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 1. Overproduction of Offspring – species produce more offspring than the environment can support “Struggle for Existence” 2. Leads to a “Struggle for Existence” Variations 3. Many Variations within a population! (def.) Differences among members of the same species Individuals with ‘best suited’ traits survive, have reproductive success 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 can act only on traits that already exist. Structures take on new functions in addition to their original function. Natural selection acts on existing variation. wrist bone five digit s

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 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.