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Fact #1
exponential increase of 

All organisms (even unicellular) have the ability to 
produce many more offspring than those 
needed to replace their parents. If all 
offspring actually survived, the every 
population would increase exponentially 
from generation to generation.


Fact #2
Steady state of 
most populations

In nature, rarely do populations actually 
increase exponentially for any length of 
time. While some populations fluctuate 
dramatically, increases are eventually offset by decreases, and the typical population from generation to generation 
is pretty close to the long term average (carrying capacity).


Fact #3
limitation of resources

The main reason populations do not 
keep on increasing forever is that they 
run out of resources. In most environments, the availability of needed 
resources keeps each population within 
fairly strict limits.


Inference #1
Competition for 
resources within 

If there are more potential members of 
a population than can be supported by 
the resources, then all members will be in competition with all other members to get the resources they need.


Fact #4
Individuals vary 
within a 

At least in sexually reproducing populations, every individual is 
unique, and even in asexual species there is some variation among 
individuals. (Darwin went to great 
lengths to show this, because people 
of the time did not recognize that 
other organisms were as individual as 


Inference #2
survival and 
reproduction of 

Some individuals will cope 
better with the environmental 
limits than others, and will 
thus leave behind more offspring than others in the 
next generation.

Fitness: ability to produce offsprings. It depend on the environmental pressure (selection is editing by exclusion)


Fact #5
Much variation is 

Offspring resemble their 
parents. As long as the advantages can be inherited, 
more individuals should have 
them in the next generation. 
Inheritance was Darwin's 
nightmare, as his model was 
inadequate to allow 
evolution, but modern 
genetics has filled that gap.


Inference #3
Change (adaptation) of 
population over time = 

If surviving organisms pass on 
the things that make them better 
competitors generation after 
generation, eventually these features will become common in 
the population. The population will appear better suited for the 
conditions it faces. The 
population has adapted.


what is adaptation

Adaptation* - A trait that gives an individual an
advantage in its environment OR
the process by which a population acquires
such a trait due to natural selection


Natural Selection

Natural selection - The effect of the environment in favouring some individuals over others in the competition to survive and replicate their genes. In fact, natural selection is an editing process, that functions more by the exclusion of certain individuals from the breeding population, rather than by favouring other individuals.
- Natural selection edits out the traits least suited to the environment.