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 variation within a species for natural selection to occur

Selective pressure Selective 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 pressure Natural 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 O 2 poor blood flow shorter life span protect against harmful malaria symptoms Healthy RBCs carry O 2 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

Monocultures Monoculture – 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