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Monkeys, Apes and Humans
Anthropology 1500
Department of Anthropology, University of Missouri-Columbia

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Some basic concepts in evolutionary theory

Darwin's reasoning(from Mayr 1982, p. 479):

Fact 1: All species have such high potential fertility that population would increase exponentially (Malthus).

Fact 2: Except for minor annual and rare major fluctuations, population size is normally stable.

Fact 3: Natural resources are limited. In a stable environment they remain constant.

Inference 1: More individuals are produced than can be supported by available resources, resulting in competition for survival.
 

 

Bonobo

bonobo

 

 

 

Fact 4: No two individuals are exactly the same; populations have great variability.

Fact 5: Much of this variation is heritable.

Inference 2: Survival is not random and depends on the heritable constitution (genetics) of the individuals. This differential survival is natural selection.

Inference 3: Over generations, natural selection leads to gradual change in the population (= evolution) and production ofnew species (= speciation).

Darwin's hostile forces -- pressures that cause differential survival and reproduction

  • Food shortages
  • Predators
  • Disease and parasites
  • Climate
  • Mate shortages/competition

Genes are replicators, whereas individuals are vehicles for gene replication. Evolution works because there are entities (things) that can produce exact copies of themselves. Without exact replication, traits would not be heritable. Genetic material (DNA) can make exact copies of itself (replicate); individuals cannot. Genes that produce individuals that survive and reproduce better than others become more common and may eventually replace less successful genes.

Multiple explanation of phenotypic traits, e.g., bird song in spring (Tinbergen):

  1. Proximate causes: (a)Environmental stimuli that cause behavior, e.g., day length, presence of potential mate or competitor. b) Physiological and morphological mechanisms underlying behavior, e.g., testosterone levels.
  2. Ultimate "function": Adaptive significance of behavior; how and why it was favored by natural selection, e.g., singing males attract more mates, defend territories more efficiently.
  3. Ontogeny: Effects of the developmental envirorunent on the behavior. Does experience affect the behavior? Is learning involved? E.g., development of queens vs. workers among honey bees, based on food and rearing differences.
  4. Phylogeny: Evolutionary history of the trait. When did it appear in the ancestry of the species? Which related species have the trait, and which do not?

Working definitions:

Adaptation: A trait that spreads through a population as a result of natural selection.

Analogs or Analogous traits: Traits in different species that are similar because of convergent or parallel evolution, e.g., night vision in Aotus (owl monkey) and Galagos. Contrast with Homologous traits.

Convergent evolution: The independent acquisition over time through natural selection of similar characteristics in two or more unrelated species.

Divergent evolution: The naturally selected changes in related species that once shared a characteristic in common (as a result of having inherited it from a common ancestor) but have come to be different.

Evolution: Change in gene frequencies over time.

Group selection: The natural selection of traits that benefit the survival and reproduction of groups or species at the expense of some individuals. Unlikely to occur.

Homologs or homologous traits: Traits in different species that are similar because a common ancestor had the traits.

Individual selection: The natural selection of traits as a consequence of differential reproduction of individuals. Replicators Any entity in the universe of which copies can be made, e.g., DNA.

Vehicles: Discrete entities that contain replicators and have been designed by selection to preserve and propagate the replicators inside it. For example, individuals or phenotypes are vehicles for gene replication.

 
 
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