“Nothing in Biology Makes Sense Except in the Light of Evolution”

• Evolutionary biology is concerned with explaining and understanding the diversity of living things and their characteristics.

  • Key Question: What has been the history that produced this diversity, and what have been the causes of this history?

• Evolutionary biology extends and amplifies the explanation of biological phenomena.

  • Proximate causes of biological phenomena are immediate, mechanical causes.

    • Ex: A male bird sings > The proximate causes include the action of hormones, the action of the singing apparatus, and the operation of center centers in the brain.

  • Ultimate causes of biological phenomena are historical causes, especially the action of natural selection.

    • Ex: A male bird sings > The ultimate causes lie in the history of events that led to the evolution of singing.

What is Evolution? Is it Fact or Theory?

• The word “evolution” comes from the Latin word evolvere, meaning to unfold or unroll, or to reveal or manifest hidden potentialities.

• Biological (or organic) evolution is the inherited change in the properties of groups of organisms over the course of generations

  • Darwin’s definition of evolution was “descent with change”.

• A population is a single group of organisms

  • Several populations can be descended from a single common ancestral population

• To diverge means to become different from each other

  • A population diverges when several different changes transpire and split the population.

• A hypothesis is an informed conjecture or statement of what might be true

• A scientific theory is a comprehensive, coherent body of interconnected statements, based on reasoning and evidence, that explain some aspect of nature.

  • Scientists believe nothing can be 100% certain, but scientific theories are as close to certain as possible.

• In “The Origin of Species”, Darwin propounded two major hypotheses:

  • (1) Organisms have descended, with modifications, from common ancestors.

  • (2) The chief cause of modification is natural selection acting on heredity variation.

• A body of ideas about the causes of evolution, including mutation, recombination, gene flow, isolation, random genetic drift, the several forms of natural selection, and other factors constitute our current theory of evolution, or “evolutionary theory.”

The Evolution of Evolutionary Biology

Before Darwin

• Plato and Aristotle developed the notion that species have fixed properties.

• Early Christian theologians took Bible literally and decided that God’s creations must be unchanging and permeant since the change would imply that there had been imperfection in the original creation.

• Carolus Linnaeus (1707-1778) established the framework of modern taxonomy

• Belief in the literal truth of the biblical story of creation started to give way in the 18th century when a philosophical movement called the Enlightenment adopted reason as the major basis of authority and marked the emergence of science.

• Uniformitarianism is the proposition that natural processes that operated in the past are the same as in the present.

• Chevalier de Lamarck proposed the most significant pre-Darwinian evolutionary hypothesis.

  • He hypothesized that different organisms originated separately by spontaneous generation from nonliving matter, starting at the bottom of the chain of being.

  • He argued that species differ from one another because they have different needs, and so use certain organs and appendages more than others.

  • The principle of Inheritance of Acquired Characteristics describes alterations, acquired during an individual’s lifetime, that are inherited.

    • The theory of evolution based on this principle is called Lamarckism.

      • Ex: Giraffes stretched their necks to reach foliage above them, so their necks lengthened, and they passed this acquired characteristic onto their offspring.

Charles Darwin

• Charles Darwin (1809-1882) is the main historical figure in any discussion on evolution; he is most known for theorizing natural selection.

  • Natural selection is the idea that although many individuals are born, not all survive; certain individuals with superior features are more likely to survive and reproduce than individuals with inferior features.

• Alfred Russel Wallace (1823 – 1913) also independently conceived the idea of natural selection and sent a manuscript to Darwin about it.

Darwin’s Evolutionary Theory

• The Origin of Species, written by Darwin, contains two major theories:

  • (1) Descent with modification: all species, living and extinct, have descended, without interruption, from one or a few original forms of life

  • (2) Natural selection: the differential survival and/or reproduction of classes of entities that differ in one or more characteristics

• Darwin’s theory is considered a variational theory of change, in which the frequency of a variant form (i.e., the proportion of individuals with that variant feature) increases within a population from generation to generation.

• Darwin’s theory of evolution includes five distinct components:

  • (1) Evolution is the simple proposition that the characteristics of organisms change over time.

  • (2) Common descent: Darwin argues that species had diverged from common ancestors and that species could be portrayed as one great family tree representing actual ancestry.

  • (3) Gradualism: Differences between even radically different organisms have evolved by small steps through intermediate forms, not by leaps.

  • (4) Populational change: Evolution occurs by changes in the proportions (frequencies) of different variant kinds of individuals within a population.

