Evolution

Only a theory?

Basic premises for this discussion

• Evolution is not a belief system. It is a scientific concept. It has no role in defining religion or religious beliefs

• Evolution is a theory…but you don’t get any better than that in science

• There is a lot of contention about evolution, but not among scientists or scientific organizations.

Some basic definitions

• Fact: an observation that has been repeatedly confirmed

• Law: a descriptive generalization about how the physical world behaves

• Hypothesis: a testable statement that can be used to build inferences and explanations

Some basic definitions

• Theory: a well-substantiated explanation that incorporates facts, laws, inferences and tested hypotheses.

In science, you don’t get any better than a theory.

What is evolution?

A basic definition of evolution…“…evolution can be precisely

defined as any change in the frequency of alleles within a gene pool from one generation to the next." - Helena Curtis and N. Sue Barnes, Biology, 5th ed. 1989 Worth Publishers, p.974

So what does the definition mean?

• Evolution is a change in the number of times specific genes that code for specific characteristics occur within an interbreeding population

• Individuals don’t evolve, populations do

• There is no implied “improvement” in evolution

Evolution’s Core Principles

Descent with modification

Present day species have accumulated differences from their ancestors through the generations

Common Ancestor

Present day species

University of California Museum of Paleontology

Evolution’s Core Principles

Natural Selection(differential

reproductive success depends on an individual’s traits that allow it to survival to reproduce thus passing trait to next generation)

University of California Museum of Paleontology

Evolution’s Core Principles

Evolutionary adaptations:

inherited characteristics that increase an organisms’ chance of survival and successful reproduction

Natural Selection Affects Individuals

Evolution Affects Populations

Darwin did not work in a vacuum

Darwin’s Voyage of Discovery

A reconstruction of the HMS Beagle sailing off Patagonia.

The Voyage of the Beagle

Darwin’s Observations and Inferences

1st Observation:

All species have such great potential fertility that their population size would increase exponentially if all individuals that are born reproduced successfully.

Darwin’s Observations continued

2nd Observation

Populations tend to remain stable in size, excepting seasonal fluctuations.

3rd Observation

Environmental resources are limited

Darwin’s Observations continued

4th ObservationIndividuals of a population vary extensively in their

characteristics; no two individuals are exactly alike.

5th ObservationMuch of this variation

is heritable.

Darwin’s Inferences

Inference #1

Production of more individuals than the environment can support leads to a struggle for existence among individuals of a population, with only a fraction of offspring surviving each generation.

Darwin’s Inferences

Inference #2Survival in the struggle for

existence is not random, but depends in part on the hereditary constitution of the individuals. Those individuals whose inherited traits best fit them to their environments are likely to leave more offspring than less fit individuals.

Darwin’s InferencesInference #3This unequal ability of individuals to survive and

reproduce will lead to a gradual change in a population with favorable characteristics accumulating over the generations.

University of California Museum of Paleontology

Adaptive Radiation

• One species gives rise to multiple other species that exploit different resources.

Evolution evidence: Biogeography

• Geographical distribution of species

Examples: Islands vs. Mainland

AustraliaContinents

Biogeography

• The range of an organism is regulated in part by their ability to migrate to a new area

• Islands tend to have fewer species because of the difficulty in migrating there

• Changes in locations of continents over geologic time helps or hinders migration

Biogeography of Species

• Apes are found only in Africa and Asia

• No apes fossil or living are found in the Americas

• Species on islands are different from but most similar to the species on the nearest mainland

Evolution evidence: The Fossil Record

• Succession of forms over time

• Transitional links between past and present forms

• See Fossil Rocks and Time PowerPoint for further discussion

University of California Museum of Paleontology

Evolution evidence: Comparative Anatomy

• Homologous structures (homology)

• Descent from a common ancestor

• Vestigial organs Ex: whale/snake hindlimbs; wings on flightless birds

Vestigial Structures

• Vestigial Structures were present in the ancestor but little purpose in the descendent organism’s current environment

• Example: ostritch wings, eyes in cave dwelling fish, pelvis bones in snakes

University of California Museum of Paleontology

Mainland cormorant gave rise to Flightless cormorants

On an island with no predatorsBirds didn’t need to fly to escape so tiny wings were not a disadvantage and a heavier body and legs were an advantage

Homologous Structures:

Different types of bones in the forelimb of these mammals show descent with modification from a common ancestor as despite their differences in shape and use these bones all arise from the same embryological structure

PBS.org

Evolution evidence: Comparative Embryology

These vertebrates show similar features during the course of development

Ex. Pharyngeal (throat) pouches and ‘tails’ as embryos

Evolution evidence: Molecular Biology

• Similarities in DNA,

proteins and genes

• Common genetic

code

Lines of Evidence Supporting Evolution

• Biogeography

• Fossil record

• Homologous structures and Vestigial structures

• Embryo development similarities

• DNA similarities among organisms

“Absence of evidence is not evidence of absence.”

Carl Sagan