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Stories of Adaptation

Examples seem to be compelling evidence of adaptation, but we:

1. Risk of hand-picking examples.

2. Didn’t show that a trait works as we suggest it does.

3. Didn’t show that a trait’s benefits exceeds its costs.

4. Didn’t show differential reproduction by different genotypes.

The trouble with “just-so” stories

Is Predator Avoidance an Adaptation?Mobbing as a Case Study

Hypothesis:

Mobbing reduces predation on animals and/or young.

Mobbing:

A behavior whereby members of a group attacks a predator

So… mobbing is functional.

But has it evolved under natural selection where it is favored?

(versus being something that members of a species learn to do).

Is Mobbing an Adaptation?

adaptationa behavior that serves a function AND evolved under natural selection

Repeated themes more compelling… can a compara-tive approach help us?

cactus wren

anemone fish

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If mobbing is an adaptation,

then species that live under strong threat of predation should show mobbing,

whereas species that do not should not show mobbing.

The Comparative ApproachFirst Line of Evidence

Related species, differing risk

herring gull

• nest IN OPEN• predation risk HIGH• MOBBING observed

kittiwake gull

• nest ON CLIFFS• predation risk LOW• NO MOBBING

STRONG support for adaptation hypothesis

Second Line of EvidenceRelated species, similar risk

herring gullblack-headed gulllaughing gull

• all nest IN OPEN• predation risk HIGH for all• MOBBING observed in all

WEAK test of adaptation hypothesis!

WHY?

herring

gull

laughing

gull

All species may mob because common ancestor did so. Effectively, a single data point!

black-headed

gull

ancestral gull species

Third Line of EvidenceUnrelated species, similar risk

herring gullbank swallowground squirrel

• predation risk HIGH for all• MOBBING observed in all

nests on groundnests in river bankslives on open plains

STRONG test of adaptation hypothesis!

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These last comparisons constitute an example of convergent evolution,

wherein distantly related species independently evolve a similar adaptation.

Contrast this with divergent evolution,

wherein closely related species evolve differences due to differences in the nature of selection.

kittiwake

gull

herring

gull

relatively recent

common ancestor

ground

squirrel

black-headed

gull

relatively distant

common ancestor

These species comparisons imply a common ancestor,

And something about recency of ancestry.

Phylogenythe evolutionary history of a group

A phylogeny usually takes form of a tree.

oldTree of Lifelogo

The only figure in The Origin of Species

time

Some Phylogenetic Basics

Node

Monophyletic group

Time

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Some Phylogenetic Basics

Node

Paraphyletic group

Time

Some Phylogenetic Basics

Node

Time

More derivedLess derived (= more basal)

1. How many times did a behavior evolve?

Issues in Phylogenetic Analysis

We said that song learning evolved 3 times.

How do we know?

Perhaps it evolved once and was lost in some groups.

To decide, we count up the # gains and # losses required in each case.

Trait gained

Trait lost

One-origin hypothesis

requires 1 gain but 3 losses,

= 4 total evolutionary events.

To decide, we count up the # gains and # losses required in each case.

Trait gained

Trait lost

Three-origin hypothesis

requires 3 gains and 0 losses,

= 3 total evolutionary events.

We choose the explanation requiring the fewest # events!

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Principle of parsimony

The best explanation is that explanation requiring the fewest number of evolutionary events.

Example. powered flight

Powered flight occurs in just three types of animals: insects, bats, and birds.

What is the most parsimonious estimate of the number of times that powered flight evolved? 1? Or, 2? Or, 3?

sponges

cnidarians

flatworms

annelids

arthropods

echinoderms

chordates

4 changes

Possibly there was one origin, but 3 losses. A total of 4 changes…

sponges

cnidarians

flatworms

annelids

arthropods

echinoderms

chordates

more parsimonious conclusion: flight evolved independently in arthropods and chordates.

But how many times within chordates? Once? Twice?

just 2 changes

birds

lizards

turtles

mammals

One hypothesis: a total of 1 origin but 3 changes.

birds

lizards

turtles

mammals

Alternative hypothesis: a total of 2 origins and 2 changes.

more parsimonious!

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Flight in bats evolved after origin of mammals, further evidence of its independent evolution.

Phylogeny of Mammalia from Tree of Life

Summarizing,

Altogether, powered flight arose 3 times, once in insects, once in birds and once in bats.

2. What behavior is ancestral?

Issues in Phylogenetic AnalysisExample: Cooperative Breeding in Scrub Jays

cooperative breeding

helper at nest brings food to offspring not its own and defends nest from predators; a form of "altruism"

singular c.b.

1 pair and usually 1 helper rear young

plural c.b.

>1 female in territorial group rear young with helpers

Which mode of breeding is ancestral?

1. non-cooperative breeding has most recent origin

2. plural cooperative breeding is ancestral!

non-cooperative breeding

singular cooperative breeding

plural cooperative breeding

Example. Evolution of eusociality* in Lasioglossumsweat bees

*eusociality: advanced form of sociality involving reproductive division of labor and cooperative brood care.

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Solitary behavior ancestral.

Eusociality lost numerous times.

3. Do traits show evolutionary trends?

Issues in Phylogenetic Analysis

Cowbird species vary in host breadth. Was common ancestor a specialist or a generalist?

Is there an evolutionary trend in specialization?

Phylogenetic trends that might influence behavior:

Body size: increases with recency of origin

Complexity: increases with recency of origin

Brain:body size ratio: increases in bird and mammal lineages

Cephalization (having a head): No obvious trend!

4. Can behavior be used to build phylogenies?

Issues in Phylogenetic Analysis

Answer might seem ‘no’ because behavior varies so much, even within a species. BUT…

Bowerbirds (=20 spp.)

Bowerbirds so-called because males build structures called bowers with which to attract females.

Shown is the satin bowerbird

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Bowers, decorations, dancing and vocalizations of males vary among bowerbird species.

Satin bowerbird

Spotted bowerbird

Two species of ‘avenue builders’

A ‘maypole builder’

Related species of bowerbirds have similar types of bowers.

From Borgia, Scientific American

Analysis based on DNA sequence data yields a similar pattern.

Maypole bower builders are monophyletic, and avenue bower builders are monophyletic.

5. How does natural selection shape behavior?

Issues in Phylogenetic Analysis

http://www.youtube.com/watch?v=4SispCYjUTE

http://www.youtube.com/watch?v=kjTXS8blGx8

Strawberry poison dart frog

Adaptations for Parental Care in Frogs

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Ranitomeya imitator, a poison frog

Ranitomeya imitator:

• Breeds in phytotelms (water in bromeliads)

• Exhibits trophic egg feeding*

• Exhibits biparental care (= both sexes care)

Question: Does breeding in phytotelms drive evolution of trophic feeding and biparental care?

* Trophic egg feeding involves deposition of egg that is used as food by developing larvae

A generally good correspondence between occurrence of trophic egg feeding and biparental care

= phytotelm breeders

= parental care present

There is a highly significant (P < 0.0001) correlation between the evolution of phytotelm breeding and evolution of parental care.

At present, few complete cases for adaptation in behavior using phylogenetic analysis.

WE MUST LOOK ELSEWHERE FOR NOW…