continued . . .
Surviving creatures of the most recent period of ecological stability, before the coming of Man's wholesale destruction, represented the fittest creatures of all time! A study of the diversity of marine life over the last 600 million years (Precambrian to present) shows that there has been a rise in diversity from just more than 50 families 600 million years ago to more than 900 families today! Several extinction events have interrupted this, always followed by an increase in diversity (Benton, 1993) (p107 of Gould). The tiger has been part of an evolving lineage through all time, functioning and interacting within an ecosystem. It performs a specific function or has a particular influence to which affected or associated animals have had to adapt. In this way it influences the prevailing structure of the whole system. The general or gross characteristics (physical, physiological and behavioural) of extant (existing) lineages must thus show what can survive this intense interactive process. When ecologists, such as Gleason, reject the idea of communities of organisms and see only shifting associations of individual organisms, they are partly correct. Associations are contingent upon the physical environment. Patterns of immigration from nearby influence them. Particular conditions such as these do influence the association and are not controlled by the community in an organismic fashion (Taylor, 1992). However the patterns observed are the result of millions of years of evolution that have defined the prevailing associations. Put an African lion population in the polar regions and the polar bear in Africa and neither will have much chance of survival. Their forms have an ecological and environmental context.
The rabbit introduced into Australia has caused ecological devastation. Many examples of such new associations exist of species that have not had time to evolve a semblance of adaptation to or coevolution with the new habitat and reflect the dynamic aspect of nature. We see the impact at the moment, while the evolved outcome could take millions of years. Evolution has by its very nature a direction; the process of interaction between species, with survival as the innate drive or impulse of each, must lead to survival or extinction. Flowers and bees are locked into an association of mutual benefit. A lion must not deplete its food resource. When the ancestor of humanity first used a tool while associates did not, the new course was set for our evolution. Gorillas do not in general use tools and are largely herbivorous, yet we have the same ancestor.
"Co-evolution occurs whenever the environment in which
creatures evolve is itself evolving. From an antelope's point of view,
part of the environment - like the weather - with the important
that lions evolve" (Dawkins, 1993). (Econ. p90 Sept.). One may say that
weather represents an inert dynamism and an associated species an
dynamism. The evolving dynamism
of living creatures has led to
the idea of progress in nature and evolution so vehemently opposed by
ecologists. An organism interacts not only with other species
but with its own kind (intraspecific). Such interactions evolve into social
systems that may be just as important to survival as
interspecific behavioural strategies.
Innovations, unique biological events, lead to evolutionary advances. The origin of a cell nucleus and then multicellular life forms in the early Cambrian period was a major evolutionary advance. Early unicellular life forms probably occupied a diversity of habitats in a stable abiotic environment with minimal biotic interactions. The evolution of multicellular organisation in this era was such a radically adaptive advantage that it led to the Cambrian "explosion" of life forms. Suddenly, new interactions, greater mobility, cellular specialisation, dominance through size, greater diversity in form and defence, and the ability to predate on smaller forms was possible. With increasing biotic diversity and complexity followed increasing biotic interactions as adaptable species occupied available niches. This Cambrian era marks the beginning of the intensification of biotic interactions and the diversification of species. Since then, animals have had to evolve strategies to counter the interactive effects of associated organisms. Coevolution has been taking place intensely since the Cambrian era.
(As a note, when I say "available niches," there are not defined niches "out there" waiting to be occupied. A niche is defined by the organism that occupies it. We can imagine possible scenarios, but they are potential niches determined only by rational thought. A niche is not predetermined, but only exists with the presence of the organism. Dinosaur niches once existed, but associations are now different and dinosaurs extinct, along with their niches.)
More recently, it was the evolutionary line leading to humanity that developed a unique adaptation. In our human lineage, it was upright walking and tools use. "Very soon new species arise from this founding member, and a cluster of new descendant species appears through time" (Leakey & Lewin, 1992). Adaptive radiation is the spread of species of common ancestry into different niches (Wilson, 1992) (where they often evolve into new species) (see books). Many forms of upright walker are found in the fossil records of this early "experiment", but for some reason we are the only survivors of this lineage.
A common objection to evolution is "bacteria and yeast are (the)
oldest known surviving organisms yet they do not show any sign of
evolution" (Nadvi, 1986). However if a niche remains unchanged
throughout ages, the species occupying such a niche need not evolve.
