chapter 22 descent with modification: a darwinian view of life...

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Section B1: The Darwinian Revolution

1. Field research helped Darwin frame his view of life2. The Origin of Species developed two main points: the occurrence of

evolution and natural selection as its mechanism

CHAPTER 22DESCENT WITH MODIFICATION:

A DARWINIAN VIEW OF LIFE

• Charles Darwin (1809-1882) was born in westernEngland.

• While Darwin had a consuming interest in nature asa boy, his father sent him to the University ofEdinburgh to study medicine.

• Darwin left Edinburgh without a degree and enrolledat Christ College at Cambridge University with theintent of becoming a clergyman.• At that time, most naturalists and scientists belonged to

the clergy and viewed the world in the context of naturaltheology.

Introduction


• Darwin received his degree in 1831.

• After graduation Darwin was recommended to bethe conversation companion to Captain RobertFitzRoy, preparing the survey ship Beagle for avoyage around the world.

• FitzRoy chose Darwin because of his education,his similar social class, and similar age as thecaptain.


• The main mission of the five-year voyage of theBeagle was to chart poorly known stretches of theSouth American coastline.

1. Field research helped Darwin framehis view of life


Fig. 22.5

• Darwin had the freedom to explore extensively onshore while the crew surveyed the coast.

• He collected thousands of specimens of the exoticand diverse flora and fauna of South America.• Darwin’s explorations ranged from the Brazilian

jungles, the grasslands of the Argentine pampas, thedesolation of Tiera del Fuego, and the heights of theAndes.


• Darwin noted that the plants and animals of SouthAmerica were very distinct from those of Europe.• Organisms from temperate regions of South America

were more similar to those from the tropics of SouthAmerica than to those from temperate regions ofEurope.

• Further, South American fossils more closelyresembled modern species from that continent thanthose from Europe.


• The origin of the fauna of the Galapagos, 900 kmwest of the South American coast, especiallypuzzled Darwin.• On further study after his voyage, Darwin noted that

while most of the animal species on the Galapagoslived nowhere else, they resembled species living onthe South American mainland.

• It seemed that the islands had been colonized by plantsand animals from the mainland that had thendiversified on the different islands.


• While on the Beagle, Darwin read Lyell’sPrinciples of Geology.• Lyell’s ideas and his observations on the voyage lead

Darwin to doubt the church’s position that the Earthwas static and only a few thousand years old.

• Instead, he was coming to the conclusion that the Earthwas very old and constantly changing.


• After his return to Great Britain in 1836, Darwinbegan to perceive that the origin of new species andadaptation of species to the environment as closelyrelated processes.• For example, among the 13 types of finches that Darwin

collected in the Galapagos, clear differences in the beakare adaptations to the foods available on their homeislands.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin CummingsFig. 22.6

• By the early 1840’s Darwin had developed themajor features of his theory of natural selection asthe mechanism for evolution.

• In 1844, he wrote a long essay on the origin ofspecies and natural selection, but he was reluctantto publish his theory and continued to compileevidence to support his theory.

• In June 1858, Alfred Wallace, a young naturalistworking in the East Indies, sent Darwin amanuscript containing a theory of natural selectionessentially to identical to Darwin’s.


• Later that year, both Wallace’s paper and extractsof Darwin’s essay were presented to the LinnaeanSociety of London.

• Darwin quickly finished The Origin of Species andpublished it the next year.

• While both Darwin and Wallace developed similarideas independently, the essence of evolution bynatural selection is attributed to Darwin because hedeveloped and supported the theory of naturalselection so much more extensively and earlier.


• Darwinism as a dual meaning.

• It refers to evolution as the explanation for life’sunity and diversity.

• It also refers to the Darwinian concept of naturalselection as the cause of adaptive evolution.

2. The Origin of Species developed two mainpoints: the occurrence of evolution andnatural selection as its mechanism


• Central to Darwin’s view of the evolution of lifeis descent with modification.

• In descent with modification, all present day organisms are related through descent from unknown ancestors in the past.

• Descendents of these ancestors accumulated diverse modifications or adaptations that fit them to specific ways of life and habitats.

• Viewed from the perspective of descent withmodification, the history of life is like a tree withmultiple branches from a common trunk.

