Darwin’s Theory of Evolution: Darwin Presents His Case – Guided Notes

  • Natural Selection: The mechanism by which individuals with favorable traits are more likely to survive and reproduce.

  • Variation: Genetic differences within a population that provide raw material for natural selection.

  • Common Descent: The concept that all living organisms share a common ancestor, leading to the diversity of life observed today.

Slide 3
In 1859, Charles Darwin presented evidence and proposed a mechanism for evolution that he called natural selection.

Darwin Recognized
All species tend to produce excessive numbers of offspring

Malthus
Human suffering (disease, famine, homelessness, war) was due to the human population’s potential to grow faster than the rate at which supplies of food and other resources could be produced

So…
Darwin concluded that this occurs with all species
Because natural resources are limited, production of more individuals than the environment can support leads to a struggle for existence, with only a few offspring surviving each generation. The rest are starved, frozen, eaten, or fail to reproduce

What is Darwin’s Theory?
Genetic variation is found naturally in all populations

Sources of Variation
Genetic diversity through mutations that are not lethal
Physical or behavioral traits
Sexual reproduction between genetically different individuals

What is Darwin’s Theory?
Struggle for existence means that members of each species must compete for food, space, and other resources

Darwin’s Observations
Overproduction of offspring
Struggle to survive due to limited natural resources, predators, drought, food shortages, and disease
Individuals in a population vary in their characteristics, and these traits are inherited
Traits are passed from one generation to the next

Natural Selection
Within a varied population, individuals whose characteristics best fit their environment are most likely to survive and reproduce
These individuals tend to leave more offspring than less fit individuals

In other words…
Differential reproductive success (natural selection) is the process by which the environment filters variations, favoring some over others

What is Darwin’s Theory?
Some organisms in a population are less likely to survive
Survival of the fittest: organisms that are better adapted to the environment will survive and reproduce, passing on their genes

Vocab
Ability of an individual to survive and reproduce in its specific environment = fitness
Any inherited characteristic that increases an organism’s chance of survival = adaptation

What is Darwin’s Theory?
Over time, natural selection results in changes in the inherited characteristics of a population
These changes increase a species’ fitness in its environment

Descent with modification: each species has descended, with changes, from other species over time
This idea suggests that all living species are related and share a common ancestor