Adaptation to the niche keeps
the bacteria as the occupier of that niche. Within the bacteria family
there is massive diversity. Also, highly specialised creatures have a
limited number of possible evolutionary pathways and are largely locked
into their role. Rates of evolution
are not constant. Evolution is not determined by time, but by intensity
of the selective pressure and the capacity to evolve. Confirmation of
this is found in nature. The larvae of sea urchins, even of closely
related species may have radically different forms, suited to different
forms and periods of dispersal by ocean currents. Some feed on plankton
and spend a long time swimming free in the ocean, others have a short,
nonfeeding planktonic phase. Adult sea urchins are largely
indistinguishable. This suggests that there is not some developmental
constraint upon the adult form making it incapable of changing, but
that the structure works well (Levington, 1992). Selective pressure has
led to a diversity of ecological specialisations in the larval forms as
a solution to dispersal. The larval phase is subject to a wide range
of variable conditions, so one larval form will be successful
occasionally and another several times and so they coexist. Different,
but more uniform selective pressures act upon the adults, so natural
selection strongly defines their form as the niche requirement and so
they look very similar. Similarly, bacteria occupy a range of niches
that were present at the origin of life and persist today.
Observe any wild creature in nature and you will see it obeying the rules necessary for its survival and perpetuation. Their form and behaviour are suited to a particular role and habitat in nature. An otter, for example, is adapted to its river habitat and natural selection will continually mould it to this mode of life. It will remain an "otter" while its habitat persists. On its specialised evolutionary branch there is no easy return route. Possible modes of life are excluded from it by its present form and the presence of specialised occupants of other niches. It cannot compete with terrestrial predators for food and often has predators on land that find it easy prey, so it tends to remain near water.
Once many species are found together, biotic interactions became
inevitable and important, such as in the otter. The principle of the
survival of the fittest is that the best of the species naturally
survive. Life forms, subjected to abiotic and biotic influences (the
latter including interspecific and intraspecific interactions) die in
proportion to their maladaptation to their habitat. Those that survive
the vagaries of life to transmit their genes to
future generations are the best adapted to their particular niche and
the most fit for that particular habitat at that time. A consequence of
is that an animal becomes adapted to a habitat that consists of
and abiotic influences. There are "accidents" that have no selective
but if these are too frequent, the species may become extinct. In
general, habitats an larger ecosystems are persistent over very lon
time periods, so the adaptations have an ecological context made up of
the biotic and abiotic environment.
Many examples of adaptive strategies are found in nature. A good example of adaptation to biotic influences is the mimicry of the noxious monarch butterfly (Danaus plexippus) by the viceroy (Limenititis archippus ) butterfly. Blue jays vomit after eating a toxic monarch butterfly and then avoid both monarchs and viceroys. Insects that look like sticks, leaves or dirt, such as the leaf-like grasshopper (Arantia ), use camouflage to escape from predators.
An indigenous form of malaria in parts of Africa has led to the increased frequency of the mutated haemoglobin in some African communities, providing immunity to this parasite. Sickle cell anaemia in human beings originates in about one in 100,000 people each generation. In areas with the malarial parasite, Plasmodium falciparum, which invades and consumes red blood cells, between five and 20 percent of the population can have the sickle cell condition. It is the inherited result of the replacement of one amino acid (glutamic acid) by another (valine) in two positions on the haemoglobin molecule. 574 amino acids make up the haemoglobin molecule. Haemoglobin is found in the red blood cells and carries oxygen from the lungs to the tissues via the blood. When these mutated molecules release oxygen, they change their shape and realign into long spindles. People who inherit this condition suffer from anaemia. Where a person is homozygous (both genes on the chromosome carry this trait), this genetic condition may lead to death from hereditary anaemia. More than one third of the cells are then found in the sickle form. When only one gene of a chromosome pair carries the mutant gene (heterozygous condition), only 1 percent of cells are in the sickle form, and anaemia is at most mild.
