Natural and Artificial Selection

(2 mechanisms of evolution)

Natural selection = survival of the fittest• When the characteristics of a population of organisms adapt over many

generations it’s called natural selection

• Individuals with favourable characteristics survive specific environmental conditions pass on their alleles to offspring

• There must be variationwithin a species for natural selection to occur

Selective pressureSelective pressure – an environmental condition that ‘selects’

for certain characteristics in some individuals and against different characteristics in others

•Selective pressures can be abiotic…• eg. temperature, colour of surroundings,

water availability, etc.

•…or biotic• eg. predators, parasites,

resource competition, etc.

Selective pressureNatural selection…• …doesn’t anticipate environmental change • …has no defined purpose/goal

•A trait that at one time/situation has no survival relevance may, at another time/situation, make all the difference in survival (or vice versa)

Sickle cell anemia

• Sickled RBCs• impaired ability to carry O2

• poor blood flow• shorter life span• protect against harmful malaria symptoms

• Healthy RBCs• carry O2 efficiently

• unimpaired blood flow• normal life span• do not protect against harmful malaria

symptoms

• How would selective pressure play a role?

Artificial selection

Artificial selection - selective pressures exerted by humans on populations in order to increase the frequency of desirable traits

• eg. dog and cat breeds

•Most of the foods we eat are the result of selective breeding• eg. grains, fruit, vegetables, meat, milk, etc.

Beef cattle

Wild mustard plant

• Wild mustard has been modified (not evolved!) over time by artificial selection to produce cauliflower, broccoli, cabbage, brussel sprouts and kale species

Corn & bananas

Artificial selection of food crops

Pros• Faster growth time• Increased nutritional value • Larger yields• Pest-resistant and drought

resistant

Cons• Faster grow times = inability

to tolerate poor soil conditions

• Decreased variation = inability to respond (adapt) to environmental change

• Selective breeding MUST be balanced and maintain genetic variation within the crop so that future adaptation and may take place

MonoculturesMonoculture – practice of growing a single plant species over a

wide area for many years

• Results in a crop (population) that share all the same weaknesses and has little potential for future adaptation• eg. Like living in a city of clones

Gene Banks• Contain seeds of every plant species

currently known (especially crop seeds)

• The genotypes contained within these seeds may enable us to help species survive in present day conditions

Try this…

• Selective breeding activity