• Closely related species, the twigs of the tree,shared the same line of descent until their recentdivergence from a common ancestor.


Copyright © 2002 Pearson Education, Inc., publishing as Benjamin CummingsFig. 22.7

• This evolutionary tree of the elephant family is based on evidence from fossils.

• The other major point that Darwin pioneered is aunique mechanism of evolution - the theory ofnatural selection.

• Ernst Mayr, an evolutionary biologist, hasdissected the logic of Darwin’s theory into threeinferences based on five observations.• These observations include tremendous fecundity,

stable populations sizes, limited environmentalresources, variation among individuals, and heritabilityof some of this variation.


• Observation #1: All species havesuch great potential fertility thattheir population size wouldincrease exponentially if allindividuals that are bornreproduced successfully.

• Observation #2: Populations tendto remain stable in size,except for seasonal fluctuations.


• Observation #3: Environmental resources are limited.

• Inference #1: Production of more individuals than theenvironment can support leads to a struggle for existanceamong the individuals of a population, with only a fractionof the offspring surviving each generation.

Fig. 22.8


• Observation #4: Individuals of a population varyextensively in their characteristics; no twoindividuals are exactly alike.

• Observation #5: Much of this variation is heritable.Fig. 22.9


• Inference #2: Survival in the struggle for existenceis not random, but depends in part on the hereditaryconstitution of the individuals.• Those individuals whose inherited characteristics best fit

them to their environment are likely to leave moreoffspring than less fit individuals.

• Inference #3: This unequal ability of individuals tosurvive and reproduce will lead to a gradual changein a population, with favorable characteristicsaccumulating over the generations.


• Darwin’s main ideas can be summarized in threepoints.• Natural selection is differential success in

reproduction (unequal ability of individuals to surviveand reproduce).

• Natural selection occurs through an interactionbetween the environment and the variability inherentamong the individual organisms making up apopulation.

• The product of natural selection is the adaptation ofpopulations of organisms to their environment.

• For example, these related species of insects called mantids have diverse shapes and colors that evolved in different environments.


Fig. 22.10


• Darwin’s views on “overreproduction” wereheavily influenced by an essay on humanpopulation by Thomas Malthus in 1798.• Malthus contended that much human suffering - disease,

famine, homelessness, war - was the inescapableconsequence of the potential for human populations toincrease faster than food supplies and other resources.

• The capacity to overproduce seems to be acharacteristic of all species, with only a smallfraction of eggs developing to leave offspring oftheir own.

• In each generation, environmental factors filter heritable variations, favoring some over others.

• Differential reproduction - whereby organisms with traits favored by the environment produce more offspring than do organisms without those traits - results in the favored traits being disproportionately represented in the next generation.

• This increasing frequency of the favored traits in a population is evolution.


• Darwin’s views on the role of environmental factors in thescreening of heritable variation was heavily influenced byartificial selection.

• Humans have modified a variety of domesticated plants andanimals over many generations by selecting individualswith the desired traits as breeding stock.

Fig. 22.11

• The Darwinian view of life has two main features.

(1) The diverse forms of life have arisen by descent with modification from ancestral species.

(2) The mechanism of modification has been natural selection working over enormous tracts of time.


• If artificial selection can achieve such majorchanges in a relatively short time, then naturalselection should be capable of major modificationsof species over hundreds or thousands ofgenerations.

• Darwin envisioned the diversity of life as evolvingby a gradual accumulation of minute changesthrough the actions of natural selection operatingover vast spans of time.


• While natural selection involves interactionsbetween individual organisms and theirenvironment, it is not individuals, but populationsthat evolve.

• Populations are defined as a group of interbreedingindividuals of a single species that share a commongeographic area.

• Evolution is measured as the change in relativeproportions of heritable variation in a populationover a succession of generations.


• Natural selection can only amplify or diminishheritable variations, not variations that anindividual acquires during its life, even if thesevariations are adaptive.

• Also, natural selection is situational.• Environmental factors vary in space and time.

• Therefore, adaptations for one set of environmentalconditions may be useless or even detrimental underother circumstances.

chapter 22 descent with modification: a darwinian view of life...

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