Download BL 1021 – Unit 5

BL 1021 – Unit 5
Evolution and Populations
5.1 Evolution
• What is evolution?
• Evolution is the gradual change in the genetics of
an organism over time.
• Organisms may change in shape, color,
proportions, size, abilities, etc…
• Organisms may split off into two or more separate
species.
• Evolution can be used to trace organisms back to
common ancestors (species that were the
forefathers of multiple modern species).
5.1 Evolution
• What evolution is not:
• An explanation of the origin of life. Evolution only
deals with the changes species undergo, it does not
include the creation of life from non-life.
• An idea created by Darwin and unchanged since
then. Evolution Theory is constantly being refined.
• “Just a theory”. In everyday language, a ‘theory’
may just mean a guess. In scientific language,
‘theory’ means a tried and tested model to
describe the world that is supported by evidence.
5.2 Evidence of Evolution
• There are several sources of data that have shaped
our modern understanding of evolution:
• Fossil Record
• Fossils are the remnants and impressions of dead
organisms. By observing extinct creatures and the
timeline they lived in, people can observe when
new types of life forms arose and trace back
ancestors.
5.2 Evidence of Evolution
• Biogeography
• This is the study of the locations of where different
types of animals live on Earth.
• There is a trend that animals of a family living close
to one other will resemble each other more than
animals of a similar environment in another part of
the world.
• For example, a bird that lives in the jungles of South
America will likely resemble birds from other habitats
in the Americas more than a bird form an African
jungle. This suggests that the American birds had
one common ancestor that split off into each type.
5.2 Evidence of Evolution
• Comparative Anatomy
• The anatomical
structure of many very
different species have
many similarities.
• This suggests that they
comes from a common
ancestor that has split
into several groups and
each have specialized
to suit their needs.
5.2 Evidence of Evolution
• Comparative
Embryology
• The earl stages of most
animals look very
similar, eventually
growing in different
directions.
• This suggests some
commonality between
all animals.
5.2 Evidence of Evolution
• Genetics
• The genetic code for all life on Earth is the same. As
well, the code for specific proteins and enzymes in
many creatures is very similar.
• Human and mouse genes for many proteins are
70% similar
• Human and chimpanzee genes are more than 95%
similar.
• This suggests that separate species were once
related but have changed over the years since
splitting off into separate organisms.
5.3 Natural Selection
• The driving force of evolution is natural selection.
• Organisms that are best adapted to their
environment are more likely to thrive and
reproduce more offspring. This means that welladapted individuals will pass on their genes more
often than poorly adapted individuals.
• Therefore the most effective traits to survive in an
environment will become more common over time
as they are passed on with a higher frequency.
5.3 Natural Selection
• Natural selection does not mean that the most
complex, largest, strongest or fastest are favored. It
favors individuals who are best adapted to their
environment, which sometimes may mean smaller,
simpler or more efficient.
• Evolution does not have a “goal” nor are some
creatures on a higher “level” – evolution by natural
selection simply encourages organism to be the
best suited to their current environment at that
current time.
5.3 Natural Selection
• Natural selection promotes adaptations, changes in
the animal to suit the specific environment they are
in.
• Adaptations can include growing longer claws,
changing color, being able to survive in new harsh
conditions, being able to process a new food
source, and blending in to the surroundings.
5.3 Natural Selection
• Humans have also forced selection in certain
directions to help making organisms more useful to
us, this is artificial selection.
• Artificial selection occurs when people purposefully
breed organisms to pass on certain desired traits.
• In natural selection, the best adapted organisms will
breed more, but in artificial selection, those
organisms with human-desired traits will reproduce
more.
5.4 Populations
• A population is a group of organisms of the same
species living in a particular area.
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Populations can be measured by:
Birth Rate
Death Rate
Average Age
Migration Patterns
Etc…
5.5 Microevolution
• Microevolution deals with the small genetic
changes that occur in populations over time.
• Microevolution is often caused by random mutation
– small changes in the DNA caused by the DNA
duplication process making a “mistake” and
changing the DNA of an individual. Random
mutation can cause good, bad or neutral effects
depending on how the DNA is changed.
5.5 Microevolution
• However, sometimes major events can cause radial
change in population.
• Genetic drift occurs when only a small percentage
of a population has offspring, meaning the full
variety and normal proportions of traits are not
passed on.
• Genetic drift will decrease the diversity of a
population. Diversity is the variety of traits seen in a
population.
5.5 Microevolution
• Genetic drift can be caused by two mechanisms:
• The Bottleneck Effect
• When a catastrophic event kills off a large portion
of the population randomly, leaving a new
proportion of traits in a population, and potentially
removing some traits entirely.
• An example would be flowers that used to come in
both red and white, after a disaster, only a few red
plants survived and thus all plants from then on will
be red.
5.5 Microevolution
• The Founder Effect
• When a new, isolated area is colonized by a small
group, only the traits of the individuals who come to
the new area will show up in future generations.
Many of the traits of the original homeland
population may not carry over to this new colony.
• An example would be a type of bird that is colored
either white or grey. If a group of grey birds move to
an isolated island, all of the birds on that island will
be grey in future generations.
5.5 Microevolution
• A force that helps increase diversity is gene flow.
• Gene flow is when genes are traded from one
population to another. This can happen through an
animal moving to a new area and finding a mate,
or by pollen in plants drifting in the air to a new area
and fertilizing far-away plants.
• If separate populations have been apart too long
they may not be able to interbreed any more (they
have become two separate species), making gene
flow impossible.
5.5 Microevolution
• Regardless of cause, a change in the population
can come in one of three types:
• Directional Selection
• The population has shifted in one direction, favoring
a new normal variety.
• An example would be mice that once varied
between white to dark brown. Over time, they are
all light brown to very dark brown, showing a trend
towards darker colors.
5.5 Microevolution
• Diversifying Selection
• This occurs when an organism starts to favor two or
more extremes instead of an average range
between the two.
• An example would be mice who are colored
anything from white to dark brown. Over time, only
mostly white and dark brown mice are common,
with light brown mice being rare.
5.5 Microevolution
• Stabilizing Selection
• When a population reduces the variety of its
population and sticks close to one trait.
• An example would be mice that vary from white to
dark brown. Over time they have become all mostly
light brown (the middle). This means that white and
dark mice have become rarer, and the middle
ground is the majority.
5.5 Microevolution
5.6 Macroevolution
• Macroevolution encompasses the large-scale
changes in populations and creation of new and
different species. It is what most people think about
when they think of the word “evolution”.
• Macroevolution is caused by many microevolution
events accumulating over time. Speciation occurs
when two populations of a species become too
different to breed and are now called two separate
species.
5.6 Macroevolution
• Macroevolution comes in two forms:
• Non-Branching Evolution
• In this case an organism may radically change but
remains as one species (it did not branch off into
multiple species).
• Branching Evolution
• In this case multiple populations have speciated
and become so different from one another that
they cannot interbreed. They have branched off
into different species.
5.7 Mechanisms of Speciation
• Speciation, the creation of multiple species from
one original species can be caused by several
factors:
• Genetic Isolation
• The genes of two populations become so different
that they cannot conceive newborns together.
• Habitat Isolation
• Two populations will not interbreed because they
live in different environments and thus will not meet
to interbreed.
5.7 Mechanisms of Speciation
• Temporal Isolation
• Reproduction does not occur because two
populations mate at a different time of day, season
or year.
• Behavioural Isolation
• Interbreeding does not occur because each
population has different courtship rituals.
• Mechanical Isolation
• Interbreeding is not possible due to the differences
in reproductive anatomy being incompatible.