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DARWINIAN EVOLUTION &
NATURAL SELECTION
Lamarck’s Explanation Lamarck was 1st to suggest that similar
species descended from a shared common ancestor
Related living species to fossil forms based on shared appearances
Hypothesized that acquired traits—those that are not determined by genes but result from organism’s experience or behavior—are passed on to offspring
Charles Darwin In mid-1800s both Charles
Darwin & Alfred Wallace independently hypothesized that evolution occurred through natural selection.
Natural Selection: the process by which organisms with favorable variations reproduce at higher rates than those without such variationsIncrease of favorable traits within
a population over time
Voyage of the Beagle Darwin embarked on a 5-yr expedition on the H.M.S.
Beagle to South America & the South Pacific as a ship naturalist in 1831
Carefully recorded observations & collected 1000s of specimen & fossils
Darwin’s interest in geology spurred by the book Principles of Geology, by Charles LyellEmpasized Uniformitarianism: principle that states that the
geological structure of Earth resulted from cycles of observable processes & that these processes operate continuously
Led Darwin to conclude that modification of environments is very slow process that occurs over long periods of time
Voyage of the H.M.S. Beagle
Analysis of Darwin’s Data
Darwin’s data & specimen collected earned him great praise when the Beagle returned to England in 1836
Analysis of 13 different finch species collected from the Galapagos Islands showed that they were very similar, aside from bill adaptations for particular food sourcesIndicated a recent common ancestorLed Darwin to reason that large differences can accumulate
between similar species over millions of yrs
1859 published his work on evolution by natural selection in famous book On the Origin of Species by Means of Natural Selection
Darwin’s Finches:
Darwin’s First Theory
Decent With Modification: states that newer forms of a species that appear in the fossil record are modified descendents of older forms of the species
Took this theory a step further than Lamarck by inferring that ALL species have evolved from one or a few original forms of life
Darwin’s Second Theory Modification by Natural Selection:
An organism’s environment limits population growth by increasing death rate or decreasing birth rate
Some organisms acquire traits that make them better able to adapt to their environment
Traits that increase reproductive success or fitness of an organism & are inherited will be passed on to offspring
Results in change of genetic makeup of population over time
Favorability of traits depends on environmental demands—favorable traits give adaptive advantage
Several broad types of Natural Selection…
Stabilizing Selection
Individuals with the average form of a trait have the highest fitness.
Directional Selection
Individuals that display a more extreme form of a trait have greater fitness than those with an average form of the trait.
Disruptive Selection
Individuals with either extreme variation of a trait have greater fitness than individuals with the average form of the trait.
Sexual Selection
The preferential choice of a mate based on a specific phenotypic trait.
In many species, the male is often more Examples: bright
coloration, heavy plumage