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Evolution, Natural Selection,
and Species Origination
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.
The Historical Road Leading to Darwin’s 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 life’s
diversity.
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 Darwin’s arguments for evolution.
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.
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 Earth’s geologic features could be explained
by gradual mechanisms currently operating in the world.
Charles Lyell (geologist, 1797-1875) incorporated Hutton’s 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 Darwin’s thinking however, Darwin was not the first to apply the principle of gradualsim
to biological evolution.
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 1) An idea of use and disuse, where the components of an organism’s
anatomy that are used extensively become larger and stronger,
while those that are not used deteriorate.
2) 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.
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 ship’s naturalist, he collects thousands of plant and animal
specimens.
Upon the Beagle’s 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.
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
Darwin’s.
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
Darwin’s, agreed that Darwin should be recognized as the primary
scientist.
In publishing his theory, Darwin developed two main ideas:
1) Evolution explains life’s unity and diversity and
2) 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
Darwin’s theory. By Darwin’s reasoning, Linnaeus’ classification
system reflected organismal history with life at various taxonomic
levels related through descent from a common ancestor.
Natural Selection:
How the heck does this work & how does it explain adaptation?
Ernst Mayr (contemporary evolutionary biologist) has synopsized
Darwin’s 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.
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
3 important caveats to remember about natural selection:
1) 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.
2) Natural selection can amplify or diminish only heritable traits - that
is, traits that are passed from organisms to their offspring.
3) 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.
Darwin’s 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.
•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
Reproductive Isolation


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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
Figure 24.4
Prezygotic barriers impede mating or hinder fertilization if mating does occur
Habitat
isolation
Temporal
isolation
Behavioral
isolation
Individuals
of
different
species
Mechanical
isolation
Gametic
isolation
Mating
attempt
HABITAT ISOLATION
Fertilization
TEMPORAL ISOLATION BEHAVIORAL ISOLATION MECHANICAL ISOLATION
GAMETIC ISOLATION
Postzygotic barriers prevent a hybrid zygote from
developing into a viable, fertile adult
Reduced
hybrid
viability
Reduced
hybrid
fertility
Hybrid
breakdown
Viable,
fertile
offspring
Fertilization
REDUCED HYBRID
VIABILITY
REDUCED HYBRID
FERTILITY
HYBRID BREAKDOWN

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

In sympatric (“same country” speciation,
speciation takes place in geographically
overlapping populations
Allopatric speciation
Sympatric speciation
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
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
organism’s form as it develops into an adult