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Chapter 7

Chapter 7 - Ecology.docx

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University of Toronto St. George
Dr.Rosada Silva

Chapter 7: Life History NEMO GROWS UP: A CASE STUDY  The offspring produced by different organisms vary tremendously  Nemo’s father would have changed sex and become a female after Nemo loses his mother to a predator  Clownfish spend their entire adult lives within a single sea anemone  In what appears to be a mutually beneficial relationship, the anemone protects the clownfish by stinging their predators  The clownfish, in turn, may help the anemone by eating its parasites or driving away its predators  Two to six clownfish typically inhabit a single anemone o The largest fish in the anemone is a female o The second largest, is the breeding male o The remaining fish are sexually immature nonbreeders o If the female dies, as in Nemo’s story, the breeding male undergoes a growth spurt and changes sex to become a female, and the largest nonbreeder increases in size and becomes the new breeding male.  The hatchling fish leave the anemone to live in the open ocean, away from the predator-infested reef  When a juvenile fish enters an anemone, the resident fish allow it to stay there only if there is room. If there is no room, the young fish is expelled and returns to the dangers of an exposed existence on the reef  Why do clownfish engage in these complicated machinations just to produce more clownfish?  These solutions are often well suited for meeting the challenges and constraints of the environment where a species lives INTRODUCTION  Human history is a record of past events  An individual organism’s life history is a record of major events related to its growth, development, reproduction, and survival  The timing and nature of life history traits, and therefore the life history itself, are products of adaptation to the environment in which the organism lives  Biologists analyze life history patterns in order to understand the trade-offs, constraints, and selection pressures imposed on different stages of an organism’s life cycle Concept 7.1 Life history Patterns Vary Within and Among Species LIFE HISTORY PATTERNS VARY WITHIN AND AMONG SPECIES  The study of life histories is concerned with categorizing variation in life history traits and analyzing the causes of that variation INDIVIDUALS WITHIN SPECIES DIFFER IN THEIR LIFE HISTORIES  Individual differences in life history traits are ubiquitous  Despite this variation, it is possible to make some generalizations about life histories in Homo sapiens: - For example. Women typically have one baby at a time, reproduction usually occurs between the ages of 15 and 45, and so on  Life History Strategy – of a species is the overall pattern in the timing and nature of life history events averaged across all the individuals in the species - Is shaped by the way the organism divides its energy and resources between growth, reproduction, and survival - Within a species, individuals often differ in how they divide their energy and resources among these activities. Such differences may result from genetic variation, from differences in environmental conditions, or from a combination of both - the life history strategy is determined by effects of natural selection, not the choices of the individual organism  Genetic Differences – Some life history variation within species is determined genetically - Heritable variation in life history traits is the raw material on which natural selection acts - Selection favors individuals whose life history traits result in their having a better chance of surviving and reproducing than do individuals with other life history traits  Geologists sometimes describe life histories as optimal in that they are adapted to maximize fitness  Fitness – the genetic contribution of an organism’s descendants to future generations  No organism has a perfect life history-that is, one that results in the unlimited production of descendants  All organisms face constraints that prevent the evolution of a perfect life history  Although life histories often serve organisms well in the environments in which they have evolved, they are optimal only in the sense of maximizing fitness subject to constraints  Environmental Differences – A single genotype may produce different phenotypes under different environmental conditions, a phenomenon known as phenotypic Plasticity - Ex. Most plants and animals grow at different rates depending on temperature  Changes in life history traits often translate into changes in adult morphology - Ponderosa pine trees in cool, moist climates allocate more re-sources to leaf production than do trees in desert climates - Desert trees are shorter than those grown in cooler climates, but for a given height, they have thicker trunks - They also have lower photosynthetic rates and consume less CO2 because they have