Evolutionary Theory A new era of biology began

Evolutionary Theory

• A new era of biology began on November 24, 1859 – The 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

Natural Selection • The basic idea of natural selection is that a population can change over generations if individuals that possess certain heritable traits leave more offspring than other individuals • The result of natural selection is evolutionary adaptation – An accumulation of inherited characteristics that enhance organisms’ ability to survive and reproduce in specific environments

Evolution • The change over time in the genetic composition of a population • A population may accumulate enough change that it constitutes a new species

Resistance to the Idea of Evolution • The Origin of Species – Shook the deepest roots of Western culture – Challenged a worldview that had been prevalent for centuries

• The Darwinian revolution challenged traditional views of a young Earth inhabited by unchanging species • In order to understand why Darwin’s ideas were revolutionary – We need to examine his views in the context of other Western ideas about Earth and its life

The Idea of Evolution • The Greek philosopher Aristotle (384 -322 B. C. ) – Viewed species as fixed and unchanging – scala naturae • The Old Testament – Holds that species were individually designed by God and therefore perfect

• James Ussher (1581 -1656), a 17 th century archbishop of Armagh, Ireland • By counting the generations of the Bible and adding them to modern history, he fixed the date of creation at Monday, October 23, 4004 B. C.

• Swedish botanist Karl von Linné or Carolus Linnaeus in Latin (1707 -1778) • He described plants and animals on the basis of physical appearance and classified them relative to each other according to the degree of their similarities. • He used a binomial nomenclature in organisms – two Latin names genus and species naming

• Carolus Linnaeus – Interpreted organismal adaptations as evidence that the Creator had designed each species for a specific purpose – Was a founder of taxonomy, classifying life’s diversity “for the greater glory of God”

• The French zoologist, George Louis Leclerc, Comte de Buffon (1707 -1788) , actually said that living things do change through time and that the Earth must be much older than 6000 years • The Count of Buffon was careful to hide his views in a 44 volume natural history book series

• Erasmus Darwin (1731 -1802 ), the grandfather of Charles Darwin • Erasmus was an English country physician, poet, and amateur scientist. • He believed that evolution has occurred in living things including humans. • He wrote of this in his poems and other relatively obscure publications.

Fossils, Cuvier, and Catastrophism • George Cuvier (1769 -1832) • French scientist • first to document extinctions of ancient animals • Paleontology, the study of fossils, was largely developed by Cuvier – Fossils are remains or traces of organisms from the past – Usually found in sedimentary rock, which appears in layers or strata

• Cuvier opposed the idea of gradual evolutionary change – advocated catastrophism, speculating that each boundary between strata represents a catastrophe – violent and sudden natural catastrophes led to new life forms • The study of fossils helped to lay the groundwork for Darwin’s ideas

Theories of Gradualism • Gradualism – Is the idea that profound change can take place through the cumulative effect of slow but continuous processes – Proposed by James Hutton (1726 -1797) in 1795

• Charles Lyell (1797 -1875) • theory of uniformitarianism – The Earth must be very old – the same slow, progressive changes caused by natural processes in the past operate today in shaping the land

• Jean Baptiste, Chevalier of Lamarck (1744 -1829) • The first evolutionist who publicly stated his ideas about the processes leading to biological change • His theory about these processes was entirely incorrect. He believed that evolution was mostly due to the inheritance of acquired characteristics.

• Charles Darwin (1809 -1882 )

Darwin’s Research • As a boy and into adulthood, Charles Darwin – Had a consuming interest in nature • Soon after Darwin received his B. A. degree – He was accepted on board the HMS Beagle, which was about to embark on a voyage around the world

The Voyage of the Beagle • During his travels on the Beagle – Darwin observed and collected many specimens of South American plants and animals • Darwin observed various adaptations of plants and animals – That inhabited many diverse environments

• Thomas Malthus (1766 -1834) • Essay on the Principles of Population – he observed that human populations will double every 25 years unless they are kept in check by limits in food supply • Darwin came to realize that all plant and animal populations have this same potential to rapidly increase their numbers unless they are constantly checked by limitations in food, water, and other resources that are essential for survival.

• Geologists Hutton and Lyell – Perceived that changes in Earth’s surface can result from slow continuous actions still operating today – Exerted a strong influence on Darwin’s thinking

• Darwin’s interest in the geographic distribution of species – Was kindled by the Beagle’s stop at the Galápagos Islands near the equator west of South America

• 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 • In June 1858 Darwin received a manuscript from Alfred Russell Wallace – Who had developed a theory of natural selection similar to Darwin’s • Darwin quickly finished The Origin of Species – And published it the next year

• 1858 – Alfred Wallace (1823 -1913) • 1859 – On the Origin of Species.

