The Origin of Species (Charles Darwin): two main ideas – descent with modification (evolution) explains life’s unity and diversity and that natural selection brings about the match between organisms and their environment.
Evolution is supported by an overwhelming amount of scientific evidence
Fossils in a particular stratum provide a glimpse of some of the organisms that populated Earth at the time that layer formed. Later, erosion may carve through younger strata, revealing older strata that may have been buried. Paleontologist, Cuvier, noted that the older the strata the more dissimilar its fossils were to current life forms. He observed that from one layer to the next some new species appeared while others disappeared. He inferred that extinctions must have been a common occurrence in the history of life.
Rivers carry sediment into seas and swamps. Over time sedimentary rock layers (strata) form under water. Some strata contain fossils. As water levels change and the seafloor is pushed upward, the strata and their fossils are exposed.
Evolution is a process of descent with modification: characteristics present in an ancestral organism are altered (by natural selection) in its descendants over time as they face different environmental conditions. As a result, a species can have characteristics with an underlying similarity even though they may have different functions = homology
Even though they have become adapted for different functions, the forelimbs of all mammals are constructed from the same basic skeletal elements. Such striking anatomical resemblances would be highly unlikely if these structures had arisen anew in each species. Rather, they are homologous structures that represent variations on a structural theme that was present in their common ancestor.
At some point in development, all vertebrate embryos have a tail located posterior to the anus, as well as structures called pharangeal pouches (which develop into structures with very different functions, such as gills in fishes and parts of the ears and throat in humans and other mammals)
Convergent evolution: the independent evolution of similar features in different lineagesConvergent evolution: Sugar glider (Australian marsupial) has a eutherian look alike, the flying squirrel in North America. But the sugar glider is much more closely related to kangaroos and other Australian marsupials than to flying squirrels and other eutherians. Although they evolved independently from different ancestors, these two mammals adapted to similar environments in similar ways. Thus species share features because of convergent evolution, the resemblance is said to be analogous, not homologous
eutherian: placental: mammals having a placenta (flying squirrel); all mammals except monotremes and marsupials (sugar glider)
Adaptations: characteristics of organisms that enhance their survival and reproduction in specific environments
The Galapagos islands are home to more than a dozen species of closely related finches, some found only on a single island. The most striking differences among them are their beaks, which are adapted for specific diets.
inference: A conclusion reached on the basis of evidence and reasoning.
Related species of the insects called mantids have diverse shapes and colours that evolved in different environments.
Heritable traits – traits that are passed from organisms to their offspring i.e. no evidence that acquired characteristics (modified during its lifetime) can be inherited by offspring even if they help the organism in it’s environment.
Speciation – an evolutionary process in which one species splits into two or more speciesAn example of macroevolutionary change is the origin of new groups of organisms, such as mammals or flowering plants, through a series of speciation events.
As diverse as we may be in appearance, all humans belong to a single biological species (Homo sapiens), defined by our capacity to interbreed.
Speciation can take place with or without geographic separation
Mutation: a change in the nucleotide sequence of an organisms DNA, ultimately creating genetic diversity.Genetic drift: chance events can cause allele frequencies to fluctuate unpredictably from one generation to the next, especially in small populations.Allele: any of the alternative versions of a gene that produce distinguishable phenotypic effects.
Geographic isolation – a lake may subside, resulting in two smaller lakes; a river may change course and divide a population of animals that cannot cross it
Polyploidy – a condition in which an organism has en extra set of chromosomesReproductive isolation because of natural selection that results from a switch to a habitat or food source not used by the parent population
Unit 5 evolution by natural selection
EVOLUTION BY NATURAL
CAMPBELL AND REECE, 2010
ORIGIN OF IDEAS ABOUT ORIGINS
DIFFERENT KINDS OF EVIDENCE EXIST:
1. Fossil records
2. Modification by descent and Homology
1. FOSSIL RECORDS
(P. 453-454, 461-462)
• The study of fossils helped to lay the groundwork for
• Fossils are remains or traces of organisms from the
past, usually found in sedimentary rock, which appears
in layers or strata.
