Darwin’s Big Idea LNW09 Berg
Dec 18, 2015
Two herds of sheep
• Start off identical• In each herd
– Half of sheep dark (black)
– Half of sheep light (white)
Shepherding 101
• 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.
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?
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
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