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Lesson 7 Evolution - Evidence of Change
Species change over time. The process through which these changes occur is known
as biological evolution. Various forms of scientific evidence, including fossil records
and biochemical, anatomical, embryological, and physiological similarities, allow
scientists to classify organisms in order to show probable evolutionary relationships and
common ancestry.
Fossil Evidence A great deal of Earth's history can be determined using fossils.
Fossils are remnants or traces of organisms that are preserved in layers of rock. If an
organism gets buried under sediment, the soft parts decay, while the hard parts (bones,
teeth, etc.) undergo a change to become preserved in the sediment. This sediment
eventually hardens to becomes rock.
Fossil records provide evidence for evolution, or changes in species over time. By
studying the fossil record of a given species, it is possible to see the many changes that
have taken place over multiple generations. These changes are called adaptations.
Paleontologists have uncovered a significant amount of information about dinosaurs by
studying their fossilized remains.
Variations in DNA sequences One of the ways in which scientists judge whether two
species are related is through comparative biochemistry, or the study of organisms'
DNA sequences. Since each species has its own unique DNA sequence, it is possible
to determine the relatedness of organisms by comparing their DNA. Sequences of
related organisms are more similar than sequences of unrelated organisms.
Scientists have also found that classifications based on variations in DNA sequences
closely match organism classifications based on comparative anatomy as well as
classifications based on evidence from fossil records. So, even though related
organisms evolved from a common ancestor, at some point, the organisms changed
to form different species.
Comparative Anatomy Much can be learned by comparing the structural similarities
and differences of living things. Homologous structures are structures found on one
species that have the same basic structure and embryonic origin as those found on
another species. If homologous structures are found on two different organisms, the
species are related, even if only distantly. Organisms with homologous structures share
a common ancestor, but at some point, mutations contributed to the rise of the new
species.
Identification of vestigial structures can also indicate a relationship between two
species. Vestigial structures are non-functional remnants of features that were once
operational in a distant ancestor. These structures help establish evolutionary pathways
for modern organisms. For example, the presence of vestigial pelvic girdles and femurs
in snakes and whales implies that snakes and whales descended from animals that
walked on land. Because the presence of these structures were not harmful to the
organisms, there was no evolutionary pressure for the structures to completely
disappear.
There are a number of vestigial structures found in humans. The following list provides
some of the major examples:

coccyx (tailbone) - attachment site for muscles that control tail movement

appendix - an organ that helped with digestion of plant material, useful for huntergatherer ancestors

wisdom teeth - remnants of a once larger jaw or replacements for lost teeth
Comparative Embryology Comparative embryology is the study of similarities and
differences in embryologic development among species. To a certain extent, the
evolutionary history of organisms can be seen in the development of embryos. By
locating similarities in development, scientists can determine if species are related, even
if only distantly.
For instance, gills are present in all vertebrate embryos at some stage in development.
This common feature likely means that all vertebrates evolved from a common fish-like
organism.
Physiological Similarities The physiology of related organisms is extraordinarily
similar. For example, certain organs, such as the liver, functions the same way in all
mammals. This indicates that all mammals evolved from a common ancestor whose
liver functioned in the same way as the livers found in current mammalian species.
Analogous & Homologous Structures
Much can be learned about the evolutionary history of an organism by comparing the
structural similarities and differences of living things.
Comparative Anatomy The study of structural differences and similarities in living
things is called comparative anatomy. Structural similarities could be due to sharing a
common ancestor, or they could be adaptations to living in similar environments.
Homologous Structures Homologous structures are traits that were inherited from a
common ancestor. The presence of homologous structures in two different organisms
implies that the species are related, even if only distantly.
Homologous structures share the same origin, but may have different
arrangements or functions. For example, the forelimb of humans, cats, dolphins, and
bats have the same skeletal elements because these species share a common
ancestor that also had these traits.
Though they all contain the same general bones, the arrangement and function of the
bones may be different because they have adapted to different lifestyles.
However, the arrangement of the bones is slightly different. Dolphins have adapted to
living in water and use their forearms for swimming. Similarly, bats have adapted to
flight, and humans have developed the ability to grip and adeptly control objects with
their hands. Selective pressures were placed on organisms in order to survive in
different types of environments. These pressures to survive in new environments led to
the different adaptations of the original structures.
Analogous Structures Sometimes, structures in different species appear to be
homologous, but they are not. These structures are said to be analogous structures.
Analogous structures have the same function, but do not share a common origin.
Analogous structures evolve separately in unrelated species, and their presence does
not imply that the organisms descended from the same ancestor. Because analogous
structures evolved separately, their structure and arrangement are also very different.
The wings of a dragonfly and the wings of an eagle both have the same function - flight.
However, they are analogous structures because they evolved separately, from two
different ancestors.
For example, birds have a single pair of wings that are composed mostly of muscle,
bone, and feathers. Dragonflies are invertebrates, whose two pairs of wings are made
mostly of integument.
Evolution - History of Life on Earth
Organisms have changed significantly since the first appearance of life on Earth.
Important Terms

