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 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’SOBSERVATIONS

DARWIN’SOBSERVATIONS

NATURALSELECTIONANDSPECIESFITNESS

• 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

SUMMARYOFDARWIN’STHEORY1. Organisms differ; variation is inherited2. Organisms produce more offspring

than survive3. Organisms compete for resources4. Organisms with advantages survive to

pass those advantages to their children5. 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 Artificial SelectionTraits that are more beneficial in the organism’s environmentbecome more common in the population

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