Download Theory of Natural Selection

Things Change Over Time
I. Why study Evolution?
A. Evolution explains the Diversity and Unity
of life
1. Diversity – the abundance of different
forms of living things on Earth
2. Unity – the biochemical, cellular, genetic,
and physiological characteristics/processes
common to all living things
B. What is Evolution?
 The change in a species over time (i.e. the
gene pool changes due to adaptations)
1. The Process: Descent thru Modification
 new life comes into existence over time
 all species come from common ancestry
 all species comes from existing species
via modification
2. The Mechanism: Natural Selection
 those best suited for env’t will survive and
pass traits to offspring
3. The Result: Diversity and Unity
II. Theories of Evolution
A. Lamarck
1. Evolution occurs thru the use and
disuse of physical features
Ex. Birds
 used their front limbs to fly so turned
into wings
 if didn’t use, they would disappear
2. He was wrong, but first to propose the
idea that things “change over time”
B. Carolus Linnaeus
1. Father of binomial nomenclature (taxonomy)
2. Believed that each species had an “ideal”
structure & fxn and had a specific place in
the scala naturae (ladder of life)
C. Malthus (English economist)
1. Stated that human population would
increase faster than food supply
 carrying capacity – only a limited number
of resources w/in an env’t
2. This idea would be applied to all species
3. This would influence one major aspect of
Natural Selection
D. Charles Darwin
1. Made observations on his voyage around
the world (H.M.S. Beagle  Galapagos
Islands)
Galapagos Islands
Darwin’s route
2. He proposed: Theory of Natural Selection
a. Those organisms who are better suited for
their env’t (due to adaptations), are more
likely to survive, reproduce, and pass on
favorable traits to offspring
b. “Survival of the Fittest”
3. In 1859, he published a book of his findings:
“The Origin of Species”
4. Four key components to his theory:
a. Genetic Variation
1. Within a species, there are heritable
differences which continue to be passed on
(ex. Some plants bear larger fruit, some
cows produce more milk)
2. Balanced polymorphism - When nat. selec.
favors heterozygotes over homozygotes
 example: sickle-cell anemia
3. Sources of variation: mutations,
crossing-over, sexual reproduction
b. Adaptation
1. Organisms respond to their env’t…change…
and increase their chance of survival
2. Increasing percentage of succeeding
generations having beneficial traits.
c. Competition
1. Env’t resources are fixed; however, indiv.
produced exceed resources
2. So, reproduction rate exceeds survival rate
3. Indiv. must “compete” for limited resources
d. Fitness/Reproduction
1. Some indiv. have better traits, thus better
suited for env’t  will likely reproduce
2. Fitness related to particular env’t
3. Relative fitness – fitness of one pheno. v.
another
Ex. Snowshoe hare: better in snowy env’ts
Ex. Darwin’s Finches…
Darwin’s Finches
5. Types of natural selection:
a. Directional – a shift in
the freq. of a trait in a
particular direction
 common in changing
env’ts
Ex. Horseracing:
pick horses that can
run fastest
b. Stabilizing – selects for
avg. pheno. and against
extreme pheno.
 common in stable,
unchanging env’t
c. Disruptive – selects for
extreme pheno. and
against avg. pheno. b/c
subject to predation
III. Evidence of Evolution
 What is the evidence for a common ancestry
and descent thru modification?
A. Five aspects:
1. Fossil Record
 radioactive dating: isotopes (Carbon 14)
 Keep in Mind:
 the fossil record is not complete
 many species live in env’ts that didn’t
allow for fossil formation (e.g. grasslands,
deserts). You need envt’s with rock, mud,
calcium deposits
 though incomplete, the record still provides
us with evidence of similarities
2. Biogeography
a. organisms evolve to adapt to their env’t, then
spread to other locations
b. there is a distribution of similar species
around the world
3. Comparative Anatomy
 Related organisms share a unity of body
plan and/or function
 Example: backbone, rib cage, and
arrangement of internal organs are
similar in fish, amphibians, reptiles,
birds, and humans
a. Homologous Structures
 organisms with similar structures, but
different functions
b. Analogous Structures – Diff. organisms have
body parts with a similar fxn, but not structure.
Insect wing
Bat wing
Bird wing
c. Vestigial Structures
 Structures/organs that no longer serve a
purpose and have become reduced in size
 Examples:
1. Cave fish – no eyes b/c don’t need to see
in dark
2. Humans – wisdom teeth, appendix, tail
bone
4. Comparative Embriology
 Related organisms develop similarly
 Example:
 All vertebrate embryos develop gill-like
structures, even though only fish have
gills as adult
lamprey
turtle
chicken
cat
human
5. Comparative Biochemistry
a. all living things use the same basic biological
molecules for similar functions;
1. DNA/RNA stores genetic info
2. proteins serve as enzymes
3. ATP, carbs, fats store NRG
4. Lipids build membranes
b. Metabolic systems are similar:
1. photosynthesis/respiration
2. gene expression, protein synthesis
c. Basic cell structure is almost universal