Download Introduction to Evolution - Springfield

to the differential survival and reproduction
they cause. This is an example of how new
variation that is produced by mutation may
be selected for by natural selection.
3.Mutations in human populations — Humans
have acquired mutations leading to inherited
disorders or traits (for example, sickle cell
anemia, lactose intolerance, albinism) and,
unfortunately, mutations are the cause of
many of the types of cancers found today.
Isn’t evolution “just a theory?”
“Theory” in science has a very different meaning than in popular use. In everyday conversation, when someone says “I have a theory,” they
are guessing as to an answer about something, which, in science, is a hypothesis. In the
sciences, a theory is a broad explanation of a
natural phenomenon that is widely supported
by many experiments. While many theories are
long-standing and consistently upheld, they
are still subject to being modified or disproved
over time in the face of new evidence. Examples of theories that explain natural phenomena include the theory of gravity, the theory of
atoms, the theory of plate tectonics, and the
theory of evolution.
What is genetic variation
and why is it important?
Variation serves as the vital “fuel” for evolution
and must be present for natural selection to
occur. Individuals within species may vary in
such traits as visual acuity, wing shape, tooth
enamel thickness, odor detection, leg length,
canine size, fur thickness, disease immunity,
and so on. This variation enables some individuals to have higher reproductive success
than others, and makes it more likely that at
least some individuals of a species will survive
to reproduce in the face of continuous environmental changes.
If biologists cannot agree on
evolution, how can I accept it?
Scientists do not disagree about whether or
not evolution occurs, but about some of the
specifics of how it occurs. Their arguments
are typically about the importance of each
evolutionary force in causing allele frequency
change, or the rates of evolutionary change
for various genes in different species. While
scientists may debate specifics of evolutionary
processes and patterns, the existence and actions of the five evolutionary forces are agreed
upon by all.
If humans evolved from apes,
why do apes still exist?
Humans did not evolve from modern apes.
The fossil record and genetic evidence indicates that we shared a common ancestor with
the apes. This common ancestor would have
been quite ape-like, however, and the earliest
members of the human family do have many
ape-like features such as smaller brain size
and a forward-jutting jaw. These early human ancestors occurred in the fossil record
soon after the split with the African apes. By
the time we see recognizable modern chimps
and humans in the fossil record, those earlier
forms had become extinct.
Why did the apes stop evolving
(unlike the humans)?
The simple answer to this is, “They didn’t.” Apes
have by no means “stopped evolving.” They
have their own unique evolutionary history,
and since the split from our common ancestor,
their environment and lifestyle have changed
less than ours, resulting in their maintaining
more similarities to the ancestral form. Evolution is not goal-directed, but is a continual
process, affected by random events as well as
environmental conditions.
For more information, visit thelibrary.org/exploringhumanorigins.
How do we know that species
are related and share a common
ancestor?
The greater the number of features that are
shared between taxonomic groups, the more
closely related they are presumed to be, and
the more recent in time their presumed shared
ancestor lies. The fossil record represents
numerous intermediate forms between extinct
and living species that serve as “snapshots of
time” and complement the evidence from genetics, comparative anatomy, embryology, and
physiology. The more fossils we find, the more
complete the record becomes and the more
gaps we fill in.
For example, it was long predicted that birds
and reptiles shared a common ancestry because of their similar anatomical and physiological features. Scientists expected to eventually discover bird-like dinosaurs (reptile-like
fossils with feathers, bird-like fossils with
teeth, or bird-like fossils with long reptilian
tails). In fact, the fossil record now includes numerous intermediate dinosaur-to-bird forms,
including Archaeopteryx as well as Eoraptor,
Compsognathus, Protarchaeopteryx, Caudipteryx and Velociraptor. We have also used new,
sophisticated genetic evidence to support the
relationship between reptiles and birds. For
example, geneticists recently discovered that
the very same genes producing scales in reptiles are responsible for feathers in birds.
Scientists are able to draw conclusions and develop scientific theories from the observations
and data available to us in the natural world.
Those conclusions and theories continue to be
questioned and evaluated by scientists with
each new observation and data set in order to
better understand the world in which we live.
Springfield-Greene County Library District
4653 S. Campbell Ave • thelibrary.org
THE
SCIENCE
OF
EVOLUTION
Prepared by
Suzanne Walker-Pacheco, Ph.D., a professor in
the Department of Sociology and Anthropology
at Missouri State University; and Kevin Jansen,
Ph.D., department chair and associate
professor of biology at Drury University.
In conjunction with the
Smithsonian Institution
traveling exhibit
Exploring Human Origins:
What Does It Mean
To Be Human?
