Main Points of Darwin’s Theory of Natural Selection 1.Over production. Most organisms produce more...
-
Upload
ruby-robbins -
Category
Documents
-
view
214 -
download
0
Transcript of Main Points of Darwin’s Theory of Natural Selection 1.Over production. Most organisms produce more...
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
© 2011 Pearson Education, Inc.
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
Per
cen
tag
e o
f P
op
ula
tio
n
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.
Nu
mb
er o
f B
ird
sin
Po
pu
lati
on
Beak Size
Population splits into two subgroups specializing in different seeds.
Beak Size
Nu
mb
er o
f B
ird
sin
Po
pu
lati
onKey
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