II. Darwin's Ideas

A. Background B. Darwin's Logic C. Evolution by Natural Selection

Charles Robert Darwin

was an English naturalist, geologist, and biologist

Voyage on the Beagle

Collected many plants and animals specimens, fossils

Witnessed earthquake in South America (saw fossils and shells get displaced, could determine place where he was was once underwater)

Post Voyage

Highly regarded naturalist

Published accounts of trip

Write multiple books

Developed theory of evolution by natural selection

Alfred Russel Wallace (1823-1913)

Alfred Russel Wallace (1823-1913), Naturalist that studied species in Indonesia

Reached same conclusions independently

Darwin published both

• Jointly presented to the Linnaean Society in 1858

  Darwin finished and published the work in 1859

  “On the origin of species by natural selection”

Evolution

descent with modification (change in alleles over time in a population)

Natural selection

one evolutionary mechanism, differential survival and reproduction

Population

a group of the same species that is interbreeding and sharing a gene pool

Darwin made 4 observations

Overproduction

Variation

Unequal survival and reproduction

Non random survival and reproduction

Observation 1: Overproduction

More offspring are produced than can survive and reproduce

Observation 2: Unequal Survival and reproduction

Some survive longer and have more offspring than other

Darwin was heavily influenced by economist Thomas Malthus

• He suggested that natural resources are limited

  There is competition for resources

  More individuals than resources = some individuals do not survive (limits to population growth)

Observation 3: Heritable Variation

Individuals have variation, offspring tend to resemble parents

Observation 4: Nonrandom survival and reproduction

Survival and reproduction is based on phenotype

Nest adapted individuals -> more offspring

Darwin’s Conclusion

Natural selection

Descent with Modification

Natural selection

Beneficial traits -> greater reproductive success

Heritable beneficial traits accumulate in populations over generations- this is necessarily true mathematically

Descent with Modification

Populations change over time as beneficial traits increase in frequency and detrimental traits decrease in frequency

All living things share common ancestry via descent with modification

Evolution by Natural Selection

Mechanism of evolution (one of several)

Better adapted individuals -> greater survival and reproduction -> heritable beneficial traits increase in frequency overtime

“Survival of the fittest”: survival, within a population, of traits that promote reproductive success

Evolutionary Fitness

Does not equal physical fitness

Equals survival and reproduction

Differences in fitness due to heritable traits overtime

Less favorable traits decrease in frequency

More favorable traits increase in frequency

In other words populations evolve

Individuals do not evolve

Individuals do not evolve

Natural selection acts on existing variation

Does not increase or induce variation

Natural selection depends on time and place

What works in one environment may not in another, cannot plan ahead for future changes

• environment drives evolution

Summary of Darwin’s Work

• Observations 

  • Overproduction

  • Unequal survival and reproduction

  • Heritable variation

  • Non-random survival and reproduction

• Beneficial traits, (which increase survival and reproduction), increase in frequency over generations leading to speciation