Lecture Outline - Evolution

1. History of Evolutionary thought

2. Principles of Evolution by Natural Selection

3. Modern Synthesis: combining Darwin + Mendelian genetics

4. Sex

5. Evidence for evolution

6. Why the controversy?

Historical ContextOriginal European belief: Genesis account of creation (6 days)

In 1500’s, explorers started bringing home animals no one had ever seen before; literal Creationism feel out of favor

Replaced by Special Creationism (held a hint of ecology):

(1) God created every living thing, exactly suited to its particular environmental niche and location

(2) When one species went extinct, an appropriate successor was created and installed in its place

- was the belief of most scientists into the 1800’s

Lamarck - “Acquired Characters”1809 - French biologist proposed that species had changed over time, and were related by common ancestry

Proposed that individuals changed to meet the needs of their environment, and that these acquired characters were then passed on to their offspring

Most famous idea - giraffes stretched their necks to reach high branches

The Birth of Evolutionary TheoryIn the early 1800’s, some enterprising young men began exploring the world’s biodiversity with an eye for pattern and process in the natural world. One was Charles Darwin - educated, from a wealthy family - destined to become a preacher, he set off to explore the world after college (1832-1836) - collected and observed animal life from islands on the Beagle

Evolution and IslandsDarwin observed that neighboring islands in the Galapagos had similar, but different, species of mockingbirds

Evolution and IslandsDarwin observed that neighboring islands in the Galapagos had similar, but different, species of mockingbirds

Over time, descendents became different species on each island

Evolution and Islands

Darwin set off on Beagle with no clear notion of evolutionary patterns

- returned with basic ideas of his theory and data to support it

However, he knew that proposing how species “came into existence” would create controversy in Victorian England

- sat on his theory for 20 years, writing Origin of Species

Alfred Russell Wallace

Wallace’s family lost their fortune, so he worked, educated himself, and took off to explore the world as a young man

While Darwin sat home writing his book, Wallace travelled the world, financing his epeditions by collecting and selling animals

Wallace’s collections gave him insight into:

- variation within a species (all individuals are not the same)

- geographical distribution of species (not random)

Alfred Russell Wallace

His observations suggested that geographical boundaries could explain species distributions

3rd species, as predicted byWallace,P. pithecia

- rivers delineated boundaries between related species

Different speciesof spidermonkeyP. monachus

one speciesof spidermonkeyP. irrorata

Darwin & Wallace

Wallace had the theory to contest Special Creationism (evolution), but not the mechanism (natural selection) 

He and Darwin then began corresponding 

In 1858, Darwin received a manuscript from Wallace outlining the basics of evolution via natural selection 

- his friends urged him to quickly write up a summary of his 20 years’ worth of thoughts + data 

- both papers were read aloud together + published  

Darwin vs. Wallace

The following year (1859), Darwin published his book On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life

Major contribution: idea that new species arose by descent with modification from ancestral species

History forgot about Wallace, but his independent derivation of natural selection helped convince other scientists of this revolutionary new theory  

2. Evolution by Natural SelectionDarwin’s theory had 4 postulates, or underlying assumptions:

(1)   Individuals within a species are variable

(2)   Some of this variation is passed to offspring (in other words, traits are heritable)

(3)   In every generation, more offspring are produced than can survive (due to limited resources)

(4)   Survival and reproduction are not random:

- individuals with the most favorable variations survive, or produce the most young

- this is the principle mechanism of evolution, called natural selection

Evolution by Natural Selection

Darwin’s 1st postulate:

(1)   Individuals within a species are variable

Evolution by Natural Selection

Darwin’s 2nd postulate:

(2)   Some of this variation is passed to offspring (in other words, traits are heritable)

Beak size of parents (mm)

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Evolution by Natural Selection

Darwin’s 3rd postulate:

(3)   Every generation, more offspring are produced than can survive

Organism Reproductive potentialaphid 524 billion in one year

elephant 19 million in 750 years

a bacterium cells cover earth 7 feet deep in 2 days

Starfish 1079 offspring in 16 years

Evolution by Natural Selection

Darwin’s 4th postulate: natural selection

Observed in natural populations all the time

Beakdepth

Evolution by Natural Selection

Darwin’s 4th postulate: natural selection

- drought favored deeper beaks that could crack the tough seeds that were available

A change in the genetic makeup of a population is…

Beakdepth

Evolution by Natural Selection

Any change in the genetic makeup of a population is termed evolution

- this can happen by chance, by natural selection, by human breeding programs, etc.

