Evolution for Ecology

The Importance of Evolution in Ecology  Ecologists study proximate or “how” questions:  How nutrients flow through ecosystems  Evolutionary biologists study ultimate or “why” questions:  Why does a stickleback fish have armor in one lake but not another?

The Importance of Evolution in Ecology  Evolutionary processes can work surprisingly fast.  This is increasingly important in agriculture and medicine.

Pink Bollworms  Farmers consistently had to spray their crops with insecticides to control the pests.

Pink Bollworms  Transgenic cotton incorporates a gene from a bacterium that causes the caterpillars to die when they eat the plant.  Few applications of insecticide needed.

Resistance  The problem with pesticides is that pests evolve resistance to them and they become less effective over time.  Would this happen with transgenic cotton?

Pink Bollworm Experiment  Did the Pink Bollworms evolve resistance to the Bt toxin in the transgenic cotton?

Pink Bollworm Experiment  Yes, in just one generation!  Evolution can be fast!  So, is transgenic cotton useful? How can we prevent resistance from becoming established?

Rapid Evolution in Sticklebacks  Armor plates protect the fish from predators.  But there is a cost to plates – slower growth, breed later, and have lower winter survival rates.

Rapid Evolution in Sticklebacks  Cutthroat trout are predators of sticklebacks.  Use vision to find prey – easiest to find sticklebacks in clear water.

Rapid Evolution in Sticklebacks  So, if a murky lake is cleaned up and is now clear, what would you expect to happen to the plate count on the fish?  Plate count will increase?  Plate count will decrease?  Plate count will stay the same?

Rapid Evolution in Sticklebacks  As the lake cleared, predation became more of a problem and plate number increased.

Rapid Evolution of Invasive Species  The snowy campion is native to Europe, introduced to North America 200 years ago.  No enemies  Smut (fungus)  Hadena bicruris caterpillars  Now a common agricultural weed.

Rapid Evolution of Invasive Species  Two possible ways to allocate resources.  Which is best will depend on levels of predation, infection etc.

Rapid Evolution of Invasive Species  Europe – more enemies so bigger investment in defense.  North America – fewer enemies so more invested in reproduction & growth.

The Logic of Evolution by Natural Selection

Sticklebacks  Plating in sticklebacks is determined genetically.  Set at birth & can’t change.

Natural Selection  Variation is essential for natural selection to occur.  Populations of sticklebacks contain a variety of plate counts.

Natural Selection  Heredity is also essential for natural selection to occur.  Traits are passed genetically from parents to offspring.

Natural Selection  Survival and reproduction are selective  In a lake with high predation, sticklebacks with more plates survive better & reproduce more.

Natural Selection in the Medium Ground Finch  Daphne Major – Galapagos  No fresh water source – rely on short spring rains.  Medium ground finches eat seeds.

Natural Selection in the Medium Ground Finch  The finch population contains:  Variation  Heritibility

Natural Selection in the Medium Ground Finch  1977 drought – birds had to rely on seeds from previous year.  Small soft seeds eaten first, larger tougher seeds left.  Many birds starved.  After drought, average beak size was larger.

Natural Selection in the Medium Ground Finch  When the survivors reproduced, their offspring also had larger beaks.

Genetics and Evolution

Some terms  Assertions:  1. Populations vary in their phenotype  What they look like  2. Differences in the phenotype are due at least in part to difference in genotype.  Which alleles are present  Alleles are different versions of a gene  3. Some phenotypes and thus some genotypes survive better & reproduce more.

Some terms  Consequence: The beneficial trait will spread through the population.  The alleles for the beneficial trait will increase in the population.  Alleles associated with failure will disappear.  Total reproductive contribution is an individual’s fitness.  Evolution involves changes in the frequencies of alleles in the population.

Genetics of Plating in Sticklebacks  Plating is controlled by a gene called Eda.  There are two versions, or alleles  c for complete plating  l for light or low plating  Codiminant

Genetics of Plating in Sticklebacks  Tradeoff between fast growth & overwinter survival vs protection from predators.  Different phenotypes (& therefore genotypes) will be selected for in different situations.  Different environmental pressures.

Genetics of Plating in Sticklebacks  You can see here how the population varied over time as the lake cleared.

Adaptation  Over time, natural selection leads to adaptation to the environment.  Traits beneficial in a particular environment have accumulated in the population.  Move the organisms to a different habitat and the tight fit may be gone.

Genetic Drift  Genetic drift is another mechanism of evolution.  Does NOT result in adaptation.  Due to random chance.  Does result in a change in the frequency of alleles in the population.  Migration can also change the frequency of alleles in the population.

Managing the Evolution of Resistance

Pink Bollworms  Recall that pink bollworms can evolve resistance to the Bt toxin in the transgenic cotton.  Resistance is recessive.  rr are resistant  sr and ss are susceptible  There are several different resistance alleles

Pink Bollworms  Tradeoff:  Susceptible worms grow faster and mature sooner, but are killed by Bt toxin.

Pink Bollworms  Resistant worms have a much higher fitness on transgenic (Bollgard) cotton, while susceptible worms have a somewhat higher fitness on normal cotton.  The latter means that resistance alleles will be rare where Bollgard has never been used.  Goal: delay evolution of resistance

Integrated Pest Management  IPM involves the use of a variety of techniques to battle pests.  Use two different types of Bt toxin.  Parasitic nematodes – more lethal to resistant bollworms.

Managing a Bollworm Infestation  High dose / refuge strategy  Part of a field planted with Bollgard cotton, part with normal cotton.  The refuge (normal cotton) allows some of the susceptible worms to survive which decreases the fitness advantages associated with resistance.