Evolution and Natural Selection

Jean-Baptiste Lamarck French biologist known for his idea that acquired traits are inheritable.

Alfred Russell Wallace a contemporary of Darwin born 1823 in England died Rio Amazonas & Rio Negro in Malay Archipelago documenting >125,000 species (>1,000 new sp.) independently discovered a theory of natural selection read Thomas Malthus toured US & Canada

Charles Darwin Charles Darwin H.M.S. Beagle convinced two colleagues to preset his and Wallace's’ work at Linnaean Society meeting. “On the Origin of Species by Means of Natural Selection, or the Preservation of Favored Races in the Struggle For Life” published in 1859

Voyage of HMS Beagle

Robert Fitzroy Captain of the HMS Beagle surveyor

The voyage of the Beagle

Major Influences geology experiences in the Galapagos Islands Thomas Malthus (English Economist)- wrote an essay on principles of population

Darwin’s Observations and Inferences Observation 1: Left unchecked, the number of organisms of each species will increase exponentially, generation to generation. Observation 2: In nature, populations tend to remain stable in size. Observation 3: Environmental resources are limited.

Inference I Production of more individuals than can be supported by the environment leads to a struggle for existence among individuals, with only a fraction of offspring surviving in each generation.

more observations Observation 4: Individuals of a population vary extensively in their characteristics with no two individuals being exactly alike. Observation 5: Much of this variation between individuals is heritable.

Inference II Survival in the struggle for existence is not random, but depends in part on the heritable characteristics of individuals. Individuals who inherit characteristics most fit for their environment are likely to leave more offspring than less fit individuals.

Inference III The unequal ability of individuals to survive and reproduce leads to a gradual change in a population, with favorable characteristics accumulating over generations (natural selection).

Linking Evolution and Genetics A major problem in Darwin’s theory was the lack of a mechanism to explain natural selection. How could favorable variations be transmitted to later generations? With the rediscovery of Mendel’s work and its vast extension in the first half of the 20th century, the missing link in evolutionary theory was forged.

Evolution "descent with modification"; change in genetically determined characteristics of populations.

Natural Selection The sustained differential contribution of offspring by different genotypes.

“sustained differential” eliminates the random element; natural selection is deterministic.

“contribution of offspring” emphasis on differential reproduction rather than survival. “survival of the fittest” -vs- “fitness”

“different genotypes” the true unit that is selected for or against.

Natural Selection Sequence 1. Variation occurs within populations. 2. Sexual reproduction is the fundamental source of variation. 3. Variation must be heritable or genetically based.

4. Organisms produce more offspring than can possibly survive and resources are limiting, therefore competition among individuals within the population occurs.

5. Individuals show differential mortality and differential reproductive success.

6. Superior survivors/reproducers increase their genotypic frequency within the population.

7. Populations become adapted to the environment. 8. The environment is constantly changing.

Stabilizing Selection extreme phenotypes are selected against

Directional Selection one extreme phenotype or the other is selected against.

Disruptive Selection average phenotype is selected against. example: two colored morphs (brown & green) in a single population of insects.

Biological Species Concept A species is a population of organisms that actively or potentially interbreed, producing viable offspring and which remain reproductively isolated from other such populations.

Isolating Mechanisms geographic isolation temporal isolation behavioral isolation mechanical isolation

Geographic Isolation

Galapagos Islands

Evidences for Natural Selection fossil record biogeography comparative anatomy comparative embryology comparative biochemistry comparative behavior comparative physiology

Fossil Record

Homologous Structures

Comparative Embryology

Convergent Evolution