aerobic - living in the presence of gaseous oxygen

anaerobic - living in the absence of gaseous oxygen

autotroph - organism that can synthesize its own food

heterotroph - organism that cannot synthesize its own food and consumes organic
material
Evolutionary Timeline

3.8 billion years ago - first cells (anaerobic prokaryotes)

3.2 billion years ago - first photosynthetic bacteria (prokaryotes)

2 billion years ago - first eukaryotes

1 billion years ago - first multicellular organisms

570 million years ago - first arthropods

500 million years ago - first fish

475 million years ago - first land plants

360 million years ago - first amphibians

300 million years ago - first reptiles

225 million years ago - first mammals and dinosaurs

150 million years ago - first birds

200 thousand years ago - first humans
First Life The geologic record indicates that the first life forms appeared approximately
3.8 bya and were anaerobic prokaryotes. The first cells were most likely heterotrophic,
feeding on organic molecules. Evidence of the first photosynthetic autotrophs were
dated back to 3.5 - 3.1 bya. A type of cyanobacterium was able to split water molecules,
releasing oxygen into the atmosphere. It was the action of these bacteria which cleared
the way for the evolution of aerobic organisms.
Endosymbiont Theory Championed by American biologist Lynn Margulis, the
endosymbiont theory describes the evolution of eukaryotic cells from prokaryotic
organisms. The theory proposes that certain eukaryotic organelles developed from
prokaryotic cells engulfed by a host cell.
As the name implies, the engulfed cells developed a mutually beneficial, symbiotic
relationship with each other and the host. Each part provided something the others
could use. Eventually, the relationship became so interdependent that the engulfed cells
and host cells could not survive independently of one another.
The host cell provided protection from predators and the environment. The chloroplast's
ancestor, likely a cyanobacterium, provided the ability to synthesize complex organic
molecules from simple compounds. The mitochondrion's ancestor, possibly a purple
bacterium, provided more energy by possessing the ability to undergo aerobic
respiration.
Evidence Chloroplasts and mitochondria:

have double membranes. The make-up of the innermost membrane for each is
more similar to the plasma membrane of a prokaryote.

have their own circular DNA, similar to prokaryotic DNA.

are similar in size to bacteria.

have ribosomes similar to those in bacteria.
Natural Selection
Some organisms have favorable traits that are well-suited to their immediate
environment. Organisms with this advantage are more likely to thrive, reproduce, and
pass their traits to future generations than organisms without those favorable traits. This
process is known as natural selection.
Favorable Traits Favorable traits are traits that promote an organism's success in a
particular environment. Organisms with favorable traits are more likely to reproduce and
pass on their traits than organisms without favorable traits. In this way, the external
forces of nature determine, or "select" which traits (or even which types of organisms)
will continue to exist in a population.
Favorable traits may include physical traits, such as the body of a sea lion storing extra
fat, or they may be instinctive behavioral traits, such as a bird building a nest to protect
its young.
If an organism dies before reproducing, then its unique traits will be eliminated from the
population. On the other hand, if an organism has favorable traits that allow it to survive
and produce many offspring, then its traits will be more numerous within the population.
Natural Selection Occurs Over Time Genetic variation combined with natural
selection allows species to survive by adapting to changes in the environment. If
enough significant changes or adaptations occur in the inherited traits of a population,
natural selection may result in a new species.
Over time, favorable traits are likely to increase within a population and unfavorable
traits are likely to decrease. In this way, natural selection plays an important role in the
way species evolve over time. When natural selection results in changes in a species,
these changes generally develop gradually, over many generations, rather than
suddenly.
The following are sources of genetic variation among living organisms.

Mutations, or changes to an organism's genetic material, are a significant source
of genetic variation.

Sexual reproduction results in an offspring that has a combination of genetic
material from its parents, which contributes to genetic variation.

