Natural Selection & Change

Chapter 1

How Did Darwin come to study natural history?

How Did Darwin come to study natural history?

• Charles Darwin originally studied medicine, but he disliked it.

• He became interested in natural history instead.

• His professor John Henslow helped him join the HMS Beagle voyage as the ship’s naturalist.

• On the voyage, Darwin collected many plants and animals and observed differences among species.

• Darwin also read Charles Lyell, who argued that Earth changes gradually over very long periods of time.

• This helped Darwin realize that species could also change gradually over long periods.

Know: Beagle voyage + observations + Lyell → helped Darwin develop his ideas about evolution.

How did animal breeding contribute to Darwin's theory of evolution?

How did animal breeding contribute to Darwin's theory of evolution?

• Darwin noticed that breeders could change animals over generations.

• Breeders choose animals with desired characteristics.

• They breed those animals together.

• Their offspring are more likely to have those characteristics.

• Repeating this over generations can produce major changes.

Artificial selection

Definition: Humans selectively breed organisms for desired traits.

Example: Breeding cows, horses, pigs, sheep, chickens, dogs, cats, and other domestic animals for particular characteristics.

Darwin realized that if humans can cause species to change through selection, nature might be able to do something similar.

How did Malthus influence Darwin?

How did Malthus influence Darwin?

Thomas Malthus argued:

• Populations can grow very quickly.

• Resources such as food are limited.

• Eventually there will not be enough resources for everyone.

• Therefore, organisms must compete for survival.

Darwin applied this idea to nature: More organisms are born than can survive.

Individuals with characteristics that help them survive are more likely to reproduce and pass those characteristics on.

Easy chain to remember:

Too many offspring → limited resources → competition → some survive/reproduce more → helpful traits spread

Natural Selection vs. Artificial Selection

Natural Selection vs. Artificial Selection

Artificial selection

• Humans decide which organisms reproduce.

• Humans select traits they want.

• Example: breeding dogs for particular characteristics.

Natural selection

• The environment determines which traits are beneficial.

• Individuals with useful characteristics are more likely to survive and reproduce.

• Those characteristics become more common over generations.

Main difference

Artificial = humans select
Natural = environment selects

Darwin called this idea natural selection.

Why are inheritance and variability important?

Why are inheritance and variability important?

Natural selection requires variation between individuals.

Variation / variations

Definition: Differences in characteristics among individuals of the same species.

Example:

• Some individuals may be lighter or darker.

• Some may run faster.

• Some may resist disease better.

For natural selection to change a species:

1. Individuals must have different traits.

2. Some of those traits must affect survival/reproduction.

3. The useful traits must be inherited by offspring.

4. Over generations, those traits become more common.

Darwin did not know exactly how inheritance worked. Gregor Mendel's work on inheritance was not widely known until later.

Key point:

No variation = natural selection has nothing to select.

How can natural selection produce complex structures like the eye and brain?

How can natural selection produce complex structures like the eye and brain?

Complex structures do not need to appear all at once.

Instead:

Small useful change → selected → another useful change → selected → repeated for millions of years

Evolution of the eye

The first eye may have been only a few light-sensitive cells.

Even this simple structure would help an animal detect:

• Light vs. darkness

• Possibly the shadow of a predator

Small variations could gradually make the eye more effective.

The pages give examples of different levels of eye complexity:

• Very simple light-sensitive structures

• More developed eyes

• Complex eyes such as those of humans

Each intermediate stage can still be useful, so natural selection can preserve it.

Brain

The same principle applies to the brain.

Examples progress from:

• Jellyfish → simple network of neurons

• Worms → primitive nervous system

• Crayfish

• Bees

• Monkeys/apes

• Humans

Natural selection does not have a final goal. Increasing complexity can develop when it improves survival or reproduction.

Examples of natural selection helping animals cope with environmental change

Examples of natural selection helping animals cope with environmental change

You should know at least 3 examples.

1. White Sands lizards

2. Peppered moths

3. Human skin color

1. White Sands lizards

1. White Sands lizards

The environment changed to white sand dunes.

• Originally darker lizards stood out.

• Lighter lizards were harder for predators to see.

• Light-colored lizards survived/reproduced more.

• Eventually the dune population became much lighter.

Environment change → camouflage becomes advantageous → lighter color spreads

2. Peppered moths

2. Peppered moths

Originally:

• Trees were light-colored.

• Light moths were camouflaged.

• Dark moths were easier for birds to see.

After pollution from burning coal:

• Trees became darker.

• Dark moths became better camouflaged.

• Dark moths increased in the population.

When pollution later decreased:

• Trees became lighter again.

• Light-colored moths increased again.

Important: The environment changed which color was most useful.

3. Human skin color

3. Human skin color

Amount of melanin is influenced by sunlight.

• Areas with intense sunlight → darker skin helps protect against strong sunlight.

• Areas with less sunlight → lighter skin helps people produce enough vitamin D.

Different environments favored different levels of skin pigmentation.

How can disease drive natural selection?

How can disease drive natural selection?

Disease can act as an environmental pressure.

Example: Black Death / plague

• Many people died.

• Some individuals were genetically more resistant.

• Resistant individuals were more likely to survive.

• Survivors reproduced and passed resistance-related traits to offspring.

• Over generations, resistance could become more common.

The same general principle applies to other diseases such as HIV/AIDS and potentially new epidemics.

Easy chain:

Disease → variation in resistance → resistant individuals survive more → reproduce → resistance spreads

How can natural selection change behavior?

How can natural selection change behavior?

Natural selection can affect behavior, not just physical traits.

Example: partridge wing flapping

Some partridges instinctively flap their wings when they hear certain sounds.

Researchers found that:

• Wing flapping may help birds escape predators.

• Birds that flap effectively are more likely to survive.

• If this behavioral tendency is inherited, it can become more common.

Key point:

If an innate behavior varies between individuals and one version improves survival/reproduction, natural selection can favor that behavior.

Natural selection:

Natural selection — Process in which individuals with characteristics that improve survival or reproduction leave more offspring, causing those characteristics to become more common over generations.

Artificial selection:

Artificial selection — Humans choose which organisms reproduce in order to increase desired characteristics.

Variation / variations:

Variation / variations — Differences in characteristics among individuals within a species.

Population

Population — A group of individuals of a species; Malthus argued populations can increase faster than available resources.

Melanin

Melanin — Substance involved in skin pigmentation; different amounts of melanin can be advantageous under different levels of sunlight.