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Facebook Instagram Twitter. Using the Understanding Science flowchart to illustrate and bring students' science stories to life.
Bulletin of the Ecological Society of America. Read it Graves Jr, J.
Reiber, A. Thanukos , M. Hurtado, and T. Evolutionary science as a method to facilitate higher level thinking and reasoning in medical training.
Read it Price, R. Andrews, T. McElhinny, L. Mead, J. Abraham, A. Such natural historians would collect, catalogue, describe and study the vast collections of specimens stored and managed by curators at these museums. Darwin served as a ship's naturalist on board HMS Beagle , assigned to a five-year research expedition around the world.
Darwin gained extensive experience as he collected and studied the natural history of life forms from distant places. Through his studies, he formulated the idea that each species had developed from ancestors with similar features. In , he described how a process he called natural selection would make this happen. The size of a population depends on how much and how many resources are able to support it. For the population to remain the same size year after year, there must be an equilibrium, or balance between the population size and available resources.
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Since organisms produce more offspring than their environment can support, not all individuals can survive out of each generation. There must be a competitive struggle for resources that aid in survival. As a result, Darwin realised that it was not chance alone that determined survival. Instead, survival of an organism depends on the differences of each individual organism, or "traits," that aid or hinder survival and reproduction.
Well-adapted individuals are likely to leave more offspring than their less well-adapted competitors.
Traits that hinder survival and reproduction would disappear over generations. Traits that help an organism survive and reproduce would accumulate over generations. Darwin realised that the unequal ability of individuals to survive and reproduce could cause gradual changes in the population and used the term natural selection to describe this process.
Observations of variations in animals and plants formed the basis of the theory of natural selection. For example, Darwin observed that orchids and insects have a close relationship that allows the pollination of the plants. He noted that orchids have a variety of structures that attract insects, so that pollen from the flowers gets stuck to the insects' bodies. In this way, insects transport the pollen from a male to a female orchid.
In spite of the elaborate appearance of orchids, these specialised parts are made from the same basic structures that make up other flowers. In his book, Fertilisation of Orchids , Darwin proposed that the orchid flowers were adapted from pre-existing parts, through natural selection. Darwin was still researching and experimenting with his ideas on natural selection when he received a letter from Alfred Russel Wallace describing a theory very similar to his own.
This led to an immediate joint publication of both theories. Both Wallace and Darwin saw the history of life like a family tree , with each fork in the tree's limbs being a common ancestor. The tips of the limbs represented modern species and the branches represented the common ancestors that are shared amongst many different species. To explain these relationships, Darwin said that all living things were related, and this meant that all life must be descended from a few forms, or even from a single common ancestor.
He called this process descent with modification. Darwin published his theory of evolution by natural selection in On the Origin of Species in The implication that all life on Earth has a common ancestor has met with objections from some religious groups. Their objections are in contrast to the level of support for the theory by more than 99 percent of those within the scientific community today. Natural selection is commonly equated with survival of the fittest , but this expression originated in Herbert Spencer 's Principles of Biology in , five years after Charles Darwin published his original works.
Survival of the fittest describes the process of natural selection incorrectly, because natural selection is not only about survival and it is not always the fittest that survives. Darwin's theory of natural selection laid the groundwork for modern evolutionary theory, and his experiments and observations showed that the organisms in populations varied from each other, that some of these variations were inherited, and that these differences could be acted on by natural selection. However, he could not explain the source of these variations.
Like many of his predecessors, Darwin mistakenly thought that heritable traits were a product of use and disuse, and that features acquired during an organism's lifetime could be passed on to its offspring.
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He looked for examples, such as large ground feeding birds getting stronger legs through exercise, and weaker wings from not flying until, like the ostrich , they could not fly at all. In the late 19th century this theory became known as Lamarckism. Darwin produced an unsuccessful theory he called pangenesis to try to explain how acquired characteristics could be inherited. In the s August Weismann 's experiments indicated that changes from use and disuse could not be inherited, and Lamarckism gradually fell from favour.
The missing information needed to help explain how new features could pass from a parent to its offspring was provided by the pioneering genetics work of Gregor Mendel.
Mendel's experiments with several generations of pea plants demonstrated that inheritance works by separating and reshuffling hereditary information during the formation of sex cells and recombining that information during fertilisation. This is like mixing different hands of playing cards , with an organism getting a random mix of half of the cards from one parent, and half of the cards from the other. Mendel called the information factors ; however, they later became known as genes.
Genes are the basic units of heredity in living organisms.
They contain the information that directs the physical development and behaviour of organisms. Genes are made of DNA. DNA is a long molecule made up of individual molecules called nucleotides. Genetic information is encoded in the sequence of nucleotides, that make up the DNA, just as the sequence of the letters in words carries information on a page. The genes are like short instructions built up of the "letters" of the DNA alphabet. Put together, the entire set of these genes gives enough information to serve as an "instruction manual" of how to build and run an organism.
The instructions spelled out by this DNA alphabet can be changed, however, by mutations, and this may alter the instructions carried within the genes. Within the cell , the genes are carried in chromosomes , which are packages for carrying the DNA. It is the reshuffling of the chromosomes that results in unique combinations of genes in offspring. Since genes interact with one another during the development of an organism, novel combinations of genes produced by sexual reproduction can increase the genetic variability of the population even without new mutations.
This can introduce genes into a population that were not present before. Evolution is not a random process.
Although mutations in DNA are random, natural selection is not a process of chance: the environment determines the probability of reproductive success. Evolution is an inevitable result of imperfectly copying, self-replicating organisms reproducing over billions of years under the selective pressure of the environment. The outcome of evolution is not a perfectly designed organism. The end products of natural selection are organisms that are adapted to their present environments.
Natural selection does not involve progress towards an ultimate goal. Evolution does not strive for more advanced , more intelligent, or more sophisticated life forms. Rapid environmental changes typically cause extinctions. Genetic drift is a cause of allelic frequency change within populations of a species. Alleles are different variations of specific genes.