Earth History GEOL 2110

Lectures 5 & 6EVOLUTION

Major Concepts• In the late 1700/early 1800’s, a number of natural scientists

had proposed that evolution of life was likely, however, without a mechanism to explain evolution, the idea was generally dismissed.

• Darwin’s Origin of Species (1859) not only provided multiple lines of evidence for evolution, but moreover, proposed a plausible cause (natural selection).

• With the discovery of genetic theory in the early 1900’s providing a mechanism for evolution, it became overwhelmingly accepted as a “fact” of science.

• The genetics within isolated, small populations provided the triggers for the “origin” of new species

• The mechanisms of evolution are still being figured out, but not the concept of evolution is not in doubt and has achieved the status of a paradigm of science.

Early Ideas about Evolution

• To the early Greeks and Romans, the notion that nature changes/evolves was well accepted

• Christian teachings required that all organisms were created together, at once, and unchanged for all of time

• The notion that God would let organisms go extinct (as proposed by Cuvier and Smith) was antithetical to genesis and heritical

Early Ideas about EvolutionGeorges de Buffon

Histoire Naturelle (1847)• defined the concept of species

(the ability to interbreed)• was the first to emphasize the

importance of environment in the changes (evolution) within species

• recognized that organism seek to adapt to their environment but did not offer an explanation how this might occur

• noted evidence for inheritance of traits, but again did not offer an explanation.

• Enough time for Evolution was a major sticking point

Early Ideas about Evolution

Erasmus Darwin (1731–1802)Charles’s Grandfather

• as an animal breeder, recognized the subtle changes from generation to generation

• argued that the fitness of an organism to feed itself, protect itself against predators, and its ability to succeed in mating were important factors controlling its survival and thereby its ability to pass on these characteristics to its offspring.

•Still the question remains - how does inheritance work?

Early Ideas about EvolutionDefined “biology” as a study of the plant and animal world• Ideas about evolution published in 1809, where he argued that the fundamental course of nature is change and this change was controlled by environment• Did not ascribe to Buffon’s fixed species concept, but rather believed that organism represent a continuum of change.•Did not believe in extinction, but rather the “loss” of an organism was due to it’s changing into or replacement by another form.

Jean Baptiste de Monet – Chevalier de LAMARCK

(1744–1829)

Inheritance of Acquired CharacteristicsLamarck’s Giraffes

But again, how are these acquired characteristics passed on?

Darwinian Evolution• Born in the year that Lamarck published

his ideas about evolution• Embarked on a 5-year voyage (1832-

1835) around the world on the HMS Beagle where he was charged with documenting the diversity of flora and fauna in the tropics and Southern hemisphere.

• Struck by: • Variety of species• Number and diversity of individuals per

species• The importance of competition for food,

safety from predators, and shelter

Charles Darwin (1809-1882)

Darwin’s Epiphany

Darwin

Captain FitzroyHMS Beagle

Ship-side ReadingMalthus (1798) Principles of PopulationLyell (1830) Principles of Geology

Natural Selection

• Natural environments change• Individuals within a population of

organisms change• Individual changes that are best fitted

(adapted) to the changes in the environment are the most likely to survive

• “survival of the fittest”

Radiation into Environmental NichesThe Finches of the Galapagos

• Only birds on the islands – no competition

• 4 different genera

• 15 different species

• Each adapted a different beak type reflecting their dietary preference

On the Origin of Species• First draft completed in 1839, but

not published until 1859

• Fearful of backlash from British scholars and clergy

• For 20 years, compiled supporting data

• Primary goals of the book:• Document evidence for

Evolution• Propose a mechanism

(Natural Selection)

Evidence of EvolutionBranching Organization of Life

Early classifications of the animal and plant kingdom show groups within groups within groups based on anatomical features

Best portrayed by a branching family tree implies related lineages

Today, genetic coding affirms that grouping

Evidence of EvolutionHomologous Elements

Diverse organisms share similar organs and features

For example, the bones of appendages in a variety of vertabrates

Green –humerusRed – radiusBlue – ulnaTan – wrist and phalanges

Evidence of EvolutionVestigal Structures and Imperfections

Unnecessary body parts

Why would such things be “created”?

HumansTail bonesAppendicies

Evidence of EvolutionEmbryonic History

All vertebrate embryos go through similar early stages of development

“Ontogeny recapitulates Phylogeny” Hackel (1866)

(i.e. embryonic development repeats evolutionary history)

Evidence of EvolutionPaleogeography

On unconnected continents, unrelated animals have evolve to occupy similar environmental niches

For example – Marsupials of Australia

“Ecological Convergence”

Next Lecture

EVOLUTION IIGenetics and the Origin of Species

Quiz #2Chapter 3

Hypothesis/Theory/Paradigm

The Scientific Method - Seeking understanding• Observations of phenomenon lead to the development of

multiple possible interpretations or explanations - hypotheses • Physical, chemical, mathematical or thought experiments are

devised that attempt to prove the various hypothesis wrong• A set of related hypotheses that withstand repeated testing

(are well corroborated) and explain a wide range of observations develop the status of theory

• Theories that continue to stand up to scrutiny evolve into paradigm status - largely unquestioned until too many anomalies develop

• Based on facts, but not a statement of certainty or truth

• Testable – holds up to repeated testing• Predictive – forward and backward in time• Explains many related natural phenomenon• Deemed by a majority of scientists to have a

high probability of being correct

Charactersitics of a Scientific Theory

Genetic Theory The Mechanism behind Evolution

• By the mid-1800, the high probability that evolution happens was generally accepted

• Based on Darwin’s (1859) evidence and well reasoned arguments, the idea that natural selection of favorable characteristics within populations of organisms drove evolution was also generally accepted.

