Notesale: Turn your study into money

Document Preview

Extracts from the notes are below, to see the PDF you'll receive please use the links above

---

Cladistics

Topic 5
...
4
■

Biology SL

When preparing a cladogram, it becomes clear that birds share a
significant number of derived characteristics with a group of
dinosaurs

➢ To decide how close a common ancestor is, researchers look at how many primitive
and derived traits are shared
○

Primitive traits are characteristics that have the same structure and
function and that evolved early on in the history of the organism being
studied

○

Derived traits are characteristics that have evolved more recently as
modifications of a previous trait

➢ Some of the derived characteristics used to put birds and dinosaurs in the same
clade are
○

Fused clavicle

○

Hollow bones

○

Flexible wrists

○

Characteristic egg shell

○

The hip and leg structure

Variations and phylogeny

➢ Phylogeny is the study of the evolutionary past of a species
○

By comparing the similarities in the polypeptide sequences of certain
proteins in different groups of animals, it’s possible to trace their common
ancestry

➢ The more differences in the DNA there are between 2 species, the less closely
related the species are
Evolutionary clock
➢ The changes in polypeptide sequences can be used as a kind of clock to estimate
how far back in time 2 related species split from a common ancestor
➢ By comparing homologous molecules from 2 related species, it’s possible to count
the number of places along the molecules where there are differences

➢ DNA hybridization is the process during which one strand of DNA from species A is
fused together with a homologous strand from species B
○

Where the base pairs connect, there’s a match
...
There are 83 nucleotide differences between A
and C and 26 between A and B
...
4

Biology SL

➢ We shouldn’t consider that mutations are constant
○

They can happen at varying rates

○

All we have is an estimation/proportion, rather than an absolute time or
date for speciation

➢ To double-check the timing, biochemical data can be compared with morphological
fossil evidence and radioisotope dating
Analogous and homologous traits

➢ Homologous characteristics are ones derived from the same part of a common
ancestor
○

Ex: some organisms have extremely primitive eyes while others have very
developed ones

---