BIO 585 Coral Reef FieldResearch Methods

Reef biodiversity and evolution

Biodiversity

“…the millions of plants, animals, andmicroorganisms, the genes they contain,and the intricate ecosystems they helpbuild into the living environment.”

—World Wildlife Fund 1989

Biodiversity

“This definition is a bunch of

hooey. What we mean when we

say biodiversity is taxonomic,

usually species diversity.”

—McCartney 2004

Measures of species diversity

• species richness = S = the number ofspecies in a community or region

– pros:

• easy to survey and compare

• no need to census individuals

– cons:

• no information on relative abundance

• an incomplete measure of diversity

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Bird surveys at two tallgrassprairie sites in DeSoto NationalWildlife Refuge, Iowa.

Data from van Dyke (2003).

Species richness for the two sites issimilar (S site A = 8, for site B = 11)

But site A is dominated by 1species. Site B has greater“diversity” (abundances are moreeven). We need an index ofdiversity that reflects this.

Convert values to pi (proportion oftotal community abundancerepresented by i th species)

H' for site A = 1.64

H' for site B = 2.25

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Site B′ = −∑H p pi i( ln )

Compute the Shannon index ofdiversity (H' )

E for site A = 0.789

E for site B = 0.938

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Si ii=

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Or Pielou’s index of evenness (E )

The history of biological diversity

• the number of living species, in many groups,has increased steadily to the present

• this has been punctuated by major extinctions

Scleractinian corals appear

Modern-dayspecies diversity

Otheranimals281,000

Higherplants248,000

Protozoa30,800

Algae26,900 Fungi

69,000

Prokaryotes4,800

Viruses1,000

Insects751,000

• >1.5 million species havebeen described

• about 2/3rd’s are animals

• nearly half are insects

• from 4 to 150 times asmany remain undescribed

From Wilson 1992

Geographicpatterns of speciesdiversity on land

• species richness oftrees, ants, birds andmammals increasestowards the Tropics

• exceptions: conifers,bees, salamanders,waterfowl

Latitudinal patterns of marinebiodiversity

• species diversityincreases towards theequator in bivalves,tunicates, othersessile invertebrates,benthic forams

• exceptions: marinealgae, brittle stars,penguins

Explanations for increaseddiversity at lower latitudes

• higher solar radiation and primary productivityprovides greater resource base

• larger contiguous geographic area enhancesspeciation and retards extinction rates

• stability/time hypothesis: speciation has beenuninterrupted by glaciations; extinctions

• predictable tropical environment: greatercompetition, parasitism anddisease—promoting specialization andspeciation

Biodiversity “hotspots”

• small areas with a concentration of species

• contain many endemics, species foundnowhere else

due to their smallrange ofdistribution,endemics are veryvulnerable toextinction

Madagascar: 93% of primate,99% of frog species are endemics

From: Spaulding, MD et al. (2001) World Atlas of Coral Reefs.UNEP/WCMC and University of California Press, Berkeley CA.

The center of greatest diversity of scleractinian corals is theIWP, with gradients of decreasing diversity to the east and west

The Caribbean Sea holds the greatest concentration of speciesin the Atlantic Ocean

The IWP supports greater species richness in all coralreef animal taxa, and in associated ecosystems

One likely explanation for this is the much greater area of

reef coverage in the IWP, compared with eastern Pacific

and Atlantic regions

Positive species/area relationships are common interrestrial biogeographic studies, particularly on islands

This was a key finding that spurred the development ofisland biogeography theory (MacArthur and Wilson 1967)

Coral reefs and tropicalrainforests

the earth’s two most biodiversehabitats

Coral Reefs TropicalRainforests

Area (km2)(% global total)

284,3001

(0.1%)11,610,3502

(7%)

Animal Phyla(% global total)

32(94%)

9(26%)

Species(undescribed)

93,0003

(1-3 million)800,0004

(5 – 30 million)

Species per100 km2 area

33 7

Fishes(% of all marine)

40005

(25%)

Birds(% global total)

26006

(30%)

Land plants(% global total)

100,0007

(40%)

Sources of biodiversity estimates: 1Spaulding et al. 2001; 2Skole andTucker 1993; 3Paulay 1997, Reaka-Kudla 1997; 4May 1992; 5Ormondand Roberts 1997; 6Diamond 1985; 7Myers 1980.

Speciation on aBig Blue Planet:an Introduction

What is a species?

• a unit of discontinuity inthe living world

• an interbreedingpopulation: afundamental unit ofevolution

• a category of diversityworth conserving

Species concepts in biology

morphological species

phylogenetic species

biological species

Morphological species• the familiar

“taxonomic”description basedon sets ofdistinguishingcharacteristics

• often works butcan be misleading

from Futuyma (1998), p. 463

Morphological species of horned lizards

Morphological species

• these morphologically similarmeadowlarks cannot interbreed: theyare true species

Morphological species• these allopatric warblers were previously

classified as distinct species

• but they freely interbreed where they meet

Audubon’s warbler Myrtle warbler

Maps from USGS

Breeding range map of myrtle warbler

Breeding range map of Audubon’s warbler

The phylogeneticspecies concept:

species aremonophyletic

groups

[tips of the tree areindividuals, letters

denote populations]

Application of the phylogenetic concepthelped uncover cryptic species in thecopepod Eurytemora affinis

photo courtesy of SW Missouri State University

And inAfricanelephants

Biological species

• “...groups of interbreeding populationsthat are reproductively isolated from

other such groups...”

