NATURAL SELECTION
STRUGGLE FOR EXISTENCE If more individuals are produced than can survive à members of a population must compete to obtain food, living space, and other limited necessities of life Called: Struggle for Existence Which individuals come out on top of struggle? Survival of the Fittest
VARIATION AND ADAPTATION Individuals are different from one another Variation! Some individuals are better at surviving in their environment than others Predatory species that are faster, longer claws, sharper teeth à catch more prey Prey species that are faster, better camouflaged à avoid being caught
SOME VARIATIONS ARE FAVORABLE Adaptation - any heritable characteristic that increases an organism s ability to survive and reproduce body parts or structures bear s claws body color à camouflage or mimicry physiological functions a plant carrying out photosynthesis behaviors avoidance strategies prey use crane flapping wings
Overproduction DARWIN S OBSERVATIONS Variation Darwin realized that species tend to produce excess offspring limited resources + excess offspring = competition Not Everyone Survives! Differences among members of the same species Most variation is heritable Siblings look more like each other than they do other people
DARWIN S OBSERVATIONS
NATURAL SELECTION AND SPECIES FITNESS Overtime, natural selection results in changes in the inherited characteristics of a population. These changes increase a species fitness Species fitness = survival rate SURVIVAL OF THE FITTEST
SUMMARY OF DARWIN S THEORY 1. Organisms differ; variation is inherited 2. Organisms produce more offspring than survive 3. Organisms compete for resources 4. Organisms with advantages survive to pass those advantages to their children 5. Species alive today are descended with modifications (changes) from common ancestors
COMMON ANCESTOR
ARTIFICIAL SELECTION Artificial Selection: selective breeding of domesticated plants and animals to produce offspring with genetic traits that humans value nature provides variation, humans select variations that are useful. Example: a farmer breeds only his best livestock
ARTIFICIAL SELECTION Humans have been modifying species for thousands of years Darwin saw this being done with dogs and pigeons He observed that a species could be changed a lot in a short period of time Thought that the same thing could be happening in nature at a slower pace
DIFFERENCES Natural Selection Traits that are more beneficial in the organism s environment become more common in the population Artificial Selection Humans choose the traits that become more common in the population
PESTICIDES: NATURAL SELECTION IN ACTION Pesticides are used to kill insects in crops and homes Insects evolve resistance to pesticides over time High doses and more potent pesticides need to be used In the 1950 s only small amounts of the pesticide malathion were used Today insects are resistant to very high concentrations of malathion
PESTICIDE RESISTANCE
SUMMARY Natural selection does not make organisms better Adaptations don t have to be perfect just good enough to enable organisms to pass its genes to next generation Doesn't have to move in fixed direction no one perfect way of doing something If local environment changes à traits that were once adaptive may no longer be useful and different traits may become adaptive If environment changes faster than species can adapt = extinction
COMMON DESCENT Natural selection depends on the ability of organisms to reproduce Every organism alive today is descended from parents who survived and reproduced Living species are descended with modification from common ancestors (descent with modification) Implies life has been on Earth for very long time Used fossil record for evidence
COMMON ANCESTORS Darwin based his explanation for diversity of life on the idea that species change over time Implies that all organisms are related Common ancestor shared by tigers, panthers, cheetahs Common ancestor shared by these felines and horses, then bats Farther back à all mammals share a common ancestor with birds, alligators, fish According to the principle of common descent, all species living and extinct are descended from common ancestors