Chapter 22 – Descent with Modification, A Darwinian View of Life

Darwin A new era of biology began on

November 24, 1859. On this day Charles Darwin published On the Origin of Species by Means of Natural Selection

Darwin made two major points in his book: He presented evidence that the

many species of organisms presently inhabiting the Earth are descendants of ancestral species

He proposed a mechanism for the evolutionary process, natural selection

Evolutionary History Linnaeus: taxonomy Hutton: gradualism Lamarck: evolution Malthus: populations

Cuvier: paleontology Lyell: uniformitarianism Darwin: evolution Mendel: inheritance Wallace: evolution

Influences on Darwin Linneaus: classification Cuvier: Fossils, the remains or traces of organisms from

the past; along with classification, helped to lay the groundwork for Darwin’s theories.

Hutton and Lyell: Gradualism, the idea that profound change can take place through the cumulative effect of slow but continuous processes (opposite of catastrophism); had a strong influence of Darwin’s thinking

Lamarck: Evolution, idea that current species are modified descendents of previous/extinct species; even though mechanism was unsupported (through acquired traits), but concept was valid

Darwin’s Voyage Naturalist on HMS Beagle 5 year voyage around the globe from 1831-1836 Job: to catalogue all of the flora and fauna they encountered Made significant discoveries to his theory in the Galapagos

Islands

Darwin’s Publication In 1844, Darwin wrote a long essay on the origin

of species and natural selection, but he was reluctant to introduce his theory publicly, anticipating the uproar it would cause, most of scientific world did not believe in evolution or gradualism

In June 1858 Darwin received a manuscript from Alfred Russell Wallace. Wallace had developed a theory of natural selection similar to Darwin’s

Darwin quickly finished The Origin of Species and published it the next year, 1859

Darwin’s Theory Descent with Modification - all organisms are

related through descent from an ancestor that lived in the remote past

Modification through Natural Selection - populations of organisms can change over generations if individuals having certain heritable traits leave more offspring than others (differential reproductive success), organisms better “fit” to the environment will reproduce more successfully thereby passing on their better adapted genes.

Darwin’s Evidence Darwin’s

Finches – looked at beaks adapted to specific diets found on different islands

Darwin’s View In the Darwinian view, the history of life is like a tree with

multiple branches from a common trunk. The tips of the youngest twigs represent the diversity of living organisms as they exist today

Darwin’s Theory Explained Evolutionary biologist Ernst Mayr – dissected

the logic of Darwin’s theory into three inferences based on five observations

Observations:1.Exponential fertility 2.Stable population size

3.Limited resources 4. Individuals vary 5.Heritable variation

Inferences:1.Struggle for existence2.Non-random survival3.Natural selection

(differential success in reproduction)

Darwin’s Theory Explained Observation #1: For any species, population sizes

would increase exponentially if all individuals that are born reproduced successfully

Observation #2: Nonetheless, populations tend to be stable in size except for seasonal fluctuations

Observation #3: Resources are limited Inference #1: Production of more individuals than

the environment can support leads to a struggle for existence among individuals of a population, with only a fraction of their offspring surviving

Darwin’s Theory Explained Observation #4: Members of

a population vary extensively in their characteristics; no two individuals are exactly alike

Observation #5: Much of this variation is heritable

Darwin’s Theory Explained Inference #2: Survival depends in part on

inherited traits. Individuals whose inherited traits give them a high probability of surviving and reproducing are likely to leave more offspring than other individuals

Inference #3: This unequal ability of individuals to survive and reproduce will lead to a gradual change in a population, with favorable characteristics accumulating over generations

Natural Selection Differential success in reproduction that results

from the interaction between individuals that vary in heritable traits and their environment

Can produce an increase over time in the adaptation of organisms to their environment

If an environment changes over time, natural selection may result in adaptation to these new conditions

Evidence for Natural Selection Examined predation

pressures Transplanted guppies

from pike-cichlid pools to killfish pools, measured ages and size over 11-year period

Average size and age at maturity of guppies of transplanted populations increased

Concluded that change in predator resulted in some population variations being favored over others

Evidence for Evolution Homology Embryology Vestigial Structures Similarities in Macromolecules Biogeography Fossils

Evolution Evidence: Homology Anatomical resemblances that represent

variations on a structural theme that was present in a common ancestor

Evolution Evidence: Embryology Comparative embryology reveals additional

anatomical homologies not visible in adult organisms

Shows common ancestry

Evolution Evidence: Vestigial Remnants of

structures that served important functions in the organism’s ancestors

Shows evolutionary history of a species and common ancestry

Evolution Evidence: Molecular Biologists also

observe homologies among organisms at the molecular level, including genes that are shared among organisms inherited from a common ancestor and the proteins encoded in those genes.

Evolution Evidence: Biogeography The geographic

distribution of species

Some mammals that have adapted to similar environments have evolved independently from different ancestors

Evolution Evidence: Fossil Record The succession of

forms found in the fossil record

Shows evolution of current species from ancestral species over time