Download Evolution through natural selection

Darwin and his
theory of evolution
Changing ideas
Who was Charles Darwin?
Artificial selection

In order to test his hypothesis, Darwin studied
plant and animal breeders

The breeders knew that individuals within a
species varied, some producing more or less
fruit, some producing more or less milk, etc.

Darwin noted that these variations could be
passed to offspring
Artificial selection

Farmers and breeders would choose the
traits they most favored and breed only
those

Artificial selection is the process of
humans choosing the traits that they find
most useful from the natural variations
already present
Small variations

Darwin applied this idea of variation to wild
species

Sometimes a trait would develop that would
allow the organism a better chance of survival

Small, hereditary variations (adaptations) are
the basis for evolution in a population
Adaptation

This neck variation is known as an adaptation

An adaptation is any inherited characteristic that
increases an organisms chance for survival
Natural selection


Organisms with adaptations favorable to
the current environment will be the ones
most likely to survive and reproduce
Since those that are better adapted for
survival leave more offspring, their
advantageous traits (adaptations) will
increase in frequency in the population.
Change Over Time

After many generations, the population
will be almost entirely made up of
individuals with the advantageous trait (all
adapted to their environment)!
Essentials to Remember …

Organisms cannot “try” to adapt, nor does it
give them what they need!!!

Variation must be present in the population!

Either an organism has the trait (adaptation) to help it
survive & reproduce or it doesn’t!!!
Common descent

Darwin stated that species can change over time,
passing favorable traits on to the next generation

This principle of change over time is called
descent with modification

With this in mind, Darwin proposed another idea. He
suggested that if species change over time and
become new species, that there must be a point in
history where there was a single, common
ancestor, that evolved and changed over time to
form all the species of life on earth today
Evidence for evolution

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There are four major pieces of evidence that
support Darwin’s theory
The fossil record
Geographic distribution of living species
Homologous structures
Similarities in embryology
The fossil record

Darwin not only collected live samples, but dead
ones as well, fossils

Some of the fossils Darwin collected resembled
organisms of his time, while others did not

Darwin saw the fossil record as a detailed record
of evolution through time
Geographic distribution of living species

Darwin noticed how several different
species of finch, all living in different areas,
shared similar traits

Some of these similar traits are known as
analogous structures - traits similar in
function, but not necessarily appearance,
due to a similar environmental influence
Example – Two different species, one living on South
America, the other living in Africa, live in similar environmental
conditions (climate, habitat, niche). Both species have similar traits
and behavior because the similar environments cause them to adapt in
similar ways.
Homologous structures

A structure is considered homologous if it
comes from the same embryonic tissue, but
has a different mature form


Example – Bird wings, human arms, and dolphin flippers all
serve different functions, but they all formed from the
same part of the embryo
Darwin noticed these similarities and differences
and suggested several things


These similarities suggest that all life descended with
modification from a single, common ancestor
We could group animals according to how recently they
shared an ancestor, ie how similar the structures are
Embryology

Vertebrate organisms at the embryo stage
of development seem to have certain
similarities

These similarities are related to the
tissues and cells all growing in similar
ways and may also be the reason for
homologous structure formation
Embryology
Evolution in populations

Evolution does not occur in individuals, it occurs in
populations

A gene pool is the entire group of genes that a population
shares

Relative frequency is the number of times an allele occurs
in a population, usually written as a %

Relative frequency has nothing to with the Law of
Dominance

A change in relative frequency implies evolution is occurring
Types of natural selection

Natural selection can affect the frequency
of phenotypes in 3 ways

Directional selection

Stabilizing selection

Disruptive selection
Directional Selection

Directional selection occurs when natural
selection favors a single phenotype and this
causes a directional shift in the allele frequency
In the graph to the
left, a food shortage
causes the supply
of small seeds to
run low, so natural
selection favors the
birds with large
beaks and we see a
shift in the allele
frequency
Stabilizing Selection

This type of natural selection
occurs when the individuals
with a middle or average
phenotype have the higher
frequency

The extreme ends will be
favored against

An example of this is human
birth weight – Babies born in
the “normal” range will have a
higher chance for survival than
those at the 2 extremes
Disruptive Selection

This happens when individuals at both
extremes have a higher frequency than
those in the average range
An example of this is
bird selection of seed
size. If the amount of
seeds with average
size decreases while
amounts of seeds with
larger and smaller size
increase, birds
specializing in those 2
sizes will increase in
frequency, while those
that do not will
decrease