2. 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

3. 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 variations
 Increase of favorable traits within
a population over time

4. 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 Lyell
 Empasized 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

5. Voyage of the H.M.S.
Beagle

6. 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
sources
 Indicated a recent common ancestor
 Led 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

7. Darwin’s
Finches:

8. 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

9. 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…

10. Stabilizing Selection
 Individuals with the average form of a
trait have the highest fitness.

11. Directional Selection
 Individuals that display a more extreme
form of a trait have greater fitness than
those with an average form of the trait.

12. Disruptive Selection
 Individuals with either extreme variation of a
trait have greater fitness than individuals
with the average form of the trait.

13. 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