Download Natural selection

What is Evolution???
Learning Target: I can explain Natural
Selection and the 4 conditions that are
required for Natural Selection to take place.
What is Evolution???
Change in the heritable (genetic)
traits of a population over time
Note: when we discuss evolution, we are talking about
populations changing, not individuals
Natural selection:
The main mechanism by
which evolution occurs
4 Conditions for Natural Selection:
1. Variation: Individuals in a population are
not identical to each other.
2. Inheritance: Traits are passed to offspring;
traits have a genetic basis
3. Environmental population limits:
Environmental limiting factors prevent all
individuals from surviving to reproduce;
some die young.
4. Environmental selection:
• Individuals in the population with more
favorable (advantageous) traits are the
ones that survive to reproduce.
• Individuals without advantageous traits
die before reproducing.
These factors result in a change in the average
trait of the population… Biologists call this
EVOLUTION!
1. VARIATION
Members of a population have traits similar to the
average trait of the entire population, but they are
not identical.
YOUR TURN:
Using height as
an example,
sketch a graph to
represent the
statement above.
Mean (average) height
Frequency
•
Height (cm)
2. INHERITANCE
• DNA determines the traits of individuals
DNA
trait
• Individuals inherit DNA from their parents
• This causes the traits of the offspring to resemble
the traits of the parents
3. ENVIRONMENTAL POPULATION LIMITS
Population
A. For all species, if every individual born into a
population were to reproduce, the population
would grow exponentially
Time
3. ENVIRONMENTAL POPULATION LIMITS
Population
B. Environmental factors (limiting factors) prevent the
majority of individuals from surviving to reproduce
Time
What are some examples of limiting factors?
4. SELECTION
•
Individuals without advantageous traits
die before reproducing.
Individuals with advantageous traits
survive to reproduce.
Frequency
•
These individuals
survive to reproduce
Characteristic
Populations change, not individuals
Frequency
• The “average” characteristic or other measure of the
population changes over generations
These individuals
survive to reproduce
Characteristic
The environment is the selective force
behind evolution
• The environment determines what
characteristics are “favorable”
• Characteristics are said to be “selected for”
• Because the environment changes over time,
the characteristic that is more favorable for a
population changes
• Therefore, characteristics of the population
change, or evolution occurs
Artificial Selection
• Humans select for the desired trait
• Example: plant & animal breeding
• Not an example of evolution
Why is genetic variation in a population
important?
•
A population without much variation limits a
species’ ability to further evolve. (Variation is
one of the 4 factors required for natural
selection to cause evolution)
•
If populations do not adapt to their
environment, they may become extinct.
Sources of genetic variation
1) SEXUAL REPRODUCTION
A. Meiosis – one copy of a gene is passed on from
each parent, combination may be different
than someone else
B. Random fertilization – only one of the millions
of sperm involved in mating will fertilize the
egg.
The randomness of sexual reproduction explains
why siblings can look so different.
Sources of genetic variation
2) MUTATION
Mutation means a change in DNA sequence.
- Can happen when DNA is copied during cellular
replication
A new DNA sequence can result in new variation (not
always).
–
–
–
Many mutations produce traits that are harmful
(e.g. Huntington’s disease)
Some mutations produce traits that are neutral
(neither helpful nor harmful)
Very, very few mutations produce traits that are
advantageous
Natural Selection
• Individuals with advantageous genes survive
to reproduce and pass on these genes to their
offspring.
• Individuals without advantages genes do not
survive to reproduce, and these genes do not
get passed on in the population.
• “Survival of the Fittest” – best fit (able to
survive and reproduce) in the environment
• Natural Selection leads to adaptation!!!
Conclusion
• Mutation does NOT cause evolution
– it is only a source of variation (just one of the factors
required for natural selection)
• Natural selection determines if the frequency of
traits will change within a population.
• Change in traits in a population = EVOLUTION