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Chapter 5 This lecture will help you understand: • • • • • • • Natural selection Evolution and biodiversity Species and mass extinction Levels of ecological organization Population characteristics that predict growth Population ecology Conservation challenges Central Case: Striking Gold in a Costa Rican Cloud Forest • The golden toad of Monteverde, discovered in 1964, had disappeared 25 years later. • Researchers determined that warming and drying of the forest was most likely responsible for its extinction. • As the global climate changes, more such events can be expected. Evolution and natural selection • Evolution = genetic change across generations • Natural selection = process by which traits that enhance survival are passed on to future generations more than those that do not • This alters the genetic makeup of populations over time. Natural selection shapes diversity • Charles Darwin and Alfred Russell Wallace each proposed natural selection as a mechanism for evolution and a way to explain the variety of living things. • A trait that promotes success in natural selection is called an adaptive trait or an adaptation. • A trait that reduces success is maladaptive. • A trait that is adaptive in one location or season may prove maladaptive in another. Natural selection and genetic variation • For a trait to be heritable, genes in an organism’s DNA must code for the trait. • Mutations are accidental changes in DNA. • Mutations that are not lethal provide the genetic variation on which natural selections act. • In these ways, variable genes and variable environments interact in adapting to environmental conditions. Biodiversity • Biodiversity, or biological diversity, is the sum of an area’s organisms, considering the diversity of species, their genes, their populations, and their communities. • A species is a particular type of organism; a population or group of populations whose members share certain characteristics and can freely breed with one another and produce fertile offspring. • A population is a group of individuals of a particular species that live in the same area. Biodiversity Costa Rica’s Monteverde cloud forest is home to many species and possesses great biodiversity. Speciation • Speciation: The process by which new species come into being • It is an evolutionary process that has given Earth its current species richness— more than 1.5 million described species and likely many million more not yet described by science. • Allopatric speciation is considered the dominant mode of speciation, and sympatric speciation also occurs. Allopatric speciation 1. Single interbreeding population 2. Population divided by a barrier; subpopulations isolated Allopatric speciation 3. The two populations evolve independently, diverge in their traits. 4. Populations reunited when barrier removed, but are now different enough that they don’t interbreed. Allopatric speciation Many geological and climatic events can serve as barriers separating populations and causing speciation. Phylogenetic trees • Life’s diversification results from countless speciation events over vast spans of time. • Evolutionary history of divergence is shown with diagrams called phylogenetic trees. • Similar to family genealogies, these show relationships among organisms. Phylogenetic trees These trees are constructed by analyzing patterns of similarity among present-day organisms. This tree shows all of life’s major groups. Phylogenetic trees Within the group Animals in the previous slide, one can infer a tree of the major animal groups. Phylogenetic trees And within the group Vertebrates in the previous slide, one can infer relationships of the major vertebrate groups, and so on… Extinction • Evolution has not always progressed in a straight-forward manner, or from simple to complex. • The Burgess Shale fauna were complex and bizarre marine animals from 530 million years ago that vanished completely. Extinction • Extinction is the disappearance of an entire species from the face of the Earth. • Average time for a species on Earth is ~1–10 million years. • Species currently on Earth = the number formed by speciation minus the number removed by extinction Extinction • Some species are more vulnerable to extinction than others: • Species in small populations • Species adapted to a narrowly specialized resource or way of life • Monteverde’s golden toad was apparently such a specialist, and lived in small numbers in a small area. It was endemic to the forest, occurring nowhere else. Extinction • Until 10,000 years ago, North America teemed with camels, mammoths, giant sloths, lions, horses, saber-toothed cats, and other large mammals. • Many scientists think their extinction was brought on by hunting after human arrival. Mass extinctions Earth has seen five mass extinction events: 50%+ of species were wiped out. Life’s hierarchy of levels • • • • • Organismal Population Community Ecosystem Biosphere Ecology • • • • Communities are made up of multiple interacting species that live in the same area. Ecosystems encompass communities and the nonliving material with which their members interact. Population ecology investigates how individuals within a species interact with one another. Community ecology studies interactions among species. • Ecosystem ecology reveals patterns by studying living and nonliving components of systems in conjunction. Habitat and niche • • • • Habitat = the specific environment where an organism lives (including living and nonliving elements: rocks, soil, plants, etc.) Habitat selection = the process by which organisms choose habitats among the options encountered Niche = an organism’s functional role in a community (feeding, flow of energy and matter, interactions with other organisms, etc.) Specialists = organisms with narrow breadth and thus very specific requirements Population ecology • Population = a group of individuals of a species that live in a particular area • Several attributes help predict population dynamics (changes in population): • • • • • • Population size Population density Population distribution Sex ratio Age structure Birth and death rates Population size Number of individuals present at a given time The passenger pigeon was once North America’s most numerous bird, but is now extinct. Population density Number of individuals per unit area In the 19th century, the flocks of passenger pigeons showed high population density. Population distribution Spatial arrangement of individuals Age structure • Or age distribution = relative numbers of individuals of each age or age class in a population • Age structure diagrams, or age pyramids, show this information. Age structure Sex ratio • • • Ratio of males to females in a population Even ratios (near 50/50) are most common. Fewer females causes slower population growth. Note human sex ratio biased toward females at oldest ages. Population growth Populations grow, shrink, or remain stable, depending on rates of birth, death, immigration, and emigration. (crude birth rate + immigration rate) – (crude death rate + emigration rate) = growth rate Survivorship curves • Type I: survival rates are high when organisms are young and decrease sharply when organisms are old. • Type II: survival rates are equivalent regardless of an organism’s age. • Type III: most mortality takes place at young ages, and survival rates are greater at older ages. Exponential growth • Unregulated populations increase by exponential growth: • Growth by a fixed percentage, rather than a fixed amount. • Similar to growth of money in a savings account Exponential growth in a growth curve • Population growth curves show change in population size over time. • Scots pine shows exponential growth Limits on growth • Limiting factors restrain exponential population growth, slowing the growth rate down. • Population growth levels off at a carrying capacity—the maximum population size of a given species an environment can sustain. • Initial exponential growth, slowing, and stabilizing at carrying capacity is shown by a logistic growth curve. Logistic growth curve Population growth: Logistic growth Logistic growth (shown here in yeast from the lab) is only one type of growth curve, however. Population growth: Oscillations Some populations fluctuate continually above and below carrying capacity, as with this mite. Population growth: Dampening oscillations In some populations, oscillations dampen, as population size settles toward carrying capacity, as with this beetle. Population growth: Crashes Some populations that rise too fast and deplete resources may then crash, as with reindeer on St. Paul Island. Density dependence • Often, survival or reproduction lessens as populations become more dense. • Density-dependent factors (disease, predation, etc.) account for the logistic growth curve. • Other factors (e.g., catastrophic weather events) occur regardless of density, and are density-independent factors. Biotic potential and reproductive strategies • Species differ in strategies for producing young. • Species producing lots of young (insects, fish, frogs, plants) have high biotic potential. • Others, such as mammals and birds, produce few young. • However, those with few young give them more care, resulting in better survival. r and K-selected species r-selected species • Many offspring • Fast growing • No parental care K-selected species • Few offspring • Slow growing • Parental care Conservation of Biodiversity • • Ecotourism = tourism focused on visiting natural areas. It provides economic incentive to local communities for conservation of nature Conclusion • The golden toad and other Monteverde organisms have helped illuminate the fundamentals of evolution and population ecology. • Natural selection, speciation, and extinction help determine Earth’s biodiversity. • Understanding ecological processes at the population level is crucial to protecting biodiversity.