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Chapter 19
Descent With Modification: A Darwinian View of Life
evolution
- processes that have transformed life from its earliest forms to the vast diversity found today
On the Origin of Species (by Means of Natural Selection) - Darwin argued 2 points:
-
species were not created in their present forms (evolved from ancestral species)
proposed a mechanism for evolution (natural selection)
natural selection
- a population can change over time as individuals with certain heritable traits leave
more offspring than others
evolutionary adaptations
- inherited characteristics that increase an organism’s ability to survive and
reproduce in a particular environment
The Origin of Species
- challenged the worldview that Earth was a few thousand years old and that
unchanging forms of life were made by God during a single week
natural theology
taxonomy
- philosophy that studied nature to discover the Creator’s plan (dominated the 1700s)
- saw adaptations of organisms as evidence that they were created specifically for their
environment
- branch of biology that names and classifies life forms
- founded by Carolus Linnaeus (1707 – 1778)
- species grouped into a hierarchy of categories
- evolutionary relationships played no role
binomial nomenclature
- 2-part system of naming organisms created by Linnaeus(Genus and species)
fossils - relics or impressions of organisms from the past preserved in rock
- most found in sedimentary rock
- show that a succession of organisms have populated Earth throughout time
paleontology
- study of fossils
- largely developed by Georges Cuvier
(1769 – 1832)
Cuvier - noted that the deeper (older) the layer of sedimentary rock, the more different organisms were from
modern organisms
- saw that extinction had been a common occurrence
catastrophism - belief advocated by Cuvier (did not believe in evolution)
- catastrophes destroyed species in areas that were then repopulated by species immigrating
from other areas
gradualism
- belief advocated by geologist James Hutton (1795)
- geological changes result from slow but continuous processes
uniformitarianism
- theory put forth by Charles Lyell (1797 – 1875)
- geological forces that shaped Earth’s surface continue today
Hutton and Lyell
- 2 conclusions strongly influenced Darwin:
- Earth must be very old
- slow, subtle processes can cause substantial change over time
Jean Baptiste Lamarck (1744 – 1829)
- published a theory of evolution (1809) which stated that evolution was
driven by a tendency toward greater and greater complexity (which he
equated with perfection)
- incorporated 2 common ideas into his theory:
use and disuse - body parts used to interact with the environment become larger and stronger
- body parts not used deteriorate
inheritance of acquired characteristics - changes organisms acquire during their lifetimes are passed
onto offspring (there is no evidence of this)
Charles Darwin (1809 – 1882)
Galápagos Islands
- sailed from Great Britain aboard the HMS Beagle as the ship’s naturalist (1831)
- observed various adaptations of species along the coast of South America
-volcanic islands 900 km of the coast of Ecuador
- unique species fascinated Darwin
- Darwin collected many species including 13 types of finches
Darwin - had read Lyell’s book and began doubting church’s positions
- acknowledged that Earth was very old and constantly changing
- believed that life had also evolved
- after returning home, he began to perceive that the origin of species was related to adaptation to the
environment
- thought new species could arise from an ancestral species by the gradual accumulation of adaptations
to a different environment (over many generations, 2 populations of one species could become different
enough to be designated as different species  Darwin’s Finches)
Darwinism
- belief that evolution explains life’s unity and diversity
- belief that natural selection is the cause of adaptive evolution
descent with modification
- Darwin believed all organisms were related through descent from an unknown
common ancestor in the distant past
- descendants spilled into various habitats over millions of years and
accumulated diverse modifications (adaptations)
- history of life is like a tree (evolutionary tree)
Summary of Darwin’s main ideas:
Natural selection is differential success in reproduction (unequal ability of individuals to survive and reproduce).
Natural selection occurs through an interaction between the environment and the variability among individuals in
a population.
The product of natural selection is the adaptation of populations of organisms to their environment.
Thomas Malthus
- wrote an essay on human population (1798)
- said that much of human suffering (disease, famine, homelessness, war) was due to the
potential for the human population to increase faster than their resources (food, etc.)
Darwin - noticed that all species have the tendency to overproduce
- only a fraction of offspring survive and reproduce
artificial selection
- process by which humans selected natural variations they found useful
- farmers would selectively breed the largest pigs, cows that gave the most milk, etc.
- has produced many diverse domestic animals and crop plants
- showed Darwin that selection can cause substantial change in a population
Some subtleties of natural selection:
- a population is the smallest unit that can evolve (an individual alone cannot evolve)
- evolution is measured as changes in proportions of variations in a population over time
- natural selection can amplify or diminish only heritable variations (acquired characteristics cannot be
passed on)
- specifics of natural selection are situational (adaptation useful in 1 situation may be useless in another)
evidence of evolution: biogeography, fossil record, comparative anatomy, comparative embryology, molecular
biology
biogeography - geographical distribution of species
- Why are tropical animals of South America more closely related to desert animals of South
America than to tropical animals of Africa?
- Why is Australia home to so many marsupials but few placental mammals?
- modern species are where they are because they evolved from ancestors that inhabited those
regions
fossil record
- succession of fossil forms is compatible with other evidence:
evidence from biochemistry, molecular biology, and cell biology identifies prokaryotes as
the ancestors of all life  oldest known fossils are prokaryotes
chronological appearance of the different classes of vertebrates in the fossil record is
consistent with the history of vertebrate descent (fish are oldest vertebrate fossils, then
amphibians, then reptiles, etc.
if all species were created at same time they would all make their first appearance in the
fossil record in rocks of the same age (this is not the case)
fossils of transitional forms that link older fossils to modern species have been
discovered
comparative anatomy - anatomical similarities between species in the same taxonomic group give evidence of
descent with modification
- example: similarities in the bones of the forelimbs of all mammals
homology
- similarity in characteristics due to common ancestry
homologous structures - anatomical signs of evolution
vestigial organs - structures of little, if any, importance to an organism
- remnants of structures that had important functions in ancestors
- example: vestiges of pelvis and leg bones in whales
comparative embryology
- closely related organisms go through similar stages in embryonic development
- example: all vertebrates go through a stage in which they have gill pouches
- vertebrates diverge more and more as development progresses
Ontogeny recapitulates phylogeny
- extreme view of late 19th century
- says that the development of an organism (ontogeny) is a
replay of the evolutionary history of the species (phylogeny)
- in reality, ontogeny can provide clues to phylogeny
molecular biology
- evolutionary relationships among species are reflected in their DNA and proteins
- common genetic code is evidence that all life is related
Darwin’s ideas: widely accepted in science
have stood up to thorough and continual testing by experiments and observations