Download Darwin and Natural Selection

8/21/2014
 Definition:
frequency of heritable traits changes from
one generation to the next
Chapter 22
Pg. 452 – 468
◦ Includes changes in a population, species, or group of
species
◦ Evolutionary change is based on the interactions between
populations & their environment which results in
adaptations (inherited characteristics) to increase fitness
 Most
important concept of biology –
links the whole subject
 An
allele is one of several (or many) varieties of a
gene
 Individuals inherit alleles that code for traits that
establish morphology (form/structure), physiology, or
behavior
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◦ Evolution is when the frequency
of these alleles in the
population changes over time
 Aristotle
(382 – 322 BCE):
◦ Example: sickle-cell disease – homozygous normal individuals are more
susceptible to malaria, and homozygous sickle-cell individuals suffer from
complications of their disorder
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Scala naturae – life-forms
can be arranged on a
scale of increasing
complexity
 Old Testament: perfect
species were individually
designed by God
Diplody: diploid eukaryotes (two sets of DNA, like YOU!) are
capable of hiding genetic variation (recessive alleles) from
selection
Heterozygote advantage: some recessive genes, while
disorder-causing, may be helpful in a way; heterozygotes have
a dominant allele and recessive allele, and would benefit from
the presence of each
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Grouped similar species into general categories reflecting what he
considered the pattern of their creation (anatomy and morphology)
Developed taxonomy – the branch of biology dedicated to the
naming and classification of all forms of life.
◦ Domain  Kingdom  Phylum  Class  Order  Family  Genus 
Species
 Domains: Eubacteria, Archaea, Eukarya
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Developed binomial nomenclature –
a two-part naming system that includes
the organism’s genus and species
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
Georges Cuvier (1769 – 1832): French geologist and
paleontologist, opposed evolution.
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◦ Advocated catastrophism – events in the past occurred suddenly and
by different mechanisms than today (catastrophe destroyed many living
species, then areas were repopulated by immigrant species).
◦ Evolution was a natural progression towards perfection, and
organisms proceeded up a ladder of complexity.
◦ Use and disuse – described how body parts can develop with
increased usage, while unused parts weaken. This idea was
correct (think athletes that train for competitions!).
◦ Inheritance of acquired characteristics – described how body
features acquired during the lifetime of an organism (like
muscle bulk) could be passed on to offspring. This was
incorrect – only changes in genetic material can be passed on.
◦ Importance: Lamarck recognized that species evolve, though
his explanatory mechanism was flawed. He developed ideas of
adaptation and heritability.
 This explained boundaries between strata and location of different species.
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Hutton: described gradualism, the slow and subtle changes in
organisms caused by slow and continuous geologic change
Charles Lyell (1797 – 1875): described uniformitarianism, the
idea that Earth’s processes have remained at the same rate
through all of time, so Earth is very old.
Malthus (1766 – 1834): when more babies are born than
deaths, war, famine, and disease are results of overcrowding.
This causes a struggle for existence.
Darwin went to med school at 16, dropped out, and
joined the clergy; he ended up on the HMS Beagle.
He traveled to South America and the Galapagos
Islands, collecting specimens of plants and animals,
as well as fossils, and constantly made
observations.
50 years after Lamarck published his ideas,
Charles Darwin published On the Origin of Species
His theory stated that natural selection, or
“survival of the fittest”, was driving the force of
evolution (“descent with modification”)
One of the earliest advocates for evolution, and the first to
support it by publishing his theories. They included:
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South America: species were related but different from European
species. Fossils didn’t match European fossils, but S. American
fossils were related to S. American species, implying some sort of
descent.
Galapagos Islands: west coast of S. America, different varieties of
species were on each island, all of which were related to mainland
S. American species.
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Published in 1809, Darwin’s birth year.
Darwin also noticed each island had unique finches
that seemed perfectly suited to its own habitat.
Reasoning: gradual accumulation of adaptations
to an environment could lead to new species over long time periods.
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Lamarck
Darwin
 Progression towards
 Adaptation to specific
perfection
environment
 Acquired characteristics
 Only heritable traits
 Inner drive by the
 Natural selection (short
organism (i.e. giraffe
giraffes don’t make as
stretches to reach tree)
many babies)
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The first published work about natural selection was not
by Darwin, but by Alfred Russell Wallace. Wallace
published a paper on natural selection in 1858.
Darwin just happened to make it to fame first. However,
he waited 30 years before publishing his ideas on
evolution. He published On the Origin of Species by Means
of Natural Selection in 1859.
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Mechanism for evolution is natural selection
Darwin didn’t use “evolution”, but rather “descent with
modification”
Descent with Modification – all species originated from the
same species, but over time, an accumulation of slight
modifications led to all the diversity we have on Earth today.
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Linnaeus’ theories on taxonomy fit this view – organisms are grouped
based on common ancestors and more similar organisms are more closely
related.
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Darwin's main focus – the mechanism of evolution
Definition: differences in survive and reproduction among individuals
in a population as a result of their interaction with the environment
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A
physical trait that
provides a benefit to an
individual in survival or
reproduction
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The quick version of natural selection:
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Natural
selection leads to an
accumulation of adaptations
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Overproduction: Populations produce more offspring than can possibly
survive. Overproduction leads to competition.
Natural Variation: Individuals in a population vary extensively from
each other, mostly due to inheritance.
Fitness: struggle to survive, individuals whose inherited
characteristics best fit to environment leave more offspring than
less fit.
Adaptation: enhance an organism’s ability to survive and reproduce
Descent with modification: Unequal ability of individuals to survive and
reproduce leads to gradual change in population, with favorable
characteristics accumulating over generations.
