THE THEORY OF EVOLUTION
NATURAL SELECTION AND EVIDENCE OF
EVOLUTION
CHARLES DARWIN AND NATURAL SELECTION
Has been said that its impossible to understand any field of biology without
understanding evolution.
The modern theory of evolution is perhaps the most fundamental concept in Biology.
A rich fossil record has been important to biological sciences since the 18th century.
It formed the basis of early evolutionary concepts
Scientist wondered where these species came from, why some no longer existed, and
what relationship, if any, they have to modern species.
Became clear that Earth was much older than expected
Early 19th Century scientist were convinced that life slowly changed over time, or
evolved.
Charles Darwin
(1809-1882) English scientist
Father of modern evolutionary theory
1831 at age of 21 recommended for position of naturalist on HMS Beagle
Ship chartered for a 5 year collecting and mapping expedition to South America and
South Pacific.
A voyage that would forever change the science of Biology
Darwin’s job was to collect, study, and store specimens discovered
The voyage took him through a number of environments with some of the World’s
greatest biodiversity
Galapagos Islands were among the most important
Group of small islands about 1000km off west coast of South America
Support a great diversity of animal and plant life
Darwin studied and compared the anatomy of many species of reptiles, insects, birds, and
flowering plants unique to the islands.
Similar species seen in other parts of the world.
By the end of the trip he was convinced that evolution occurs- species change over time.
Darwin returned from his voyage in 1836
Spent next 22 years studying his collection and conducting experiments
Darwin was also intrigued by an essay written at the time that suggested human
population was growing faster than the food supply on Earth.
He knew many species produce large numbers of offspring, and since Earth was not
covered with such species, their must be a struggle for existence among individuals.
He envisioned many kinds of struggles = comp for food and space, escape from
predators, and need to find shelter.
Only some of individual in population survive the struggles to produce offspring.
His answer came from his own pigeon breeding experiments
Individual pigeons have different variations of traits that can be inherited.
Breed pigeon with desirable variations, he was able to produce offspring with these same
features.
Artificial Selection = a breeder selects particular traits
Same force in nature?
Using data he gathered from natural world he began his well known idea of evolution by
natural selection.
Natural Selection = a mechanism for change in populations that occurs when organisms
with favorable variations for a particular environment survive, reproduce, and pass these
variations on to the next generation.
Organisms with less favorable variations are less likely to survive and pass on traits to the
next generation.
“Survival of the Fittest”
Therefore, each new generation is made up largely of offspring from parents with the
most favorable variations.
Fig 18.3
On the Origin of Species by Natural Selection: 1859
Convinced many scientist that evolution does occur through natural selection
Unifying theory for all biology (today)
NATURAL SELECTION AND ADAPTATION
Why do some plants have thorns?
Why some animals have distinct colors?
Adaptation = any trait that aids the chances of survival and reproduction of an organism.
Mole-rat possesses a number of features that adapt them to life underground
Ancestors lived above ground and didn’t have these characteristics
Fig 18.4
Structural Adaptation:
= changes in the structure of body parts
Some obvious ones are used for defense against predators
Thorns on rosebushes
Spines on sea urchins
Mimicry:
= structural adaptation that provides protection for an organism by enabling it to copy the
appearance of another species
Fig 18.5
Camouflage:
= a structural adaptation that enables an organism to blend in with its surroundings
A defense adaptation that involves changes to the color of organism
More likely to escape predators and survive to reproduce
Structural adaptations may take millions of years to develop
Depends on type and rate of reproduction and environmental factors
Some evolve relatively quickly
Camouflage adaptations in moth’s evolve in 100 years
Physiological Adaptation:
= changes in an organisms metabolic process
Can develop rapidly
Some medicines developed during this century are no longer useful
50 years ago antibiotic drug penicillin “wonder drug”
Could kill many types of disease-causing bacteria
Not as effective today
Many species of bacteria have evolved physiological adaptations make them resistant to
penicillin.
“Multi-drug resistance”
Rapid changes in pest organisms such as insects and weeds
Resistant to chemicals after being exposed to pesticides
EVIDENCE OF EVOLUTION
Most of the evidence of evolution comes from indirect sources
Ancestors of whales once land-dwelling, wolf-like animals?
Study their fossils.
Although the fossil record in incomplete, it does provide evidence that evolution has
occurred.
Provides a record of earlier life.
“Completing a jigsaw puzzle with missing pieces”
May still be able to understand the overall picture
Same with fossils
Can’t trace each and every step because intermediate forms cannot be found.
Still understand the general pathway of evolution
Can show a step-by-step sequence of evolution
Fig 18.7: Camal
Used skulls, teeth, and limb bones
40-50 mya camals were a small, rabbit sized animal
Homologous Structures:
= modified structure that is seen among different groups of descendants
Forelimbs of animals look the same in Fig 18.8
From outside, they would look different and vary in function, but they are similar in
skeletal features.
Evolved from a common ancestor with the same basic limb structures
Over the course of evolution, vertebrates moved into different environments.
Within each environment, animals faced very different needs for survival
Limbs more useful in environment = survival
Became adapted to different ways of life.
Modified structures are still the same even though they aren’t used for same function.
Often similar in structure, in function, or both
However, similarity of function doesn’t always mean that the 2 organisms are closely
related.
Homologous structures probably had a common evolutionary origin.
Analogous Structures:
= any body part that is similar in function but different in structure
Butterfly wing and Bird wing
Fig 18.9
Evolved independently in 2 distantly related groups of ancestors
Can’t be used to indicate evolutionary relationships, but they do provide evidence of
evolution.
Insect and bird wings most likely evolved when their different ancestors independently
adapted to similar ways of life.
Vestigial Structures:
= any body structure that is reduced in function in a living organism, but may have been
used in an ancestor.
Functionless structures indicate evolutionary pathways.
What is appendix used for?
Evolutionary biologist can tell you what an appendix WAS for.
Organisms contain structures that have reduced functions but were once used by ancestral
organisms.
Appendix = intestine, different diets, secrete hormones/enzymes/bile?
Particular structures become vestigial as species change in form and behavior
The structure, although it may serve no function, continues to be inherited as part of the
body plan for that species.
The eyes of sightless species, common Mole-Rat and Cave Fish, may be considered
vestigial because eyes were most likely functional in ancestors.
Fig 18.10 = Ostrich wings
Embryonic development shows evolution from a common ancestor.
Look at embryos
Fig 18.11: Vertebrates
Tail, gills slits, can be seen in all species
As development continues, become more distinct
Common Ancestors
Genetic comparisons may reveal hidden relationships
Help tell us how different species are related
Comparison of DNA and RNA within a taxonomical group
More reliable that anatomical relationships
Use nucleotide sequencing to indicate levels of relationships
Since the 1970’s research has shown that by comparing nucleotide sequence in the DNA
and RNA of species, its possible to constrict hypothetical evolutionary trees showing
levels of relationships
Fig 18.12