Natural Selection & Adaptation

Unit 97C

Natural Selection

• Gradual process by which biological traits become either more or less common in a population

Nature basically “selects” certain individuals to be successful and

reproduce and over time the population changes due to this

selection.

What has happened to the mice population below?

The population has changed from 50% brown & 50% white to 80% white & 20% brown. WHY?

Adaptations Cause Fitness• Adaptation – any inherited characteristic that

increases an organism’s chance of survival. (Remember the source of any new trait is a

mutation.)• Adaptations make an organism better suited to

the environment. So in the struggle for survival, organisms with adaptations suited to the environment will be more likely to survive and reproduce.

• Fit offspring then survive and reproduce and even more offspring are “born” that have the adaptation.• Over generations, the DNA in the

whole population changes, so that all the population has the adaptation that makes the population fit for its environment.

Fitness • The ability of an organism to survive

and reproduce

Which lion is more fit?

If fitness is the ability to survive

& reproduce

This lion will hunt for more food, have the energy to reproduce, and pass on his genes to his offspring.

Traits that help an organism survive

1. Survive a predator2. Survive disease3. Compete for food4. Compete for territory

Traits that help individuals reproduce

• Attracting a mate• Compete for nesting sites• Successfully raising young

“Survival of the Fittest”

• Faster, stronger, good hunter, better camouflaged• Best organism that survives and

reproduces

Mesquite have the longest tap root of any desert species

Adaptation

• Any characteristic that increases fitness

Types of Selection

Types of Selection

Disruptive Selection

• Describes changes in population genetics in which extreme values for a trait are favored over medium values.

• The variance of the trait increases and the population is divided into 2 distinct groups.

Types of Selection

Stabilizing Selection

• Genetic decreases as the population stabilizes on a particular trait or value. It varies medium traits. “Average Joe wins”• Opposite of disruptive selection.

Types of Selection

Directional Selection• A single phenotype is favored, causing

the allele frequency to continuously shift in 1 direction.• Occurs under environmental changes

when populations migrate to new areas with different environmental pressures.

Natural Selection Summary Points

• Inherited variation – there are genetic variations (differences) between individuals of a population

• The more genetic diversity, the more variations of alleles that are suited for the environment. The more offspring will survive and reproduce.

• The population will then continue.

Natural Selection Summary Points

• Overproduction of offspring – there are more offspring are produced than can survive

• Struggle for existence – there is competition for survival among the members of the population

Witness to Evolution• Peppered Moth– 2 types: dark vs. light

Peppered moth

Peppered moth• Why did the population change?– early 1800s = pre-industrial England• low pollution• lichen on trees = light colored bark

– late 1800s = industrial• factories = soot coated trees• killed lichen = dark colored bark

– mid 1900s = pollution controls• clean air laws• return of lichen = light colored

bark

Peppered moth: Evolution in actionYear % dark % light1848 5 951895 98 21995 19 81

clean air, light-colored bark

pollution, dark-colored bark

Clean Air Act, light-colored bark

industrial melanism

Natural Selection & Fitness

Unit 97D

Genetic or Inherited Variation

• variation in alleles of genes that occurs both within and among populations

What determines these different traits or variations?

• Mutations in our DNA• crossing over & independent

assortment in meiosis.

Let’s remember…

• Genotype is the individuals set of alleles for a trait…

BB, Bb, or bb • Phenotype is the appearance or

expression of the trait…Homozygous black, heterozygous black, or

homozygous brown

48%

16%

36%Bb heterozygous blackBB homozygous blackbb homozygous brown

• 64% of the total mouse population has black fur• The black allele (B) occurs 40% of the

time while the brown allele (b) occurs 60% of the time. • The brown allele occurs more

frequently than the black. What change in the habitat could change the frequency of alleles?

Elements of Natural Selection

1. Inherited Variation2. Producing More offspring than can

survive3. Limited (finite) supply of

Environmental Resources

1. Inherited Variation• In an environment that favors black

fur, mice with brown fur will be eaten, and the frequency of the allele for brown fur (b) will decrease. Black mice will have a better chance of surviving and passing on their alleles to their offspring.

The frequency for the while allele in the mouse population below will decrease.

2. Producing More offspring than can survive

• Most populations produce more offspring than can survive. Many offspring are lost due to predation, starvation, etc… The offspring that do survive to adulthood will have phenotypes that give them an advantage within that environment over those that didn’t survive & reproduce.

3. Limited (finite) Supply of Resources• Individuals compete for food, water,

space, & shelter.• When resources become scarce, such

as during a drought, competition increases. Populations decline, and the individual with advantageous traits will survive and reproduce.

The ground finches with larger

beaks can eat a wider variety of

seeds & nuts.

If a drought occurs, the finches with smaller beaks will run out of food & the population of finches with larger

beaks will increase.

Natural Selection & Diversity

Unit 97E

Speciation

• the evolutionary process by which new biological species arise

Remember…

• Every species has different DNA.• Species can breed and produce

fertile offspring.

• How has this population changed over time?

Body has become red with spots

• What has caused this change?Mutations with redder body & blacker spots have been beneficial & allowed these insects to survive and reproduce. The mutations in the nucleotide sequences have now changed the DNA and a new species has evolved.

How do new species evolve?

Due to natural selection, individuals with adaptations that give them an advantage over other individuals in the same environment are more likely to produce and pass the adaptations to their offspring. If the adaptation (mutation) continues to be beneficial, over time, the trait is passed on to generations, and a larger portion of the population with have that adaptation.

Adaptation

Let’s remember, individuals within a species are genetically different.

Diversity in a Species

If the snow & ice melt, will this polar bear be able to grow thin brown fur?

NO!

But if a few mutated polar bears are born with thin, brown fur…

Diversity among Species

• The process of natural selection and other mechanisms of evolution have lead to the millions of species of living organisms living on Earth today and millions of different species that lived in the past.

How does speciation occur?Originally, there was 1 species of tortoise on 1 of the Islands.

How does speciation occur?

• An event occurred, like a storm, that stranded some tortoises on another island. These individuals started a new population in a different environment.

• Over time, mutations occurred, & the genetic variation increased so the 2 populations couldn’t mate & produce fertile offspring and were now 2 separate species.

How does speciation occur?Now, each island has at least 1 different species of tortoise.

Reproductive Isolation

• occurs when two populations cannot interbreed and a new species is formed (speciation occurs).

• A single species has evolved through natural selection into new and different species

Things that CAN (but don’t always) cause reproductive isolation:

1. Habitat isolation2. Behavioral isolation3. Geographic isolation4. Temporal isolation

Habitat Isolation

• 2 populations breed in different areas of a habitat • Ex. snakes that breed in the water vs.

the land

Habitat Isolation(a)

Water-dwelling Thamnophis Terrestrial Thamnophis

Behavioral Isolation• 2 populations can breed, but

behavior causes them not to. • Ex. Eastern & Western meadowlarks

using 2 different songs to attract mates

http://bio1100.nicerweb.com/sketches/sounds/Chordata/Aves/Melospiza_melodia.mp3

Behavioral IsolationCourtship ritual of different populations of

Blue-Footed Boobies

Geographic Isolation

• 2 populations are separated by a geographic barrier (river, mountain, etc.). • Ex. Colorado River separated

squirrels

Geographic Isolation

Geographic Isolation

Geographic Isolation

Temporal Isolation

• 2 populations breed at different times.• Ex. orchids that pollinates once a

year

Wood & Leopard Frog