Darwin’s theory of natural selection, combined with the new understanding of genetics (the means by which characteristics are transmitted from one generation to the next) provided the mechanism for understanding the link between organisms and their environment, which is the focus of ecology.

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Ecology 1.1
Ecology is the scientific study of the relationships between organisms and their environment. The environment includes the physical and chemical conditions and biological or living components of an organism’s surroundings. Relationships include interactions with the physical world as well as with members of the same and other species.

Ecosystems 1.2
Organisms interact with their environment in the context of the ecosystem. Broadly, the ecosystem consists of two components, the living (biotic) and the physical (abiotic), interacting as a system.

Hierarchical Structure 1.3
Ecological systems may be viewed in a hierarchical framework, from individual organisms to the biosphere. Organisms of the same species that inhabit a given physical environment make
up a population. Populations of different kinds of organisms interact with members of their own species as well as with individuals of other species. These interactions range from competition for shared resources to interactions that are mutually beneficial for the individuals of both species involved. Interacting populations make up a biotic Comunità.
The community plus the physical environment make up an ecosystem. All communities and ecosystems exist in the broader spatial context of the landscape—an area of land (or water) composed of a patchwork of communities and ecosystems.

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5.5 Genetic Variation Occurs at the Level of the Population
Adaptations are the characteristics of individual organisms—a reflection of the interaction of the genes and the environment. They are the product of natural selection. Although the process of natural selection is driven by the success or failure of individuals, the population—the collective of individuals and their alleles—changes through time, as individuals either succeed or fail to pass their genes to successive generations. For this reason, to understand the process of adaptation through natural selection, we must first understand how genetic variation is organized within the population. A species is rarely represented by a single, continuous interbreeding population. Instead, the population of a species is typically composed of a group of subpopulations—local populations of interbreeding individuals, linked to each other in varying degrees by the movement of individuals (see Sections 8.2 and 19.7 for discussion of metapopulations). Thus, genetic variation can occur at two hierarchical levels, within subpopulations and among subpopulations. When genetic variation occurs among subpopulations of the same species, it is called genetic differentiation.
The sum of genetic information (alleles) across all individuals in the population is referred to as the gene pool. The gene pool represents the total genetic variation within a population. Genetic variation within a population can be quantified in several ways. The most fundamental measures are allele frequency and genotype frequency. The word frequency in this context refers to the proportion of a given allele or genotype among all the alleles or genotypes present at the locus in the population.

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The phenotypic trait that selection acts directly upon is referred to as the target of selection; in this example, it is beak size. The selective agent is the environmental cause of fitness differences among organisms with ­ different phenotypes, or in this case, the change in food resources (abundance and size distribution of seeds). The increased survival rate of individuals with larger beaks resulted in a shift in the distribution of beak size (phenotypes) in the population (Figure 5.13). This type of natural selection, in which the mean value of the trait is shifted toward one extreme over another (Figure 5.14a), is called directional selection. In other cases, natural selection may favor individuals near the population mean at the expense of the two extremes; this is referred to as stabilizing selection (Figure 5.14b). --- although not necessarily to the same degree, it can result in a bimodal distribution of the characteristic(s) in the population (Figure 5.14c). Such selection, known as disruptive selection, occurs when members of a population are subject to different selection pressures.

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A cline is a measurable, gradual change over a geographic region in the average of some phenotypic character, such as size and coloration. Clines are usually associated with an environmental gradient that varies in a continuous manner across the landscape,

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Each species has a range of abiotic environmental and resource conditions under which it can survive, grow, and reproduce. The primary factor influencing the distribution of a population is the occurrence of suitable environmental and resource conditions—habitat suitability.
Within the geographic range of a population, individuals are not distributed equally. Individuals occupy only those areas that can meet their requirements (suitable habitat). Because organisms respond to a variety of environmental factors, they can inhabit only those locations where all factors fall within their range of tolerance

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Whereas distribution defines the spatial extent of a population, abundance defines its size—the number of individuals in the population. In Figure 8.4, the population abundance is the total number of red dots (individuals) within the blue line that defines the population distribution. Abundance is a function of two factors: (1) the population density and (2) the area over which the population is distributed. Population density is the number of individuals per unit area.
Distribution:
Individuals of a population may be distributed randomly, uniformly, or in clumps (aggregated; Figure 8.10). Individuals may be distributed randomly if each individual’s position is independent of those of the others. In contrast, individuals distributed uniformly are more or less evenly spaced. A uniform distribution usually results from some form of negative interaction among individuals, such as competition, which functions to maintain some minimum distance among members of the population (see Chapter 11). Uniform distributions are common in animal populations where individuals defend an area for their own exclusive use (territoriality) or in plant populations where severe competition exists for belowground resources such as water or nutrients (Figure 8.11; see also Figures 11.17 and 11.19).

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abundance describes the number of individuals in the population but provides no information on their characteristics—that is, how individuals within the population may differ from one another. Unless each generation reproduces and dies in a single season, not overlapping the next generation (such as annual plants and many insects), the population will have an age structure: the number or proportion of individuals in different age classes. Because reproduction is restricted to certain age classes and mortality is most prominent in others, the relative proportions of each age group bear on how quickly or slowly populations grow (see Chapter 9).
Populations can be divided into three ecologically important age classes or stages: prereproductive, reproductive, and postreproductive.

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At some stage in their lives, most organisms are mobile to some degree. The movement of individuals directly influences their local density. The movement of individuals in space is referred to as dispersal, although the term dispersal most often refers to the more specific movement of individuals away from one another. When individuals move out of a subpopulation, it is referred to as emigration. When an individual moves from another location into a subpopulation, it is called immigration. The movement of individuals among subpopulations within the larger geographic distribution is a key process in the dynamics of metapopulations and in maintaining the flow of genes between these subpopulations (see Chapters 5 and 19)

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The resulting pattern of population size as a function of time is shown in Figure 9.3 and is referred to as geometric population growth.

For populations, such as the hydra, where birth and death are occurring continuously (not daily intervals), population ecologists often represent the processes of birth and death as instantaneous rates and Population growth as a continuous process rather than on defined time steps (such as one day). The model is then presented as a differential equation:

dN/dt = rN

The term ΔN/Δt is replaced by dN/dt to express that Δt (the time interval) approaches a value of zero, and the rate of change becomes instantaneous. The value r is now the instantaneous per capita growthrate (sometimes called the intrinsic rate of population growth), and the resulting equation is referred to as the model of exponential population growth (in contrast to geometric population growth based on discrete time steps).
The model of exponential growth (dN/dt = rN) predicts the rate of population change over time. If we wish to define the equation to predict population size, N(t), under conditions of exponential growth N(t) at any given value of t, it is necessary to integrate the differential equation presented previously. The result is:
N(t) = N(0)ert

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The stochastic (or random) variations in birthrates and death rates occurring in populations from year to year are called demographic stochasticity, and they cause populations to deviate from the predictions of population growth based on the deterministic models discussed in this chapter. Besides demographic stochasticity, random variations in the environment, such as annual variations in climate (temperature and precipitation) or the occurrence of natural disasters, such as fire, flood, and drought, can directly influence birthrates and death rates within the population. Such variation is referred to as environmental stochasticity. We will discuss the role of environmental stochasticity in controlling the growth of populations later in Chapter 11 (Section 11.13).

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Population growth