– Ernst Mayr

Biological species• the most widely

accepted and usefulspecies concept

• problems:

– useless for asexualorganisms

– sometimes difficult to test(e.g. allopatric species)

Biological species

• reproductive isolation: anobjective criterion that isbiologically significant

• application of the concept hasborn a research program

Understanding speciation?

study the evolution of reproductiveisolation

Reproductive isolatingmechanisms

• prezygotic mechanisms act priorto hybrid zygote formation

– behavioral (mate choice)

– gamete recognition

• postzygotic mechanisms act onhybrid zygotes

– hybrid inviability/sterility

– ecological inferiority of hybrids

Prezygotic: behavioral isolation

• courtship and mating behaviors diverge dramatically,even between closely related species

• Hawaiian Drosophila do not interbreed: females rejectcourtship of heterospecific males

Mating behaviors can evolve rapidly

• Drosophila heteroneura and D. silvestris are sisterspecies, sympatric on Hawaii

D. heteroneura has dumbbell-headed males that butt headswith other males to gain displayterritories on leks

from Fr eeman and Herron 2002

D. silvestris has roundy-headed males that fight othermales, Greco-Roman style,for territories

Behavioralisolation

• mating calls of frogsand insects arespecies-specific

• females recognizeconspecific calls

Prezygotic isolation due togamete incompatibility• marine

invertebrates andother organismsthat lack matingbehavior: speciesrecognition occursbetween egg andsperm

Sea urchin bindin: a gamete recognitionprotein involved in speciation

• Bindin is an adhesiveprotein that binds spermto the egg surface.Without this, fertilizationdoes not occur.

• Rapid change of bindinproteins creates barriersto fertilization betweenspecies.

Bindin

eggsurface

Postzygotic reproductiveisolation

• hybrids areoften inviable orsterile

Postzygotic reproductiveisolation

• hybrids are sometimes ecologically inferior

bird predation on hybridscreates abrupt boundariesbetween Heliconius colorraces

Yet hybridization in natural populations is widespread. Themost striking examples are where true species interbreedalong defined “hybrid zones.”

Hybrid zones are narrow and stable for many years. How doesthe zone persist, and how do the “parental species” maintaintheir integrity?

Mytilus edulis

Mytilus trossulus

Fig. 1. Frequency of M. edulis and M. trossulus i n the coastal Gulf of Maine (af terRawson et al. 2001). Insert: locati on of Gulf of Maine map and future proposed samplesi tes. A = Lunenburg, Nova Scoti a, B = Cobscook Bay, Maine, C = Dam ariscotta River,Maine, D = Nahant, Massachusetts, and E = Mysti c, Connecti cut.

Hybridzone

AllopatricM. edulis

Blue mussels in theMytilus edulis speciescomplex hybridizewherever species overlap

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Cobscook Bay, Maine,showing typical interstitialhabitat for mussels

These sibling species aretyped using 3 codominant

nuclear DNAs

The geography of speciation

• most speciesform whengeographicbarriers blockgeneticexchange

Allopatric speciation• arguably the most common way species form

• evidence for allopatric speciation is common inbiogeography:

– related species often occupy nearby, non-overlappingranges

Allopatric speciation• Isthmus of Panama closed

~ 3.1 MYA

• Isthmus closure split ~150“geminate” (twin) species

Evidence for allopatric speciation in snapping shrimps

Knowlton et al.(1993) created aphylogeny of Pacific (P) andCarribean (C) species pairs ofAlpheus

In 6 out of 7 cases, the closestrelative of a species was on theother side of the Isthmus

dispersal stops

An important avenue for allopatricspeciation is through “dispersal andcolonization”

genetic and reproductivedivergence of the colonists

Evidence from patterns ofspeciation in Hawaiian Drosophila

D. heteroneura

D. silvestris

• thought to explain species radiations on islands

Founder effect speciation

• divergence of a small population isolated onperiphery of range

Founder effectspeciation

• the genetic “founder effect” of small populationsize itself is thought to drive speciation

Sympatric speciation

• no geographic barrier

• much rarer

• yet, studies of sympatricspeciation show how ecologicalfactors, varying within acontinuous habitat, may drivespeciation by selecting forassortative mating

Sympatric speciation

• valid cases:

– cichlids in Camerooncrater lakes

– host races of Rhagoletis

(apple maggot fly)

Sympatric speciation: an example• Rhagoletis pomonella: the apple maggot fly

• larvae feed on natural host: hawthorn fruits

apples hawthorns

• a “host race” infesting apple trees appeared inNorth America ~150 years ago

Speciation in progress

• the apple race and hawthorn raceshave evolved reproductive barriers

striking geneticdifferences between theraces mark the earlystages of speciation

Adults mate and oviposit on their host trees

The classic “three stage”model of allopatric speciation• 1st stage: a geographic barrier

divides a large population into twoor more isolates

• 2nd stage: divergence of mating and/orhabitat preferences occurs in thedescendant populations

The classic “three stage” model ofallopatric speciation

• 3rd stage: reproductive isolation is completed, or “perfected”

– thought to be a crucial step, why?

• secondary contact is common on Earth

• without complete reproductive barriers, partially isolated species will fuse

During the “third stage” ofallopatric speciation...

• reproductive isolation can be completed intwo distinct ways

– as a byproduct of the divergence process

– via selection against hybridization(reinforcement)

Coyne and Orr’s (1997) surveyof Drosophila species pairs

• collected the following data from over 150species pairs, worldwide

– whether the species are allopatric or sympatric

– genetic distance between the species pair (basedon allozymes) as an estimate of its age

– the level of postzygotic and prezygotic isolation

Their results...

Prezygotic isolation increases steadily with genetic distance

Genes for prezygotic isolation diverge over time, just like the restof the genome

Their results...

Full isolation evolves in allopatry

Secondary contact is not necessary to complete the speciationprocess

Coyne and Orr’s (1997) survey supports reinforcement

Prezygotic isolation evolves faster in sympatry than in allopatry

Implies that selection against hybridization is powerful