5.2 Natural Selection
Understanding: Natural selection can only occur if thereis variation amongst members of the same species
If all individuals of a population were identical, there would be no way ofsome individuals being favored over others
Natural selection depends on variation within a population
Understanding: mutation, meiosis & sexual reproductioncause 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 makean 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 (increasessurface area for a sufficient amount of nutrient absorption)
•Adaptations = process where a population becomes better suited to survive in itsenvironment over generational time
•develop by natural selection
•do not develop during the lifetime of one individual (individuals do not ADAPT or TRY toadapt to their environment
•Populations adapt, not individuals
…not to be confused with
•Acquired characteristics = traits that develop duringthe lifetime of one individual; cannot be inherited
•a no longer accepted explanation for evolution. It refers to offspring inheritingsome trait developed or modified during the lifetime of the parents (Lamarck)
… not to be confused with
Acclimation = process where an organism adjustsphysiologically to changing environmental conditions overa short period of time. No change in the genes of theindividual occur, but the expression of genes may bemodified.
e.g. humans adjusting to high altitude, cold temps, orhumid summers
e.g. adjusting fish to a new tank
Understanding: species tend to produce moreoffspring than the environment can support
Organisms vary in the number of offspring they produce
slow-breeding, e.g. southern ground hornbill – 0.3young/year
fast-breeding, e.g. bacteria double every 20 min; fungusproduces giant puffball with 7 trillion spores
Despite this variation, overall trend: more offspringproduces than environment can support
Leads to competition for resources and struggle forsurvival
Understanding: individuals that are better adaptedtend to survive & produce more offspring while lesswell adapted tend to die or produce fewer offspring
This is the definition of Natural Selection!
Example: in dry season, when only available leaves ontrees are up high, giraffes with longer necks are betteradapted than those with shorter necks, so they willsurvive and have a chance to reproduce offspring alsowith longer necks
Understanding: individuals that reproducepass on characteristics to their offspring
Much of variation between individuals is heritable (can bepassed from parents to offspring through genes)
Examples: skin color in humans and behavior such asdirection of migration & birdsong
Non-examples: acquired traits, such as dark skin obtainedthrough tanning
Understanding: natural selection increases the frequency ofcharacteristics that make individuals better adapted &decreases frequency of other characteristics leading tochanges within the species
This is the definition of evolution by natural selection!
Over generations, the characteristics of the POPULATIONgradually 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 cansupport.
Individuals that are better adapted tend to survive & producemore offspring while less well-adapted tend to die or produce lessoffspring.
Individuals that reproduce pass on favorable traits to theiroffspring.
Frequency of more favorable traits increase while less favorabletraits decrease, leading to changes within species.
Application: Changes in beaks of finches onDaphne 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 naturalphenomena: the theory of evolution by natural selection canexplain 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 antibioticresistance in bacteria
Rapid (a few decades)
Causes of antibiotic resistance:
Widespread over-use of antibiotics (treating diseases & in animalfeeds)
Bacteria reproduce rapidly (every 20 min)
Populations of bacteria are usually huge
Antibiotic resistance form either by MUTATION or one bacterialspecies PASSING ITS PLASMID containing antibiotic resistance geneon to another bacterial species
