Chapter 15Darwin’s Theory of Evolution

Theory vs. Hypothesis

• Theory: unifying idea which is proven through repetitious experimentation

• Hypothesis: conjectures or plausible explanations that may be testable

• Evolution is a theory and not a hypothesis.

Darwin’s observations

Pinta IslandIntermediate shell

Pinta

Isabela IslandDome-shaped shell

Hood IslandSaddle-backed shellHoodFloreana

Santa Fe

Santa Cruz

James

Marchena

Fernandina

Isabela

Tower

Characteristics varied noticeably among the

different islands

Who influenced Darwin’s thinking?

• James Hutton• 1795, published work

that geological forces have shaped Earth

• Rock layers form slowly so Earth must have formed over a period of time longer than a few thousand years.

Movement of Earth’s crust

Sea level

Sedimentary rocks form in horizontal layers.

When part of Earth’s crust is compressed, a bend in a rock forms, tilting the rock layers.

As the surface erodes due to water, wind, waves, or glaciers, the older rock surface is exposed.

New sediment is then deposited above the exposed older rock surface.

Sea level

Who influenced Darwin’s thinking?

• Charles Lyell• Wrote Principles of

Geology• Stressed the principle

of uniformitarianism, that events in the past can be explained from occurrences today

• (the present is the key to the past)

Jean Baptiste Lamarck

• Part of why Darwin’s theory of evolution was controversial is that many people still confuse Darwinian evolution with Lamarckian evolution.

• Lamarck’s ideas about evolution are considered to be unsound.

Tendency toward perfection

• Populations evolve as a consequence of adaptations that are beneficial for the time and place that the organisms reside and not to become perfect.

• There is no such concept as perfection in biological understanding of nature.

Use and Disuse

• Lamarck believed that if a behavior is useful and beneficial to the organism, then the body parts used to carry out the behavior become prominent and developed.

Example: Giraffe’s have long necks because they needed the longer necks in order to eat leaves from trees.

Use and Disuse

• Example of disuse: Horses and cows eat grass from the ground and therefore do not use their necks in feeding. This is why horses and cows have short necks.

Inheritance of acquired traits

• Lamarck believed that organisms can inherit acquired traits from parents.

• Example: If Michael Phelps learns to swim extraordinarily well, then his child will be born with the ability to swim extraordinarily well. Is this true?

Influence of Thomas Malthus

• 1798, economist Thomas Malthus published a work describing the nature of population growth.

• Populations that continue to grow exponentially will eventually run out of resources.

Effect of Competition on Population Growth

• Darwin observed that few species are overpopulated.

• Competition for resources and limitations on birth rate must select out some individuals.

Influence of Artificial Selection

• Darwin also observed that for the time of human existence, humans have selected plant and animal traits which benefit mankind.

• Ex: dog domestication, increased crop yield per plant

Darwin’s conclusions

• Earth is very old.

• Events observed today must have also happened in the past.

• Organisms struggle for resources and mates.

• Some organisms are better competitors than others.

• Traits can be selected.

Natural Selection

• The forces of nature create a selective pressure so that traits which are better adapted in a particular environment enable the organisms within a population to survive to reproductive age.

• The ability to reproduce is called fitness.• Alfred Wallace coined the term “survival

of the fittest.”

Survival of the fittest

Ability to pass on genes (NOT only the strongest survive)

The force that drives evolution…The force that drives evolution…Individuals better adapted to the environment, Individuals better adapted to the environment, survive & leave more offspringsurvive & leave more offspring

Natural selectionNatural selection

Factors of Natural Selection• For the environment to effect “selection” of

which organisms may survive, thus influencing the gene pool, then three factors must exist:

• 1) gene pool must have variety (from mutations,

immigration & emigration)• 2) variety must be inheritable (able to be passed

on)• 3) there must be competition among organisms

of the same species• What are some things they may compete for??

Effect of Natural Selection on Populations

• The organisms selected and survive to reproductive age are more likely to pass on their genes and influence the gene pool than those that die before mating.

Descent with modification• With sexual reproduction, each generation

inherits genes that are varied from the previous generation. These descents are modified from the parental generation.

Evidence for evolution

• Gene and protein comparisons

Evidence for evolution

• Homologous structures

• Vestigial organs

• Homolgous organs (structures)– Organs that are similar in structure, but

different in function: shows common ancestry

• Vestigial organs (structures)– Structures that are reduced & no longer function in

related species

• *Analogous organs (structures)– organs that are similar in function, but

different in structure– i.e. wings of birds & butterflies

Evidence for evolution• Embryology

• Embryology– Ex: early in development, all vertebrate embryos are

similar; shows that new genetic instructions have been layered on top of older ones

• Atavistic structures in individual members of a species arise from a defect in the individual's genetic development. A gene may not turn on (or off) at the right time.

• These abnormalities differ from vestigial organs, that are genetic holdovers from a species' ancestors. These organs (e.g. appendix) occur in all members of a species, but have lost their original functionality.

Evidence for evolution

• Fossils showing extinct and transitional species

Cooksonia, earliest evidence of

branching plant

Speciation

• If one population becomes isolated and inbreeds, their gene pool becomes smaller.

• This population may not be able to breed with another population of the same species, thus leading to speciation (evolution of new species).

Debate over Evolution• Many people confuse Darwin’s

ideas with Lamarck’s ideas.• People proclaim that to believe in

evolution is to disregard God.• Darwin does not propose how man

came to be but how all organisms have the capacity to change.

• We know today that mutations and genetic recombination can lead to differences in DNA from one generation to the next, making some organisms better adapted to the environment than others.