities between individuals of the same species or among different species, that arose from common ancestry

NOTES:
Darwin’s theory = many modern species of organisms are descendents of ancestral species that were different from the species. As the descendents of that ancestral organism spilled into various habitats over millions of years, they accumulated diverse modifications, or adaptations, that fit them to specific ways of life.

2(a)
Classification
1. Classification is placing organisms into groups
2. The classification is hierarchical with each successive group containing more diverse kinds of organism
3. Each group possesses unique features
4. The lowest or more exclusive taxon is the species, the highest or most inclusive is the kingdom
5. The taxa used in order of decreasing size are:
Kingdom, Phylum, Class, Order, Family, Genus and Species
6. A binomial naming system was used to name the species, to avoid confusion by common name
7. The first part of a binomial is the genus, the second part is the genus the species belong to
Phylogeny
8. Phylogeny was based on evolution and involved passing genes from ancestors to descendants
9. A phylogenetic tree can be constructed to reflect the evolutionary history
10. Use of DNA base sequences to assess relationships between species or by comparing their anatomy

(b)
1. To assess phylogenetic relationships that cannot be measured by comparative anatomy and other nonmolecular methods.
2. To compare species too closely related to display much divergence in morphology.
3. To trace evolutionary relationships of species that are so different that there is little morphological homology
4. Each nucleotide position along a stretch of DNA represents an inherited character in the form of one of the four DNA bases: A (adenine), G (guanine), C (cytosine), or T (thymine). Thus homologous regions of DNA from two species that are 1,000 nucleotides long provide 1,000 points of comparison.
5. A systematist may compare several DNA regions to assess the relationship between two species.
6. Molecular character states are unambiguous: A, C, G and T are easily recognizable and one cannot be confused with another.
7. Possible to analyse the extensive quantity of genetic information and provides a quantitative tool for constructing cladograms.
8. Molecular data are easily converted to numerical form and hence are amenable to mathematical and statistical analysis.
9. Use of molecular clock to date the time of divergence.
10. These methods avoided the pitfalls of convergent evolution.
11. Can be used on living or dead material

(c)
1. as the viruses depend on cells for their propagation therefore there is no common ancestor  polyphyletic evolution
2. viruses originated from fragments of cellular nucleic acids that could move from one cell to another
3. they replicate very quickly, providing a great deal of material for the engine of natural selection
4. rapid genetic variation is acted on by powerful selective pressures provided by the host's adaptive immune system and by modern medicine, which destroy pathogens that fail to change
5. error-prone replication mechanisms e.g. retroviruses acquire on average one point mutation every replication cycle, because the viral reverse transcriptase that produces DNA from the viral RNA genome cannot correct nucleotide misincorporation errors
6. plasmid and transposon are found in viral genome which help to transfer genome between cells
7. there are different types of virus, that viruses evolve with their host, to escaped genes, or to degenerate cells