Anthropology 101: Human Biological Evolution

Lecture 2: Evolutionary Fact and Theory

Dr. Leanna WolfeLAWolfe@aol.com

Office AHSDrop In Hours:5:00-6:30 PM AHS 303

Darwin and the history of evolutionary thinking

• Historical Context

• Darwin’s theory of natural selection

Organisms are well-suited to their environment

• Viceroy butterfly

• Monarch butterfly

Complex adaptations require special explanation

The Scientific Revolution in Europe

• Global exploration

• Round Earth - 1492 etc.

• Biological Diversity

• Earth orbits the sun - 1541 Copernicus

• Solar system in motion - 1600’s Galeleio

• physics, medicine, chemistry advanced

• Scientific methods

• Measuring instruments

A problem emerges from the growing body of knowledge

• Early Geology = Study of stratigraphy• Stratum (strata) = Layers of earth

Within these layers

find fossils of non-

existing animals

And many resemble

living ones

A solution is proposed: James Hutton (1794)

• Strata = gradual process erosion accumulation

• Proposed “Deep Time” Earth profoundly old

The solution is resisted: Georges Cuvier (1796)

• Proposed “Catastrophism”

Strata formed by series of divine catastrophes

Species go extinct

Multiple creation events

The solution is refined: Lyell (1830)

• Proposes “Uniformitarianism”

• Natural processes same in past

and present

• Slow accumulation of changes

Requires deep time

• Darwin’s friend & mentor

So if the earth isn’t young…maybe life isn’t either

• Jean-Baptiste Lamarck

• First to propose an evolutionary mechanism for species change

• He was wrong in the details, but important nonetheless

Prior to Lamarck: Fixity of Species

• Species: A group of organisms that can interbreed to produce viable off-spring

Biblical understanding:• Every species was a unique

act of creation• Once created, never changes

…so perhaps species aren’t fixed

Lamarckian Evolution – Inheritance of Acquired Characteristics• Traits determined by their usefulness in an environment.• If environment changes, organisms acquire new traits• pass new traits on to their offspring

Meanwhile… • Thomas Malthus (1798)• An Essay on the Principle

of Population• Population growth outstrips

food supply• Population limited by

limited resources• Writing about human

population

Charles Darwin influenced by these ideas

• Born to rich family in 1809• Grandfather Erasmus proposed

early idea of species evolution

• Dropped out of med school• Studied religion• 1832 sailed on HMS Beagle• 5 year voyage to map coast of

South America

The Voyage of the Beagle(1831-1836)

The HMS Beagle visited the Galapagos Island• Animals unique to each

island

• Similar to mainland species

• Islands are volcanic, recent formations

• Appearance of NEW FORMS

Darwin collected many birds on the Galapagos • A variety of finch species

• Similarity in beak structure, body shape, and coloring

• Perfect gradation between species

• One species modified for different purposes

Adaptive Radiation

Darwin returned to England in 1836

• Sort collections • Talked to scientific

colleagues • Started a series of notebooks• Talked to plant and animal

breeders• Growing evidence of

gradation of forms in the fossil record

• Read Thomas Malthus

Enter Wallace• Alfred Russel Wallace

• Working-class family, b.1823

• little formal education

• Expedition to Amazon in 1848

• Developed similar ideas in

response to variety of natural

phenomena

Publication

• 1858: Enter Wallace… Yawn…

• 1859: Darwin published On the Origin of Species.

People noticed

The Theory of Evolution by Natural Selection

• Evolution: Change over time Can apply to many things besides biological organisms

• Natural Selection: Process by which species evolve Analogous with artificial selection

• Darwin interested in change over time of biological forms via the process of natural selection

• What is the process? How do biological forms change?

Darwin’s Theory of Natural Selection

1. The ability of a population to expand is infinite, but the ability of the environment to support populations is always finite.

