Transcript Darwin`s idea ppt

Darwin’s Big Idea
LNW09 Berg
How sheep herds change
• Go back to Biblical
times
• Ancient way of doing
things
• Still being done
Two herds of sheep
• Start off identical
• In each herd
– Half of sheep
dark (black)
– Half of sheep
light (white)
Two tribes of shepherds
One prefers dark clothing
One prefers light clothing
Shepherding 101
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Herd of sheep
Dark sheep have dark kids
Light sheep have light kids
Limited herding area
– Lambs born every spring
– Must sell or eat excess
– Can’t support more than regular herd size
Spring of 2009 BC
• Good winter
• Your herd doubles
• A few die of wolves,
theft, clumsiness,
disease
– Lose equal black &
white
• Which ones do you
sell or eat?
Selection of sheep
I like dark sheep. I’ll get rid
of mostly the light ones.
I like light sheep. I’ll get rid
of mostly the dark ones.
Late 2009 BC herds
Selected herds are different
Spring of 2008 BC
• Flock got big again
• Not enough grazing
for all
• A few die of wolves,
theft, clumsiness,
disease
– Equal black & white
• Which ones do you
sell or eat?
2008 BC herds
Repeatedly selected
herds are more
different
2007 BC herds
More selection gives
more difference.
Artificial selection
• Human decides
– Who lives (reproduces)
– Who dies (or doesn’t reproduce)
– Therefore what is in next generation
• Individuals don’t change
– Group (herd) changes
• Works for plants, animals, fungi, pets,
birds…all reproducing biology
Goats in nature
• Two herds of wild
goats
• Start with
– Half black, half white
• Herds migrate to
different places to
have their babies
• Some places are
light, some are dark
Herd in dark environment
Babies born
Herd in light environment
Babies born
The problem
Predator 101
• You can only grab small things
– No full-grown healthy sheep
• You can only grab what you can see
• Eat all you can today
– Small, easily spotted lambs (awwww)
• There is more here than you can eat
What happens?
• Babies get eaten (some of them)
– Light babies in dark area
– Dark babies in light area
• Next year the dark area herd will be darker
– Many light babies eaten
• Next year the light area herd will be lighter
– Many dark babies eaten
• Herd changes, not individuals
Natural selection
• Group changes due to selecting one kind over
another
– Individuals aren’t changing
• Humans not directly involved in making choice
– Pollution can still cause natural selection
• Examples
– Dirty cities favor darker prey on buildings
– Microbes become antibiotic resistant when
antibiotics are present
For change to occur
• Need a population with variation
• Need an environmental factor to favor some
and discriminate against others
• Need a selection agent (who decides?)
– Humans: artificial selection
– Nature: natural selection
• Need time (generations)
• Artificial selection is thousands of years old
• Natural selection was Darwin’s big idea
• Practiced by Mother Nature for eons
Variations
• Strong selection pressures (deadly disease)
can change population fast
– Weak pressures (minor parasite) have small
effects (hard to see)
• The longer you wait, the more change you will
see
• We see this happening in the natural and
human world all the time
Natural Selection
• Darwin’s big idea
• He never even used the word “evolution”
• He had no idea how the variation
happened in the first place
• His idea was about HOW change
happened
• Still looks good 150 years later