Evolution  Lecture 26

Chapter 16

Topics for today:

•         Modes of speciation and supporting evidence 

1.      Allopatric speciation (finished last lecture)

2.      Peripatric speciation

3.      Parapatric speciation

4.      Sympatric speciation

•         Definition of a subspecies

•         Rates of speciation

EvoBeaker: Snails

Exercise 6

 

 

B. Peripatric speciation: founder effect

  • Most controversial
  • Mayr observed that isolated populations are highly diverged to the point of species designation
  • Allele frequencies differ in founding group because of drift
  • New epistatic relationships formed
  • Selection alters gene frequencies accordingly
  • Rapid change yields reproductive isolation”
  • Founding population has different gene frequencies with new epistatic relationships
  • Climbs a different peak in adaptive landscape
  • Hybridization breaks up each population’s positive epistasis
  • No intermediates should show up in fossil record
  • Underlying idea behind “puncutated equilibrium”

Fig. 16.12

 

Example 1 From natural populations

o       Compare phylogenetic patterns of widespread species and closely related peripheral species

o       Peripheral species should be nested in widespread species

o       Confounded with divergence due to selection

§         Lab population bottlenecked to 1, 4, or 16 pairs

§         Behavioral attributes altered by the founder event

Fig. 16.14

           

Example 2: Experimental evidence for peripatric speciation

o       2 fruit fly species bottlenecked for 40-50 generations

o       Do they chose their own founder population (positive assortative mating)?

o       Do they chose other founder populations (negative assortative mating)?

o       No evidence that genetic drift results in reproductive isolation

Fig. 16.14

 

C. Perapatric speciation: differences in selection

·        Different habitats with strong selection and a sharp boarder

·        Individuals with wrong genotype fail

·        Probably common but difficult to demonstrate

Example 1 – from natural population

·        Grass has evolved tolerance to heavy metals in mine tailings, Anthoxanthum odoratum

·        Metal tolerant plants flower earlier

·        Evolved capacity for self-fertilization

Fig. 16.15

 

D. Sympatric speciation

·        Highly controversial

·        Barrier to gene flow arises within a population with no spatial barrier to gene flow

·        Most likely caused by strong disruptive selection

·        Strong selection for different resources cause heterozygotes to have low fitness

Example 1 – From natural population

·        Apple maggot fly’s native host is hawthorn

·        Emerge in late summer, mate, lay eggs on fruit

·        Larvae hatch, drop to ground, overwinter

·        First observed as pest on apples in eastern North America ~150 yr ago

·        Apples set fruit ~ 3 wks earlier

·        Gene exchange is limited (disruptive selection)

Fig. 16.18

 

Example 2 - Experimental evidence for sympatric speciation

·        Fruit flies subjected to disruptive selection

·        Most studies show no reproductive isolation

·        Exception when selection is based on trait that causes assortative mating:

o       Development time

o       Habitat choice

·        Developed partial reproductive isolation

Fig. 16.17

 

Definition of a subspecies

·        Leopard frogs previous considered one species with subspecies

·        Studies at overlap show reproductive isolation

·        Phylogenetic studies show two clades with 27 species

·        A named geographic race; a set of populations of a species that share one or more distinctive features and occupy a different geographic area from other subspecies (pg 552 Futuyma)

Example: Six subspecies of Box turtles (Terrapene carolina)

1) Eastern    2) Desert   3) Three Toed  

4) Florida   5) Gulf Coast   6) Ornate

 

How fast does speciation occur?

Two different measurements

1.      Time for speciation (TFS)

·        Time from beginning of differentiation to reproductive isolation

2.      Biological speciation interval (BSI)

·        Time between branch points

Fig. 16.21

 

Estimate of Time for Speciation (TFS)

How much sequence divergence is there between young sister taxa?Molecular divergence in birds

·        2% @ 1 myr

·        Young species pairs have @ 4% sequence divergence so TFS @ 2 myr

·        Ranges from 0.23 - 5 myr (average 2.6 my)

·        Upper bound because don’t know exact time of speciation

Fig. 16.22

 

How to measure Biological Speciation Interval (BSI)?

Need to know

·        Time since most recent common ancestor (t)

·        Current number of species in the clade (Nt)

·        Solve for R (diversification rate,  species per unit time)

·        Time between branching points (BSI) = 1/R (time per species)

Assumptions

·        Rate of speciation is exponential

·        No extinction

Nt = eRt

Fig. 16.21

 

Estimates of TFS & BSI a have wide range

Fig. 16.23

Why are speciation rates so variable?

 

The mechanism of speciation matters

·        Very slow if speciation occurs by mutation and drift of neutral alleles

·        Faster if driven by ecological or sexual selection

·        Extremely fast in cases of polyploidy or speciation due to recombination

The stability of the environment matters

·        Divergent populations are likely to come into contact eventually

·        Divergence lost through interbreeding if reproductive isolation has not yet evolved

 

Relationship between environmental stability and rates of speciation

·        Speciation rates higher in a stable environment

o       Higher speciation of trilobites during stable environments

o       No speciation of beetles during glacial era