Earth History GEOL 2110

Lecture 6EVOLUTION

Part II

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.

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• Questioning and skepticism are evidence of

healthy science, not evidence of doubt and uncertainty

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, could control their pollination, and they had simple patterns of heredity.

• Mendel’s math-rich results were published in 1865 in an obscure journal, but was not recognized until ~1900.

Pisum sativum

Genetic Theory The Mechanism behind Evolution

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

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

• Devised the notion of dominant and recessive hereditary traits. Each flower had 2 possible genetic factors (alleles). The recessive trait would only appear when two recessive alleles would merge .

Genetic Theory The Mechanism behind Evolution

• Mendel’s laws of heredity formed the observational foundation of genetic theory, but the biochemical basis for heredity was still a mystery

• In mid-century, the carrier of genetic information was discovered in the form of DNA, which occurs in the nucleus of all cells

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

• Variations could form by mutation of the genes, alteration of the DNA strands, the way that daughter DNA recombine in the offspring

Double Helix model for DeoxyriboNucleic AcidWatson and Crick (1953)

The Genetic CodeUniversal to ALL Life

20 Amino Acids are coded bysequences of 3 NucleotidesUracilCytosineAdenineGuanine

Revisiting Lamarck’s Giraffes

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 mathematical and statistical manner was called - Synthetic Theory of Evolution or Neo-Darwinism• 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 to 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 - allopatric 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?

Traditional Representation of Horse Evolution (Matthew, 1926)

Later 12 species of Horse were discovered to coexist at this time!

Or Punctuated Equilibrium?

Current Representation of Horse Evolution

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 EvolutionReappearance of Features in Different Species

OligoceneHoplophoneus

(relative of dogs and bears)

PleistoceneSmilodon

(cat family)

PlioceneMachairodus(cat family)

Pliocene (SA)Thylacosmilus(opossum family)

Recent Challenges to Neo-DarwinismNeutralism

• 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

• Consequently, many mutations are simply accidents of nature that have no advantage or disadvantage

African Rhino Indian Rhino

Recent Challenges to Neo-DarwinismInheritance 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.

Recent Challenges to Neo-DarwinismMacroEvolution

• 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

• It is clear that some organisms simply die out because oaf their inability to rapidly adapt, but some do adapt quickly

• 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