  • (5) Natural selection accounts for adaptions (features that appear designed to fit organisms to their environment)

• According to the idea of blending inheritance, variation should decrease, not increase.

  • When you blend black and white paint, gray is produced, not more black and white paint.

• Particulate inheritance is Gregory Mendel’s theory that proposed inheritance is based not on blending fluids, but on particles that pass unaltered from generation to generation, so that variation can persist.

Evolutionary Biology After Darwin

• Neo-Lamarckism includes several theories based on the old idea of inheritance of modifications acquired during an organism’s lifetime.

• Theories of Orthogenesis (also known as “straight-line evolution”) say that the variation that arises is directed towards fixed goals, so that a species evolves in a predetermined direction by some kind of internal drive, without the aid of natural selection.

• Mutationist theories were advanced by some geneticists who observed that discretely different new phenotypes can arise by a process of mutation.

Evolutionary Synthesis

• Evolutionary synthesis (or modern synthesis) is the reconciliation of Darwin’s theory with the findings of modern genetics, which gave rise to a theory that emphasized the coaction of random mutation, selection, genetic drift, and gene flow.

• Neo-Darwinism is the theory of natural selection of inherited variations, that denied that acquired characteristics might be inherited; often used more broadly to mean the modern theory that natural selection, acting on randomly generated particulate genetic variation, is the major, but not the sole, cause of evolution.

• Microevolution is slight, short-term evolutionary changes within species.

• Macroevolution is the evolution of the major alternations that distinguish higher taxa (genera, families, orders, and classes).

Evolutionary Biology Since the Synthesis

• Molecular evolution is the analysis of the processes and history of changes in genes.

• Neutral theory of molecular evolution says that most of the evolution of DNA sequences occurs by chance (genetic drift) rather than by natural selection

• Evolutionary developmental biology is devoted to understanding how the evolution of developmental processes underlies the evolution of morphological features at all levels, from cells to whole organisms.

• Evolutionary genomics is concerned with variation and evolution in multiple genes or even entire genomes.

How Evolution is Studied

• The causes of evolution, such as genetic drift and natural selection, are often studied by comparing data, such as patterns of variation in genes, with theoretical models.

• Throughout science, causes are not seen; rather they are inferred. You can’t see the evolution or the causes of evolution in action, but you can infer they are happening based on the results.

• Hypothetico-deductive method is the method where hypotheses are tested and are rejected, modified, or provisionally accepted.

Philosophical Issues

• Before Darwin, both philosophers and people, in general, answered questions such as “Why do plants have flowers?” or “Why are there apple trees?” by imagining the possible purpose God could have in creating them

  • Darwin changed this and made biology a science, rather than a supernatural explanation of natural things

    • The adaptations of organisms could be explained by purely mechanistic causes.

      • Ex: The pink petals of a flower have a function (attracting pollinators), but not a purpose, and doesn’t need a purpose.

Ethics, religion, and evolution

• The creationist movement believes in God’s creations and opposes the teaching of evolution in public schools, or at least demands equal times for creationist beliefs.

• The theory of evolution is very controversial around the world, particularly among religious and philosophical groups.

• Evolution provides no basis for human ethics.

• “Like other sciences, evolutionary biology cannot be used to justify beliefs about ethics or morality. Nor can it prove or disprove theological hypotheses such as the existence of a deity. Many people hold that evolution is compatible with religious beliefs. However, evolution is incompatible with a literal interpretation of some passages in the Bible. Evolutionary biology and other sciences can test and reject claims for supernatural causes of observed phenomena.”

• The naturalistic fallacy is the philosophical supposition that what is “natural” is “good”.

• Evolutionary theory does not provide a basis for moral theory or human ethics. It only describes the world as it is, not as it “should” be based on human ethics.

Extra “Boxes” of Information

Box 1A: Fundamental Principles of Biological Evolution

• An individual’s phenotype (its observed traits) is distinct from its genotypes (its DNA). Phenotypic differences among individuals are caused by both genetic differences and environmental effects.

• Acquired characteristics are not inherited.

• Evolution is a change in a population, not an individual

• Changes in allele frequencies may be random or nonrandom

• Natural selection can account for both slight and great differences among species.

• Natural selection can alter populations beyond the original range of variation when changes in allele frequencies generate new combinations of genes.

• Populations usually have considerable genetic variation.

• Speciation, the origin of two species from a single ancestor species, usually occurs by the genetic differentiation of geographically isolated populations.