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Life Science: GED Test Prep (page 3)

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Updated on Jul 5, 2011

Biological Evolution

Mutations cause change over time. The result of a series of such changes is evolution, or as Darwin put it, "descent with modification." The great diversity on our planet is the result of more than 3.5 billion years of evolution. The theory of evolution argues that all species on Earth originated from common ancestors.

Evidence for Evolution

Several factors have led scientists to accept the theory of evolution. The main factors are described here.

  • Fossil record. One of the most convincing forms of evidence is the fossil record. Fossils are the remains of past life. Fossils are often located in sedimentary rocks, which form during compression of settling mud, debris, and sand. The order of layers of sedimentary rock is consistent with the proposed sequence in which life on Earth evolved. The simplest organisms are located at the bottom layer, while top layers contain increasingly complex and modern organisms, a pattern that suggests evolution. The process of carbon dating has been used to confirm how old the fossils are and that fossils found in the lower layers of sedimentary rock are indeed older than the ones found in the higher layers. This helps scientists to chart evolutionary history based on time. And new fossils are turning up all the time; for example, the fossil called Tiktaalik, which was found in 2006, is believed to mark the transition from fish to land animals.
  • Biogeography. Another form of evidence comes from the fact that species tend to resemble neighboring species in different habitats more than they resemble species that are in similar habitats but far away.
  • Comparative anatomy. Comparative anatomy provides us with another line of evidence. It refers to the fact that the limb bones of different species, for example, are similar. Species that closely resemble one another are considered to be more closely related than species that do not resemble one another. For example, a horse and a donkey are considered to be more closely related than are a horse and a frog. Biological classifications (kingdom, phylum, class, order, family, genus, and species) are based on how organisms are related. Organisms are classified into a hierarchy of groups and subgroups based on similarities that reflect their evolutionary relationships. The same underlying anatomical structures of groups of bones, nerves, muscles, and organs are found in all animals, even when the function of these underlying structures differ.
  • Embryology. Embryology provides another form of evidence for evolution. Embryos go through the developmental stages of their ancestors to some degree. The early embryos of fish, amphibians, reptiles, birds, and mammals all have common features, such as tails.
  • Comparative molecular biology. Comparative molecular biology confirms the lines of descent suggested by comparative anatomy and fossil record. The relatedness of two different species can be found by comparing their DNA.

Darwin also proposed that evolution occurs gradually, through mutations and natural selection. He argued that some genes or combinations of genes give an individual a survival or reproductive advantage, increasing the chance that these useful combinations of genes will make it to future generations. Whether a given trait is advantageous depends on the environment of the organism. We can witness the changes in populations of living organisms: antibiotic-resistant bacteria, super lice that are resistant to chemical treatments, and the increased frequency of dark-colored moths versus the lightly colored variety (Biston betularia) after the industrial revolution in Britain. These are all examples of evidence of natural selection.

Natural selection is only one of several mechanisms by which gene frequency in a population changes. Other factors include mating patterns and breeding between populations.

Interdependence of Organisms

The species in communities interact in many ways. They compete for space and resources, they can be related as predator and prey, or as host and parasite.

Plants and other photosynthetic organisms harness and convert solar energy and supply the rest of the food chain. Herbivores (plant eaters) obtain energy directly from plants. Carnivores are meat eaters and obtain energy by eating other animals. Omnivores eat both meat and plants. Decomposers feed on dead organisms. The flow of energy can then be represented as follows:

    Sun → Photosynthetic organisms → Herbivores → Carnivores or Omnivores → Decomposers

The food chain is not the only example of the interdependence of organisms. Species often have to compete for food and space, so that the increase in population of one can cause the decrease in population of the other.

Organisms also may have a symbiotic relationship (live in close association), which could be classified as parasitism, mutualism, or commensalism. In a parasitic relationship, one organism benefits at the expense of the other. Commensalism is symbiosis in which one organism benefits and the other is neither harmed nor rewarded. In mutualism, both organisms benefit.

Under ideal conditions, with ample food and space and no predators, all living organisms have the capacity to reproduce infinitely. However, resources are limited, limiting the population of a species.

Humans probably come closest to being a species with seemingly infinite reproductive capacity. Our population keeps increasing. Our only danger seems to come from viruses and bacteria, which at this point we more or less have under control. When we need more food, we grow more, and when we need more space, we clear some by killing off other biomes. By doing this, humans modify ecosystems and destroy habitats through direct harvesting, pollution, atmospheric changes, and other factors. This attitude is threatening current global stability, and has the potential to cause irreparable damage.

Behavior of Organisms

Even the most primitive unicellular organisms can act to maintain homeostasis. More complex organisms have nervous systems. The simplest organism found to have learning capability is a worm, suggesting a more complex nervous system. The function of the nervous system is the collection and interpretation of sensory signals, messages from the center of the nervous system (brain in humans) to other parts of the body. The nervous system is made of nerve cells, or neurons, which conduct signals, in the form of electrical impulses. Nerve cells communicate by secreting excitatory or inhibitory molecules called neurotransmitters. Many legal and illegal drugs act on the brain by disrupting the secretion or absorption of neurotransmitters, or by initiating a response by activating the receptors that neurotransmitters would normally bind to. Behavior can also be affected by hormonal control. Hormones are produced in one part of the body and are transported by the circulatory system to another part of the body where they ultimately act.

Many animals have sense organs that enable them to detect light, sound, and specific chemicals. These organs provide the animals with information about the outside world. Animals engage in innate and learned social behavior. These behaviors include hunting or searching for food, nesting, migrating, playing, taking care of their young, fighting for mates, and fighting for territory.

Plants also respond to stimuli. They turn toward the Sun and let their roots run deeper when they need water.

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