Change is Change is constant…constant…

Donna May D. PapaDonna May D. PapaCollege of ScienceCollege of ScienceUniversity of Santo TomasUniversity of Santo Tomas

Chapter 6Chapter 6

Pre-Darwinian Evolutionary Pre-Darwinian Evolutionary IdeasIdeas

Before 18th century - origin of species was not Before 18th century - origin of species was not scientific – “Creation Event”scientific – “Creation Event”

Irreversible change & mythologyIrreversible change & mythology Xenophanes, Empedocles and Aristotle developed Xenophanes, Empedocles and Aristotle developed

early ideas about evolutionearly ideas about evolution Fossils were recognized as former life destroyed Fossils were recognized as former life destroyed

by natural catastropheby natural catastrophe Archbishop Ussher calculated 4004 bc as date of Archbishop Ussher calculated 4004 bc as date of

life’s creationlife’s creation Louis Buffon (1707-1788) – stressed environmental

influences modified animal types

Jean Baptiste de LamarckJean Baptiste de Lamarck offered first offered first complete explanation of evolution (1809complete explanation of evolution (1809))

argued that fossils were remains of extinct argued that fossils were remains of extinct animalsanimals

mechanism was mechanism was inheritance of acquired inheritance of acquired characteristics – characteristics – transform to produce transform to produce evolutionevolution

Lamarck’s concept is Lamarck’s concept is transformationaltransformational; ; individuals transform their own traits to evolveindividuals transform their own traits to evolve

In contrast, Darwin’s theory is In contrast, Darwin’s theory is variationalvariational or or due to differential survival among offspringdue to differential survival among offspring

Principle of UniformitarianismPrinciple of Uniformitarianism two important principles of Uniformitarianism :two important principles of Uniformitarianism :1)1) Laws of physics and chemistry remain the same Laws of physics and chemistry remain the same

throughout earth’s historythroughout earth’s history2)2) Past geological events occurred by natural processes Past geological events occurred by natural processes

similar to those observed todaysimilar to those observed today

Natural forces acting over long periods could explain Natural forces acting over long periods could explain formation of fossil-bearing rocksformation of fossil-bearing rocks

Geological changes are natural and without direction; Geological changes are natural and without direction; both concepts made marks on Darwin’s theoryboth concepts made marks on Darwin’s theory

Darwinian Evolutionary Darwinian Evolutionary Theory: The Evidence Theory: The Evidence

Perpetual Change Common Descent Multiplication of species Gradualism Natural Selection

perpetual changeperpetual change Species are not fixed, but

are often changing (commonly at different rates)

Through the generations, most common forms change, as shown in the fossil record

Fossil - remnant of past life

Geological TimeGeological TimeSedimentary Rock LayersSedimentary Rock Layers LLaw of stratigraphyaw of stratigraphy dates oldest layers at the dates oldest layers at the

bottom and youngest at the topbottom and youngest at the top Time is divided into eons, eras, periods and Time is divided into eons, eras, periods and

epochs epochs (See inside back cover of your books)(See inside back cover of your books)

Evolutionary TrendsEvolutionary Trends Fossil record allows observation of evolutionary Fossil record allows observation of evolutionary

change over broad periods of timechange over broad periods of time Animal species arise and become repeatedly Animal species arise and become repeatedly

extinct.extinct. Animal survival = 1–10 million yearsAnimal survival = 1–10 million years

Horse Evolution Shows Horse Evolution Shows Clear TrendClear Trend From Eocene to Recent periods, From Eocene to Recent periods,

genera and species of horses genera and species of horses were replacedwere replaced

Earlier horses - smaller sized and Earlier horses - smaller sized and fewer grinding teeth, and more fewer grinding teeth, and more toestoes

Reduction in toes and increase in Reduction in toes and increase in size and numbers of grinding teeth size and numbers of grinding teeth correlate with environmental correlate with environmental changeschanges

Common descent

All forms of life had a common ancestor.

Phylogeny – Evolution produces a branching structure and the form, this takes is called a phylogeny

Multiplication of speciesMultiplication of species

New species form by splitting and transforming older species

Typically the result of isolation or separation, and based primarily upon reproductive isolation

GradualismGradualism Evolution is a gradual and slow process (by human standards)

and large, rapid changes are usually not common in the evolution on a group of organism

Most large changes over time occur through a series of small changes

Large changes tend to be harmful for organisms, although not always

Natural SelectionNatural SelectionFive Observations forming Natural Selection

1.Organisms have a great potential to reproduce

2. Natural populations normally remain constant in size, with small fluctuations

3. Natural resources are limited. Therefore, a continuing struggle for

existence occurs among members of a population

4. All organisms show variation

5. Variation is heritable.

Differential survival and reproduction occur among varying individuals in a population, and over many generations, generates new adaptations and new species.

Evidence for Perpetual ChangeEvidence for Perpetual Change Seen in the fossil record Fossils are deposited in stratified layers Dating possible based on position in the

rock Fossils tend to illustrate trends in characteristic change

Ex. Evolution of horses

Evidence for common descentEvidence for common descent

Homology – different adaptations of the same structure from a shared ancestor

Used to form phylogenies based on shared characteristics Used in ontogeny, or the history of development. “ontogeny (history of development of an organism throughout its

lifetime) recapitulates “phylogeny”

Comparison of gill arches of Different embryos. All shown Separated from the yolk sac.Note similarity of embryos.

Evidence for Common Evidence for Common DescentDescent

Homologous structuresHomologous structures Arm of a human Arm of a cat Wing of a bat Fin of a whale Leg of a horse

Multiplication of SpeciesMultiplication of Species Ancestral species

speciate into diverse forms through isolation

End result can be multiple species.

How new species ariseAllopatric speciation Speciation due to a geographic barrier

- Separated by an ocean, mountain range or a road

Sympatric speciation Speciation in the same place without

separation

Classic Example – Darwin’s finches

Adaptive radiation – production of many ecological species from a common ancestral stock.

gradualismgradualism

A theory of some debate

today Fossil record does not

show perfect intergrades from form to form Sporting mutations can cause dramatic

changes that may be important to evolution

Disturbances to Disturbances to EquilibriumEquilibrium

Random Genetic Drift Nonrandom Mating Recurring Mutation

Migration Natural Selection

Nonrandom matingNonrandom mating

Positive assortative mating – Individuals selecting a particular genotype to mate with

Inbreeding – mating between close relatives