Determining the relatedness of species

Question 1

Homologous structures

Answer 1

Homologous structures are structures that have been derived from a common ancestor and thus show similarities in structure, but have different functions.
Example: The bones of a bat’s wing are homologous to those of a whale’s flipper, even though one structure is specialised for flying and the other for swimming.

Question 2

Vestigial structures

Answer 2

Vestigial structures are structures that are non-functional remnants of structures that were functional in ancestral species.
Example: Whales are mammals evolved from a terrestrial mammalian ancestor, but are now specialised for life in the seas. Their skeletons show the presence of a reduced pelvis and, in some cases, vestiges of the bones of the hind limbs.

Question 3

Molecular homology

Answer 3

The similarity of patterns in the nucleotide sequence of DNA or the amino acid sequences of polypeptides from different organisms as evidence for a common evolutionary origin → can show two species are closely related even if their structural morphology is different.

Question 4

DNA hybridisation.

Answer 4

1. DNA from two species is heated to 87°C, and the strands separate
2. At 55°C and the strands join back together. Sometimes, the original strands of each species will join together and sometimes, a strand of species 1 will hybridise with the complementary strand from species 2. When hybrid if formed, some base pairs do not match correctly and then have less hydrogen bonds → not stuck together as tightly and thus will separate at a lower temp.
3. The DNA is heated slowly from 55°C to 87°C. At some point (below 87°C), the hybrid DNA strands will separate.
   The temperature at which the DNA becomes single stranded indicates the degree of complementary base pairing → so the higher the temperature the more closely related the two species are.

Question 5

DNA sequencing (whole genome comparison)

Answer 5

DNA sequencer that shows the base sequence and therefore the number of base differences between species. Can be for a gene or genome. (for a specific gene need PCR and gel electrophoresis)
These comparisons can help clarify the evolutionary history of species as species that have diverged from a common ancestor more recently have fewer base differences in their DNA, because there has been less time for mutations to accumulate in their genome than for species that share a more ancient common ancestor.

Question 6

Amino acid sequence

Answer 6

The number of amino acids differing between a pair of species provides a guide for the evolutionary distance between the two species.
Used for species that are less closely related
There are challenges associated with amino acid sequencing… The degeneracy of the genetic code (where DNA triplet sequences can code for the same amino acid) → even if the amino acid sequence is the same, the nucleotide sequence may be different, showing more evolutionary changes.

Question 7

Phylogenetic trees

Answer 7

Diagrams that represent the evolutionary relationships between species. Phylogenetic trees are not fixed, but are subject to change as new research and observations are made.

Question 8

Homoplasy / analogous structure

Answer 8

a feature found in two unrelated species, having evolved independently in each due to similar selection pressures

Question 9

Convergent evolution

Answer 9

Is the independent evolution of similar features in species creating an analogous structure that have similar form or function but were not present in the last common ancestor (leads to homoplasy)

Question 10

Divergent evolution

Answer 10

Species with a common ancestor that change to become increasingly different over time