5.2 Natural Selection

Understanding: Natural selection can only occur if there is variation amongst members of the same species  If all individuals of a population were identical, there would be no way of some individuals being favored over others  Natural selection depends on variation within a population

Understanding: mutation, meiosis & sexual reproduction cause variation between individuals in a species Variation can be caused by: CO of NS HC in PO of M RO of HC in MO of M RF of E by S M Which one(s) occur via mutation? Which one(s) occur via meiosis? Which one(s) occur via sexual reproduction? What is the only source of variation for asexual species?

Understanding: adaptations are characteristics that make an individual suited to its environment and way of life  Structure is related to function e.g. structure of bird’s beak is related to its diet e.g. structure of small intestine (thousands of villi) is related to its function (increases surface area for a sufficient amount of nutrient absorption) Adaptations = process where a population becomes better suited to survive in its environment over generational time develop by natural selection do not develop during the lifetime of one individual (individuals do not ADAPT or TRY to adapt to their environment Populations adapt, not individuals

…not to be confused with Acquired characteristics = traits that develop during the lifetime of one individual; cannot be inherited a no longer accepted explanation for evolution. It refers to offspring inheriting some trait developed or modified during the lifetime of the parents (Lamarck)

… not to be confused with  Acclimation = process where an organism adjusts physiologically to changing environmental conditions over a short period of time. No change in the genes of the individual occur, but the expression of genes may be modified.  e.g. humans adjusting to high altitude, cold temps, or humid summers  e.g. adjusting fish to a new tank

Understanding: species tend to produce more offspring than the environment can support  Organisms vary in the number of offspring they produce  slow-breeding, e.g. southern ground hornbill – 0.3 young/year  fast-breeding, e.g. bacteria double every 20 min; fungus produces giant puffball with 7 trillion spores  Despite this variation, overall trend: more offspring produces than environment can support  Leads to competition for resources and struggle for survival

Understanding: individuals that are better adapted tend to survive & produce more offspring while less well adapted tend to die or produce fewer offspring  This is the definition of Natural Selection !  Example: in dry season, when only available leaves on trees are up high, giraffes with longer necks are better adapted than those with shorter necks, so they will survive and have a chance to reproduce offspring also with longer necks

Understanding: individuals that reproduce pass on characteristics to their offspring  Much of variation between individuals is heritable (can be passed from parents to offspring through genes)  Examples: skin color in humans and behavior such as direction of migration & birdsong  Non-examples: acquired traits, such as dark skin obtained through tanning

Understanding: natural selection increases the frequency of characteristics that make individuals better adapted & decreases frequency of other characteristics leading to changes within the species  This is the definition of evolution by natural selection!  Over generations, the characteristics of the POPULATION gradually change  3 examples:  Evolution of melanistic moths in polluted areas  Evolution of Finch beaks on Galapagos Islands  Evolution of antibiotic-resistant bacteria

Summary of Natural Selection: (Study this slide!)  There is variation among members of a species.  Species tend to produce more offspring than the environment can support.  Individuals that are better adapted tend to survive & produce more offspring while less well-adapted tend to die or produce less offspring.  Individuals that reproduce pass on favorable traits to their offspring.  Frequency of more favorable traits increase while less favorable traits decrease, leading to changes within species.

Application: Changes in beaks of finches on Daphne Major  Darwin studied finches (“Darwin’s finches”) on Galapagos Islands in 1835  Published Origin of Species in 1859  Darwin observed 14 different species with different beak sizes & shapes and different diets  Darwin did not think one could observe evolution happening  Peter & Rosemary Grant studied Darwin’s finches in 1970s-1980s and observed evolution happening!  Studied Geospiza fortis on the island Daphne Major  Eats small and large seeds – small body and beak size  1977 drought = shortage of small seeds = many G fortis died  1982 severe El Nino rain = more small seeds = increase in G fortis  1983 dry & less small seeds  1987 G fortis had longer and narrower beaks than 1983 

Nature of Science: use theories to explain natural phenomena: the theory of evolution by natural selection can explain the development of antibiotic resistance in bacteria  These trends show evolution by natural selection taking place:  When new antibiotic used on patients, bacteria become resistant within a few years.  Antibiotic resistance spreads to more species of pathogenic bacteria.  In each species, proportion of infections caused by resistant strain increases.

Application: evolution of antibiotic resistance in bacteria  Rapid (a few decades)  Causes of antibiotic resistance:  Widespread over-use of antibiotics (treating diseases & in animal feeds)  Bacteria reproduce rapidly (every 20 min)  Populations of bacteria are usually huge  Antibiotic resistance form either by MUTATION or one bacterial species PASSING ITS PLASMID containing antibiotic resistance gene on to another bacterial species