Introduction to a Darwinian View of Life

Bio 204

Winter 2005

Lapsansky

Charles Darwin’s radical idea (or was it?)

Meticulous observation

Considered numerous possible relationships, especially b/w animals and their environment

2 main points:Species evolved from ancestral species

Natural selection was the mechanism for this evolutionary change

Figure 22.1

A desire to assign order (for practical and other purposes) is at the heart of human understandingLinnaeus developed the system of binomial nomenclature

Figure 22.1

Early explanations for the great diversity of life on earth did not utilize scientific methodsDarwin was trained as a natural theologian after failing medical school

Figure 22.1

Observations of some species hinted at historical connections and indicated that some change had occurred over timeTransmutation (Erasmus Darwin)

Erasmus DarwinOrganic life beneath the shoreless wavesWas born and nurs'd in ocean's pearly caves;First forms minute, unseen by spheric glass,Move on the mud, or pierce the watery mass;These, as successive generations bloom,New powers acquire and larger limbs assume;Whence countless groups of vegetation spring,And breathing realms of fin and feet and wing.

Erasmus Darwin. The Temple of Nature. 1802.

(This poem hints at the fact that Lamarck and Erasmus Darwin were contemporaries.)

Figure 22.1

Collecting evidence of changes affecting life on earth, over long periods of time

…but was it gradual or catastrophic change?

James HuttonScottish geologist, 1795Geologic mechanisms are slow and continuous, producing gradual change= Geologic gradualism

Jean Baptiste LamarckFrench invertebrate natural historian, 1809Proposed that life forms evolved along with Earth’s changes

used the fossil recordtoward greater complexity

Two Principles:use / disuseinheritance of acquired characteristics

How did Lamarck explain simple life forms?

Thomas MalthusEnglish political economist, 1798Essay on human condition as related to over-reproduction and limited resources

Georges CuvierFrench anatomist, 1813“Essay on the Theory of the Earth” and founded the science of paleontologyCatastrophism, to explain earth’s history

Fig 22.3

Charles LyellScottish geologist, 1830Incorporated Hutton’s gradualism into this widely read book, “Principles of Geology” The same slow geologic mechanisms at work today have always been at work = UniformitarianismDarwin concluded that the earth must be very old, and had a copy of Lyell’s book with him on the Beagle

Voyage of the BeagleVoyage of the Beagle1831-18361831-1836

Fig 22.5

Voyage of the BeagleVoyage of the Beagle1831-18361831-1836

Alfred Wallace

British naturalist, 1858Letter from West Indies with article to review and forward to LyellDeveloped a theory of evolution identical to the one Darwin was working on

Wallace and Darwin jointly present their ideas (1858) to the Linnaean Society of London, 32 years after the end of Darwin’s voyage“On the Origin of Species” is published for the first time in 1859

On the Origin of SpeciesDescent with modification (aka evolution) is the explanation for life’s unity and diversity

all organisms are related via a common ancestor

adaptations developed as descendents from a common ancestor moved into new habitats (or the habitat changed)

Darwin’s metaphor for the history of life was a branching tree.

On the Origin of SpeciesNatural selection is the mechanism

3 inferences based on 5 basic observations (as characterized by Ernst Mayr, 1982) Too many individuals for limited resources

Those that have favorable traits are “more fit”, survive and reproduce

The traits of the survivors dominate in a population and cause it to evolve

Darwin had synthesized some of the basic concepts in population ecology and inheritance, at the dawn of the

Cell Theory (1839) and prior to Mendel’s genetics experiments (1866).

Observation 1

Species have a great potential for reproduction

Populations would increase exponentially if all individuals survived and reproduced

Fig 22.8Fig 22.8

Observation 2

Populations tend to remain stable over time, except for mild seasonal fluctuations and occasional severe fluctuations

Fig 52.17 Moose population on island in Lake Superior

Observation 3Natural resources are limited

Fig 52.19

Discussion in Malthus, “Essay on the Principle of Population” helped to clarify this for Darwin

Since more individuals are produced than can be supported by the available resources but population size remains stable, it means that there must be a fierce struggle for existence among individuals of a population, resulting in the survival of a part, often a very small part, of the offspring of each generation

In populations, more individuals are born than can be supported by the environment

Only a fraction will survive to the next generation (there is a struggle for existence)

Inference 1:

Observation 4

No two individuals are exactly the same; rather, every population displays enormous variability.

Fig 22.9

Observation 5

Much of this variation is heritable

However, Darwin did not know the mechanism of inheritance

Fig 1.3b

Inference 2:

Those with better traits survive and reproduceSurvival depends on an organism’s characters that are, at least in part, inheritedMore fit individuals will leave more offspring than less fit individuals

Survival in the struggle for existence is not random but depends in part on the hereditary constitution of the surviving individuals.  This unequal survival constitutes a process of natural selection.

Inference 3:

Unequal ability of individuals to survive and reproduce leads to gradual change in a population

More favorable characters accumulate = Microevolution (i.e. evolution in a population)

Macroevolution = origin of new species and other taxonomic groups due to accumulation of change over long periods of time.

Over generations this process of natural selection will lead to a continuing gradual change of populations, that is, to evolution and to the production of new species.

Peter and Rosemary Grant have been working on Galapagos finches since 1973 on Isla Daphne Major.

Geospiza fortis is a seed-eater that cracks seeds by grasping at the base of the bill and applying force.Beak size is correlated with consumed seed size.

Are characteristics variable in a population? YES.

Is some of the variation among individuals heritable? YES.

Is there an excess of offspring, only some of which survive to reproduce (due to limited natural resources)? YES.

Is survival and reproduction nonrandom? YES.

Other Evidence for Evolution

Organisms are adapted to their environments

Camouflage is an example of evolutionary adaptation

Fig 22.10a A floral mantid.

Other evidenceArtificial selection

Fig 22.11b Vegetables developed by humans from wild mustard plant

Other evidence

Examples of natural selection over short periods of time

Fig 22.12 Evolution of resistance to insecticides in insect populations

Is it fair to call this“natural” selection?

How is it similar to antibiotic resistance among pathogenic bacteria?

Other evidenceHomologous structures

Fig 22.14 Forearm bones in mammals

Other evidence

Molecular “record” – molecular homologies

Fig 17.16 Ribosomal structure (large subunit of bacterial ribosome)

Other evidenceBiogeography

Fig 34.32 Adaptive radiation of Australian marsupials compared to similarities among other mammals

Other evidenceFossil record

Fossil Trilobites

Fig 22.7 Elephant evolution based on fossils