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How Populations Evolve
Jean-Baptiste Lamarck Traits acquired during
lifetime are inheritable Charles Lyell
Geologic change is the steady accumulation of changes over a long period of time
Alfred Wallace Contemporary of
Charles Darwin Conceived theory very
similar to Darwin
Attended Cambridge for religious studies
Biologist on HMS Beagle in 1831Studied plants, animals,
fossils Influenced by Lyell’s geology
ideas Consulted with Wallace in
1850s
1859—Published On the Origin of Species by Means of Natural Selection
Immediate controversy, continues today
Members of a population vary in traits, traits are inheritable
Species produce more offspring than an environment can support
Individuals with traits that give them a better chance of survival & reproduction will tend to leave more offspring
Unequal production of offspring will cause these traits to increase in a population over generations
FossilsOlder fossils deeper in
strata of rocksOlder fossils show less
variationOlder fossils show less
complex featuresYounger fossils show more
variation and more complex features
Younger fossils have characteristics more like living organisms
BiogeographyStudy of geographical
distribution of species If species have similar
characteristics but are in different locations, can infer that there was a common ancestor from which each evolved
Used with idea of continental drift
Comparative AnatomyHomologous structures—different functions but
similar structure. Theorized to be due to common ancestry
Comparative AnatomyVestigial structures—structures with little or
no importance to organism. Theorized to be previously important organ that organisms evolved out of needing Some structures now seen to have uses and may
not be “vestigial”
Molecular BiologyMany organisms have similar
DNA Comparable DNA to humans….
Other humans 99+%Chimpanzees—95-98%Fruit flies—60%Bananas—50%
However, the similarities depend on how the analysis is performed (see this article or this one)
New alleles and phenotypes originate by mutation
Most mutations are harmful or neutral Some may confer some benefits
For examples, see this article Mutations provide the variety in traits
needed by Darwin’s description of natural selection
Beneficial mutations may increase survivability
Genetic Drift—change in a gene pool due to chanceSmaller populations more quickly and strongly
affected Bottleneck Effect—Sudden and extreme
event that leaves a small populationFlood, fire, earthquake, disease, etc.Unlikely to have same genetic makeup of original
population
Any given trait has variations within a populationHeight, hair color, tail length, beak
thickness, immune response, etc. Variations tend to follow a “bell curve”
Stabilizing SelectionFavors intermediate phenotypesSelects against extremesHuman birth weight
Too large or too small increase risk for baby’s survival
Directional SelectionMoves traits in direction of one phenotype
extremeSelects against other extremeColoration to allow better camoflague
Disruptive Selection Individuals at both extremes selected for Intermediate forms selected against.Galapagos finch beak size
Organisms with certain physical traits are more likely than others to obtain mates
Some traits may compete with other factors of natural selectionFighting for a mate may
result in injury and deathSome physical traits (e.g.
extremely large or long tails) may make survival difficult