Biology 5C ~ Lecture 22 ~ Macroevolutionary Rates

what are "darwin's finches"?

species that diverged so rapidly that it is difficult to determine their precise relationships

*vary in many traits
*radiated into distinct species across the galopages

adaptive radiation:

the rapid evolution of ecological differences among species in a diversifying lineage

some species stay relatively...

unchanged over generations (stasis)

*ginko
*coelocanth

some species diverge...

rapidly over short timescales

*anolis lizards on isolated islands diverged to occupy distinct ecological niches

not all lineages...

speciate or go extinct at the same rate

*different diversification rates

investigate with computational tools

-phylogenetic comparative methods involve mapping species and their traits along with evolutionary relationships

-time-calibrated phylogenies mean that the branches are proportional to elapsed time

often use "genetic difference" for elapsed time

-compare nucleotide sequences in the same gene

*percent difference in ibs rRNA
*branch length represents genetic distance

net diversification rate:

rate of speciation --- rate of extinction

(can compare this value across different lineages)

Speciation and extinction are measured in clades (vary enormously)

-average rates of speciation + extinction per lineage, per million years

-all the descendants of some common ancestors

birth-death models

-mathematical models for describing how lineages may speciate or go extinct

-homogenous birth-death model assumes each lineage has a constant probability of splitting or extinction

-often used as a reference point

*does not fit observed data very well

rates of speciation and extinction estimates with fossils

-fossil occurrence data: appearance and disappearance of lineages in the fossil record

-often cannot identify individual species

-fossil preservation rates vary dramatically across groups

*the bottom two are methods to estimate rates from incomplete records

what these models show (slide 15):

1. speciation and extinction rates tend to be roughly equal over long periods and large spatial scales (high turnover)

2. the total number of species in individual clades waxes and wanes through time (rare clades become more common, and vice versa)

3. both speciation and extinction rates show tremendous variation, both through time and across clades (more than expected from birth-death models)

rates of speciation and extinctions estimated with phylogenies

-using time-calibrated evolutionary relationship data

-only have data for extant species

-missing data on the timing and number of extinctions

*tree balance -- how evenly tips are distributed across sister nodes

what the following models show:

1. phylogenies are more imbalanced than would be expected from a homogeneous birth-death model (variation in speciation + extinction rates)

2. rates of speciation and extinction can vary dramatically among closely related groups

3. diversification rates slow as lineages approach the present day (maybe biases in sampling)

two ways to estimate trait evolution (avg. rates over time):

-allochronic
-synchronic

*quantitative traits

allochronic:

measuring difference in mean trait at the beginning and end of a lineage

*divide by time to get change per unit time

synchronic:

measuring mean trait in two extant lineages that diverged from a common ancestor at a known point

*divide by time since common ancestor to yield rate of divergence

trait evolution and phylogenetic trees

assume that trait evolution mimics a "random walk"

*model evolution of a quantitative trait
*statistical process called "brownian motion"
*can estimate the rate that traits "wonder" through time

short-term and long-term patterns in trait evolution

-trait change under short timescales suggests strong selection (rates too high to explain by drift alone)

-but long timescales, differences across species are less than those expected under the hypothesis of genetic drift (many species experience stasis)

*ecological opportunity can drive rapid evolution

Traits can influence speciation and extinction rates (state-dependent speciation and extinction)

-some traits are only apparant above the level of the individual, like species (one species cannot have a 'range')

-what if a trait conferred the ability for a group to speciate faster than it went extinct? (differential speciation and extinction... based on heritable traits?)

quick review:

*just like birth and death, speciation and extinction occur at some rate over time

*the rate of speciation and extinction is measured through fossil and phylogenetic data

*these rates (and the rate of trait evolution) vary dramatically across the tree of life

*traits themselves may also influence the propensity for lineages to diversify or go extinct