Evolutionary Ecology In what ways do plants adapt to their environment? Evolutionary Ecology Natural selection is a constant Individuals are continuously challenged by their environment Populations are constantly undergoing selection (directional, stabilizing, divergent) Perspective on evolution Tendency to think of evolution as phylogenies and speciation Considerable selection without speciation Alterna+ve phenotypes Alterna+ve species Alterna+ve life histories, physiology, life form Ecological Selec+on Differen+al Fitness Differen+al Performance? Individuals and their environment Light and temperature Wind, fire, damage Neighbors and their impact on resources Mutualists involved in reproduction or dispersal Herbivores and pathogens Other conspecifics for reproduction Soil moisture patterns and Soil geology and chemistry Mineral nutrients Soil mutualists and action of soil organisms on mineral cycling 1
Selection->Differential Success Selection is a concept incorporating the probabilistic success of individuals or populations in a given environment Success refers to survival and reproduction Geographic differences in coevolution: example of an invasive species, Centaurea diffusa This native of central Eurasia was introduced to western North America. It associates with similar plants in both places (genera). In central Eurasia this is a relatively uncommon plant. But in North America this has become a terrible invasive. Root exudates directly affect neighbors, and directly affect soil microbes and mycorrhizae, influencing NAm native hosts Selection on Centaurea In North America, Centaurea was selected to produce more root exudates More exudate in N. Am. More invasion w/ N. Am. Natives & soil Phenotypes with differen+al rates of exudate produc+on Centaurea evolved in the context of new plants and soil organisms The changes in Centaurea increased the success of its populations Differen+al Fitness of those phenotypes 2
Diffuse reciprocal evolution of the physiology of plants and soil organisms to each other The Centaurea example also shows that local communities may reciprocally evolve tolerances to each other s impact on the environment (in this case chemical root exudates) Selection->Differential Success For example, a population adapted to an unusual soil type that contains toxic metals should differentially be successful (compared to other non-adapted populations): Invading and establishing on those soils Reproducing and maintaining populations on those soils While many of these species are restricted to serpentine, others are not and represent populations that have adapted to this soil. Arctostaphylos montana Calamagrostis ophitidis Serpentine soils act as an environmental constraint to plants because of their low N & P and high heavy metal %. These soils, therefore, select from available species, those that can tolerate their conditions. Calachortus uniflorus The Pygmy Pine forest occurs on very acidic soils that are low in nutrients, with a clay pan beneath the surface. A number of species are found only in these soils. Organisms can act as agents of ecological selection, as in the trees invading a chaparral area, shading out the shrubs. 3
Selection can be positive as well. The Douglas-fir above invaded shrubs that share the same mutualist fungi for mycorrhizae. In some cases, ecological evolution refers to specific cases of speciation or patterns of evolution within a population. Arctostaphylos gabilanensis is adapted to a habitat intermediate between its two ancestral parents, A. glauca and A. luciana. Adaptation Adaptation is a change in the characteristics of a population that leads to higher probabilities of survival and reproduction Dimensions of Adapting Physiologically Shifts in aspects of metabolism, biochemistry, or sensitivity to environmental conditions Structurally Size and shapes of leaves, density of wood, aspects of flower size, color or display, size of plant or location of buds Life History variations Relative proportion of life span spent in one stage or another Physiology Life form/ Structure Life History These are not independent categories because plants are integrated organisms. Keep that in mind even while we focus on these independently. But they are the sources of differential responses 1. Physiological Adaptations Qualitatively (as in C3 vs C4 vs CAM PS) Quantitatively (as in shade vs sun-adapted leaves = shifts in proportions of light reactions vs dark reactions Plants are a world of hormone gradients, each hormone influenced by different factors, such as light, mineral status, temperature, drought, daylength, damage. Plants are constantly responding to their world in this way. 4
2. Structurally Variation in shape/thickness of leaves Variation in density of wood in trees Variation in life form (annuals vs perennials; tree vs shrubs; deciduous vs evergreen) Long-term, this is a sorting process of the habitat. Short-term, as plants develop, structurally each node/internode/leaf unit adjusts to their local environment. 3. Life Histories Life history is the different stages of development of individuals Because life history patterns involve multiple generations, these usually reflect long-term selective forces on populations Adaptation Different components of fitness may be under conflicting selective pressures Resolution of compromises in the allocation of energy and materials to competing demands means more than one solution exists for any environment Contrast Annuals vs Perennials Perennial life histories are favored by high and relatively constant adult survival Annual life histories are favored by high fecundity and habitats in which adult survival is difficult or seedling establishment rates can be high 5
Interactive nature of Life Histories usually yields collections of adaptations selected to be synergistic with other adaptations. r-selection and K-selection as an example These can be thought of as life history syndromes. Ultimately, think of this as a resolution of conflicting demands, an allocational tradeoff. J. P. Grime incorporated R. H. Whittaker s ideas about stress-selection in his R, S, C system Interaction among Life History Stages leads to the Principle of Allocation Assume there are finite resources of energy and materials available to individuals Assume that different stages of life history sometimes represent competing demands (e.g. maximizing seed production may minimize adult survival) Allocation occurs among competing demands (trade-offs?) Life History Evolution Stages of a life cycle interact which means that stage-specific survival will influence evolution of life history patterns Imagine two populations: adults have high survival rates adults have low survival rates selection will be strongly different on seeds and juveniles for these two populations Semelparity (big bang reproduction) vs. Iteroparity (multiple reproductive cycles) If seedling establishment has a low probability while adult survival is high, then slow frequent reproduction may be favored. If seed establishment has a high probability while adult survival is low, then big-bang reproduction may be favored. 6
Other examples of trade-offs Seed size vs seed number Increasing seed size may reduce the time from seedling to reproduction (and the reverse). Increasing seed # comes at the expense of reducing or parental survival Patterns observed among plants: trade-offs between growth rate and shade tolerance trade-offs between seed number and seed size monocarpic (semelparous) vs polycarpic (iteroparous) reproduction wood density (strength) increases at a cost to growth rates chemical defenses to herbivory can be produced at a cost to growth rates What patterns do assemblages display? Do all species at a site have the same life form, physiology, life histories? At the other extreme, are life histories or life forms randomly dispersed across a landscape? Conclusions 1. Natural selection constantly impacts ecological systems 2. Sorting occurs at the scale of individuals, populations and communities 3. Organisms are selected by differential fitness Conclusions 4. Different components of fitness may be under conflicting selective pressures 5. Resolution of compromises (trade-offs) means more than one solution exists for any circumstance 7