Main Points of Darwin’s Theory of Natural Selection

1. Over production.Most organisms produce more offspring than can survive.

2. Competition.Organisms compete for food and resources.

3. Variation.There is variation among individuals of a species.

4. Adaptation.Individuals with traits best suited to the environment will survive.

How Gene Frequencies Change

Sources of Variation• Ways that new adaptations and

gene frequencies arise– S – sexual reproduction– C – crossing over during meiosis

(recombination of genes)– A – arrangement of chromosomes

(alleles) during meiosis

– M – mutations of DNA

KEY CONCEPT

• Evolution by Natural Selection, causes changes in POPULATIONS!

Population EvolutionPopulation genetics genetic principles as they apply to entire populations of organismsPopulation group of organisms of the same species living in the same area

Genotype the representation

on the gene of an organism

Phenotype the physical trait

shown by a genotype

Allele different form of a gene

Gene pool combined genetic

info. for all members of a

population

Population Genetics

• Natural selection acts on individual’s phenotypes not genotypes.

• Populations evolve– Individual’s genes

will stay the same– Population’s gene

pool may change over time due to Natural Selection

• Which trait was a better adaptation and selected for?

• How were frequencies of different alleles affected?

Population Genetics

Gene Traits:

A) Single gene trait: controlled by single gene with two alleles

♦ Examples: widow’s peak, hitchhiker’s thumb, tongue rolling

B) Polygenic trait: controlled by 2 or more genes, each with 2 or more alleles

♦ Examples: height, hair color, skin color, eye color

Most human traits are polygenic.

This type of variation can cause different types of selection of one phenotype over others

Directional, Disruptive, and Stabilizing Selection

• Three modes of Natural Selection:– Directional selection favors individuals at one

end of the phenotypic range

– Disruptive selection favors individuals at both extremes of the phenotypic range

– Stabilizing selection favors intermediate variants and acts against extreme phenotypes

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Three Types of Natural Selection Three Types of Natural Selection

Stabilizing Selection = maintains an already existing system.

> eliminates organisms that deviate from the norm.> environment must remain unchanged.> explains why there are “living fossils.”> as long as environment doesn’t change,

organisms won’t change. Horseshoe CrabGinkgo Tree

Key

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Birth Weight

Selection against both

extremes keep curve narrow and in same

place.

Low mortality, high fitness

High mortality, low fitness

Stabilizing Selection

Stabilizing Selection

Directional Selection = favors one extreme or the other.

> eliminates organisms that are not in that extreme.

> eventually leads to changes in the population.

> occurs when organisms must adapt to a change in their environment.

> may develop into a RESISTANCE (the ability of an organism to withstand a harmful agent).

MALARIA

Directional Selection (page 398)

Food becomes scarce.

Key

Low mortality, high fitness

High mortality, low fitness

Directional Selection

Disruptive Selection = favors two extremes at one time.

> eliminates organisms that are more common.

> eventually leads to changes in the population.

African butterflies can range from red to blue.The red and blues are foul-tasting to predators.

The other colors are eaten more often resulting in a selection in favor of the extreme colors.

Disruptive Selection (pg 399)

Disruptive Selection

Largest and smallest seeds become more common.

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Beak Size

Population splits into two subgroups specializing in different seeds.

Beak Size

Nu

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Low mortality, high fitness

High mortality, low fitness

ORGIN OF SPECIES

What is a Species? A Group of interbreeding organisms that can produce fertile offspring.

The Origin of Species The Origin of Species (Macroevolution)(Macroevolution)

Macroevolution

• the formation of new species between organisms (speciation) and accompanying events

Microevolution • refers to changes in allele frequencies in a

gene pool from generation to generation. Represents a gradual change in a population.

• Macroevolution Microevolution

How Do New Species Form? Speciation- formation of a new species

Causes of Speciation:

1. Geographic Isolation2. Temporal Isolation3. Behavioral Isolation4. Ecological Isolation

Geographic Isolation

• Physical separation of members of a population

(by formation of a canyon, mountain, river, etc.)– Leads to different adaptations on

each side of barrier, and eventually new species (allopatric speciation)

Geographic Isolation = New Species: An Example

Temporal Isolation

• Species isolated because they reproduce in different seasons or times of day

• Prevents species from ever breeding together – keeps two species separate but usually arises after species have formed.

Behavioral Isolation

• Species Differ In Their Mating Rituals (e.g. different bird songs, mating colors, dances, pheromones, etc.)

Ecological Isolation

– Species inhabit the same area, but different habitats, so they don’t encounter each other

Other Factors Can Affect Genetic Variation In A Population

• Other factors that increase variation in the genetic material (gene pool) of a population on which natural selection acts:– Random/nonrandom mating =

sexual selection– Gene traits-single/polygenic– Isolation– Genetic Drift– Fitness is the relative ability

of genotypes to survive and reproduce

Microevolution refers to changes in allele frequencies in a gene pool from

generation to generation. Represents a gradual change in a population.

 Causes of microevolution:                        1)  Genetic drift

2) Natural selection

3) Gene flow (migration of genes from one population to another)

2) Mutation

GENETIC DRIFTDNA frequency in a population

changes simply by CHANCE not fitness

FOUNDER'S EFFECT:

some individuals of a population colonize a new habitat

ex: Columbus bringing organisms to USA

Spain: DNA Frequency66% pink33% red

S. America: DNA Frequency25% pink75% red

Bottleneck Bottleneck Effect:Effect:

Small representation of the original population after a catastrophic event. Alleles in gene pool are not equally represented and species may evolve.

HOW DOES GENETIC DRIFT AFFECT THE DIVERSITY OF A

POPULATION OVER TIME?

Genetic drift decreases diversity. In this case yellow and pink DNA are gone, and red DNA is now the majority.

Genetic drift = decrease in DNA diversity

Patterns of Evolution

• Convergent Evolution

• Divergent Evolution

• Coevolution

Convergent Evolution• Organisms appear similar, because of similar habitat and

selection pressure. These organisms aren’t closely related

• Why?– Similar environments select for similar traits

Divergent Evolution•  accumulation of differences between

groups which can lead to the formation of new species; evolving from a common ancestor

• Related populations or species become less and less alike

A Second Type Of Divergent Evolution

• Artificial Selection– Humans, rather than

nature, select traits

Coevolution

• Two or more species in close association w/one another change together– Predator/Prey– Parasite/Host– Plant/Pollinator