BSC2010 Unit 3 Lecture 8

What are phylogenetic trees built based on?

Synapomorphies

Convergent evolution

When superficially similar traits may evolve independently in different lineages

Evolutionary reversal

When a character reverts from a derived state back to an ancestral state

Homoplasy

Same trait develops in different lineages

Strengths of morphological characters

-Allow inclusion of extinct species which have almost exclusively morphological characters

-Fossil evidence can order traits and date divergences

Weaknesses of morphological characters

-Some taxa show few morphological differences

-Difficult to compare distantly related species

-Some morphological variation is caused by environment

Strengths of developmental characters

-Powerful approach for clarifying deep relationships where significant divergence in form occurs in adults

-We simultaneously learn about mechanisms of diversification in form

Weaknesses of developmental characters

Sometimes developmental mechanisms are “reused” for different functions

Strengths of behavioral characters

Often as heritable as morphology

Weaknesses of behavioral characters

-Trait expression is often plastic

-Precise measurements of comparable behavior may be difficult

Strengths of molecular characters

-Nuclear, chloroplast, and mitochondrial DNA sequences used → lots of characters

-Lots of knowledge on DNA trait change

-Rates of change vary among loci

Weaknesses of molecular characters

Only 4 trait states per character, so homoplasies are common.

Molecular clock

In DNA sequences under natural selection, rate of evolution through mutation and drift is relatively constant over time.

Rate of fixation of new neutral mutations =

mutation rate