Evolution By Natural Selection
Populations and species evolve, meaning that their characteristics change through time. More precisely, evolution may be
defined as changes in allele frequencies over time..
o Exceptions exist (e.g overdominace, u-s balance and m-s balance and evolution is much richer than merely changes
in allele frequencies.
Natural selection occurs when individuals with certain alleles survive or produce the most offspring in a population. An
adaptation (at the micro evolutionary scale) is a genetically based trait that increases an individual's ability to survive or
produce offspring in a particular environment.
Evolution by natural selection in not progressive and it does not change the characteristics of the individuals that are selected it
changes only the characteristics of the population. Animals don't do things for the good of the species (nut might behave
altruistically under particular conditions) and mot all traits are adaptive. All adaptions are constrained by trade-offs as well as
genetic and historic factors.
Scientific theories are often made up of observations about a natural pattern and 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 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
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.
o Typological: idea that species are unchanging types and that variations within species are unimportant or even
In 1809 Jean Baptiste de Lamarck was the first to propose a formal theory of evolution.
o Lamarckian Evolution: is progressive in the sense of always producing larger and more complex, or “better”
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 the pass on those changes to their offspring.
o By that mechanism there is an ascesion up the ladder over time.
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.
Darwin claimed that instead of being unimportant or an illusion, variation among individuals in a population was the key to
understanding the nature of species.
Population: consists of individuals of the same species that are living in the same area.
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 survive and produce more offspring than do individuals without those traits
leading to changes in the makeup of the population.
The Pattern of Evolution
Darwin described evolution as decent with modification, meaning that change over time produced modern species from
o We aknist never know what the ancestoral speciece are.
The pattern component of the theory of evolution by natural selection makes two claims about the nature of species:
o they change through time
o they are related by common ancestry
Evidence for Change Through Time
Fossils: are traces of organisms that lived in the past.
Fossil Record: the many fossils that have been found and described in the scientific lecture.
Sedimentary Rocks: form from many layers of sand or mud and where most fossils are found.
Geologic Time Scale: a relative time scale based upon fossil content.
o Geologic time is divided into eons, eras, periods and epochs.
Researchers now have used radioactive isotopes to assign absolute ages to the geologic time scale
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 Evolution By Natural Selection
At first, many scientists insisted that living examples of these species would be found in unexplored regions of the globe.
But as the research continued and the number and diversity of fossil collections grew, the arguement became less and less
Many fossils are 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 time.
Early scientists observed that extinct fossil species are typically succeeded in the same region, by similar living species. This
pattern became known as the “law of succession”
Darwin interpreted this is as evidence that extinct forms and living forms are related, that they represent ancestors and
As the fossil record has become more complete, many transitional forms have been discovered with traits that are
intermediate between earlier and later species.
For example, intensive work over the past several decades has yielded fossils that document a grdual change change over time
from land-dwelling mammals that had limbs to ocean-dwelling mammals that had reduced limbs or no limbs.
These transitional forms provide strong evidence for change through time.
Vestigial Traits: 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.
For example, the human appendix is a reduced version of the cecum- an organ found in other vertebrates that functions in
Evidence that the characteristics of species have changed over time.
Evidence That Specives Are Related
Overall data from the fossil record and contemporary species refute the hypothesis that species are immutable.
One line of evidence comes from similarities among island species. For example, Darwin collected mockingbirds from the
Galapagos 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 common ancestor.
o His first recorded expressed doubt about fixed species
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.
Homology: is a similarity that exists in species descended from a common ancestor. Homology can be recognized and studied
at three interacting levels: genetic (molecular), developmental and structural.
o Means the study of likeness
Genetic Homology: is a similarity in the DNA sequences of different species. A main example is the genetic code itself.
The genetic code is described as “universal”
Some amino acids in some organisms are assigned uncommonly to codons
A more-appropriate decriptor than “universal” for the almost universal genetic code is „standard‟
Evolutionary Developmental Biologists propose that fruit flies and humans shared a common ancestor that possessed a gene
that was similar to eyeless and Aniridia and that gene was invovled in forming a light-gathering organ.
Visual organis have diverged as the lineages leading to fruit flies and that gene was involved in forming a light gathering
Visual organs have diverged as the lineages leading to fruit dlies and human evolved, but the same gene remained involved in
determining where eyes are located.
Eye homology is complicated to assess, as are all homologies.
Is a similarity in embryonic traits. An example is the gill pouches found during embryonic development in chicks, humans,
Refers to similarities in adult morphologies. One example is the common structural plan found i