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Evolution
Unit 11
Theories of Evolution
• Evolution – change in a species over time
• Theory of evolution – explains how the variety of
living things on earth has developed from their
ancestors over the past 3 billion years.
• Originally there were two theories:
– Jean Baptiste de Lamarck – changes in species
occurred because of acquired traits
• An organism’s experience or behavior would determine the
traits of its offspring.
• Example – Giraffes stretch their necks because they want to
get food from tall trees and their babies are born with longer
necks.
– Charles Darwin – naturalist in 1859 takes a
trip on the HMS Beagle and makes some
observations that leads him to come up with
his theory of evolution.
• Natural selection – the name of Darwin’s theory of
evolution. It states that environmental conditions
determine which organisms survive and reproduce
– Adaptations or special characteristics that make an
organism well suited to a particular environment help
certain organisms survive over others
– The adaptations an organism has can give it an
advantage over others helping it to survive better then
others this is called “survival of the fittest”
Natural Selection Continued . . .
– If the organism have good adaptations and survived as a
result they would be more likely to reproduce and pass their
genetic information on to the next generation.
– If they had poor adaptations they wouldn’t survive to
reproduce and pass on their genetic information to the next
generation.
– Eventually only the individuals with the good traits would be
found in that environment because they are the only ones
who survived long enough to reproduce.
– Darwin came up with this theory based on two major
observations
» There is a struggle for existence which limits the
number of offspring that survive
» There are differences among organisms due to their
individual inherited variations
• Example: White mice and black mice living in an
environment with black soil. Which type of mouse
is more likely to survive if owl’s are their main
predators?
*Black mice would survive better so the white
mice are naturally selected by the owl not to
survive.
*Only black mice would live long enough to
reproduce. So their traits will be passed on to the
offspring.
*White mice will not live long enough to
reproduce so they will not pass on their traits to
the offspring.
*Eventually all mice in this environment will be
black so they can survive attacks by owls.
Struggle for Existence
• Darwin observed that the Earth could not
support huge increases in populations
• Darwin concluded there is a struggle for
existence so only a few offspring of any
species survive and mature to reproduce
• There is competition among organisms for
various resources like food, water, space
and shelter
Genetic Variations
• When two parents reproduce all of their offspring
are different this is called genetic variations
• We already know that the parent’s DNA
separates to recombine during sexual
reproduction that is what gives us such variation
among the offspring
• Only traits that are inherited from the parents
DNA can be passed on to the offspring
– Example: Eye color
• Acquired traits like muscle growth cannot be
passed on to the offspring
• Mutations in the DNA can also cause new traits
to form
Example of Genetic Variation
• Penicillin was the first antibiotic for
bacterial infections.
• In recent years there have been new
strains of bacteria that are resistant to
penicillin
• This happened because the DNA in the
bacteria mutated to become resistant to
penicillin so it could survive better in its
environment
Evidence for Evolution
• There are several types of evidence on
earth that prove evolution to be true:
– Fossils: traces or remains of dead organisms
that have been preserved by natural
processes
– Formed when an organism’s remains are replaced by
another substance
» Example: organisms are buried in sediment, tissues
get hard and are slowly replaced by minerals found
in ground water. Over time these minerals harden to
make an exact copy of the organism.
Fossils continued . . .
• Fossils are found in layers of sedimentary
rock called strata.
– The layers of strata on top are younger then
the layers below.
– Similarities between older and more recent
fossils prove that there is an evolutionary
relationship among species
– By studying fossils scientists can see that
species have changed over time and that
most ancient life forms no longer exist
Comparative Anatomy
• Similar parts among organisms are called
homologous structures
– They are inherited from common ancestors
– Have similar forms but different functions
– Example: wing of a bat, flipper of a whale,
front leg of a cat and arm of a human are
different but are all made up of the same type
of bones
– These bones are attached to each other and other bones
in a similar way
– The limbs of these animals indicate that they evolved
from the same ancestor long ago.
Comparative Anatomy
continued . . .
• A structure that has little or no function in
one organism but is clearly related to a
structure that does function in another
organism is called a vestigial structure
– Example: human appendix does not have a
function in humans but there is a similar
structure called the cecum found in rabbits
and other plant eating animals. It helps them
to digest their food!
– The fact that this organ exists in other animals proves that
humans probably evolved from an organism that once
used its appendix
Comparative Embryology
• Similarity between embryos provides
evidence that all vertebrates follow a
common plan in their early stages of
development.
• This is because these animals have
similar sets of genes and this similarity
comes from having a common ancestor
Comparative Biochemistry
• The similar chemistry or biochemistry between
organisms provides some of the strongest
evidence for evolution
• All organisms store their genetic information in
DNA molecules in almost exactly the same way
• DNA shows that all organisms are related in a
basic way
• The more similar two organism’s DNA
sequences are the more closely related they are
Comparative Biochemistry
continued . . .
• The same proteins in organisms are also
made up of similar amino acids
– A small number of amino acid differences in
the same protein for two different organisms
means they are closely related
– A large number of amino acid differences in
the same protein for two different organisms
means they are not closely related.
Mechanisms for Evolution
• Organisms are adapted to their
environment
• These adaptations are specific traits that
allow them to survive in those
environmental conditions
• Since the environment is always changing
the evolution of living things is still going at
a very slow rate
Mechanisms for Evolution
continued . . .
• Different adaptations in living things occur
by chance as a result of genetic variations
in a population
• Sometimes adaptations work well for an
organism and sometimes they don’t
• Adaptive value of a trait is determined by
the conditions of the environment
Example
• Dark moths in an environment with white
trees is a bad adaptive value
• However if pollution darkens the white
trees now the dark moths have a high
adaptive value because their trait for color
will help them survive
• Through the process of natural selection a
species evolves over time
Different types of adaptations
• Physical adaptations – size, shape, color
and structure of organisms or parts of
organisms
– Example: camels have extremely wide two
toed feet to avoid sinking in sand
Different Adaptations continued . . .
• Behavioral adaptation – include the
functions and behaviors of an organism
– Example: birds building a nest or bears
hibernating
Reproductive isolation
• A population is physically separated from other
populations of its kind for a long time
• The population changes so much from being
isolated that it can no longer reproduce with
similar members of any other population
• The population undergoes speciation or
becomes a new species
– Sometimes they will evolve to fill a niche
– Niche is everything a species needs to survive such
as how it gets its food, water, shelter and avoids
predators
– This new species will likely have some physical or
behavioral advantage over other species surviving in
that same niche
• When a group from a specific population develops into a
new species this is called divergent evolution
Geographic isolation
• Most common type of separation that leads to
the formation of a new species
• An actual physical barrier like a river or
mountain prevents organisms from moving
between populations
• Adaptive radiation – those individuals that have
the best traits for that environment survive and
reproduce. Over time each environment causes
the development of a new species
– Example: Darwin’s finches
Convergent Evolution
• Sometimes organisms in different places
evolve in similar environments
• Because they face similar challenges in
their environments they may form similar
structures although they are not closely
related
– Example: Grassland birds like emus, rheas
and ostriches
• They are not closely related but all three have
long, strong legs that allow them to run through tall
grass. Also they are all flightless
Endosymbiotic Theory
• About 2 billion years ago some ancient
prokaryotic cells began evolving internal
cell membranes
• These were the ancestors of eukaryotic
cells
• According to the endosymbiotic theory
prokaryotic cells entered those eukaryotic
ancestors and lived inside them