Evolution Darwin Natural Selection Notes BSC 2011

Chapter 22 Evolution, Darwin, Natural Selection
Evolution = Heritable change in one or more characteristics of a population or species from one
generation to the next.
Charles Darwin: British Naturalist (1809-1892)

Raw material for Darwin’s theory of natural selection had been known for decades
Theory shaped by several different fields of study and experiences such as: geology,
economics, and the Beagle
The traits of the organisms that are best at surviving and reproducing will be the ones
that are most present in the next generation
Several species of finch adapted to different environmental niched
Evolution had to be depicted as a branching tree for Darwin’s original ide
At the same time, Darwin’s theory undermined the old idea that species were idealized
typed, fixed elements in a clearly defined natural order.
Species had to be treated as populations of varying individuals, with no fixed limit on the
range of possible variation.
Early Trees of Life: The depiction that Darwin hypothesis of species forming
relationships over na period of time
Scholars had debated evolution for generations before Darwin:

Individuals in a species are basically identical and species are unchanging.

“Species change as they spread from their original location”

Changes in nature are gradual, uniformitarianism

Species reappear after catastrophes; fossils represent extinction
Jean-Baptiste Lamarck (early 1800s)

He believed in “The inheritance of acquired characteristics”
He was also the first to suggest that animals could change or become extinct in response
to interactions with their environment
Example: Necks of giraffes got longer as they reached higher foliage

He developed his own and first comprehensive evolutionary theory in Philosophie
Zoologique in 1809. (50 years before Darwin’s origin of species)
He proposed there was an “alchemical complexifying force” that drove organisms up the
“complexity ladder”
He paired this with a second “environmental force” that adapted them to environments
though use and disuse of traits
He also came up with the term, biology and used it!
*** Darwin and Wallace both conceive of the idea of natural selection ***
Alfred Wallace independently conceived mechanism of natural selection

Naturalist in the Amazon and Southeast Asia
Interested in biogeography
Communicated and presented with Darwin
Darwin receives most of the credit for the theory of natural selection
Darwin found inspiration in economics and human population growth
Thomas Mathus: Essay on the Principle of Population (1797)

Most policies designed to help the poor are doomed because of the pressure of
population growth. If a nation doubles its population in a few decades, it will lead to
famine and misery.
Animals and plants should experience the same population pressure

Food supply is finite; individuals must compete, with many dying before becoming

Populations change over time and traits vary within a population
One population can split and branch in different directions as they adapt to their
There are pressures on populations due to limited resources

Natural Selection:
1. Variation in traits, many ae inherited
2. There are more offspring than can survive; resources are limited; organisms compete.
*** Better traits = higher survival and reproduction ***
Natural Selection is a mechanism of evolution
1. Variation within a given species
• Traits heritable = passed from a parent to offspring (genetic basis not yet known)

Natural Selection
More offspring produced than can survive
Competition for limited resources
Individuals with better traits survive and reproduce
Traits that can enhance survival and reproduction become more common
Natural Selection: Ground Finch

1978 drought: fewer small seeds
Small beaked birds struggled to find small seeds, and suffered heavy mortality
After one generation, the beak size is projected to increase
Evidence of Evolutionary Change:

Fossil Record
Biogeography (What Wallace is really interested in)
Observations of natural and artificial selection
o Anatomical
o Developmental
o Molecular
Fossils are traces of past organisms usually preserved in sedimentary rock
Sedimentary rock layers (strata)
Younger stratum with more recent fossils
Older stratum with older fossils
Fossil record is incomplete, but shows gradual evolutionary change
(Horses adaptive change)

Foot anatomy
Tooth morphology
Transitional Form: intermediate between ancestral form and descendant form
1. Endemic species = Species that only exist in a specific region or island
2. Fossils and biogeography show evolutionary patterns
3. Similar species on continents that are far apart suggest past physical connection
Observations of natural and artificial selection: (Selective breeding/artificial selection)

Breeders choose parents, over time can lead to large changes in morphology
Made possible by genetic variation
Some characteristics can change in only a few generations of selective breeding
Kale, broccoli, cabbage, and Brussel sprouts all originate from the same plant!
*** Homology: fundamental similarity due to descent from a common ancestor ***
Anatomical Homology

Humans, Cats, Whales, and Bats all shared a common ancestor because we all have a
humerus, radius, ulna, carpals, metacarpals, and phalanges
Whales have no hind limbs, but many still have hind limb bones because their terrestrial
ancestors had legs.
o These types of structures are referred to as “vestigial structures” (doesn’t have a
purpose anymore over time)
Developmental Homology

How one develops over time because of the common ancestor
Molecular Homology

All living species use DNA to store information
Sequences of closely related species tend to be more like each other than to distantly
related species
Convergent Evolution
2 species from different lineages show similar characteristics because they occupy similar
environments (converge at this point)

Based on environmental similarities and received them independently
Can cause many issues when scientists are trying to classify organisms
Homology vs Homoplasy
Homology: A shared trait that has been inherited from a common ancestor
Homoplasy (aka analogous structure): A shared but independently derived trait = convergent

Wings of birds and insects are an example of an analogous structure (similarity due to
convergent evolution)
The BONES are an example of homology, but the WING fur (actual physical flat part)
like structure is an example of convergent evolution (analogous structure)

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