Evolution, Natural Selection,

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Evolution, Natural Selection, and Species
Origination
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Evolution - all the changes that have transformed
life on Earth from its earliest beginnings to
the diversity that characterizes it today. Taken
on a grand scale to mean the gradual appearance
of all biological diversity, from the earliest
microbes to the enormous variety of organisms
alive today. Evolution has been and continues
to be such a monumental concept because its study
illuminates biology at every level including the
molecular, microscopic, and macroscopic.
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The Historical Road Leading to Darwins Ideas on
Descent with Modification In ancient times,
several Greek philosophers proposed that life
gradually evolved, but Aristotle (384-322 B.C.),
who had more influence on early Western science,
viewed species as unchanging. Through his
observations, Aristotle devised the scala naturae
(or scale of nature) on which he arranged
life-forms according to their increasing
complexity. This idea that each organism had its
allotted place persisted well into the 1700s with
scientists now seeing species as having been
perfectly designed by the Creator to fulfill a
particular role/purpose. One such scientist was
Carolus Linnaeus (1707-1778) who sought to
classify lifes diversity.
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Linnaeus founded the branch of biology concerned
with naming and classifying organisms, known as
taxonomy. Although he subscribed to the
unchanging view of species, Linnaeus recognized
similarities among organisms and adopted a
nested classification system in which similar
species are grouped into increasingly more
general categories (unlike the hierarchical
linearity of the scala naturae). Additionally,
Linnaeus developed the two-part system of naming
organisms according to genus and species
(binomial nomenclature) that is still in use
today. Even though Linnaeus did not think that
the observations concerning the resemblance of
species implied evolutionary kinship, a century
later, his work would aid in Darwins arguments
for evolution.
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Influence of Paleontology on Darwin Paleontology
- the study of fossils developed largely by
Georges Cuvier (1769-1832). Cuvier observed that
fossils were more dissimilar from current life as
one examined deeper (older) layers of the
sedimentary strata and that from one stratum to
the next, new species appeared while others
disappeared. Still, Cuvier strongly disavowed
any support for an idea of gradual evolutionary
change rather, he speculated that each boundary
between strata was reflective of a catastrophe
(catastrophism), accounting for the demise of
species alive at the time with subsequent
repopulation by species immigrating from other
areas not affected by the catastrophe.
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In contrast to the view of catastrophism, James
Hutton (geologist, 1769-1832) subscribed to a
concept of gradualism (profound change occurs
through the cumulative effect of slow but
continuous processes), proposing that Earths
geologic features could be explained by gradual
mechanisms currently operating in the
world. Charles Lyell (geologist, 1797-1875)
incorporated Huttons thinking and derived a
theory of uniformitarianism, offering that the
same geological processes are operating today as
in the past, and at the same rate. The ideas of
Huton and Lyell greatly influenced Darwins
thinking - however, Darwin was not the first to
apply the principle of gradualsim to biological
evolution.
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• The Lamarckian Theory of Evolution
• Jean Baptiste de Lamarck (1744-1829) was a
  biologist who developed a comprehensive model
  concerning how life evolves
• He explained gradual evolutionary descent based
  on two principles -
• An idea of use and disuse, where the components
  of an organisms anatomy that are used
  extensively become larger and stronger, while
  those that are not used deteriorate.
• The belief in the inheritance of acquired
  characteristics in which an organism could pass
  the modifications from the above principle to its
  offspring.
• We now know this reasoning to be faulty, but his
  thoughts deserve merit as they recognized the
  influence of gradual evolutionary change over
  time as the best explanation regarding his
  observations.

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Darwin (1809-1882) and the Origin of Species In
1831, Charles Darwin sets out as part of the crew
on the HMS Beagle for a voyage around the
world. As the ships naturalist, he collects
thousands of plant and animal specimens. Upon
the Beagles stop at the Galapagos Islands,
Darwin observes several kinds of finches seeming
to be different species, but quite similar in
appearance. The most striking differences among
the finches were their beaks, each one adapted
for a specific diet. It would not be until
several years after his return home (in 1836)
from the voyage that Darwin realized that an
explanation for such adaptations was critical to
understanding evolution.
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In 1844, Darwin composed an essay on the origin
of species and natural selection as the mechanism
of evolution, but refrained from having it
published in anticipation of the controversy it
would cause. In 1858, Darwin receives a
manuscript from Alfred Wallace, another
naturalist who has developed a theory of natural
selection similar to Darwins. Darwin is now
spurred to quickly finish his book, On the of
Origin of Species by Means of Natural Selection,
and has it published in 1859. Darwin and Wallace
are both credited with the development of the
theory concerning natural selection, but Wallace,
a great admirer of Darwins, agreed that Darwin
should be recognized as the primary scientist.
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• In publishing his theory, Darwin developed two
  main ideas
• Evolution explains lifes unity and diversity and
• Natural selection is a cause for adaptive
  evolution
• Darwin saw unity in life with all organisms
  having descended from a common ancestor that
  lived in the far distant past - when descendants
  of that ancestral organism spread forth into
  various habitats (over millions of years), they
  accumulated adaptations that best suited them to
  cope with their environments.
• As stated earlier, Linnaeus saw that some
  organisms resemble each other more closely than
  others, but had not identified this as a result
  of evolution. Still, his taxonomic scheme
  largely fit with Darwins theory. By Darwins
  reasoning, Linnaeus classification system
  reflected organismal history with life at various
  taxonomic levels related through descent from a
  common ancestor.

