Adaptation or Exaptation?An experimental test of hypotheses on

the origin of salinity tolerance in Bufo calamita

Ivan Gomez-mestre and Miguel Tejedo

2005

• Introduction

• Materials and methods

• Results

• Discussion

• Conclution

• Introduction• Materials and methods

• Results

• Discussion

• Conclution

Adaptation

Differences in phenotype among populations if have a genetic basis.

Confer fitness advantage in environment.

(Endler 1986; Sinervo and Basolo 1996).

Exaptation

As a trait evolved for other usages, and later co-opted for its current function.

feather

Gould & Vrba(1982)

Trait

Some traits may share a developmental history in spite of metamorphosis

Postmetamorphic traits may be affected by the environment experienced during premetamorphic stages

(Goater, 1994; Tejedo et al., 2000;Relyea, 2001; Relyea & Hoverman, 2003)

• A trait under certain environmental in one life stage could be affected by selection acting on an associated trait from another life stage experiencing a different environment.

(Deban & Marks, 2002)

Local adaptation of salinity tolerance

• linked to fitness

• genetic basis underlying the trait

(Gomez-Mestre & Tejedo 2003)

Adaptation or Exaptation?

Salinity tolerance may have – direct action of selection– a correlated response to selection acting on s

ome other trait

• Spain populations (even freshwater populations)have shown higher embryonic salinity tolerance than any of the UK populations

• steep South-to-North decreasing gradient in genetic diversity

(Beebee,1985)

North-to-South in western Europe

• decreasing rainfall

• increasing evapotranspiration

• increasing summer drought

• freshwater is the standard larval environment

• drought is a more common selective pressure than salinity

Exaptation

salinity tolerance during embryonic and larval phases

drought tolerance during the terrestrial phases

• The salinity tolerance traits could have evolved as an exaptation.

(Arnold, 1994)

Hypothesis

• relies on association between – salinity tolerance in the aquatic stages,– drought tolerance of the terrestrial juvenile an

d adult stages.

• Introduction

• Materials and methods• Results

• Discussion

• Conclution

Bufo calamita

http://www.herpetofauna.at/amphibien/bufo_calamita.php

Three populations

• freshwater environments– Donana– Pedroso

• brackish environments– Jarales

• Exposed juveniles to either humid or dry conditions for 5 weeks– survival– growth – behaviour： burying and prey capture

Two hydric potentials

• Humid (12 replicates per population)

-150 kPa

• Dry (15 replicates per population)

-1150 kPa.

vermiculite

Burying and prey capture

• Introduction

• Materials and methods

• Results• Discussion

• Conclution

Survival

• Weight at metamorphosis significantly affected survival

• Neither population of origin nor humidity significantly affected survival

Growth

• Growth was not the same across treatments.

• The dry environment significantly reduced growth rate.

• However, neither population of origin nor its interaction with humidity affected growth rate.

Burying

• Humidity had a very significant effect on proportion of time spent buried.

• Toadlets under dry conditions spent more time buried.

Morphological changes

• Body length relative to body mass did not vary significantly between humidity treatments

• Population of origin did not affect relative changes in morphology.

Prey capture success

Toadlets from the humid treatment – more attempts at prey capture– higher efficiency

• Introduction

• Materials and methods

• Results

• Discussion• Conclution

Decreased growth rates

• physiological adjustments,

• shifts in behaviour

• osmotic stress interfere with the control of tongue

Decreased growth rates

• smaller size

• reduced ability to capture prey

Lack of association between drought and salinity tolerance

• the reaction of the three populations across environments were remarkably paralle

• high similarity in drought tolerance among populations

Explain why

• First– the level of drought used in the experiment m

ay not have been stressful enough

– may be expressed only at the hardest conditions

• Secondly– populations are not isolated

– substantial variation in salinity tolerance within freshwater populations

(Gomez-Mestre & Tejedo, 2003)

• Third– different pathways for osmoregulatory physiol

ogy may simply

– however,dismiss the coupling hypothesis

• the information available on tadpole osmoregulation is scarce

• aquatic and terrestrial stages may have different physiological responses to osmotic stress

• each evolving independently of the other.

• Introduction

• Materials and methods

• Results

• Discussion

• Conclution

Conclution

• Salinity tolerance in the aquatic phase of

B. calamita is more likely to have evolved in these populations as an adaptation, rather than an exaptation from drought tolerance.

• Thanks for your attention

• Environmental heterogeneity tends to increase the phenotypic plasticity of traits (when populations exchange migrants)

• However, when migration is restricted, selection under extreme conditions tends to favor local adaptation over plasticity

(Pigliucci 2001)

(Rawson and Hilbish 1991; Sultan and Spencer 2002)

common garden experiments