Download Applied Biology 14.3 Natural Selection as a Mechanism

14.3
Natural Selection the
Mechanism for Evolution
14.3
14.3 Darwin proposed natural selection as
the mechanism of evolution
Population: a group of individuals of the same species living in the
same area at the same time.
 Darwin’s finches- isolated finches adapt to their local
environment. (Differences in beaks)
(13 species of finches unique to Galapogos Islands) – Finch wkst.
How Can This be Explained? Darwin’s Explanation
 Descendants of the earliest organisms spread into various
habitats over millions of years. In these habitats, they
accumulated different modifications, or adaptations, to
diverse ways of life. (Descent With Modification)
 Finches better suited for their environment survive better than
others. The organisms that are better suited reproduce
offspring that are also better suited. Less suited organisms die
and do not reproduce. (Natural Selection)
Darwin’s Observations Leading to
Natural Selection
1. Overproduction of Offspring – species
produce more offspring than the
environment can support
2. Leads to a “Struggle for Existence”
3. Many Variations within a population!
 (def.) Differences among members of the
same species
4. Individuals with ‘best suited’ traits survive,
have reproductive success, and pass on
their advantageous traits.
Darwin's theory of
natural selection.
Natural Selection
Natural Selection: individuals with traits/adaptations
well suited for an environment will be survive and
produce more offspring than individuals without these
traits.
 Over time, there will be a higher % of
individuals with this advantageous trait.
 Nature selects, not humans.
Natural selection: Beetles with brown genes escaped
predation and survived to reproduce more frequently than
beetles with green genes, so that more brown genes got into
the next generation.
Natural selection acts on existing variation.
 Natural selection can act only on traits that already
exist.
 Structures take on new functions in addition to
their original function.
five
digit
s
wrist
bone
Misconception: “Natural selection involves
organisms ‘trying’ to adapt.”
Response: Natural selection leads to adaptation, but
the process doesn’t involve “trying.” Either an individual
has genes that are good enough to survive and reproduce,
or it does not—but it can’t get the right genes by “trying.”
Misconception:
“Natural selection
gives organisms
what they ‘need.’ ”
Artificial Selection vs
Natural Selection
Artificial selection: selective breeding of
domesticated plants and animals to produce
offspring with genetic traits that humans value.
 Plant breeder-Improved grain production, disease
resistance, or protein content.
 Animal breeder- temperament, coat, growth rate
 Different from natural selection because they are
traits that humans choose, not nature.
 Artificial selection can produce a great deal of
change in a much shorter time period
The Bottom line is….
Natural Selection – Nature “chooses”/selects and
favors traits more suitable for their
particular environment.
Result?  Evolutionary adaptation to
the environment.
Pesticides- Natural Selection in Action
Pesticide: poisons used to kill insects that are pests in crops and homes.
 Initial pesticide spraying kills most of the insects. Those that survive
have a mutation- a gene for pesticide resistance.
 Resistant survivors reproduce passing on the resistant gene.
 In each generation the number of resistant insects increases.
 Eventually that pesticide is ineffective.
 The population underwent evolutionary change.
 The pesticide does not create resistant individuals.
 The pesticide naturally selects for resistant insects that are already
present in a population.