• However, Darwin did not explain how new characteristics (and thus new species) originated

Genetic Theory The Mechanism behind Evolution

Gregor Mendel (1822-1884)

• Austrian monk, plant breeder

• Devised experiments cross-breeding pea plants with purple and white flowers, examining about 28,000 plants in the course of his experiments.

• He chose peas for his experiments because he could grow them easily, develop pure-bred strains, protect them from cross-pollination, and control their pollination.

Pisum sativum

Genetic Theory The Mechanism behind Evolution

• Mendel discovered that when cross-breeding purple and white flower colors, a blended color did not result, but rather colors remained distinct with the purple color being more common

• Morever, the first generation would produce only purple colors and in the second generation 25% of the flowers would be white, 75% purple

• Devised the notion of dominant and recessive hereditary traits

Genetic Theory The Mechanism behind Evolution

• Mendel’s results were published in 1865 in an obscure journal, but was not publicized until ~1900

• Mendel’s laws of heredity formed the observational foundation of genetic theory

• In mid-century, the molecular coding of genes was discovered in the form of DNA, which occurs in the nucleus of all cells

• Variations in genotype (genetic code) gives rise to variations in phenotype (physical appearance)

Double Helix model for DeoxyriboNucleic AcidWatson and Crick (1953)

Neo-DarwinismAn Explanation for the Origin of Species

• Natural Selection could explain gradual changes in the phenotype of an organism (phyletic gradualism), but what gave rise to abrupt changes and new species?

• The development of population genetics in the 1930’s & 40’s began looking at evolution by natural selection in a statistical manner - Synthetic Theory of Evolution• 1938 – Genetics and the Origin of Species (Dobzhansky)

integrated genetic theory with evolutionary biology• 1942 – Systematics and the Origin of Species (Mayr) applied

Natural Selection Theory and current species• 1944 – Tempo and Mode of Evolution (Simpson) showed

evidence evolution in the fossil record

Neo-DarwinismAn Explanation for the Origin of Species

Population Genetics shows: • The genetics of large populations of

are stable because new character traits are muted by the large gene pool; little chance of genetic differentiation into new species

Ernst Mayr (1904-2005)Systematics and the Origin of Species (1942)

• In small, isolated populations, genetic anomalies can become more prominent quickly (founder effect)

• Genetic isolation of a subpopulation that is separated from the main population, by migration, geographic barriers, or lack of mobility, causes genetic divergence - allotropic speciation

Neo-DarwinismAn Explanation for the Origin of Species

Punctuated Equilibrium The theory proposes that most evolution is marked by long periods of evolutionary stability, which is punctuated by rare instances of branching evolution. This would be expected from periodic events of genetic isolation producing allotropic speciation. The theory was contrasted against phyletic gradualism, the popular (Darwinian) idea that evolutionary change is marked by a pattern of smooth and continuous change in the fossil record.

Stephen J. Gould (1941-2002)Punctuated equilibria: an alternative to phyletic gradualism, with Niles Eldredge (1972).

Phyletic Gradualism?

Or Punctuated Equilibrium?

Evolutionary Changes in the Fossil Record

• Some species in the fossil record persist for millions of years relatively unchanged despite significant environmental changes

• Differentiation of species (divergence) is typically subtle• At certain times when many new ecological niches were

abruptly created (e.g., K-T extinction of the dinosaurs), divergence resulted in rapid and extreme speciation – adaptive radiation

• Availability of new niches, can also lead to distinct species converging on a common body form and dietary preference (e.g. marsupial evolution in Australia)

• In some cases, certain species can re-acquire traits that were lost in previous manifestations (iterative evolution), and in other cases, different species can acquire a similar trait (parallel evolution)

Parallel Evolution

OligoceneHoplophoneus

(relative of dogs and bears)

PleistoceneSmilodon

(cat family)

PlioceneMachairodus(cat family)

Pliocene (SA)Thylacosmilus(opossum family)

Challenges to NeoDarwinismNeutralism

• Apparently, organisms have more genetic variability than they need

• Some of the extra traits may be unaffected by natural selection, i.e., they are neutral

African Rhino Indian Rhino

Challenges to NeoDarwinismInheritance of Acquired Characteristics (revisited)

• Natural selection implies that rapid responses to environmental change is not likely

• However, some evolutionary change are remarkably rapid – immune system

Exposure to disease triggers the immune system to create (acquire) antibodies

However, some experiments suggest that immunity can be passed on to offspring.

Challenges to NeoDarwinismMacroEvolution

• Natural selection is good at explaining incremental changes (microevolution), but can it explain dramatic changes, like making an eye or a wing?

• Possibly not as abrupt as they appear in the fossil record; many intermediate stages

• Pre-adaptation – organs adapted for one function change for a new function

Challenges to NeoDarwinismMacroEvolution

Evolutionary stages of the mollusk’s eye

Squid

Challenges to NeoDarwinismMacroEvolution – Pre-Adaptation

Transform-ation of gill arches into jaw bones

The Mexican salamander (Ambystoma) prefers to stay in its embryonic state submerged and breathing with juvenile gills. If stressed, however, it will emerge and breath with adult lungs

Evolution and Creation “Science”A False Equivalence

Evolution is a scientific theory because it is:• Testable• Predictable• Self-correcting• Open to further refinementCreationism/Intelligent Design is not a scientific

theory because it is none of the above

Next Lecture

Fundamentals of StratigraphyChapter 4