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