May 9–June 2 at the Library Center,
4653 S. Campbell Ave.
Exploring Human Origins: What Does It Mean To Be Human?
was organized by the Smithsonian’s National Museum of
Natural History in collaboration with the American Library
Association. This project was made possible through the
support of a grant from the John Templeton Foundation and
support from the Peter Buck Human Origins Fund.
Smithsonian Institution
Questioning, probing, and critiquing are
qualities in the nature of scientists, and we
continue to engage these qualities to better provide explanations for how our natural
world works.
That makes this document a helpful
supplement to the Smithsonian Institution
traveling exhibit, Exploring Human Origins:
What Does It Mean to Be Human? May 9–June
2 at the Library Center, 4653 S. Campbell
Ave. The exhibit of panels, interactive kiosks,
hands-on displays and videos explore
milestones in the evolutionary journey of
becoming human. See a complete list of
programs accompanying the exhibit at
thelibrary.org/exploringhumanorigins or in the
Bookends calendar of events.
What is Science?
Science involves seeking out reliable explanations for what we see in the world around us,
using three basic approaches: observation,
experimentation, and logic. Scientists use a
series of steps in the scientific method to: 1)
observe some phenomenon empirically, using
at least one of our five senses; 2) ask ourselves
a question about the phenomenon — how/
what/why types of questions; 3) propose a
hypothesis that is a testable statement to
investigate the question; and 4) design an
experiment or other alternate study to collect
data that will test the reliability of our hypothesis. Similar conclusions to repeated testing
of similar hypotheses can lead to the development of a theory. We derive benefits from the
scientific method every day. Most household
products we use, and all the medications we
take, have been subject to rigorous testing as
part of this process.
What is Evolution?
Biologists define evolution as a change in
allele frequency in a population over time.
A population is considered to be a breeding
group, and alleles are simply variant forms
of a gene. For example, the ABO blood type in
humans is determined by one gene, three possible alleles (A, B, or O), and four possible phenotypes (types A, B, AB, and O). If a population
experiences a change in the frequencies of the
three allele variants, we conclude that evolution has occurred. We continually observe and
document the occurrence of allele frequency
change, and thus evolution, in populations all
around us, both in nature and in the laboratory.
How Does Evolution Work?
Five main factors cause evolution to occur.
These evolutionary forces are mutation, natural selection, sexual selection, gene flow, and
genetic drift.
• Mutations are heritable changes in the DNA,
occurring randomly in the genome of all species. Mutation is the only evolutionary force
to produce new variation.
• Natural selection was proposed independently by both Charles Darwin and Alfred
Russel Wallace to explain how evolution
works to allow organisms to become better
adapted to their environment. The steps
are: 1) More individuals of each species are
produced than survive; 2) variation exists
among individuals such that some have
inherited traits allowing them to be better
suited to their environment than others; 3)
those better suited to their environment are
more likely to survive and produce more offspring than those with traits that make them
less well-suited; and 4) alleles for the wellsuited traits will be found in more offspring
in the next generation, causing a change in
allele frequency, or evolution.
• Sexual selection can be considered a type
of natural selection that takes the form of
choosing a mate or competing for mates.
• Gene flow occurs when individuals move
from one population to another, taking their
alleles with them. This causes a change in
allele frequency in both the population they
leave and the one they go to.
Cover image and above: Smithsonian’s Human Origins Program
• Genetic drift is also referred to as “random
genetic drift” because the “drift” (or change)
in allele frequency that occurs from one
generation to the next results from chance.
The smaller the population, the greater the
effect genetic drift will have. Remember Gilligan’s Island? A boat is lost at sea, leaving
its passengers stranded on an island. The
fateful trip of the SS Minnow resulted from a
chance event, a storm. The gene pool of that
new population of seven individuals obviously would have been a very limited one,
without all the variation found in the mainland population from which they came. That
random shift in allele frequency from their
old population (on the mainland) to their
new population (on the island) is genetic
drift.
The effects of evolutionary forces are often
combined; mutation produces new genetic
variation, and the other evolutionary forces
redistribute or eliminate it within and among
populations.
Are there examples of evolution
that affect humans?
Yes, some common examples of evolution that
directly affect humans include:
1. Changes in the influenza virus each year —
Scientists found the rate of mutation in the
influenza virus to be high enough that we
must develop new flu vaccines each year.
2.Development of bacterial resistance to antibiotics — Some strains (MRSA, for example)
of Staphylococcus aureus, a type of bacteria
found normally on humans and often without ill effects, have developed resistance to
at least four antibiotics, including penicillin,
over the past several decades and currently
have no known treatment. A particular mutation may allow some individuals to survive at least one round of antibiotics; when
these individuals reproduce, they pass on
those new alleles responsible for its hardy
nature and confer the antibiotic resistance
to a portion of the next generation. These
new alleles will increase in frequency, due