Natural selection causes evolution that results in adaptation, producing organisms that are better suited to their particular environment

adaptation increases..

Problems with Darwin’s Explanation

There were two major problems in Darwin’s theory: (1) Darwin didn’t have any knowledge of Mendel’s work on genetics, so he didn’t get how heritability worked

(how were favorable traits passed from parents to offspring?) (2) mutation had not been discovered, so Darwin couldn’t explain where variation came from  

3. The Modern Synthesis

This was taken care of between 1932-1953 in a series of books that integrated genetics with the theory of evolution by natural selection

- this is termed the Modern Synthesis

- explained things in terms of alleles, which are different versions of a given gene 

The Modern SynthesisModern synthesis restated Darwin’s 4 postulates:

(1) Individuals in a population are variable for traits, because in every generation, mutation creates new alleles, and

sexual reproduction creates new allele combinations (2) Individuals pass some of their alleles to their offspring (3) More offspring are produced than can survive (4) Individuals that survive, or reproduce the most, have allele

combinations that best adapt them to their environment

4. Sex and Selection

Nearly every organism has sex at some point in its life - why?

Sex is less efficient than asexual reproduction (cloning) - sex requires a mate - wastes half of resources on males, who don’t have babies

Principle advantage of sex: generates variation among offspring,creating new allele combinations not present in either parent

Because you don’t know what conditions will be like for the next generation, you make a bunch of non-identical offspring & hope at least a few have the right traits to survive

Sexual selection and female choice

Males have traits important in mating displays (color, tail, claws) whereas females lack such ornamentation

- male energy goes into looking sexy, attracting mates

- females invest energy in making large, healthy offspring

Fiddler crabs

male

Peacocks

Sexual Selection

Darwin’s sequel, The Descent of Man and Selection in Relation to Sex, was dedicated to this phenomenon

How could traits involved in male display or competition persist in the face of natural selection? - shouldn’t natural selection weed out brightly colored individuals (by predation), or those wasting energy on big tails?

Sexual SelectionSexual selection can be:

(a) intersexual - males appeal to female preferences

Females may choose mates based on traits that indicate his health, or superior genes..

... or possibly just based on arbitrary preferences of the females of that particular species (what color their eyes see most clearly, for instance)

Sexual SelectionSexual selection can be:

(a) intersexual - males appeal to female preferences

(b) intrasexual - males compete with each other for access to females

Whereas natural selection makes a population more adapted to its environment, sexual selection does not-- it makes one sex more successful in mating with the other

- increases fitness by increasing reproductive success

- does not increase adaptation or survival

Darwin’s 3 Forms of SelectionOver his career, Darwin wrote about 3 kinds of selection:

(1) Natural selection- makes a species better adapted to its environment

(2) Sexual selection- makes one sex more appealing to the other- increases mating opportunities

(3) Artificial selection- we choose desired traits and amplify them through selective breeding of domestic organisms

In last 150 years, no one else has found another kind

Evidence for Evolution

Evidence for the theory of evolution is found in:

(1) homology among related organisms

(2) the fossil record (intermediate forms in a given lineage)

(3) vestigial structures

(4) molecular evidence - relatedness of DNA sequences - conserved action of genes - conserved genetic code, transcription/translation 

- modifications of this shared “scaffold” show how natural selection altered the ancestral arrangement, increasing adaptation for different mammal lineages

Homologous Structures

same set of bones constructs hands & forearms in all mammals revealsa shared ancestry

Evidence for Evolution: the Fossil Record

Evolutionary history of whales is well recorded in fossils showing progressive modification over time, resulting in forms similar to modern whales

Long-predicted fossils of ancestral whales with legs were recently discovered

Evolution is science - we make testable predictions like, “one day we will find a fossil whale with legs”

Evidence for Evolution

Evidence for the theory of evolution is found in:

(1) homology among related organisms

(2) the fossil record (intermediate forms in a given lineage)

(3) vestigial structures- molar teeth in vampire bats- tailbones in humans- reduced hind legs in snakes

(4) molecular evidence