Download Evolution - Parma City School District

Evolution
Evolution= change in a species, populations, or
groups of species over time
• Populations vary by the frequency of
inheritable traits from one generation to the
next
• Traits are represented by alleles
• Alleles may modify morphology (form or
structure), physiology, or behavior
• Evolution = changes in allele frequency over
time
Two areas of Evolutionary Study
exist:
• MICROEVOLUTION:describes how populations
change from one generation to the next
• describes how new species originate
• focus is on individuals
• MACROEVOLUTION: describes changes in groups
of related species
• covers broad periods of time
• determines relationships between species
Jean Baptiste Lamarck: early theory of
evolution (1809)
• 3 important ideas:
• Use and Disuse: body parts of organisms can
develop with increased usage, while unused parts
weaken
• Inheritance of Acquired Characteristics: body
features acquired during the lifetime of an organism
(muscle bulk) could be passed on to offspring.
THIS IS NOT CORRECT!
• Natural Transformation of Species: offspring
produced with changes, future generations will be
different
Charles Darwin
• Charles Darwin (1860) published “The Origin of
Species”; proposed the idea of Natural Selection
= “Survival of the Fittest”
• Studied as a Naturalist aboard the H.M.S. Beagle
• Studied extensively in the Galapagos Islands
• Darwinism = Natural selection is the driving force
• NEO–Darwinism = today’s view, incorporates
genetics
Natural Selection
• the differences in survival and reproduction among
individuals in a population due to interaction with the
environment
• some genotypes provide better traits for coping with
the environment
• More successful individuals produce more offspring
• Superior traits = Adaptations  increase fitness
• Favorable traits are adaptive (selection acts for the
trait)
• Unfavorable traits are maladaptive (selection acts
against the trait)
Darwin’s arguments for Natural
Selection:
• Populations possess an enormous reproductive potential
(Darwin calculated that 2 elephants would produce a
population of 19 million after 750 years if all offspring
survive!)
• Population sizes remain stable
• Resources are limited
• Individuals compete for survival
• There is variation among the members of a species
• Variation is heritable
• Only the most fit individuals survive
• Evolution occurs as advantageous traits accumulate
• Result = small changes accumulate over large
periods of time
• Evidence for evolution does exist and can be
observed.
• The mechanisms by which evolution occurs
have been the cause for debate through the
ages!
Some of the other beliefs for the origin of life are
not based upon science:
• Extraterrestrial origin
• “Intelligent Design” – religion
• “Big Bang” theory
• Others?
KINDS OF SELECTION: Natural Selection
may act upon a population in a variety of
ways…
• 1. Stabilizing Selection= eliminates
individuals that have extreme or unusual
traits
• Individuals with the most common traits are
the best adapted
• Frequency of common traits is maintained
• Other trait variations are selected against
• 2. Directional Selection= favors traits
that are at one extreme of a range of traits
• Traits of the opposite extreme are selected
against
• Favored traits become more frequent
• Examples: Insecticide resistance, Industrial
Melanism (peppered moth)
• 3. Disruptive Selection (Diversifying
Selection) = occurs when the
environment favors extreme or unusual
traits, selects against common ones
• Examples: weed in Nature are tall, in lawns
short
• 4. Sexual Selection = the differential
mating of males (sometimes females) in a
population
• Females choose superior males for mates
(Female Choice)
• Male Competition: strongest males
reproduce
• Leads to sexual dimorphism = differences
in the appearance of males and females
• This can lead to disruptive selection
SOURCES OF VARIATION
• In order for Evolution to occur variation must
be present
• CONSIDERABLE variation exists in all
populations
• Variation arises from the following
mechanisms:
Sources of Variation
1. Mutations = provide the raw
material, can invent alleles that never
existed before
Sources of Variation
2. Sexual Reproduction = new
combinations of alleles, genetic
recombination
• Crossing Over – exchange of DNA
between non sister chromatids
• Independent Assortment of homologous
pairs
• Random joining of gametes
Sources of Variation
3. Diploidy = copies of each
chromosome in a cell; recessive alleles
can be hidden
Sources of Variation
4. Outbreeding = mating of unrelated
partners
Sources of Variation
5. Balanced Polymorphism = maintaining
different phenotypes in a population
• Heterozygote Advantage: Sickle Cell
Anemia
• Hybrid Vigor: superior quality of offspring
resulting from 2 different strains of inbred
plants
• Frequency Dependent Selection (Minority
Advantage): when the least common
phenotypes have the biggest advantage
Important things to remember:
• Not all variation has selective value!
