Chapter 22

Descent with Modification: A Darwinian View

Charles Darwin (1809-1882)

• Traveled on the ship the H.M.S. Beagle and served as an observer and naturalist.

• Had an interest in geologic distribution of species and fossils.

• Reassessed his trip and theorized that new species arise from ancestral forms by gradual accumulations of adaptations to a different environment.

“The Origin of Species”Darwin’s book focused on 2 main ideas:1. “descent with modification” explains life’s

unity and diversity.2. Natural Selection – competition for

limited resources results in differential survival. Individuals with favorable phenotypes are more likely to survive and produce more offspring, passing traits to the next generation.

Natural Selection• Simplistically, evolution is defined as a

change in gene frequency within a population.

• Gene frequencies change in response to changes in the environment. These changes act as a selective mechanism on populations.

• Environments can be more or less stable, affecting the evolutionary rate and direction. Different genetic variations can be selected in each generation.

• An adaptation is a genetic variation that is favored by selection and manifested as a trait that provides the organism with an advantage in a particular environment.

• Phenotypic variations are not directed by the environment but occur through random changes in the DNA and through new gene combinations.

• Some variations significantly increase or decrease the fitness of an organism and the population.

African cowpea seeds

Artificial Selection• Humans impact variation in other species.

Evidences for evolution

Scientific evidence of biological evolution uses information from geographical, geological, physical, chemical and mathematical applications.Morphological homologyBiochemical/genetic similaritiesFossil recordMathematical models and simulations

Homologous Structures

• Morphological homologies are features shared by common ancestry.

• Vestigial structures are remnants of functional structures, which can be compared to fossils and provide evidence for evolution.

Biochemical/genetic similarities

• DNA nucleotide and protein sequences provide evidence for evolution and ancestry.

Mathematical models

• Phylogenetic trees, graphical analyses of allelic frequencies can be used to support evo relationships.

Key Points to Remember1. Individuals DO NOT evolve, a population

evolves through its interaction with the environment.

2. Evolution can only be measured as changes in relative proportions of heritable variations in a population over a succession of generations.

3. Natural selection can amplify or diminish heritable traits only.

4. Environmental factors vary from place to place and time to time.