5.4 Cladistics

Question 1

What are clades

Answer 1

A group of organisms that have evolved from a common ancestor.

Question 2

What are nodes?

Answer 2

The branching points on cladograms.
- Nodes denote a speciation event when a common ancestor splits into two, or more, species.

Question 3

What are clades based on?

Answer 3

The molecular similarity of DNA base sequences or amino acid sequences.
- The more similarities that exist between the DNA (or protein) sequences of two species an animal, the closer the relationship and the more recently these species diverged from each other.

Important: a comparison of amino acid sequences is not as accurate as a DNA comparison. Changes at the DNA level need not always result in a different protein.

Question 4

Molecular clock

Answer 4

A technique where the rate at which mutations occur is used to calculate how long ago species diverged

Question 5

What does the length of the lines connecting phylogenetic tree (a cladogram where time is shown) represent?

Answer 5

The length of the lines separating species on cladograms is often used to represent the estimated time since they diverged.

Question 6

What is the relationship between the number of differences between two species and the time since they diverged from a common ancestor?

Answer 6

The longer the time period since the two species separated, the more differences there will be when the DNA of the two species is compared.

Question 7

Key limitations to using morphological differences (structural characteristics) as a basis for classification:

Answer 7

• Closely related organisms can exhibit very different structural features due to adaptive radiation (caused by divergent evolution)
• Distantly related organisms can display very similar structural features due to convergent evolution

Question 8

Classification of the figwort family

Answer 8

• Until recently, figworts were the 8th largest family of flowering plants (angiosperms)
• The classification of the figworts was based on observable traits such as a tube-shaped flower structure
• When DNA sequence analysis began, plant scientists discovered that the shared features of the figwort family were in fact analogous and not evidence of shared ancestry
• It was discovered that the original figwort family was not a true clade
• Therefore several genera were moved to other existing families and new families were also created
• They were reclassified from 1 clade to 5 different clades

Question 9

Analogous and homologous structures

Answer 9

Homologous structures: similar structure but different function (exists due to common ancestry through divergent evolution)
Analogous structures: different structure but similar function (exists due to selective environmental pressures through convergent evolution)