Populations
Populations
Animals exist in nature as members of populations. As we saw in Organic Evolution, a population is a reproductively interactive group of animals of a single species. A species of animal may comprise a single, cohesive population or may contain many geographically disjunct populations, often called demes. Because members of a deme interbreed, they share a common gene pool. Migration of individuals among demes within a species can impart some evolutionary cohesion to the species as a whole.
Each population or deme has a characteristic age structure, sex ratio, and growth rate. The study of these properties and the factors that influence them is called demography. Demographic characteristics vary according to the lifestyle of the species under study. For example, some animals (and most plants) are modular. Modular animals, such as sponges, corals, and bryozoans, consist of colonies of genetically identical organisms. Reproduction is by asexual cloning, as described for hydrozoans in Radiate Animals. Most colonies also have distinct periods of gamete formation and sexual reproduction. Colonies propagate also by fragmentation, as seen on coral reefs during severe storms. Pieces of coral may be scattered by wave action on a reef, becoming seeds for formation of a new reef. For these modular animals, age structure and sex ratio are difficult to determine. Changes in colony size can be used to measure growth rate, but counting individuals is more difficult and less meaningful than in unitary animals, which are independently living organisms.
Most animals are unitary. However, even some unitary species reproduce asexually. Clonal species are found in many animal groups, including insects, reptiles, and fish. Clonal groups contain only females, which lay unfertilized eggs that hatch into daughters genotypically identical to their mothers. This kind of cloning is called parthenogenesis. The praying mantid Bruneria borealis, common in the southeastern United States, is a parthenogenetic unitary animal.
Most metazoans are biparental, and reproduction follows a period of organismal growth and maturation. Each new generation begins with a cohort of individuals born at the same time. Of course, individuals of any cohort will not all survive to reproduce. For a population to retain constant size from generation to generation, each adult female must replace herself on average with one daughter that survives to reproduce. If she produces more than one viable female offspring the population will grow; if fewer than one, the population will decline.
Animal species have different characteristic patterns of survivorship from birth until death of the last member of a cohort. The three principal types of survivorship are illustrated in Figure 40-2. Curve a, in which all individuals die at the same time, probably occurs only rarely in nature. Curve b, in which the rate of mortality as a proportion of survivors is constant over all ages, is characteristic of some animals that care for their young, such as birds. Human populations generally fall somewhere between curves a and b, depending on the state of nutrition and medical care.
The survivorship of most invertebrates, and of vertebrates like fish that produce great numbers of offspring, resembles curve c. For example, a mature female marine prosobranch snail, Ilyanassa obsoleta, produces thousands of eggs each reproductive period. Zygotes develop into freeswimming planktonic veliger larvae, which can be scattered far from the mother’s habitat by oceanic currents. They form part of the plankton and experience high mortality from numerous animals that feed on plankton. Furthermore, the larvae require a specific, sandy-bottomed substrate on which to settle and metamorphose into an adult snail. The probability of a larva surviving long enough to find a suitable habitat is very low, and most of the cohort dies during the veliger stage. We therefore see a rapid drop in survivorship in the first part of the curve. The few larvae that do survive to become snails have improved odds of surviving further, as reflected by the more gentle slope of the curve for older snails. Thus, high reproductive output balances high juvenile mortality in such animals.
Many animals survive to reproduce only once before they die, as seen in many insect species of the temperate zone. Here, adults reproduce before the onset of winter and die, leaving only their eggs to overwinter and repopulate the habitat the following spring. Similarly, Pacific salmon after several years return from the ocean to fresh water to spawn only once, after which all adults of a cohort die. However, other animals survive long enough to produce multiple cohorts of offspring that may mature and reproduce while their parents are still alive and reproductively active. Populations of animals containing multiple cohorts, such as robins, box turtles, and humans, exhibit age structure. Analysis of age structure reveals whether the population is actively growing, stable, or declining. Figure 40-3 shows age profiles of two idealized populations. On a global scale, humans exhibit an age structure similar to curve a in Figure 40-2, although age structures vary among regions.
Animals exist in nature as members of populations. As we saw in Organic Evolution, a population is a reproductively interactive group of animals of a single species. A species of animal may comprise a single, cohesive population or may contain many geographically disjunct populations, often called demes. Because members of a deme interbreed, they share a common gene pool. Migration of individuals among demes within a species can impart some evolutionary cohesion to the species as a whole.