Sickle-cell anaemia also provides protection from malignant malaria, a parasite-induced anaemia. If too many people in the population have the sickle-cell condition, the occurrence of the potentially lethal homozygous condition increases in their progeny. Two parents with the homozygous sickle cell condition have all their genes in the mutant form and can only give birth to children with both their gene pairs with the mutant gene and severe hereditary anaemia. Parents heterozygous for this condition can give birth to a child without sickle cell anaemia, or with it. Where malaria is killing people without sickle-cell anaemia, people with the heterozygous condition survive and reproduce and so the incidence of sickle cell anaemia increases. A balance in the population occurs with malaria killing people without sickle-cell anaemia and people dying from hereditary anaemia (Wilson, 1992).
Distributions of Falciparum malaria, especially in Africa, show that sickle-cell anaemia is an adaptive strategy. This malarial parasite is also found from Europe, through India to Asia and northern Australia, but a different adaptation matches this distribution. It is a gene for B-thalassemia. Another adaptation called g-6-pd deficiency matches the malarial parasite's distribution in various parts of the world and confers some resistance against this disease. In each case, natural selection maintains the polymorphic state, as those without the genes have a higher chance of dying from malaria and those homozygous for the gene die of genetic diseases (Strickberger, 1985). Malaria must have caused much misery for a long part of our evolutionary history.
Camels have adapted to two abiotic factors in their habitat, high day temperatures and a shortage of water. Its body temperature rises as high as 40 degrees Celsius during the heat of the day and falls down to 34 degrees Celsius during the night (McFarland, 1993). A camel unable to do this would die before it reached a reproductive age. Normally, mammals regulate their body temperatures within a much narrower temperature range through evaporative heat loss, but this requires that the animal has enough water.
An extreme example of evolution, based on gene action is found in the social system of ants. Ants illustrate allometry, the differential growth rate of body parts. One part of the body grows faster than another, producing a variety of body forms using the same basic morphological structures. The cast system is based upon a single genetic code as an ant colony is a single genetic unit with all the individuals produced by a single queen. Ant forms vary physically from queens to big-headed soldiers to small-headed workers. What form develops depends on the food and chemical stimuli received as larvae (Wilson, 1992). Different ant forms have evolved as a response to the environment through natural selection and so are genetically based, yet each ant is genetically identical to the other. Individual ants are the units that interact with the environment, and succeed or die based on the fitness of their adaptations. Understanding how natural selection operates here is difficult. Each ant is genetically identical (identical genotypes), all originating from the same queen, but many ant casts that differ physically (different phenotypes) make up the colony. As the colony functions as a reproductive unit with a single queen, natural selection "tests" the colony, while the ant casts are the agents of the colony. Just as our hands serve as tools for our whole body, so the worker ants serve the whole colony. If, through a genetic change, a queen produces workers with unusually small jaws so that the workers are unable to collect the usual food, then the whole colony fails. Genetic changes or variations originate with the queen. Mutations that improve the performance of one ant form, improves the survival potential of the whole colony. They pass this adaptation onto the next generation through the survival of the whole colony. The amazing variety of body structures in an ant colony is the best evidence that evolution does not take place through the evolution of acquired characteristics, because environmental influences upon individual ants cannot be transmitted back to the queen. Only the testing of the whole colony through natural selection can determine the success of the queen's genes via her progeny. This is proof that natural selection must be operating to weed out unsuitable variation.
Foraging bumblebees (Bombus sp.) have to cope with a combination of factors within their habitat. Climatic changes, access to and location of flowers, nectar abundance, and available time requires behavioural solutions. When flowers are abundant, bumblebees expend energy to maintain a high thoracic temperature, allowing activity in cold climates. When circumstances require, they conserve energy by reducing their activity level and conserving heat. If flowers are scarce, they forage slowly, using less energy and achieving a net energy gain. A bumblebee optimises its time budget to maximise the time foraging, so spends only two to four minutes inside the nest between foraging trips (McFarland, 1993). Bumblebees need to thermoregulate below 25 degrees Celsius, so to offset the energy required at lower temperatures, they concentrate on flowers with more nectar as temperatures fall. In warm weather bumblebees visit lambkill and wild cherry, while in cooler weather they prefer the Rhododendron. They also devote different amounts of time to different flower species. To maximise foraging efficiency the bumblebee has to budget its time and energy while interacting with its habitat. This behaviour of the bumblebee enables it to coexist with the honey bee. It can work faster, for longer hours and in worse weather than honey bees (Lampkin, 1992). A bumble bee will attend to flowers a bee ignores or cannot use. Long-tongued bumble bees are more effective at pollinating specific crops such as field beans.
