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