• Much is Neutral Variation i.e. fingerprints
in humans
• The environment often determines whether a
variation is neutral or whether it has selective
value.
Causes of Changes in Allele
Frequencies
• Natural Selection was Darwin’s mechanism
for evolution
• With the understanding of genetics, the picture
becomes more clear
• Other factors can change allele frequency
other than natural selection
Causes of Changes in Allele
Frequencies
1. Natural Selection: increase or
decrease in allele frequency due to the
impact of the environment
2. Mutations: Introduce new alleles that
may provide a selective advantage; most
however are deleterious (harmful)
3. Gene Flow:alleles introduced by
emigration (leave) vs. immigration (enter)
Causes of Changes in Allele
Frequencies
4. Genetic Drift: random increase or decrease
of alleles; the effect is strongest in small
populations
• The Founder Effect: Occurs when the allele
frequencies in a group of migrating individuals
are, by chance, not the same as their
population of origin (Amish & Polydactyly)
• Bottleneck: occurs when a population
undergoes a dramatic decrease in size; the
small group left becomes vulnerable to genetic
drift (environmental catastrophes)
Causes of Changes in Allele
Frequencies
5. Nonrandom Mating: mates chosen
based on particular traits similar to their
own, choose nearby individuals
• Inbreeding: individuals mate with
relatives
• Sexual Selection: females choose
males based upon attractive appearance,
behavior etc.
GENETIC EQUILIBRIUM
• Occurs when allele frequencies remain
constant from generation to generation
• This is known as The Hardy-Weinburg
Principle
• AT GENETIC EQUILIBIUM THERE IS NO
EVOLUTION!
In order for this to occur the following
conditions must be met:
• No Natural Selection
• Mutations do not Occur
• The population must be isolated from other
populations (No gene flow)
• The Population is large (No Genetic Drift)
• Mating is Random
IF ANY OF THESE CONDITIONS IS NOT
MET, EVOLUTION WILL TAKE
PLACE!
Hardy-Weinburg Equations
• Genetic Equilibrium is determined by
evaluating the following values:
• Allele frequencies for each allele (p, q)
• Frequencies of homozygotes (p , q )
• Frequency of heterozygotes (pq + qp = 2pq)
2
2
– Also the following two equations hold:
● p + q = 1 (all alleles sum to 100%)
● p + 2pq + q = 1 (all individuals sum to 100%)
2
2
Example Problem
• Suppose a plant population consists of 84%
plants with red flowers and 16% with white
flowers. The red allele (R) is dominant over
the white recessive allele (r).