fewer leaves  Allocation describes the relative amounts of energy or resources that an organism devotes to different functions  The result of allocation differences in ponderosa pines is that trees grown in different environments differ in their adult shape and size  Phenotypic plasticity that responds to temperature variation often produces a continuous range of sizes  In other types of phenotypic plasticity, a single genotype produces discrete types, or morphs, with few or no intermediate forms  The differing body shapes of omnivores and carnivores result from differences in the relative growth rates of different body parts: carnivores have bigger mouths and stronger jaw muscles because of accelerated growth in those areas  Field studies show that the proportion of omnivore and carnivore morphs is affected by food supply  The more slowly growing omnivores are favored in ponds that persist longer because they metamorphose in better condition and thus have better chances of survival as juvenile toads  changes in the environment affect the relative growth rates of different body parts  In the pines. The relative growth rates of leaves and sapwood determine body shape, while in the toads, the relative growth rates of the jaw and the rest of the body determine whether the tadpole is a carnivore or an omnivore - These patterns are examples of allometry, or differential growth of body parts that results in a change in shape or proportion with size - Allometry is a very common mechanism of variation within and among species  Adaptation must be demonstrated rather than assumed  In some instances, phenotypic plasticity may be a simple physiological response, not an adaptive response shaped by natural selection - changes in growth rate due to temperature variation may occur because chemical reactions are slower at lower temperatures, and thus metabolism and growth are necessarily slower. MODE OF REPRODUCTION IS A BASIC LIFE HISTORY  At the most basic level, evolutionary success is determined by successful reproduction  Despite this universal reality, organisms have evolved vastly different mechanisms for reproducing— from simple asexual splitting to complex  Asexual Reproduction – The first organisms to evolve on Earth reproduced asexually by binary fission (―dividing in half’) - The sexual reproductive processes of meiosis, recombination, and fertilization arose later - Today, all prokaryotes and many protists reproduce asexually - While sexual reproduction is the norm in multicellular organisms, many can also reproduce asexually  Ex. after they are initiated by a (sexually produced) founding polyp, coral colonies grow by asexual reproduction  Each polyp is a genetically identical copy, or clone, of the founding polyp  Once the colony has grown to a certain size and conditions are right, the polyps reproduce sexually, producing offspring that develop into polyps that start their own new colonies of clones  Sexual Reproduction and Anisogamy – Sex has some clear benefits including recombination, which promotes genetic variation and hence may increase the capacity of populations to evolve in response to environmental challenges such as drought or disease - Sex also has some disadvantages. Because meiosis produces haploid gametes that contain half the genetic content of the parent, a sexually reproducing organism can transmit only half of its genetic material to each offspring, whereas asexual reproduction allows transmission of the entire genome - He growth rate of sexually reproducing populations is only half that of asexually reproducing ones, all else being equal  Sexual reproduction originated in single-celled protists  Isogamy – The production of equal-sized gametes  Anisogamy – In most multicellular organisms, the two types of gametes are different sizes  differences between the sexes in gamete size can influence other reproductive characteristics, such as the timing of sex changes LIFE CYCLES ARE OFTEN COMPLEX  The small, early stages of many animal life cycles look and behave completely differently from adult stages - Ex. Chromis atripectoralis start life as hatchlings only a few millimeters long. The hatchlings live and grow in the open ocean, feeding on planktonic algae. When they have grown to about a centimeter in length, they return to the reef and begin to eat larger food items - This life cycle may have evolved in response to high levels of predation )n young fish that stay on the reef; young fish that spend more time growing in the open ocean may have better chances of survival  Complex Life Cycle – is a life cycle in which there are at least two distinct stages that differ in their habitat, physiology, or morphology  In many cases, the transitions between stages in complex life cycles are abrupt - Ex. Many organisms undergo metamorphosis, an abrupt transition in form from the larval to the juvenile stage that is sometimes accompanied by a change in habitat  Complex