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

• 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