• In The Origin of Species, Darwin proposed that species change through natural selection • As the 19 th century dawned – It was generally believed that species had remained unchanged since their creation – Darwin’s ideas were revolutionary

Darwin’s Focus on Adaptation • As Darwin reassessed all that he had observed during the voyage of the Beagle – He began to perceive adaptation to the environment and the origin of new species as closely related processes

• From studies made years after Darwin’s voyage – Biologists have concluded that this is indeed what happened to the Galápagos finches

The Origin of Species • Darwin developed two main ideas – Evolution explains life’s unity and diversity – Natural selection is a cause of adaptive evolution

Descent with Modification • Darwin used the phrase descent with modification in The Origin of Species – all organisms are related through descent from an ancestor that lived in the remote past – Overt time, the descendants have accumulated modifications (adaptttions) that allow them to survive and reproduce in specific habitats

• In the Darwinian view, the history of life is like a tree – With multiple branchings from a common trunk – Closely related species, the twigs on a common branch of a tree, share the same line of descent

Natural Selection and Adaptation • Evolutionary biologist Ernst Mayr (1904 -2005) – dissected the logic of Darwin’s theory into three inferences based on five observations

• 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

• 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

• 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

Artificial Selection • In the process of artificial selection – Humans have modified other species over many generations by selecting and breeding individuals that possess desired traits

Summary of Natural Selection • Natural selection is differential success in reproduction – That results from the interaction between individuals that vary in heritable traits and their environment

• Natural selection 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

Evolution by Natural Selection • A population is the smallest unit that can evolve; evolution can only be measured as changes in relative proportions of heritable traits in a population over successive generations • Natural selection can only act on heritable traits • Environmental factors vary from place to place and from time to time

• Darwin’s theory explains a wide range of observations • Darwin’s theory of evolution – Continues to be tested by how effectively it can account for additional observations and experimental outcomes

Natural Selection in Action • Two examples – Provide evidence for natural selection

Differential Predation in Guppy Populations • Researchers have observed natural selection – Leading to adaptive evolution in guppy populations • Endler and Reznick – Poecilia reticulata

• Observations: – There are significant differences between populations in the average size at which the guppies reach sexual maturity – The variations are correlated with the type of predator most active on that population

• In some pools, the main predator is the small killfish – Preys predominantly on juvenile guppies • In other pools, a larger fish is the main predator, the pike-cichlid – Preys primarily on larger sexually mature guppies • Guppies in populations preyed on by pikecichlids begin reproducing at a younger age and are smaller at maturity than guppies that are preyed by killfish

• Conclusions after eleven years and periods of 30 -60 generations of transplanted guppy populations – In pools with pike-cichlids, the guppies with the greatest reproductive success are those that mature at a young age and small size • Can produce at least one brood before growing to a larger size – In ponds with killfish, guppies that survive predation can grow slowly and produce many broods of youngs

The Evolution of Drug-Resistant HIV • In humans, the use of drugs selects for pathogens that through chance mutations are resistant to the drugs’ effects – 3 TC interferes with reverse transcriptase • HIV’s reverse transcriptase pick-up 3 TC instead of C terminating elongation of DNA

• 3 TC-resistant HIV varieties – Their reverse transcriptase discriminates between 3 TC and C • In the absence of 3 TC, these viruses have no advantage, in fact they replicate more slowly • If 3 TC is present, it becomes a selecting agent favoring reproduction of resistant individuals

• Natural selection can only act on existing variation; it cannot create favorable traits • Natural selection favors traits that increase fitness; what is adaptive in one situation is not adaptive in another

Homology, Biogeography, and the Fossil Record • Evolutionary theory – Provides a cohesive explanation for many kinds of observations

Homology • Homology – Is similarity resulting from common ancestry

Anatomical Homologies • Homologous structures between organisms – Are anatomical resemblances that represent variations on a structural theme that was present in a common ancestor

• Comparative embryology – Reveals additional anatomical homologies not visible in adult organisms

• Vestigial organs – Are some of the most intriguing homologous structures – Are remnants of structures that served important functions in the organism’s ancestors

Comparative Anatomy • Evolution is a remodeling process – It can only modify existing structures – Produces structures “less than perfect” • Back and knee problems in humans

Molecular Homologies • Biologists also observe homologies among organisms at the molecular level – Such as genes that are shared among organisms inherited from a common ancestor

Homologies and the Tree of Life • Homologies result in a nested pattern – All life sharing the deepest layer – Each smaller group adding homologies to those they share with larger groups

Homologies and the Tree of Life • Anatomical resemblances among species – Are generally reflected in their molecules, their genes (DNA), and their gene products (proteins) • Hemoglobin polypeptide

Biogeography • Darwin’s observations of the geographic distribution of species, biogeography – Formed an important part of his theory of evolution

• Species tend to be more closely related to other species from the same area • than to species with the same way of life that live in different areas

• Some similar mammals that have adapted to similar environments – Have evolved independently from different ancestors – Convergent evolution • Marsupial and North American mammals – Australian sugar glider and the North American flying squirrel

• Islands generally have many species of plants and animals that are endemic – these species are typically more closely related to species living in the nearest mainland that to species from other island groups • Diversification of finches in the Galapagos Islands

The Fossil Record • The succession of forms observed in the fossil record – Is consistent with other inferences about the major branches of descent in the tree of life

• The Darwinian view of life – Predicts that evolutionary transitions should leave signs in the fossil record • Paleontologists – Have discovered fossils of many such transitional forms

What Is Theoretical about the Darwinian View of Life? • In science, a theory – Accounts for many observations and data and attempts to explain and integrate a great variety of phenomena

• Darwin’s theory of evolution by natural selection – Integrates diverse areas of biological study and stimulates many new research questions