Transcript A basic definition of evolution…

Evolution
Only a theory?
Basic premises for this
discussion
• Evolution is not a belief system. It is a
scientific concept. It has no role in
defining religion or religious beliefs
• Evolution is a theory…but you don’t get
any better than that in science
• There is a lot of contention about
evolution, but not among scientists or
scientific organizations.
Some basic definitions
• Fact: an observation that has been
repeatedly confirmed
• Law: a descriptive generalization about
how the physical world behaves
• Hypothesis: a testable statement that
can be used to build inferences and
explanations
Some basic definitions
• Theory: a well-substantiated
explanation that incorporates facts,
laws, inferences and tested
hypotheses.
In science, you don’t get any
better than a theory.
What is evolution?
A basic definition of evolution…
“…evolution can be precisely defined as any
change in the frequency of alleles within a
gene pool from one generation to the next."
- Helena Curtis and N. Sue Barnes, Biology, 5th ed. 1989 Worth Publishers, p.974
So what does the definition
mean?
• Evolution is a change in the number of
times specific genes that code for
specific characteristics occur within an
interbreeding population
• Individuals don’t evolve, populations do
• There is no implied “improvement” in
evolution
Evolution’s Core Principles
Present day
species
Descent with
modification
Present day species
have accumulated
differences from their
ancestors through the
generations
University of California Museum
of Paleontology
Common Ancestor
Evolution’s Core Principles
Natural Selection
(differential
reproductive
success depends
on an individual’s
traits that allow it
to survival to
reproduce thus
passing trait to
next generation)
University of California
Museum of Paleontology
Evolution’s Core Principles
Evolutionary adaptations:
inherited characteristics that
increase an organisms’
chance of survival and
successful reproduction
Natural Selection Affects
Individuals
Evolution Affects
Populations
Darwin did not work in a vacuum
Darwin’s Voyage of Discovery
A reconstruction of the HMS Beagle sailing off Patagonia.
The Voyage of the Beagle
Darwin’s Observations and
Inferences
1st Observation:
All species have such great potential
fertility that their population size would
increase exponentially if all individuals
that are born reproduced successfully.
Darwin’s Observations
continued
2nd Observation
Populations tend to remain stable in size,
excepting seasonal fluctuations.
3rd Observation
Environmental resources are limited
Darwin’s Observations
continued
4th Observation
Individuals of a population vary extensively in their
characteristics; no two individuals are exactly alike.
5th Observation
Much of this variation
is heritable.
Darwin’s Inferences
Inference #1
Production of more individuals than the
environment can support leads to a
struggle for existence among individuals
of a population, with only a fraction of
offspring surviving each generation.
Darwin’s Inferences
Inference #2
Survival in the struggle for
existence is not random, but
depends in part on the hereditary
constitution of the individuals.
Those individuals whose
inherited traits best fit them to
their environments are likely to
leave more offspring than less fit
individuals.
Darwin’s Inferences
Inference #3
This unequal ability of individuals to survive and
reproduce will lead to a gradual change in a
population with favorable characteristics
accumulating over the generations.
University of California
Museum of
Paleontology
Adaptive Radiation
• One species gives rise to multiple other
species that exploit different resources.
Evolution evidence:
Biogeography
• Geographical
distribution of
species
Examples:
Islands vs.
Mainland
Australia
Continents
Biogeography
• The range of an organism is regulated
in part by their ability to migrate to a
new area
• Islands tend to have fewer species
because of the difficulty in migrating
there
• Changes in locations of continents over
geologic time helps or hinders migration
Biogeography of Species
• Apes are found only in Africa and Asia
• No apes fossil or living are found in the
Americas
• Species on islands are different from
but most similar to the species on the
nearest mainland
Evolution evidence:
The Fossil Record
• Succession of forms
over time
• Transitional links
between past and
present forms
• See Fossil Rocks and
Time PowerPoint for
further discussion
University of California Museum
of Paleontology
Evolution evidence:
Comparative Anatomy
• Homologous structures
(homology)
• Descent from a
common ancestor
• Vestigial organs
Ex: whale/snake
hindlimbs; wings on
flightless birds
Vestigial Structures
• Vestigial Structures were present in the
ancestor but little purpose in the
descendent organism’s current
environment
• Example: ostritch wings, eyes in cave
dwelling fish, pelvis bones in snakes
University of California Museum of
Paleontology
Mainland cormorant gave rise to
Flightless cormorants
On an island with no predators
Birds didn’t need to fly to
escape so tiny wings were not a
disadvantage and a heavier
body and legs were an
advantage
Homologous
Structures:
Different types of
bones in the forelimb
of these mammals
show descent with
modification from a
common ancestor as
despite their
differences in shape
and use these bones
all arise from the same
embryological structure
PBS.org
Evolution evidence:
Comparative Embryology
These vertebrates
show similar
features during the
course of
development
Ex. Pharyngeal (throat)
pouches and ‘tails’ as
embryos
Evolution evidence:
Molecular Biology
• Similarities in DNA,
proteins and genes
• Common genetic
code
Lines of Evidence Supporting
Evolution
• Biogeography
• Fossil record
• Homologous structures and Vestigial
structures
• Embryo development similarities
• DNA similarities among organisms
“Absence of evidence is not evidence of
absence.”
Carl Sagan