If two species occupy the same niche, ecologists say they are competing for the same resources. Competition for a resource such as food is a biotic selective pressure. Many factors interact, leading to the dominance of one species. A simple example would be two species of algae growing in a test tube of water with nutrients and that is regularly diluted with fresh nutrient medium. When alone in the test tube, they survive, but when combined, the species with the fastest growth rate under the prevailing conditions becomes dominant, utilising available resources. The slower growing species forms less and less of the test tube community until it disappears during the dilution and competitive process. By changing a single parameter such as light intensity before the slow growing species disappears, it may gain dominance if it is better able to use the available light and grows faster under the new environmental conditions. If the two species persist in stable coexistence, their niches are in some way separated. If dilution is continued while there is no light for growth, the niches become unsuitable and both species disappear. It is the interaction of two species or many species with each other and their environment that defines the association.
During the slow colonisation of a new island, the new arrivals alter conditions, biotic and abiotic, allowing new associations. To exclude a species, the other species must actively use some niche component. A species may also be excluded if a niche component is not present or is not accessible. Biotic selective pressure is a more objective term than the anthropomorphic "competition" when discussing interactions. Organisms interact with their living (biotic) and nonliving (abiotic) environment. As a simple example, if a seed germinates under the shade of a cliff, we say this is an abiotic selective pressure to which the young plant is subject. It may be adapted to this type of habitat and thrive. If the same seed germinates amongst other species of plant that shade it, we say that the plant is "competing" with the other plants for light. Why? This is not competition, but merely a biotic selective pressure to which the plant is subject.
Mobility allows behavioural responses to both abiotic and biotic influences, so an animal may seek shade during the heat of the day or flee from a predator. Physical adaptations are possible to both biotic and abiotic influences; thus the myriad of amazing camouflages to be found in nature or the efficient kidney of some desert animals allaying the need to drink water (The latter physical expression may be categorised as physiological ). The physical environment usually provides a constant though perhaps cyclical force, such as from seasonal changes in temperature. Migration, as by birds, is one way to escape climatic changes. A similar predictable condition develops in response to some biotic influences, so predatory pressure maintains the colouration of some animals. They more easily spot and catch the more visible prey. Gradations of this response to predatory pressure can range from this to physical-behavioural such as where the potential prey releases a bad odour to fully behavioural such as flight into an inaccessible hole.
Another form of biotic interaction occurs in nature, defined in terms of perpetuity and compatibility. The above described adaptations enhance the survival and perpetuity of the species. To survive, the animal has to perpetuate its kind through reproduction. Individual survival is the first requirement, reproductive success of individuals and its offspring, the second. Traditional Darwinian evolution defines the creature that passes its genes onto the next generation in greater numbers as the fittest. Genes are however, not the actual replicators as some geneticists such as Dawkins claim, as reproduction requires mating and mating requires individual wholes.
Traditional fitness is represented by the greatest number of progeny (and therefore genes) in the next generation. A creature maximises its genes in the next generation and so its perpetuation through the reproductive success of its progeny. Dawkins (1983) has defined many forms of fitness, but the above suffices for this book. Some critics of fitness and natural selection, accuse evolutionists of tautology. As the "fittest" are those that "survive", they claim that there is no empirical meaning to the term "survival of the fittest", just as to say "swift lizards run quickly" is meaningless (Beck, Liem & Simpson, 1991). This is countered by the evidence of good design rather than survivorship as an indication of fitness. Mammalian evolution shows improved design and adaptation to different habitats. This progressive improvement in design is not teleological, but empirical. Random drift from an original mouse-like ancestor could not lead to the "lock and key fits" to niches such as the anteater's claws or the giraffes neck.
If one picks out a single trait or feature, such as eyesight, to define fitness, it is still the contribution to reproductive success that is all important. A feature or trait has its origin as a genotype at a single locus and its "success" will lead to its increased representation and a change in gene frequencies. There are some more technical expressions of fitness that consider the promotion of one's relative's genes into the next generation on the rationale that these are shared genes (Dawkins, 1983). I will show that we find something slightly different and may call it eco-fitness (ecofitness) . An organism's perpetuity requires the habitat to which it is adapted. Maximising numbers in the next generation can quickly degrade the environment through pollution, resource depletion and denudation, so is a short term remedy - perpetuity without the necessary compatibility.