life cycles and metamorphosis can be found even among vertebrates, including some fishes and most amphibians  Over the course of evolution, complex life cycles have been lost in some species that are members of groups in which such cycles are considered the ancestral condition  The resulting simple life cycles are sometimes referred to as direct development because development from fertilized egg to juvenile occurs within the egg prior to hatching and no free- living larval stage occurs  Many parasites have evolved intricate and complex life cycles with one or more specialized stages for each host that they inhabit  Complex life cycles also occur in many types of algae and plants, reaching some of their most elaborate forms in these group o Some algae and all plants have complex life cycles in which a multicellular diploid sporophyte alternates with a multicellular haploid gametophyte o the sporophyte produces haploid spores that disperse and grow into gametophytes, and the gametophyte produces haploid gametes that combine in fertilization to form zygotes that grow into sporophytes = alternation of generations  In mosses and a few other plant groups, the gametophyte is larger, but in most plants and some algae, the sporophyte is the dominant stage of the life cycle Concept 7.2 Reproductive Patterns Can be Classified Along Several Continua LIFE HISTORY CONTINUA  Several classification schemes have been proposed for organizing patterns of reproduction SOME ORGANISMS REPRODUCE ONLY ONCE WHILE OTHERS REPRODUCE MULTIPLE TIMES  One way of classifying the reproductive diversity of organisms is by the number of reproductive events in an individual’s lifetime  Semelparous – species reproduce only once in a lifetime - Ex. Giant Pacific octopus: lies shortly after the eggs hatch, having exhausted herself in this intense period of parental investment  Iteroparous – species have the capacity to for multiple bouts of reproduction  Many plant species typically complete their life cycle in a single year or less = annual plants - such species are semelparous: after they germinate from a seed, they reproduce once and die  Most organisms do not invest so heavily in single reproductive events - Iteroparous organisms have multiple bouts of reproduction over the course of a lifetime. LIVE FAST AND DIE YOUNG OR SLOW AND STEADY WINS THE RACE?  Wilson coined the terms r-selection and K-selection to describe two ends of a continuum of reproductive patterns  r-selection – r in the term r-selection refers to the intrinsic rate of increase of a population, a measure of how rapidly a population can grow - Refers to selection for high population growth rates - This type of selection can occur in environments where population density is low  disturbed habitats that are being recolonized - In this type of habitat, genotypes that can grow and reproduce rapidly will be favored over those that cannot - Short life spans, rapid development, early maturation, low parental investment, and high rates of reproduction - ―Live fast, die young‖ - Ex. Small insects, mice, weedy plants  K-selection - refers to selection for slower rates of increase, which occurs in populations that are at or approaching K, the carrying capacity or stable population size for the environment in which they live - Occurs under crowded conditions, where genotypes that can efficiently convert food into offspring are favored - Do not have high population growth rates because they are already near the carrying capacity for their environment and competition for resources can be intense - Long-lived, develop slowly, delay maturation, invest heavily in each offspring. And have low rates of reproduction - ―slow and steady‖ - Ex. large mammals such as elephants and whales, reptiles such as tortoises and crocodiles  The r-K continuum tends to emphasize the extremes  Distinction between r-selection and K-selection is perhaps most useful in comparing life histories in closely related species or species living in similar environments  the species that occurs most r-selected characteristics, including rapid development, early reproduction, production of many small eggs, and rapid population growth  Species found in more predictable, wet forest habitats have more K-selected characteristics PLANT LIFE HISTORIES CAN BE CLASSIFIED BASED ON HABITAT CHARACTERISTICS  The success of a plant species in a given habitat, Grime argued is limited by two factors: 1. Stress – external abiotic factor that limits growth  extreme temperatures, shading, low nutrient levels, water shortages, and any other characteristics of the abiotic environment that reduce vegetative growth 2. Disturbance – my process that destroys plant biomass  under Grime’s definition, disturbance can result from biotic sources such as outbreaks of herbivorous sects and abiotic sources such as fire.  If we consider that in a given habitat, stress and
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