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Biology I
Evolution by Natural Selection
► Darwin
lived during a time of great scientific study
► Exploration and Geology helped form his ideas
► Most people in Darwin’s day believed the Earth and all
it’s inhabitants were only a few thousand years old
► They believed that creatures did not change (wrong)
► Also, geologic features were formed during a
catastrophe that humans did not witness
► Scientists began to study many of these ideas and
challenge the established thoughts by Darwin’s trip
1795, James Hutton explained the Earth was OLD
He thought layers of rocks, mountains, and valleys
were formed very slowly over millions of years
Darwin read a copy of Charles Lyell’s book, Principles
of Geology, while on his sea voyage
He wondered, if the Earth changes, maybe species do
An ancient Earth is essential to evolutionary theory
► The
fossil record challenged the idea that creatures have
not changed and have existed since creation
► In Geology, the farther down in the soil you travel, the
farther back in time you are studying
► Fossils show two interesting patterns in the rocks
► One, there have been several mass extinctions
► Fossils are found everywhere and then they disappear
for very large periods of time (happening again)
► Many of these species are never seen again (extinct)
► Two, fossils show gradual changes over time
► 1831,
Charles Darwin sets sail on the HMS Beagle
► He worked as a naturalist on board the ship
► In the Galapagos Islands, he collected 14 different
species of finches (each w/a different shaped beak)
► The island had the same climate, but they had different
food (seeds, nuts, insects, fruit, etc)
► The beaks of each finch were adapted to only one
specific island and one food source
► Evolution
happens to populations of organisms and not
individuals (no changes within their lifetime)
► These changes occur through natural selection
► Each organism within a population has diff. DNA
► Some are faster, stronger, more poisonous, etc…..
► Those organisms that are best adapted survive and pass
on their genetic information. Those that don’t survive
can not reproduce and their genes die off
► 1859,
Darwin published The Origin of Species
► He received a manuscript from Alfred Wallace, which
was nearly identical to what he had discovered
► This prompted Darwin to go ahead and publish
► Immediately, the book was controversial (still is!!!)
► The book does NOT focus on humans, but Darwin
thinks they are subject to nature like other organisms
► Within
a population there exists a variety for each trait
(hair color, height, etc.)…not all are visible
► Different combinations of parents produce different
offspring (evolution works with what’s available)
► Darwin knew that species varied from one to another
and that traits could be passed from parents to children
► He determined this by talking w/plant & animal
breeders, however he knew nothing about DNA
► Breeders
use artificial selection to determine which
traits they want offspring to inherit from parents
► Nature provides the variation and breeders select traits
► Darwin thought similar processes happened in nature
► Organisms compete for food, living space, mates, etc.
► Those w/the greatest fitness survive and reproduce
► Organisms have different adaptations which increase
the chances for survival (strength, camouflage, etc.)
► Survival
of the fittest is known as natural selection
► Nature, or the environment selects the best traits
► In both artificial selection and natural selection, only
certain organisms in a population get to reproduce
► Ex: Show quality vs. Pet quality animals from breeders
► Over time, certain traits become more prevalent in the
population and others diminish or disappear altogether
► Evolution happens with no goal or end point in mind
► Distribution of
phenotypes happens in 3 different ways
► 1) Directional Selection – Individuals at one end have
better fitness than those in the middle or other end
► The curve will be shifted to the far right or left
► Ex: Darwin’s finches had beaks from small to large
► If a food shortage resulted in only large seeds being
available, then the curve would shift to the right
► If only small seeds – then it would shift to the left
Stabilizing Selection – Occurs when individuals at
the center of the curve have the greatest fitness
► The graph appears to squish in towards the middle
► Ex: Imagine a group of lizards. The large ones are
easily spotted. The small ones might not have enough
speed to escape, but the ones in the middle are just right
► 2)
Disruptive Selection – Individuals at the upper and
lower ends of the curve have the greatest fitness
► Selection eliminates those with the average trait
► Natural selection is not the only cause of change
► In small populations, there can be a change in allele
frequencies simply by chance & probability
► Ex: Some individuals might have lots of offspring
► This random change is known as genetic drift
► 3)
► When
studying evolution, sometimes it helps to see
what happens when no change is taking place
► Hardy-Weinberg Principle – allele frequencies will
remain constant unless acted on by one or more factors
► This constant state of existence is genetic equilibrium
► 5 conditions must exist for equilibrium to happen
► Population must be large, mating must be random, no
immigration or emigration, no mutations, and no natural
selection taking place (If not - then evolution happens)
► Remember,
a species is a group of organisms that are
capable of breeding with one another
► For that population to become two different species, the
population must be split into two separate groups
► Each group will then adapt to their local environment
► When they can no longer reach the original group, then
they are reproductively isolated and speciation occurs
► There are several ways to become isolated populations
Behavioral Isolation – The groups are not physically
isolated, but don’t breed together for 2 different reasons
► This typically occurs as a result of disruptive selection
► Prezygotic Isolation – This occurs before fertilization
► Ex: Mating calls that are not recognized
► Postzygotic Isolation – This occurs after fertilization
► Ex: They die early or are infertile and can’t breed
► Result’s the same as if geographic isolation happened
► 1)
Geographic Isolation – The two populations are
separated by rivers, mountains, lakes, etc.
► With two separate gene pools, changes in one group do
not get transferred to the other group
► Ex: Albert Squirrel(gray) and Kaibab Squirrel(black)
► Separated by the Colorado River& Grand Canyon
► Rivers may separate land animals, but not birds!!!
► 3) Temporal Isolation – Reproduce at different times
► 2)
►3
different species of orchids release their pollen only
on a single day during the entire year. They can’t
pollinate the wrong flower since each day is different
► Scientific evidence supports Darwin’s assumption that
living species evolved from common ancestors
► Peter and Rosemary Grant studied the same Finches on
the Galapagos Islands that started Darwin’s theory