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MECHANISMS OF EVOLUTION
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After studying Malthus, Darwin realized that if more individuals are produced than can survive, members of a population must compete to obtain food, living space, and other limited necessities Described as the “struggle for existence” TARGET #3- I CAN EXPLAIN HOW VARIATION CONTRIBUTES TO AN ORGANISM’S SURVIVAL
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Variation plays a vital role in the struggle for existence Individuals have natural variations among their heritable traits Darwin hypothesized that some of these variations are better suited to life in their environment than others Ex: a lion with longer claws and higher endurance is a better hunter than a lion without those traits
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Adaptation: any heritable characteristic that increases an organism’s ability to survive and reproduce in its environment. Includes Body parts Claws Wings Coloration Bright colors Camouflage Mimicry Physiological functions Photosynthesis Brain capacity TARGET #4- I CAN DESCRIBE AN ADAPTATION
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Darwin recognized that there must be a connection between the way an organism “makes a living” and the environment. Differences in adaptations affect an organism’s fitness Fitness: how well an organism can survive and reproduce in relation to others of the same species within it’s environment Individuals with adaptations that are well suited to their environment can survive and reproduce more offspring Individuals with characteristics that are not well suited to their environment either die without reproducing or leave few offspring The difference in rates of survival and reproduction is called “survival of the fittest” TARGET #5- I CAN EXPLAIN WHY ORGANISMS WITH GREATER FITNESS GENERALLY LEAVE MORE OFFSPRING THAN ORGANISMS THAT ARE LESS FIT
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Natural Selection The process by which organisms with variations most suited to their local environment survive and leave more offspring The environment influences fitness
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Natural selection occurs in any situation in which more individuals are born than can survive, there is a natural heritable variation, and there is variable fitness among individuals Does not make the organism “better” Does not move in a fixed direction A process that enables species to survive and reproduce in a local environment If local conditions change, some traits may become adaptive If conditions change faster than a species can adapt, the species may become extinct TARGET #6- I CAN SUMMARIZE HOW NATURAL SELECTION ALLOWS ORGANISMS WHICH ARE MORE ADAPTED SURVIVE IN THEIR ENVIRONMENT
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Adaptations and variations among individuals in populations of species does not begin in the environment Step 1: a mutation occurs within the DNA of an organism Step 2: that mutation leads to the production of a protein that either does not function, over functions, or has a different function than originally intended Step 3: the mutation is passed down to an offspring where it is expressed in the offspring’s phenotype TARGET #7- I CAN EXPLAIN HOW A POPULATION OF ORGANISMS IN A SPECIES UNDERGOES NATURAL SELECTION
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Step 4: the mutated phenotype is naturally selected by the environment as being more advantageous, and that individual has more offspring than the non-mutated individuals Step 5: the mutation is passed down, generation to generation, until the population of organisms in that species retain that mutation **Step 6: if enough mutations collect within a population of individuals in a given species, then a new species is formed
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Within the species black bear (Ursus americanus), there is a subspecies (Ursus americanus kermodei) There are about 400 individuals in this subspecies About 10% of this subspecies is homozygous recessive for a mutation that turns their coats whitish Caused by a mutation in a pigment gene, known as MC1R (Melanocortin receptor) MC1R is involved with the synthesis of the pigment eumelanin A mutation in this gene in humans results in red hair) Evolutionary Advantage Better at catching fish during the day than their dark bear counterparts Are considered sacred by the local people, the Tsimshian, so they are hunted less EXAMPLE- THE SPIRIT BEAR
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Natural selection depends on the ability of organisms to reproduce, or to leave descendants Lineage can be traced back over many generations Over many generations, adaptation could successfully lead to the evolution of new species Darwin proposed that living species are descended with modification from common ancestors Known as descent with modification TARGET #8- I CAN EXPLAIN WHAT DARWIN’S MECHANISM FOR EVOLUTION SUGGESTS ABOUT LIVING AND EXTINCT SPECIES
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Means that life as been on earth for a long time Basis for the explanation of the diversity of life According to the principle of common descent, all species, living and extinct, are descended from ancient common ancestors
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