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Natural Selection How the heck does this work
how does it explain adaptation? Ernst Mayr
(contemporary evolutionary biologist) has
synopsized Darwins theory into 3 inferences
based on 5 observations Observation 1 - For
any species, pop. sizes would increase
exponentially if all offspring reproduced
successfully. Observation 2 - However, pops.
tend to remain stable in size, not counting
seasonal fluctuations. Observation 3 - Resources
are limited. Inference 1 - Production of more
individuals than the environment can sustain
leads to an existence struggle among individuals
of a pop. with only a fraction of their offspring
surviving each generation. Observation 4 -
Members of a pop. vary extensively in their
characteristics no 2 individuals are exactly
alike. Observation 5 - Much of this variation is
heritable.
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• Inference 2 - Survival depends in part on
  inherited traits. Individuals whose inherited
  traits give them a high probability of surviving
  and reproducing in a given environment have
  higher fitness (an advantage) and are likely to
  leave more offspring than less fit
  (disadvantaged) individuals.
• Inference 3 - This unequal ability of
  individuals to survive and reproduce will lead to
  a gradual change in a pop., with favorable
  characteristics accumulating over generations.
• Summary
• Natural selection is the differential success in
  reproduction among individuals that vary in their
  heritable traits. These reproductive differences
  emerge as each individual interacts with its
  environment.
• Over time, natural selection can increase the
  adaptation of organisms to their environment.
• If an environment changes over time, or if
  individuals of a species move to a new
  environment, natural selection may result in
  adaptation to these new conditions, sometimes
  giving rise to new species in the process.

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• 3 important caveats to remember about natural
  selection
• Although natural selection occurs through
  interactions between individual organisms and
  their environment, individuals do not evolve the
  smallest unit that can evolve is a population.
• Natural selection can amplify or diminish only
  heritable traits - that is, traits that are
  passed from organisms to their offspring.
• Environmental factors vary from place to place
  and from time to time. A trait that is favorable
  in one situation may be useless - or even
  detrimental - in different circumstances.

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Darwins theory endures due to its ability to
explain so many different types of observations
including Anatomical and molecular homologies
(similarities in characteristics resulting from a
shared ancestry) that match patterns in space
(biogeography) and time (the fossil
record). Remember, the term theory is not used
in science as it is in colloquial terms where its
meaning is more like a hypothesis. Evolutional
theory has withstood the skepticism of science
and tests of experimentation, enduring as the
most logic/plausible explanation for our
observations in biology today. By attributing
the diversity of life to natural processes,
Darwin gave biology a sound, scientific basis.
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• Speciation, the origin of new species, is at the
  focal point of evolutionary theory
• Evolutionary theory must explain how new species
  originate and how populations evolve
• Microevolution consists of adaptations that
  evolve within a population, confined to one gene
  pool
• Macroevolution refers to evolutionary change
  above the species level, for example
• - Appearance of major new changes/features in
  organisms
• - Impact of mass extinctions on diversity of life

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Reproductive Isolation

• Absence of gene flow is one contributing factor
  to speciation
• Reproductive isolation is the existence of
  biological factors (barriers) that impede two
  species from producing viable, fertile hybrids
• Two types of barriers prezygotic and postzygotic

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Figure 24.4
Prezygotic barriers impede mating or hinder
fertilization if mating does occur
Gametic isolation
Mechanical isolation
Behavioral isolation
Temporal isolation
Habitat isolation
Individuals of different species
Mating attempt
Fertilization
TEMPORAL ISOLATION
HABITAT ISOLATION
BEHAVIORAL ISOLATION
MECHANICAL ISOLATION
GAMETIC ISOLATION
Postzygotic barriers prevent a hybrid zygote
from developing into a viable, fertile adult
Reduced hybrid fertility
Reduced hybrid viability
Hybrid breakdown
Viable, fertile offspring
Fertilization
REDUCED HYBRID VIABILITY
REDUCED HYBRID FERTILITY
HYBRID BREAKDOWN
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• Speciation can occur in two ways
• Allopatric speciation
• Sympatric speciation

• In allopatric (other country) speciation, gene
  flow is interrupted or reduced when a population
  is divided into geographically isolated
  subpopulations
• One or both populations may undergo evolutionary
  change during the period of separation

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• In sympatric (same country speciation,
  speciation takes place in geographically
  overlapping populations

Allopatric speciation
Sympatric speciation
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Allopatric and Sympatric Speciation A Summary

• In allopatric speciation, a new species forms
  while geographically isolated from its parent
  population
• In sympatric speciation, a reproductive barrier
  isolates a subset of a population without
  geographic separation from the parent species

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Concept 24.3 Macroevolutionary changes can
accumulate through many speciation events

• Macroevolutionary change is cumulative change
  during thousands of small speciation episodes

• Most novel biological structures evolve in many
  stages from previously existing structures

• Many large evolutionary changes may have been
  associated with mutations genes that regulate
  development.
• Genes that program development control the rate,
  timing, and spatial pattern of changes in an
  organisms form as it develops into an adult