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Chapter 21 Populations Evolve in Ecosystems The theory of evolution The theory of evolution is the foundation of all biology Charles Darwin is credited with developing the theory of evolution Based on two important points o All living organisms descended from a common ancestor and therefore, share a common chemistry o Living organisms have the ability to adapt to their environment; the ability of species of organisms to adapt to their environment is evolution Natural Selection Darwin based his theory on four general statements Organisms show differences that can be inherited All organisms produce more offspring than can survive and reproduce in subsequent generations Variations among organisms can increase or decrease each individual’s ability to reproduce Variations that increase the likelihood of successful reproduction will be passed onto future generations Those individuals in a population with phenotypes better suited to the environment will have a selective advantage, better able to survive and produce more offspring We now refer to change in allele frequencies in a population over time as evolution Although there are different means by which allele frequencies can change, natural selection is the only one that causes to evolutionary adaption Although we often hear the term “survival of the fittest”, it is really “survival of the fit enough” Fitness Reproductive fitness is the ability of an organism to successfully reproduce o The key to an organisms’ fitness is leaving more copies of its genes for the next generation o It’s all about the number of offspring produced Adaptation Evolutionary adaptation is the result of natural selection It is an adjustment or series of adjustments a population or species makes in a given environment over time Given enough time, these evolutionary adaptations may result in a new species Descent with modification Evolution occurs at the population level and above At the population level, it refers to changes in allele frequency over time At higher levels, over much greater periods of time, these changes in a population’s allele frequencies can lead to new species Over even greater periods of time, it can lead to major evolutionary changes, such as the transformation of a fish into a terrestrial tetrapod Hardy-Weinberg equilibrium In order to determine if a population is evolving, we need to have a point of reference o We need to know what a population that is not evolving looks like Requirements for Hardy-Weinberg equilibrium o Population must be very large o Mating must be random o No mutations, immigration, or emigration o No selective pressure so no natural selection o So, there can’t be a natural population that meets all of those requirements Hardy-Weinberg equilibrium refers to an idealized situation - a reference point With this reference point, any population that does not fit the Hardy-Weinberg equation must be evolving Consider a single gene with two alleles in the gene pool (A and a) o Since there are only the two alleles, the sum of their frequencies must be 1 o If p = frequency of A and q = frequency of a, then p + q = 1 o If we square both sides (p + q)2 = 12 we get p2 + 2pq + q2 = 1 o So p2 = frequency of AA o 2pq = frequency of Aa o q2 = frequency of aa Practical applications Consider cystic fibrosis o Incidence rate among North American Caucasians = 1/2000 o q2 = 1/2000 = 0.0005; q = 0.02 o p = 0.98, 2pq = 2(0.02)(0.98) = 0.0392 o Therefore, ~4% (1/25) of Caucasians are carriers Evolution Evolution does not have a direction; it does not march toward a perfect organism In any particular environment, there are likely to be many organisms that are well-suited to fill various niches These make up communities Over time, change is normal Environmental conditions can change as well Species that were very well-suited to the old environment can now be ill-suited to the new one The species can either adapt via evolutionary process or become extinct o Humans can cause very rapid environmental changes Evolutionary relationships The concept of descent with modification would predict that we share common macromolecules with other life forms All living organisms have DNA as their genetic material, use RNA as a messenger, and use the same genetic code to make their proteins But DNA can mutate Evolutionary theory predicts that more closely related species will have fewer differences in their DNA sequences; more distantly related species will have more differences Genomic analysis indicates that humans and chimpanzees shared a common ancestor about 7 million years ago o Since that time, the lineage including humans and the lineage including chimpanzees have diverged o Research indicates that one of the biggest differences between chimps and humans is the composition and functioning of the brain Natural selection Natural selection acts on populations subjected to various pressures from the environment o Climate pressures, intra- and interspecific competition for limited resources, others Those individuals with variations that provide a selective advantage will produce more offspring and make a larger contribution to the gene pool The genetic variation is due to mutation to produce new alleles and the genetic recombination that occurs with sexual reproduction Genetic bottlenecks Stable populations can be devastated by natural catastrophes When large portions of populations are suddenly removed from the gene pool, this promotes evolution without regard to fitness or genetic makeup The frequency of alleles in the surviving population is often very different from the original population Bottleneck effect Sichuan earthquake in 2008 killed >69,000 