Notes From Reading
CHAPTER 24:E VOLUTION BY N ATURAL SELECTION (PGS.505-526)
- Populations and species evolve, meaning that their characteristics change through time.
More precisely, evolution is defined as changes in allele frequencies over time.
- Natural selection occurs when individuals with certain alleles produce the most offspring in
a population. An adaptation is a genetically based trait that increases an individual’s ability
to produce offspring in a particular environment.
- Evolution by natural selection is not progressive, and it does not change the characteristics
of the individuals that are selected – it changes only the characteristics of the population.
Animals do not do things for the good of the species, and not all traits are adaptive. All
adaptations are constrained by trade-offs as well as genetic and historical factors.
- Scientific theories are often made up of observations about a natural pattern and a
proposed process that explains that pattern.
- In the theory of evolution by natural selection, Charles Darwin and Alfred Russel Wallace in
1858 made the claim that evolution has occurred, that species have changed through time.
Then they proposed natural selection as a process to explain the pattern of evolution.
- Evolution by natural selection has become one of the best-supported and most important
theories in the history of scientific research.
24.1 The Evolution of Evolutionary Thought
- The Greek philosopher Plato claimed that every organism was an example of a perfect
essence or type created by God and that these types were unchanging.
- Aristotle ordered these types of organisms into a linear scheme called the great chain of
being. In this chain, species were organized into a sequence based on increasing size and
complexity, with humans at the top.
- This theory of special creation led to typological thinking, in which species are thought of as
unchanging types and variations are considered unimportant.
- In 1809 Jean-Baptiste de Lamarck was the first to propose a formal theory of evolution.
- He proposed that simple organisms originate at the base of the great chain of being by
spontaneous generation and then evolve by moving up the chain over time.
- Lamarck suggested that the process responsible for this pattern was the inheritance of
acquired characters. His idea was that individuals change in response to their environment
and then pass on those changes to their offspring.
- Darwin and Wallace proposed that change in species through time does not follow a linear,
progressive pattern but instead is based on variation among individuals in populations.
- A population consists of individuals of the same species that are living in the same area at
the same time.
- Darwin and Wallace proposed that evolution occurs because of natural selection—the
process by which individuals in a population with certain heritable traits tend to produce
more offspring than do individuals without those traits, leading to changes in the makeup of
the population. Notes From Reading
CHAPTER 24:E VOLUTION BY N ATURAL SELECTION PGS .505-526)
24.2 The Pattern of Evolution: Have Species Changed through Time?
- Darwin described evolution as descent with modification, meaning that change over time
produced modern species from ancestral species.
- The pattern component of the theory of evolution by natural selection makes two claims
about the nature of species:
- (1) they change through time, and
- (2) they are related by common ancestry.
Evidence for Change through Time
- Fossils are traces of organisms that lived in the past.
- The many fossils that have been found and described in the scientific literature make up the
- Most fossils are found in sedimentary rocks, which form from layers of sand or mud.
- Layers of sedimentary rock are associated with different intervals in the geologic time
scale—a relative time scale based upon fossil content.
- Geologic time is divided into eons, eras, periods, and epochs.
- Researchers now have used radioactive isotopes to assign absolute ages to the geologic
- Geologic data show that Earth is about 4.6 billion years old.
- The earliest signs of life are found in rocks about 3.4 billion years old.
- Many fossils provide evidence for extinct species, those that are no longer living.
- Darwin interpreted extinction as evidence that species are dynamic. He reasoned that if
species have gone extinct, then the array of species living on Earth has changed through
- Early scientists observed that extinct fossil species are typically succeeded, in the same
region, by similar living species; Figure 24.4a shows an example with sloths. This pattern
became known as the “law of succession.”
- Darwin interpreted this as evidence that extinct forms and living forms are related, that they
represent ancestors and descendents.
- As the fossil record has become more complete, many transitional forms have been
discovered with traits that are intermediate between earlier and later species.
- Figure 24.4b shows an example from the ancestry of whales.
- These transitional forms provide strong evidence for change through time. Notes From Reading
CHAPTER 24:E VOLUTION BY N ATURAL S ELECTION PGS .505-526)
- A vestigial trait is a reduced or incompletely developed structure in an organism that has no
function or reduced function, but is clearly similar to functioning organs or structures in
closely related species.
- Figure 24.5 shows two examples of vestigial traits in humans, including the functional traits
in related primates.
- Vestigial traits are evidence that the characteristics of species have changed over time.
Evidence That Species are Related
- Overall, data from the fossil record and contemporary species refute the hypothesis that
species are immutable.
- Let’s look at contemporary scientific evidence of the relatedness of species by common
- One line of evidence comes from similarities among island species. For example, Darwin
collected mockingbirds from the Galápagos islands. The mockingbirds were superficially
similar, but different islands had distinct species.
- Darwin proposed that the mockingbirds were similar because they had descended from a
- The mockingbird species are part of a phylogeny, a family tree of populations or species.
- The relationship between different species can be shown on a phylogenetic tree.
Genetic and Developmental Homology
- Another line of evidence comes from homologies. Homology is a similarity that exists in
species descended from a common ancestor. Homology can be recognized and studied at
three interacting levels: genetic, developmental, and structural.
- Genetic homology is a similarity in the DNA sequences of different species. A main example
is the genetic code itself.
- Developmental homology is a similarity in embryonic traits. An example is the gill pouches
found during embryonic development in chicks, humans, and cats.
- Structural homology refers to similarities in adult morphologies. One example is the
common structural plan found in the bones of the limbs in vertebrates (animals with a
- In many cases, traits are similar in different species because the species in question were
related to each other by common descent.
- If species were created independently o