Populations possess an enormous reproductive potential. Two elephants could produce 19 million
individuals after 750 years if all offspring survived to reproductive maturity, and fostered a normal
number of offspring.
Populations fluctuate around a constant size, remaining stable.
Resources, like food, water, or light, are limited and do not increase as populations grow larger.
Individuals compete for survival. Eventually, the needs of a growing population will exceed available
resources. This causes competition.
There is variation among individuals in a population. Most traits have considerable variety (hair, skin,
and eye color, height, etc.)
Much variation is heritable. Darwin recognized that traits were passed parents  offspring,
contrasting with the “acquired characteristics” view.
Only the most fit individuals survive. “Survival of the fittest” occurs because individuals with
adapted traits are able to out-compete those that aren’t so adapted.
Evolution occurs as favorable traits accumulate in the population. The individuals with the best traits
usually make the most offspring. Over time, the best traits and alleles that generate them
accumulate in the population.
Some have alleles that generate traits enabling them to cope better in
their environment than others
More successful individuals produce more offspring
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Everyone wants to reproduce.
They don’t or can’t.
There is competition.
Everyone is different, and differences are genetic.
Survive and reproduction depend on genes.
Adaptive genes survive and become common.
Populations evolve, not
individuals.
Fitness is determined by
the environment.
In summary:
Natural Selection = Differential success in reproduction
Product of natural selection = Adaptations of populations to
environment
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Superior inherited traits are adaptations,
and increase an individual's fitness
◦ Fitness is the relative ability to survive and
leave offspring (REPRODUCTIVE SUCCESS)
 It means the most capable of reproduction, NOT bigger, stronger, faster, or smarter
 Survival of the most fit genes or traits
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When the environment favors a trait (when the trait increases
the survival of its bearer) selection is said to act for that trait
Selection is said to act against
unfavorable traits – favorable ones are
adaptive, while unfavorable traits are
maladaptive
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Natural selection may affect populations in a variety of ways.
In ALL cases, natural selection acts on individual phenotypes
already present in the population.
Alleles may be expressed with other alleles in new combinations (as
a result of genetic recombination) to crease novel phenotypes, or
new phenotypes may appear in the population as a result of
mutations.
It is important to note that natural selection does NOT cause
mutations or create new phenotypes – it only
SELECTS phenotypes already present in the
population – that maximize fitness.
 Stabilizing
selection
selection
 Disruptive selection
(diversifying selection)
 Sexual selection
 Artificial selection
 Directional
Favors traits that are at one extreme of a range of
traits – traits at the opposite extreme are selected
against
 If directional selection continues for many generations,
favored traits become more and more extreme, leading to
distinct changes in the allele frequencies of the population
Eliminates individuals that have extreme or unusual traits
 Individuals with the most common form of a trait are the
best adapted, while individuals who differ from the common
form are poorly adapted
 Stabilizing selection maintains the existing population
frequencies of common traits
while selecting against all
other trait variations
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Also called diversifying selection
 Occurs when environment favors extreme or unusual traits,
while selecting against the common traits
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◦ Example: weeds – tall forms dominate, but because of disruptive
selection, weeds in people’s yards only grow short because they
escape mowing; weeds are
mostly tall in the wild
because tallness makes
them better competitors
for sunlight
◦ Examples: Darwin finches, peppered moths (industrial melanism),
insecticide resistance, season
creep (short winter/early spring)
Differential mating of males (sometimes females) in a population
◦ In reproduction, females usually make a greater energy investment – they increase fitness
by increasing the quality of their offspring with superior males; males just try to make
more babies
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Traits (physical qualities or behaviors) that allow males to increase their
mating frequency lead to an increase in frequency of these traits within the
population
◦ This leads to two kinds of sexual selection:
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 Male competition: contests of strength that award mating opportunities to the strongest males
(antlers, horns, large stature, etc.)
 Female choice: traits or behaviors in males that are attractive to females (bird plumage, elaborate
mating behaviors)
Sexual selection often leads to sexual dimorphism, differences in the appearance of males and females –
when this occurs, sexual selection is a form of disruptive selection
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Natural Selection
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Form of directional selection carried out by humans when they
sow seeds or breed animals that possess desirable traits
It is not “natural” selection, but is given for comparison
◦ Various breeds of dogs have originated as a result of humans breeding
animals with specific desirable traits
◦ Brussels sprouts, broccoli, cabbage, and cauliflower are all varieties of a
single species of wild mustard after artificial selection of offspring
possessing specific traits
Mutations
– spontaneous
accidental changes in an
organism’s DNA
◦Very rarely beneficial, but does
occur
Or
new combinations of existing
genes
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Artificial Selection
•Nature decides
•“Man” decides
•Works on individual
•Selective breeding
•Inbreeding occurs
•eg. Finch beaks
Disney Version
Populations
change over
time and everyone is happy!
Organisms become more
complex and better
Everything gets better
and the world is happy
•eg. Breeds of dogs
Real Talk
Most
individuals die off without
reproducing, especially when
the environment changes
The survivors are different
from the original population
Estimated 99.9% of species
that ever existed are extinct
It creates new traits in times of need.
◦ Selection can only edit existing variations. Also, they are not
helpful or convenient traits, just those that affect survival or
reproduction.
Individuals evolve or change over time.
It is a mindset of trying to survive and reproduce.
 It’s all about getting bigger, faster and stronger etc.
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◦ Adaptations are often compromises rather than improvements.
Organisms won’t go extinct because they will evolve.
 Evolution is random.
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