A highly specialised species may become extinct if its habitat changes. Perhaps all extinctions occur due to changes to an organism's habitat. If extinction or rapid change affects an ecosystem, the species dependent on that ecosystem may become extinct. A real-life extinction example is the blue butterfly, Maculinea arion. Ecologists declared it extinct in Britain in September 1979 (Allaby, 1986). Larvae of this insect fed on thyme and were camouflaged to look like the flowers of this plant. After three moults this creature left the thyme to the grass where ants of a particular species would take them to their nests. When stroked by the antennae of the ant, the caterpillar would secrete a liquid upon which the ants fed. In return for this liquid, the caterpillar received shelter through the winter and the ants would even feed it their own larvae! The animal would only pupate in spring and leave the ant nest as an adult in midsummer. The thyme needed sunlight, so would not grow in the shade of long grass. However, sheep and rabbits grazed grasslands intensively. This must have been the situation for the evolutionary history of this butterfly during its association with thyme. With changes in sheep farming, resulting in a reduction of sheep and the onset of myxomatosis (a disease that depleted the rabbit population), grasslands became taller, the shaded thyme became scarce (but not extinct) and the large blue butterfly became extinct!
Ecologists call the form of population regulation about to be discussed density-dependent regulation. Smith (1974) describes it concisely: "Among many organisms the mechanisms for the regulation of populations within the limits imposed by the environment are largely density dependent. Through such mechanisms organisms avoid the hazard of overexploiting their environment and even cause their own extinction. It appears that the regulation of populations is a homeostatic process." Wynne-Edwards (1986) recognises that an "equilibrium has been reached in the majority of well established populations" and maintains that this "shows that there are stabilising processes at work, though there is no obvious indication of what the controlling forces are." As with Smith above, he admits that the controlling forces must be density-dependent in their action. To Darwin he ascribed the recognition of some of these checks, namely, amounts of food, or for plants, nutrients, serving as food for others as in predation, climate, and disease or epidemics. Of these, climate is a density independent factor. Wynne-Edwards proposed an alternate hypothesis : that animals are preserving the balance between population and resources through their behaviour.
Individually, the more genes passed on to the next generation, the fitter is the individual compared with the whole population. Changes in the nucleotide sequence that make up the code of the gene, result in behavioural, physical or physiological changes in the animal (Wilson, 1992). Technicians cannot easily demarcate gene units on a chromosome, but geneticists accept that each gene is composed of several thousand nucleotide pairs. Three nucleotide pairs in a row on a pair of chromosomes codes for a single amino acid. At least five genes control skin colour in humans, while the ear's structure and skin texture involves at least 100 genes of the 100,000 genes found in humans. They reflect changes at the level of the gene (genotypic changes within the nucleus) of the phenotype - behaviour, physiology and anatomy. These may have an effect upon survival and reproduction. Our ancestor's first manipulation of a tool, which led to its survival while its companions died, would have had a genetic basis. This beneficial effect of new behaviour confers a higher rate of survival and reproduction and so spreads through the population. Natural selection is taking place, reflected as a change in the gene frequencies within the population.
It is a simple matter of numbers, that the more genes the individual passes onto the next generation, the fitter the individual is. This even applies to humans: "In short, we evolved, like other animals, to win the reproduction game. The contest has a single aim, to leave as many descendants as possible. Much of the legacy of that game strategy is still with us" (Diamond, 1992).
However, in nature we see behaviour that limits the reproductive
potential of an individual, while still enabling the selection of the
the species through competition. Wynne-Edwards (1986) expresses this
"Because of the fact that life is vested in individuals and its
is hereditary, it is beneficial for individuals to be tested by
and the less fit phenotypes eliminated from their ranks; and it is
true that the fitness value of an individual can be measured by the
it makes to the viability of its group, and of the higher organisations
of which the group is a part." What he is saying is that, in a
structured population, the penalty consumers pay when they allow the
local demand for food resources to exceed the latter's carrying
capacity (compatibility) continuously opposes the inherent tendency
for selection to maximise individual fecundity (perpetuity). There is a
feedback response from the abiotic environment to the presence of each
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