villagers Earthquake in Haiti has greatly affected allele frequencies Gene flow The movement of individuals from one population to another can also affect allele frequencies Alleles are removed from one population and added to the other This can introduce or remove disadvantageous alleles Ecology The field of ecology attempts to identify and explain the interactions between the biotic (living) and abiotic (nonliving) components of ecosystems o It’s all about energy and resources Individuals of the same species occur in populations Populations, in turn, are organized into communities, which include more than one species The nature of ecosystems Biosphere – the regions of the Earth’s waters, crust, and atmosphere inhabited by living organisms Biomes – large areas where organisms interact between each other and their environment o These are determined primarily by climate o Terrestrial - several distinct types based on temperature and rainfall o Aquatic - freshwater and marine Succession Ecological communities undergo constant change o The composition of communities are always shifting with changing conditions dominant populations change in a largely predictable manner o We can consider primary and secondary succession Primary succession occurs in areas where there is no life o This can be any new land, such as areas formed from lava flows, beaches, river deltas, or areas recently cleared or exposed by the movement of glaciers o Initially, pioneer species hold newly formed soil in place and add organic materials, allowing grasses and then large plants to take over o The dominant species change - succession Secondary succession occurs when a stable community is disrupted o Organisms associated with an earlier stage of succession once again become dominant o The land is not newly formed but the pioneer species begin the process again o Secondary succession usually occurs much more rapidly than does primary succession Climax communities are stable, mature, equilibrium communities that have resulted from succession If it’s all about energy and resources, what is the path of energy in an ecosystem? o Energy flows through an ecosystem What about nutrients, a major resource? o Nutrients cycle through an ecosystem Energy flow o There must be a constant supply of energy into ecosystems o Most ecosystems rely on sunlight energy o Producers convert sunlight energy to chemical energy in the form of chemical bonds o Photosynthesis and cellular respiration are intimately linked o Photosynthesis requires sunlight, CO2, and H2O and produces glucose and O2 o Cellular respiration requires glucose and O2 and produces CO2 and H2O o As consumers eat producers or other consumers, energy and chemicals are transferred from one organism to the next o In a food chain, energy is transferred as feeding occurs o However, most of the energy is lost as heat In fact, only about 10% of the energy at any one level is used for adding biomass (growth) of consumers at the next level The 10% rule has some major implications for numbers of organisms and biomass at each level o These are our ecological pyramids o Most ecosystems can sustain no more than five trophic levels o Even then, most 5th level consumers can also feed at lower levels o Understanding the ecological pyramid for a community can provide valuable information about the stability of that community o Ecosystems rarely involve a single food chain o Food webs more accurately depict the myriad of feeding interactions Cycling of nutrients Although essential nutrients have a tendency to remain within ecosystems, they can be readily accessible or trapped in an inaccessible form The availability of these nutrients determines the amount of productivity within an ecosystem Ecologists study biogeochemical cycles of these essential nutrients - water, phosphorus, nitrogen, carbon Population growth Population growth is dependent on two major rates* Growth rate = birth rate - death rate o If growth rate > 0, that population has the potential to grow exponentially over time o It can grow at its biotic potential o No natural population can grow at its biotic potential indefinitely Population-limiting factors will prevent a population from continuing to expand indefinitely Carrying capacity is the number of individuals of each population that the ecosystem can support indefinitely without permanently reducing the productivity of the ecosystem o It is a balance between competition for available resources and population growth o Carrying capacity varies with each species, ecological conditions, and time o In natural ecosystems, populations often stabilize around their carrying capacities o With successful species, we have usually seen population growth slow as the population nears its carrying capacity What about humans? When we compare human survival vs. other organisms, we can develop survivorship curves The type of survival is linked to reproductive and parental care strategies What effects have humans had on the planet? Pollutants from factories can lead to acid rain o But recognizing the problem can sometimes lead to a solution o Regulatory controls on sulfur emissions were enacted in 1990 Increased amounts of CO2 in the atmosphere from the burning of fossil fuels and deforestation has led to global warming More people means less resources and more waste By destroying natural habitats, we have also changed whole ecosystems This may well be increasing the risk of extinction of many species Humans may well be responsible for the sixth mass extinction in all of earth’s history Changing natural habitats rather than minimizing our impact In nature, there are neither rewards nor punishments - there are consequences. Roger G. Intersoll