• The fossil record provides evidence of the extinction
of species, the origin of new groups, and changes
within groups over time
2. MODIFICATION BY DESCENT AND
• Homology is similarity resulting from common
• Homologous structures are anatomical
resemblances that represent variations on a
structural theme present in a common ancestor.
• Comparative embryology reveals anatomical
homologies not visible in adult organisms
• Examples of homologies at the molecular level are
genes shared among organisms inherited from a
• Modification by descent is explained by: Convergent
• Convergent evolution is the evolution of similar, or
analogous, features in distantly related groups.
• Analogous traits arise when groups independently
adapt to similar environments in similar ways
• Convergent evolution does not provide information
• Darwin’s observations of biogeography, the geographic
distribution of species, formed an important part of his
theory of evolution.
• Islands have many endemic species that are often closely
related to species on the nearest mainland or island.
• Earth’s continents were formerly united in a single large
continent called Pangaea, but have since separated by
• An understanding of continent movement and modern
distribution of species allows us to predict when and
where different groups evolved.
HYPOTHESIS AND THEORY
• Hypothesis: A tentative answer to a
well-framed question, narrower in
scope than a theory, and subject to
• Theory: An explanation which is broad
in scope, and is supported by a large
body of evidence.
OVERVIEW OF THE HISTORY OF DIFFERENT
THEORIES OF DEVELOPMENT
• Spontaneous creation
• Neo Darwinism
• Punctuated Equilibrium
THEORY OF SPONTANEOUS
• The theory of spontaneous generation held
that complex, living organisms may be
produced from nonliving matter.
• It was a popular belief that mice occur
spontaneously from stored grain, or
maggots spontaneously appear in meat.
THE THEORY OF ONTOGENY
• Ontogeny is the origin and development of an
individual organism from embryo to adult.
• Within biology, ontogeny pertains to the
developmental history of an organism within its
own lifetime, as distinct from phylogeny, which
refers to the evolutionary history of species
• Lamarck hypothesized that species evolve
through use and disuse of body parts and the
inheritance of acquired characteristics.
• The mechanisms he proposed are unsupported
EXAMPLE USED TO EXPLAIN
• Giraffes stretching their necks to reach leaves
high in trees (especially Acacias), strengthen
and gradually lengthen their necks. These
giraffes have offspring with slightly longer
necks (also known as "soft inheritance").
• Darwinism as modified by the findings of
modern genetics, stating that mutations due
to random copying errors in DNA cause
variation within a population of individual
organisms and that natural selection acts
upon these variations.
PUNCTUATED EQUILIBRIUM (P. 502)
• The fossil record includes examples of species that
appear suddenly, persist essentially unchanged for
some time, and then apparently disappear
• Niles Eldredge and Stephen Jay Gould coined the term
punctuated equilibrium to describe periods of
apparent stasis punctuated by sudden change
• The punctuated equilibrium model contrasts with a
model of gradual change in a species’ existence.
• As a boy and into adulthood, Charles Darwin had a
consuming interest in nature
• Darwin first studied medicine (unsuccessfully), and
then theology at Cambridge University
• After graduating, he took an unpaid position as
naturalist and companion to Captain Robert FitzRoy
for a 5-year around the world voyage on the
DARWIN’S: VOYAGE OF THE BEAGLE
• During his travels on the Beagle, Darwin collected
specimens of South American plants and animals
• He observed adaptations of plants and animals that
inhabited many diverse environments.
• His interest in geographic distribution of species
was kindled by a stop at the Galápagos Islands near
the equator west of South America.
DARWIN’S FOCUS ON ADAPTATION
• In reassessing his observations, Darwin perceived
adaptation to the environment and the origin of new
species as closely related processes
• From studies made years after Darwin’s voyage,
biologists have concluded that this is indeed what
happened to the Galápagos finches
DARWIN’S BOOK: ON THE ORIGIN
• Darwin developed two main ideas:
• Descent with modification explains life’s
unity and diversity
• Natural selection is a cause of adaptive
• Darwin never used the word evolution in the
first edition of The Origin of Species
• The phrase descent with modification
summarized Darwin’s perception of the unity of
• The phrase refers to the view that all organisms
are related through descent from an ancestor
that lived in the remote past.
