Theory of Evolution
Darwin's Theory of Evolution
From his observations of animal breeding, he recognized
that, even within species, individuals showed variation in
traits and that the variations could be passed to offspring.
Darwin defined natural selection as the "principle by which
each slight variation [of a trait], if useful, is preserved,"
Everyone is Different Scientists learned that mutations in DNA can change genes
and produce variations in traits.
He expressed considerable concern that his own health
problems might be heritable, especially when his beloved
daughter Annie grew ill and died.
Overproduction of Offspring
Malthus argued that human populations grow
exponentially if unchecked, but that disease,
starvation, or war will limit population growth
eventually.
Survival of the Fittest
Darwin reasoned that death was not random. Offspring
which, by chance, had variations which “fit” or adapted them
to their environment would have a greater chance to survive
to maturity and a greater chance to reproduce.
"Differential survival and reproduction" is a cornerstone of
natural selection.
Gradually, Species Change
It is the gradual accumulation of many adaptations
that, over many generations within one lineage of
organisms, results in a new species. These
adaptations occur through genetic change.
Through chance variation, overproduction of offspring, and
differential survival and reproduction, the proportion of
individuals with a favorable trait (or favorable phenotype) will
increase.
Changes accumulate and a new species is formed.
Natural Selection vs. Inheritance of Acquired
Characteristics
Heritable variation: The change in the size of giraffe's necks.
Overproduction of offspring: Giraffes had a lot of babies that
they could not support.
Differential survival and reproduction: Giraffes with long
necks were more likely to survive than those with short
necks.
Species change: The next generation of giraffes had long
necks because they were more likely to survive.
The Present Arises From The Past
Darwin studied the life found across continents and saw, in
addition to tremendous variation, that species had changed in
response to the changes in their environment, over that vast
amount of time.
Descent with Modification
Darwin delighted in the great diversity of life, but also saw
unity within that diversity. He saw striking patterns in the
similarities and differences.
Adaptations
Adaptations are logical because the environment imposes
limits on organisms, selecting against those who are not “fit.”
Adaptations arise through gradual accumulation of chance
variations, so they cannot be predicted, despite the fact that
they appear to be goal-directed or intentional.
Adaptations relate to every aspect of life: food, water,
oxygen, nutrients, shelter, growth, response, reproduction,
movement, behavior, and ability to learn. Adaptations
connect organisms to the resources in their environments.
