Coevolution & Mutualism

1. Coevolution

2. Host-parasite systems

3. Coevolution among competitors

4. Character displacement

5. Mutualisms & symbioses

• Coevolution – evolution of one species in response to characteristics of another

• Differs from other types of evolution in two ways:

1. Coevolution involves mutual responses in the two species

– predator species evolves to be faster, allowing it to catch more

– results in greater selection on speed in the prey

– for “regular” evolution, there isn’t any response by the environment

2. Coevolution promotes diversity of adaptations

– evolution often converges on the same solution for the same problems

– coevolution involves specific, unique responses to specific challenges

The confusing definition of coevolution

Species A evolves an

adaptation in response to species B

Species B evolves in

response to the adaptation of

species A

This isn’t coevolution

Species A has some trait

unrelated to species B

Species B evolves in

response to that trait in species A

Not coevolution…

Coevolution in host-parasite systems

• Hosts and parasites change evolutionarily in response to each other

• Resistance – the ability of the host to combat the parasite

• Virulence – the ability of the parasite to harm the host

Hosts evolved resistance to the parasitoids

• Both resistance (in the host) and virulence (in the parasite) are costly– if the host is susceptible, selection favors

virulence– this causes selection for resistance in the

host…– leading to selection for less virulence in the

parasite (because virulence is costly)…– leading to selection for less resistance in

the host (because resistance is costly)…

Parasites that are too virulent may reduce their own fitness by killing their hosts.

Coevolution & Mutualism (Chapter 20)

1. Host-parasite interactions

2. Coevolution among competitors

3. Character displacement

4. Mutualisms & symbioses

Host-parasite interactions are due to genotype-genotype interactions– newly evolved pathogen strains may

overcome host resistance– host resistance may be specific to certain

genotypes

Red Queen Hypothesis

• Parasites are constantly evolving into new forms to avoid host resistance

• Hosts are constantly under selective pressure to evolve new resistance genes

• Result is a “coevolutionary arms race” in which both parasite and host must constantly evolve just to stay in place…

Plant-herbivore Coevolution

• Plants produce toxic “secondary” chemicals that reduce herbivory

• Some herbivores have evolved to detoxify the toxic chemicals.– herbivores may specialize on the hosts

whose defenses they have overcome– plants may evolve new defenses, and the

cycle continues (Red Queen)

• Generic defenses against herbivory (spines, cellulose, etc.) and predation (speed, hiding, lack of smell, etc.) are not coevolution

• They don’t involve responses to an adaptation specific predator/herbivore

Coevolution among competitors

Character Displacement• According to the competitive exclusion law,

no two species can share the same niche

• When living in allopatry (spatially separated), two species may use the same resources (food, nest sites, etc.)

• Where these species live in sympatry (in the same geographic area), they will evolve to use different resources to avoid competition.

Greater divergence in morphology in sympatry than in allopatry, is character displacement

• Mutualism – mutually positive interaction between two species

• Commensalism – interaction in which one species benefits and the other is unaffected

• Symbiosis – association in which two species live together as one distinctive entity

Mutualisms and commensalisms can be– obligate – neither can survive without the

other– facultative – nonessential

Three Kinds of Mutualisms

1. Energetic & nutritional – transfer of energy/nutrients from one organism to another

2. Protective – defense against predators or herbivores

3. Transport – movement of seeds, pollen, or adult organisms

Energetic Mutualisms

Gut symbiont– bacteria in animal guts digest plant

materials and produce vitamins/amino acids

– benefit to bacteria is a stable environment and a steady food source

Mycorrhizal fungi – fungi that live intermingled with plant roots– fungi uptake nutrients from soil for plants– plants provide carbohydrates to the fungi– mycorrhizal fungi are found in almost every

plant– very important where nutrients are limited

Honeyguides (energetic mutualism)

Protective Mutualisms

Ants/acacias – – acacias provide food, hollow stems for

nesting– ants attack herbivores and kill encroaching

plants– wrens nest in the acacias and are

protected from predators by the ants (commensalism)

Transport MutualismsPollination

– flowers trade nectar for pollen transport– can be general or species-specific

Seed dispersal

Plants provide nutritious fruit, seeds to encourage dispersal by animals– many seeds require gut passage to

germinate– some plants trade-off consumption of some

seeds to ensure dispersal

Seed dispersal

Why does seed dispersal benefit plants? (why not just drop seeds?)

1. Avoid pathogens that the parent may harbor

2. Colonize new habitats where conditions may be better than near the parent

3. Find locations where germination is more likely (ant nests)

• Facultative mutualisms– mixed-species flocks in birds– clownfish-anemone– associational defense with toxic plant

species

• Commensalism– birds’ nesting in tree cavities – epiphytes’ growing on trees

Symbiosis – “living together”

• Lichen – alga and fungus

• Coral and zooxanthellae– coral capture food, nutrients– zooxanthellae photosynthesize and provide

sugars, energy– coral “signal” the algae to release sugars