• In the Darwinian view, the history of life is
like a tree with branches representing life’s
• Darwin noted that humans have modified
other species by selecting and
breeding individuals with desired
traits, a process called artificial
THESE DIFFERENT VEGETABLES HAVE ALL BEEN
SELECTED FROM ONE SPECIES OF WILD MUSTARD. BY
SELECTING VARIATIONS IN DIFFERENT PARTS OF THE
PLANT, BREEDERS HAVE OBTAINED THESE DIVERGENT
• Darwin then described four
observations of nature and from these
drew two inferences;
1. Observation 1: Members of a population
often vary greatly in their traits.
2. Observation 2: Traits are inherited from
parents to offspring.
3. Observation 3: All species are capable of
producing more offspring than the
environment can support.
4. Observation 4: Owing to lack of food or
other resources, many of these offspring do
• Inference 1: Individuals whose inherited traits give
them a higher probability of surviving and reproducing
in a given environment tend to have more offspring
than other individuals.
• Inference 2: This unequal ability of individuals to
survive and reproduce will lead to the accumulation of
favourable traits in the population over generations.
• Individuals with certain heritable characteristics
survive and reproduce at a higher rate than other
• Natural selection increases the adaptation of
organisms to their environment over time
• If an environment changes over time, natural
selection may result in adaptation to these new
conditions and may give rise to new species
EXAMPLES OF EVOLUTIONARY ADAPTATION:
• Note that individuals do not evolve;
populations evolve over time
• Natural selection can only increase or
decrease heritable traits in a population
• Adaptations vary with different environments.
FORMATION OF NEW SPECIES
• Speciation: the origin of new species.
• Microevolution consists of adaptations that
evolve within a population, confined to one
• Macroevolution refers to evolutionary
change above the species level.
BIOLOGICAL SPECIES CONCEPT
• The biological species concept states that a
species is a group of populations whose members
have the potential to interbreed in nature and
produce viable, fertile offspring; they do not breed
successfully with other populations.
• Speciation can occur in two ways:
• Allopatric speciation
• Sympatric speciation
• In allopatric speciation, gene flow is interrupted or
reduced when a population is divided into
geographically isolated subpopulations. (barriers
• The definition of barrier depends on the ability of a
population to disperse.
• Separate populations may evolve independently
through mutation, natural selection, and genetic
• In allopatric speciation, geographic isolation
restricts gene flow between populations
• Regions with many geographic barriers
typically have more species than do regions
with fewer barriers
Allopatric speciation of antelope squirrels
on opposite rims of the Grand Canyon
• Reproductive isolation between populations
generally increases as the distance between
• In sympatric speciation, speciation takes place
in geographically overlapping populations
• Sympatric speciation can also result from the
appearance of new ecological niches
• For example, the North American maggot fly
can live on native hawthorn trees as well as
more recently introduced apple trees.
• In sympatric speciation, a reproductive barrier
isolates a subset of a population without
geographic separation from the parent species.
• Sympatric speciation can result from
polyploidy, natural selection, or sexual
• Reproductive isolation is the existence of
biological factors (barriers) that impede two
species from producing viable, fertile
• Hybrids are the offspring of crosses
between different species
MECHANISMS OF REPRODUCTIVE ISOLATION
• Breeding at different times of the year
• Species specific courtship behaviour (animals)
• Adaptation to different pollinators (plants)
• Incompatibility of external reproductive organs
• Prevention of embryonic development
• Prevention of fertilisation
• Infertile offspring
EVOLUTION IN PRESENT TIMES
• Important example of natural
selection and evolution:
• HIV resistance to anti-retrovirals
• The use of drugs to combat HIV selects for viruses resistant
to these drugs.
• HIV uses the enzyme reverse transcriptase to make a DNA
version of its own RNA genome.
• The drug 3TC is designed to interfere and cause errors in
the manufacture of DNA from the virus.
• Some individual HIV viruses have a variation that allows
them to produce DNA without errors.
• These viruses have a greater reproductive success and
increase in number relative to the susceptible viruses.
• The population of HIV viruses has therefore developed
resistance to 3TC
• The ability of bacteria and viruses to evolve rapidly poses a
challenge to our society.