5.4 Natural Selection (by Jane Choi)


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5.4 Natural Selection (by Jane Choi)

  1. 1. Jane Choi B IB Biology HLNATURAL SELECTION
  2. 2. Natural Selection: the non-random process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers.Natural Selection= A key mechanism of Evolution
  3. 3. CHARLES DARWIN’S ORIGIN OF SPECIESSome kinds of organisms survive better in certain conditions than others do; such organisms leave more progeny and so become more common with time. The environment thus “selects” those organisms best adapted to present conditions. If environmental conditions change, organisms that happen to possess the most adaptive characteristics for those new conditions will come to predominate.
  4. 4. Observation Deduction Organisms vary- there are differences between individual organisms even if they are members of In the struggle for existence, the less well-the same species. These differences affect how well adapted individuals will tend to die and the suited or fitted an organism is to its environment better adapted will tend to survive. This is and mode of existence. This is called adaption. natural selection Some individuals are better adapted than others because they have the favorable variations. Because the better-adapted individuals survive, they can reproduce and pass on their characteristics to their offspring. The greater survival and reproductive success of Much of the variation between individuals can be these individuals leads to an increase in the passed on to offspring – it is inheritable. proportion of individuals in the population that have the favorable variations. Over the generations, the characteristics of the population gradually change- the evolution by natural selection.
  5. 5.  As a result of the natural selection, the genetic variation within a population of organisms may cause some individuals to survive and reproduce more successfully than others. Natural selection acts on the phenotype, but the genetic basis of a phenotype that gives a reproductive advantage will become more common in a population.
  6. 6. 5.4.7 EXPLAIN HOW NATURAL SELECTIONLEADS TO EVOLUTION Natural selection is a process (not a thing) which requires:  production of variation. (the random part)  the actual selection (non-random) When a population evolves there is a cumulative change in the heritable characteristics of the population. Natural selection can act on a population without speciation occurring. In effect the genetic profile of the population is adapting to changes in local conditions. Every phase in the process of evolution is affected by variation and by selection.