Natural Selection

Natural Selection

Natural selection is the mechanism by which

evolution happens.

Thus, evolution and natural selection are inextricably

woven together.

Natural SelectionThe unequal survival and reproduction of organisms due to

environmental forces, resulting in the preservation of

favorable adaptations.

It is a two step process:1. The production of variation in a population

2. Non-random aspects of survival and reproduction

Natural Selection is a two step process:

Step One: Production of variation (Chance)

Mutations

- review 1 gene = 1 protein protein synthesis

- form can’t be separated from function

Meiosis

- recombination due to crossing-over in 1st division

- random movement of chromosomes in both

Step Two: Non-random aspects of survival and

reproduction

Superior success of certain phenotypes

Nonrandom mate choice

Recall the four basic aspects of natural selection:

1.Populations produce more offspring than the

environment can support.

Recall the four basic aspects of natural selection:

2.Overpopulation leads to competition for limited

resources.

Recall the four basic aspects of natural selection:

3.There is (genetic) variation within a population.

Recall the four basic aspects of natural selection:

4.Those most able to secure resources (fittest)

disproportionately pass on their adaptive traits to the

next generation (survive to have more offspring.)

Remember to Forget…

All your common misconceptions about evolution.

• evolution has a goal or end in mind

• evolution leads to perfection

• a single individual can evolve

• fittest means biggest, strongest, fastest

• it’s only about survival

• humans came from apes

Selection pressure: force that tends to increase allele frequencies

associated with adaptive phenotypes.

• Competition for food

• Competition for a mate

• Changes in the environment

• Predators

• Parasites

Normal Distribution

When graphed using a

histogram, data sets can

have different

distributions.

When data has no left

or right bias and is not

jumbled, it is said to

have a “normal

distribution” or bell

curve.

Normal DistributionsThey aren’t always “perfect”

but you can see the trend.

Main Types of Selection Pressures• Directional Selection

– Natural selection favors one extreme of the population for that trait

– often happens when environment changes in a consistent way- e.g.

climate gets colder.

• Disruptive Selection

– Natural selection favors both extremes selected

– Causes species to diverge

• Stabilizing Selection

– Natural selection favors the average for population selected

• Sexual Selection

– Mate choice

Directional Selection• Neck of Giraffe

• Antibiotic resistance of bacteria

• Moth color (melanin)

• Camouflage/Mimics

• Many sexually selected traits

Directional Selection

Stabilizing Selection• When the extremes of the

trait aren’t as well suited

Examples

• Bird clutch size

• Elk Antlers size

• Human birth weight

• Tail length in birds

– Example of exception?

Explain!

Stabilizing Selection

Disruptive Selection• Causes divergence within the

species

• Occurs when two different

types of resources in one

area

• Results in specialization for

each branched group

• May lead to formation of

new species

Disruptive Selection

Every species’ ultimate fate…

Adapt to changing conditions and evolve into new species.

OR

Extinction without leaving decedents.