Transcript Document

Unit2: Life Over Time
Chapter 6: Evolution of Living Things
6.1 Earth has been home to living things for about 3.8
billion years
6.2 Species change over time
6.3 Environmental changes can affect populations
6.4 Many types of evidence support evolution
Natural Selection explains how living things evolve
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Overproduction:
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A plant or animal that reproduces usually makes more offspring than can possibly
survive (than the environment can support):
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These are passed on from one generation to the next
Genetic material may change – new variation (mutation)
Reproduction: passes these traits to offspring
Adaptation
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A mutation can make an individual better able to survive
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Genetic variation: differences in DNA in a population
Species have natural differences/variations in traits – ex: fin size
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Ex: several thousand salmon eggs, not all hatch, a few hundred survive disease or
predation, several dozen reach adulthood, and few will successfully reproduce
Variation
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Population: group of individuals of the same species that live
in a particular environment or area
An advantage in the particular environment; ex: tail shape – swim faster
Selection
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Environmental factors: conditions that affect survival – include
food supply, habitat, predators, and disease
Particular adaptations lead to survival long enough to reproduce
That adaptation becomes more common in the next generation of offspring
6.3 Environmental changes can affect
populations
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Before, you learned:
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Species change over time
Organisms need resources such as energy and space
Organisms in an ecosystem affect one another
Now, you will learn:
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How populations increase and decrease in size
What factors affect population size and survival
How a species can become extinct
Population size is limited by
environmental conditions
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What if a population reproduced without limits?
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In nature, Darwin notice:
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Pair of fruit flies  200 eggs in two weeks…
> eight months, the mass of fruit flies would be more than the
mass of Earth
All populations have the ability to grow rapidly over time
Populations tend to remain about the same size
Populations could increase greatly but usually
didn’t…why?
Population size is limited by
environmental conditions
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Resources – food, water, shelter, mates, etc – are
limited
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Natural limits to population growth
Populations must be stable in order to survive
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Cannot have too many or too few members for very long
Populations increase through births
and immigration
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More than enough resources  population
size increases
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Key factor is high birth rates
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# births > # deaths
Immigration: movement of individuals into a
population
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Wind blows seeds, deer travel to new forest
Can also help a population that is decreasing in size to
become stable
Many factors limit population size
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Decline can happen two ways:
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# death > # births
Emigration: movement of individuals out
of a population
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Possibly to an area with more resources
or fewer individuals
Can help stabilize a population growing
too quickly
Limiting factors: environmental
factors that limit population size
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Include food, water, light, space, shelter,
and air
Also includes other organisms, and
sudden changes in the environment
(storm, fire, etc.)
Competition
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Can be a matter of life and death
Can be between the same species or
different species
Ex: wolves competing for food
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Too many wolves, not enough resources:
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some may emigrate
Unhealthy, very young, or very old, may starve to death
or fall prey to predators
Predators
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Prey on other animals
Major role in limiting population size
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keep the population of their prey stable
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Can keep the prey from outgrowing resource supplies
Can keep rodents and other pests in check!
Disease
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As a population grows, close contact
increases chance of disease spreading
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Those lacking natural defenses may die or grow
sick
Ex: 1900s – American chestnut trees infected by
fungus
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By 1950, almost all had died
Catastrophic Events
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Wildfires, hurricanes, floods, droughts, etc
Can wipe out an entire population
Ex: drought affecting certain plants – impacts animals
that feed on those plants
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Could also affect animals that use it as shelter
The survival of a species depends on
its ability to adapt to change
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Extinction can occur when a species can no
longer survive in their environmental
conditions
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Limiting factors - resources, predators, etc.
Usually, one population at a time
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Individuals better adapted to change are more
likely to survive and produce offspring
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Genetic variation assists in there being a chance that
some survive environmental changes
How Extinction
Happens
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Happens when ALL populations of a SPECIES die out
Maybe caused by a change in the environment
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Ex: disease, loss of habitat
Loss of population members decreases
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genetic variation
Chances members of finding a mate
A disease can change a species’ environmental
condition
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Ebola virus
Some individuals may have a natural resistance (due to genetic
variation) – they survive
Repopulation depends on how long it takes a species to
reproduce
Extinction and Species
Replacement
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Extinction is part of the cycle of life on earth
Makes room for other species to thrive
Mass extinctions are usually followed by the sudden
appearance of new species
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Ex: The Cretaceous (K-T) extinction 65 million years ago wiped
out the dinosaurs…
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Some small mammals that had existed survived and were then able
to thrive
(K-T extinction is thought to be caused by an asteroid impact
triggering earthquakes and volcanic eruptions, leaving a lot of dust in
the air – decreased sunlight:
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decreased photosynthesis – affecting food supply
Lowered temperatures
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Mammal’s fur helped them adapt to a cooler climate
Select marine species