Transcript Evolution

Evolution A change in a population over
time.
Chapters 22-24
Pre-Darwin Theories:
1. “Idealism” (Essentialism) -States that there are 2
worlds: an ideal, real world and an illusionary world
the humans perceive w/ senses.
– Variations in plants & animals were imperfect
representations of ideal forms.
– Evolution is counterproductive since ideal
organisms are already perfectly adapted to
environments.
(Plato - who disagreed w/ gradual evolution)
Pre-Darwin Theories (Continued):
2. “Scala Natura” - Stated that organisms vary from
simple to complex and that species are fixed and do not
evolve. Prevailed for over 2000 years.
(Aristotle - Questioned Plato but excluded evolution)
Pre-Darwin Theories (Continued):
3. “Creationist - essentialist” - Based on the Old
Testament account that species were individually
created and fixed. Embedded in Western / JudeoChristian culture. Fortified prejudice against evolution.
– “Natural Theology” - the philosophy that the
Creator’s plan could be revealed by studying nature.
– Adaptations were thought of as being created for
specific purposes.
Pre-Darwin Theories (Continued):
4. “Taxonomy” - Grouped organisms by categories and
ranked categories by similarity. This lead to “binomial
nomenclature” still used today.
– Clustering species into groups did not imply
evolutionary relationships since he believed species
were permanent fixtures.
– Was done only to reveal “God’s” plan.
(Linneaus, 1707 - 1778, “Natural Theologian” - sought
to order the diversity of life “for the greator glory of the
creator”)
Charles Darwin
Published the 1st convincing case for evolution in a
book “On the Origin of Species by Means of
Natural Selection” in 1859 based on his voyage on
the HMS Beagle.
Darwin (Continued):
The book:
• Synthesized seemingly unrelated facts to
create a conceptual framework that accounts
for both the unity & diversity of life.
• Discussed important biological issues about
organisms, such as why there are so many
different kinds of organisms, their organs and
relationships, similarities & differences,
geographic distribution, & adaptations to their
environment.
Darwin (Continued):
The book made 2 major points:
– Species evolved from ancestral species & were
not specially created.
– “Natural selection” is a mechanism that could
result in this evolutionary change.
Natural Selection - "The survival of the fittest" the first
proposed explanation of variation of life on Earth. It is
now considered the mechanism of evolution. It states that
survival is based on fitness of the organism.
– Fit is now defined as best able to pass on genetic
material to offspring.
(Charles Darwin, 19th century)
Types of Natural Selection:
1. Directional - One phenotype is favored in the
environment.
(ex. "dark" peppered moths have higher survival rates
in soot covered forests)
2. Stabilizing - Organisms with extreme phenotypes are
eliminated.
(ex. birth weight in humans, low survival rate for
extremes)
3. Disruptive - Organisms with common traits are
eliminated, extremes are favored.
(ex. small female and large male elephant seals)
4.Artificial - A breeder chooses which traits to favor.
(ex. seedless grapes)
Types of Evolution:
1. Divergent - Closely related species have different
behaviors and traits. (ex. Color variations in peppered
moths or beak types in finches)
2. Convergent - Unrelated species come to have similar
traits. (ex. aardvarks, anteaters, and pangolins all have
sharp teeth and claws, long snouts, and sticky tongues
but evolved from 3 different mammals)
Fossil - Based Theories:
1. “Catastrophism” - The theory that major changes in
the Earth’s crust are the result of catastrophic events
rather than gradual change. This was the first to
mention extinction.
– Differences between fossil strata correspond to
catastrophic events
– New species in younger rocks explained by periodic
local catastrophies. Foreign species would replace
local extinct species.
(Couvier, 1769 - 1832, Who developed “palentology” the study of fossils).
Fossil - Based Theories (Continued):
2. “Gradualism” - Theorized profound change
is the cumulative product of slow,
continuous processes. This competed with
catastrophism.
(James Hutton)
Fossil - Based Theories (Continued):
3. “Uniformitarianism” - Theorizes that geological
processes are so uniform that their effects balance out
over time. This was an expansion of gradualism that
was rejected by Darwin but influential to him.
Ex. Mountains & erosion
Observations:
– Slow processes can cause substantial change.
– Earth is ancient if geology is a slow process.
(Charles Lyell)
Fossil - Based Theories (Continued):
4. "The Law of Use & Disuse" - states that acquired traits
are inheritable. Now proven incorrect - changes in
somatic cells are not passed on through germ cells.
Darwin also dismissed this theory.
(Jean-Baptiste de Lamarck, 18th century).
Modern Theories:
1. Punctuated Equilibrium - Long periods of stasis
punctuated by episodes of relatively rapid speciation
and change.
(In geologic time, a few thousand years for a species
to evolve is small compared to the few millions of
years a successful species may exist)
*Long periods of stasis may be the result of stabilizing
selection in an unchanging environment.