Meiosis is the process by which sexually-reproducing organisms produce unique
gametes, or sex cells. The unique gamete of one parent can fuse with the unique
gamete of another parent to produce a unique offspring, resulting in increased
genetic variation.
Natural Selection
Natural selection is a process by which organisms with traits well suited to an
environment survive and reproduce at a greater rate than organisms less suited to that
environment.
Basic Criteria for Natural Selection The survival of an organism depends on its ability
to sense and respond to the external environment. In all environments, organisms with
similar needs compete for the same resources. These two facts fuel the process of
natural selection.
Natural selection can occur if the following criteria are met:
1. Overproduction:
A species produces more offspring than will survive to maturity.
2. Genetic Variation:
The individuals within the species' population are all genetically slightly different
from each other.
3. Struggle to Survive:
Individuals of the population must struggle to access resources and/or avoid
predators.
4. Successful Reproduction:
Individuals that are successful at surviving are able to reproduce and pass on their
genes to their offspring.
Results of Natural Selection Natural selection results in the following phenomena at
both the molecular level and the population level:
Diversity Diversity can refer to either the number of different characteristics present in
a population or the number of different species in an area. On the environmental level,
diversity creates stability in ecosystems and biomes. This is called biodiversity. A
community that has biodiversity will not be seriously affected by the extinction of one or
two species. On the species level, diversity allows the species to survive in a changing
environment.
Humans affect the diversity of a species through the following two processes:

Selective breeding; the breeding of organisms that have a certain desired trait

Genetic engineering ;the manipulation of genes that allows scientists to put genes
from one organism into another organism
Adaptation An adaptation is a structure or behavior that helps an organism survive in
its environment long enough to reproduce.
Structural adaptations can include traits that improve the ability to find food, protection,
or move from place to place. Most adaptations take long periods of time to develop fully.
However, some species are capable of fairly significant change over only a few
generations.
Many forms of behavior, including the evasion of predators, the ability to cooperate with
other individuals of the same species, and the desire to mate, are governed by traits.
Organisms with traits that produce beneficial behaviors are more likely to survive long
enough to reproduce. Therefore these traits are more likely to be passed down through
the generations and spread throughout a population.
Speciation Speciation is the process by which two populations of the same species
become so different that they can no longer interbreed. Speciation can occur in a few
ways, but the end result always increases biodiversity. The following process describes
the general steps that occur in most cases of speciation.
1. Separation Different populations of the same species become isolated from one
another.
2. Adaptation The separated populations develop different responses, or
adaptations, to the different selective pressures found in the newly isolated
environments.
3. Division Over a long period of time, the divided populations develop enough
changes to their genetic material that they are no longer able to breed with one
another.
Speciation can also occur due to a mistake during meiosis, in which too the
chromosomes do not separate correctly. The result is an offspring with an extra set or
sets of chromosomes, known as polyploidy. This is common in plants. In this case
reproductive isolation is immediate, even if the individual is not separated from the rest
of the population.
Extinction Extinction refers to the process of a species dying out completely. The
extinction of a species occurs when environmental changes and adaptive
characteristics are insufficient to permit survival.
Phylogeny A phylogeny illustrates the evolutionary history and development of a
species. While natural selection works on the individual level, it eventually affects the
species as a whole. Therefore natural selection is one of the main instruments of
phylogeny.
Genetic Drift & Gene Flow
Genetic drift and gene flow are two mechanisms involved in evolutionary processes.
Genetic Drift Genetic drift is a change in the allelic frequency within a population due
to random chance. It is especially apparent within small breeding populations.
Genetic drift occurs when random chance results in certain alleles being passed on to
future generations, while others are passed on at a lower rate, or even lost from the
population.
For example, consider a population that has individuals with either brown or blue eyes.
Suppose that eye color has no impact on reproductive success. If a random event, such
as a natural disaster, resulted in the death of all blue-eyed individuals, then this allele
would no longer be present in the population. This is an example of genetic drift
because there was no selection involved in the loss of the allele from the population.
The founder effect is an extreme example of genetic drift. In this case, a relatively
small population with fewer genetic variations becomes reproductively isolated. The
result is a new population in which variation remains significantly reduced, and
members tend to share many traits with the original founders of the population.
Gene Flow Gene flow is the transfer of genetic information from one breeding
population to another. Gene flow can be an important source of genetic variation when
unique genetic information from one population is introduced into a different population.
Gene flow can be caused by many different events. For example, a storm with strong
winds could blow pollen from one population of plants to another.