• q = 0.16 = White flowered plants (rr trait)
2
• p + 2pq = 0.84 = red flowered plants (RR and
Rr traits)
2
• To determine the allele frequency of the
white flower allele, calculate q by finding the
square root of q
q = .016 = 0.4
2
Since p + q = 1, p must equal 0.6
• You can also figure out the percentages of
individuals with the homozygous dominant
and heterozygous condition
2pq = (2)(0.6)(.4) = 0.48 or 48% =
heterozygotes
p = (0.6)(0.6) = 0.36 or 36% = homozygous
dominant
2
Speciation: the formation of a new species,
can occur by way of 3 processes
• 1. Allopatric Speciation= a population is
divided by some geographic barrier
preventing interbreeding
• Gene frequencies in each population diverge
• If gene pools diverge enough, interbreeding
will not take place if the barrier is removed
• As a result, new species form
Speciation
• 2. Sympatric Speciation= formation of a
new species without the presence of a
geographic barrier
• Balanced Polymorphism: ex. Camouflaged
insects
• Hybridization: two distinctly different forms
of a species mate and produce offspring along
a geographic boundary; the hybrid eventually
diverges from both parent populations
Speciation
• 3. Adaptive Radiation = relatively rapid
evolution of many species from a single
ancestor (Darwin’s Finches 14 varieties)
Maintaining Reproductive Isolation:
mechanisms at work if no physical barrier exists
Prezygotic Isolating Mechanisms = mechanisms
that prevent fertilization
• Habitat isolation: species don’t encounter each other
• Temporal isolation: species mate at different seasons
or at different times of the day
• Behavioral isolation: when a species does not
recognize a potential mate because it does not perform
the correct mating ritual, display the proper visual
signals etc.
• Mechanical isolation: male and female genitalia are
structurally incompatible
• Gametic isolation: male gametes don’t survive in the
environment of the female gamete
Postzygotic Isolation Mechanisms = prevent the
formation of fertile offspring
• Hybrid inviability: zygote fails to develop properly
and is aborted
• Hybrid sterility: Functional adults but they are
reproductively sterile (mule)
• Hybrid breakdown: offspring have reduced
viability or fertility
Patterns of Evolution
• 1. Divergent Evolution: 2 or more species
originating from a common ancestor
• 2. Convergent Evolution: 2 unrelated species that
share similar traits; occur because each species has
independently adapted to similar ecological
conditions (not due to shared ancestry)
• 3. Parallel Evolution: 2 related species that have
made similar evolutionary changes after their
divergence from a common ancestor
• 4. Coevolution: evolution of one species in
response to another (plants & insects, predator &
prey)
Divergent Evolution
Convergent Evolution
Parallel Evolution
Coevolution
Patterns of Evolution
5. Macroevolution: patterns of evolution
occurring over extended periods of geologic
time
• Gradualism: gradual accumulation of small
changes
• Punctuated Equilibrium: long periods of
“status quo” followed by short periods of rapid
evolution
Evidence for Evolution
• 1. Comparative Anatomy = similar anatomy
in different organisms
•Homologous Structures = similar
structures shared due to inheritance from a
common ancestor (limb structure)
•Vestigial Structures = traces of a structure
that no longer serves any useful purpose (hip
bones in snakes)
•Analogous Structures = similar in purpose
but not derived from a common ancestor
(wings in birds vs. insects)
Homologous Structures
Vestigial Structures
Analogous Structures
Evidence for Evolution
• 2. Comparative Embryology = embryos of
related organisms develop in similar ways as
embryos (vertebrates - gill pouches)
• “Ontogeny recapitulates phylogeny”. This
means that during an organisms
embryological development they will go
through stages that resemble the stages
they went through during their
evolutionary development.
Evidence for Evolution
3. Comparative Biochemistry = chemical
processes (metabolism)
• -genetic code, organic compounds,
dependence on water
• - 98% of DNA is shared between humans and
chimps
Evidence for Evolution
4. Paleontology: Transitional fossil
forms = show intermediate stages of
change (horses)
Evidence for Evolution
5. Genetics provides variation - mutations
6. Environmental changes can cause change
(Industrial Revolution - Peppered Moth)
Evidence for Evolution
7. Normal Distribution = bell shaped curve
showing variation in a species , few extremes
(rabbit color ranges from light to dark)
• Directional Selection = shift in one direction
(dark coats)
• Disruptive Selection = acts on middle of
curve (predation of medium coats)
• Stabilizing Selection = selection against the
extremes in stable conditions
Directional Selection
Disruptive Selection
Stabilizing Selection
Evidence for Evolution
8. Biogeography: unrelated species living
in similar environments but in different
geographic areas look alike