Modern Theories (Cont’d):
2. Biological Species Concept - Defined species as a
population or group of populations whose
members have the potential to interbreed with one
another in nature and produce viable, fertile
offspring.
(Earnst Mayr, 1942)
There’s a catch to the biological
species concept!
It does not work in all situations.
It cannot be applied to:
• Completely asexual organisms, ex. clones.
• Extinct organisms represented only via fossils.
There are other possibilities:
Morphological species concept
Recognition species concept
Cohesion species concept
Ecological species concept
Evolutionary species concept
Speciation - The origin of new species
Patterns of Speciation:
1. Anagenesis - (phyletic evolution) Involves the
transformation of an entire species into a new
species.
2. Cladogenesis - (branching evolution) New
species arise from a parent species that continue to
exist.
Modes of Speciation:
Allopatric - population becomes separated from rest of
species by a geographic barrier.
Leads to:
• Colonization of new areas
• Founder effects - the loss of genetic variation that occurs when a
new population is established by a very small number of individuals
from a larger population
• Genetic drift - the change in the relative frequency with which an
allele occurs in a population that results from the fact that alleles in
offspring are a random sample of those in the parents, and because of
the role of chance in determining whether a given individual survives
and reproduces. Genetic drift may reduce genetic variability.
• Differing selection pressures - due to different habitats.
• Adaptive radiation on island chains - the evolution of many
adapted species from a common ancestor.
Modes of Speciation (Cont’d):
Sympatric - Formation of a new species without a
geographic barrier.
•
•
Reproductive isolation evolves w/o geographic isolation.
Can occur quickly if genetic change results in reproductive barrier
between mutants and parent population.
Varieties:
1. Autopolyploid - An organism w/ two or more complete sets of
chromosome within each cell, all derived for a single species.
Ex. 4n - Potatoes, 3n - Apples & Bananas
2. Allopolyploid - n organism w/ two or more complete sets of
chromosomes within each cell, derived from different species.
(More common than autopolyploidy)
Ex. 6n - Triticale, from 4n from wheat and 2n from rye
Barriers isolate the gene pools of
biological species
Types of Barriers:
Prezygotic (Before Fertilization):
1. Habitat Isolation - 2 species in different habitats.
2. Behavioral Isolation - Signals/behaviors attract mates.
3. Temporal Isolation - Species breed at different times,
day/season/years.
4. Mechanical Isolation - Anatomical incompatibility.
5. Gametic Isolation - Gametes of different species fail to
attract one another.
Types of Barriers (Cont’d)
Postzygotic (After Fertilization):
1. Reduced Hybrid viability - hybrids lack vigor and
rarely, if ever, reach sexual maturity.
2. Reduced Hybrid Fertility - hybrids are usually
sterile.
3. Hybrid Breakdown - hybrids can reproduce but
their offspring have either reduced fertility or
reduced viability.
Evolution is not goal oriented,
but trends do occur…
•It is possible to isolate a single evolutionary
progression. Ex. Horse evolution
•Branching evolution can produce a trend even if some
new species counter the trend.
•Species are analogous to individuals (Steven Stanley
of John’s Hopkins)
•The presence of an evolutionary trend does not mean
there is an intrinsic drive toward a preordained state.
Most evolutionary novelties are modified versions of
older structures
Exaptation - a structure that evolves in one context and
becomes co-opted for another function
– Cannot be proven, but explains how novel designs
can arise gradually.
– Higher taxa such as families and classes are defined
by evolutionary novelties (Ex. Birds are adapted to
flight but ancestors are earthbound)
– Natural selection can not anticipate the future, but it
can improve an existing structure.
– Reflects the Darwinian tradition of large changes as a
result of natural selection through many small
changes.
Genes that control development play a
major role in evolutionary novelties.
Terms to Recognize:
• Allometric growth - Differences in relative rates of growth
of various parts of the body. Ex. Human head size
compared to body size, large in babies/small in adults
• Heterochrony - Evolutionary changes in the timing or rate
of development. Ex. Head/brain development in humans
Vs. chimps, same rate but chimp growth stops after
birth,human growth continues.
• Paedomorphosis - Retention of ancestral juvenile
structures in a sexually mature adult organism. Ex. 1.
Salamanders retain gills in adulthood, unlike most
amphibians. 2. Docility is a juvenile behavior retained in
domesticated pets.
• Homeosis - Alteration in the placement of different body
parts. Ex. Fruit fly legs develop in place of antennae.
Population Genetics
Hardy - Weinberg Law - The relative genotypes of
a population remain constant over time if the
following 5 conditions are met:
1. Large population
2. No mutations
3. No immigration or emigration
4. Random mating
5. No natural selection
Hardy-Weinberg Equations:
The sum of the frequencies of each allele,
dominant (p) and recessive (q), in a population
add up to 1.
p+q=1
The frequency of the genotypes in a population
may be determined using the following
equation:
p2 + 2pq + q2 = 1
homozygous dominants (p), heterozygotes (pq),
homozygous recessives (q)