2. Organisms within populations vary, and this variation affects the ability of individuals to survive and reproduce.

3. Variations are transmitted from parents to offspring.

Darwin called this process natural selection

Of course, long before you mature, most of you will be eaten

“Bummer of a birthmark, Hal.”

The Theory of Evolution by Natural Selection

1. Competition:

Malthus: Capacity of populations to increase is unlimited. Resources are not

2. Variation:

In all species, individuals vary in some of their physical or behavioral characteristics

3. Fitness:

Some members of a population have a variant that grants an advantage over others

The Theory of Evolution by Natural Selection

4. Heritability: Some portion of this trait variation is inherited5. Differential Reproductive Success:Individuals with favorable traits produce more offspring thanthose who don’t, leading to more members of the population whoinherited the trait6. Adaptation:Highly favorable traits will eventually become widespread in thepopulation, building new species-typical features7. Speciation:Over time, as new variants accumulate and old ones disappear, new species develop

Time

x

NO

YES

Evolution changes the characteristics of a population, NOT an individual

The Vocabulary of Evolution by Natural Selection

• Evolution: Change over time

• Natural Selection: Process by which species evolve

• Reproductive Success: The number of offspring an individual produces and rears to reproductive age

• Fitness: A measure of relative reproductive success• Adaptation: An anatomical, physiological, or behavioral trait

which improves an organism’s fitness in a given environment• Selective Pressures: Forces in the environment that influence

reproductive success in individuals

Birds of the Galapagos show us how natural selection works

Daphne Major

Medium Ground Finch

Postulate 1: Environment constrains population growth• Severe drought

occurred 1976-78

• Drought affected seed availability and quality

• Many birds died of starvation

Postulate 2: Individuals vary in ability to survive and reproduce

• Beak size varies• Small beaked birds have trouble with large seeds• During drought, larger beaked birds were at advantage

Postulate 3: Variation is transmitted from parents to offspring• Beak size is inherited

Characteristics of population changed over time

• Large beaked adults survived better• Large beaked birds had large beaked offspring• Mean beak size increased in population

But, natural selection can: produce change (Directional Selection) or maintain status quo (Stabilizing Selection)

• Small beaked birds can’t

find enough food

• Large beaked birds have

higher juvenile mortality

• Selection favors

intermediate beak size

• At equilibrium, selection

will maintain stasis (no

change)

Adaptations evolve in many small steps each favored by natural selection

• The key to natural selection is variation + selective retention

• Small changes occur

• Selection retains beneficial changes

• Complexity emerges

• Each step must be favored by natural selection

We can see these steps in the fossil record and among living creatures

Abalone:

Crude image

Spiny Murex:

Better image

Limpet: Directional information

Beyrich Split Shell:

Better directional information

Evolution can produce rapid change

• Examples in nature Radiation of Darwin’s

finches ( 500,000 yrs) Radiation of African

cichlids ( 12,000 yrs)

Loss and Imperfection

• Natural Selection can remove complexity

Cave fish lose eyesight Penguins have small

wings

• Imperfect features Human fondness for

sugar, fat, salt Peacock’s tail Human appendix

MYTH: ‘Survival of the fittest’ justifies everyone for themselves

• Survival of the fittest is NOT natural selection• Fittest does NOT always mean most aggressive, strongest,

most selfish

• Don’t use the NATURALIST FALLACY “because its natural its the right way to behave, or its

moral or good”

• Natural selection describes what happens in the world, NOT how we should chose to live our lives

MYTH: Evolution cannot be disproved

What would disprove evolution?

• Major jumps or different ordering in the fossil record

• Novel combinations that counter the predicted trajectory of

evolutionary history

• Animals & plants that don’t change

• Young earth = not enough time for all that we see to have

evolved

• Variety in the building blocks used by life and perfect forms

Key points

• Without variation, there can be no evolution

• Characteristics acquired during life are not heritable

• Whether a trait is favorable or not is determined by the

environment

• Natural selection acts on individuals

Even at a cost to the population or species

No “good of the species”

• Populations evolve

• Small steps add up to more complex traits