Each population or deme has a characteristic age structure, sex ratio, and growth rate. The study of these properties and the factors that influence them is called demography. Demographic characteristics vary according to the lifestyle of the species under study. For example, some animals (and most plants) are modular. Modular animals, such as sponges, corals, and bryozoans, consist of colonies of genetically identical organisms. Reproduction is by asexual cloning, as described for hydrozoans in Radiate Animals. Most colonies also have distinct periods of gamete formation and sexual reproduction. Colonies propagate also by fragmentation, as seen on coral reefs during severe storms. Pieces of coral may be scattered by wave action on a reef, becoming seeds for formation of a new reef. For these modular animals, age structure and sex ratio are difficult to determine. Changes in colony size can be used to measure growth rate, but counting individuals is more difficult and less meaningful than in unitary animals, which are independently living organisms.
Most animals are unitary. However, even some unitary species reproduce asexually. Clonal species are found in many animal groups, including insects, reptiles, and fish. Clonal groups contain only females, which lay unfertilized eggs that hatch into daughters genotypically identical to their mothers. This kind of cloning is called parthenogenesis. The praying mantid Bruneria borealis, common in the southeastern United States, is a parthenogenetic unitary animal.
Most metazoans are biparental, and reproduction follows a period of organismal growth and maturation. Each new generation begins with a cohort of individuals born at the same time. Of course, individuals of any cohort will not all survive to reproduce. For a population to retain constant size from generation to generation, each adult female must replace herself on average with one daughter that survives to reproduce. If she produces more than one viable female offspring the population will grow; if fewer than one, the population will decline.
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| Figure 40-2 Three types of theoretical survivorship curves. See text for explanation. |
Animal species have different characteristic patterns of survivorship from birth until death of the last member of a cohort. The three principal types of survivorship are illustrated in Figure 40-2. Curve a, in which all individuals die at the same time, probably occurs only rarely in nature. Curve b, in which the rate of mortality as a proportion of survivors is constant over all ages, is characteristic of some animals that care for their young, such as birds. Human populations generally fall somewhere between curves a and b, depending on the state of nutrition and medical care.
The survivorship of most invertebrates, and of vertebrates like fish that produce great numbers of offspring, resembles curve c. For example, a mature female marine prosobranch snail, Ilyanassa obsoleta, produces thousands of eggs each reproductive period. Zygotes develop into freeswimming planktonic veliger larvae, which can be scattered far from the mother’s habitat by oceanic currents. They form part of the plankton and experience high mortality from numerous animals that feed on plankton. Furthermore, the larvae require a specific, sandy-bottomed substrate on which to settle and metamorphose into an adult snail. The probability of a larva surviving long enough to find a suitable habitat is very low, and most of the cohort dies during the veliger stage. We therefore see a rapid drop in survivorship in the first part of the curve. The few larvae that do survive to become snails have improved odds of surviving further, as reflected by the more gentle slope of the curve for older snails. Thus, high reproductive output balances high juvenile mortality in such animals.
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| Figure 40-3 Age structure profiles of the human populations of Afghanistan and Belgium in 1995 contrast the rapidly growing, youthful population of Afghanistan with the stable population of elgium, where the fertility rate is below replacement. Countries such as Afghanistan with a large fraction of the population as children are strained to provide adequate child services. With so many children soon to enter their reproductive years, the population will continue to grow rapidly for many years to come. |
Many animals survive to reproduce only once before they die, as seen in many insect species of the temperate zone. Here, adults reproduce before the onset of winter and die, leaving only their eggs to overwinter and repopulate the habitat the following spring. Similarly, Pacific salmon after several years return from the ocean to fresh water to spawn only once, after which all adults of a cohort die. However, other animals survive long enough to produce multiple cohorts of offspring that may mature and reproduce while their parents are still alive and reproductively active. Populations of animals containing multiple cohorts, such as robins, box turtles, and humans, exhibit age structure. Analysis of age structure reveals whether the population is actively growing, stable, or declining. Figure 40-3 shows age profiles of two idealized populations. On a global scale, humans exhibit an age structure similar to curve a in Figure 40-2, although age structures vary among regions.
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