Principles of Evolution

Evolution is the change in inheritable traits in a population over generations. Change in traits is caused by changes in the genes (in DNA) that code for those traits. Natural selection tends to increase favorable traits in a population and decrease unfavorable traits. What is Evolution?

What Evolution is NOT Many people argue against evolution based on statements they have heard. Yet many of those statements are ideas that are not part of evolutionary theory.

Evolution is NOT about things getting “better and better” or more and more complex.

Evolution is NOT about organisms “trying” to adapt.

Evolution does NOT give organisms what they “need.”

Evolution does NOT mean that individuals in a population slowly change.

Evolution is NOT a theory of how life began.

Early Ideas While many people associate evolution with Darwin, other scientists were talking about organic change long before Darwin published his theory of natural selection.

Aristotle ( B.C.) viewed the living world as fixed and unchanging. All living things could be arranged in a ladder from inferior to superior, with humans on top. Aristotle’s views influenced thought for over 2000 years.

Curiosity about fossils around the 16th century led to new questions. Were fossils the remains of organisms? If they were, what did this say about species being unchanging?

James Hutton ( ) Charles Lyell ( ) These two naturalists are best known for the theory of Uniformitarianism, proposed by Hutton and expanded by Lyell. Uniformitarianism states that the physical laws that now shape the earth have always done so, and that the past can be understood by studying the present. Both suggested that the Earth is very old.

Jean-Baptiste de Lamarck ( ) Lamarck sought a naturalistic explanation for the diversity of modern organisms and the animals seen in the fossil record. Lamarck used the theory of Inheritance of Acquired Characteristics, a widely-held belief at the time, to explain change in organisms. Blacksmiths, for example, were thought to pass their well-developed right arms on to their sons.

Lamarck’s Theory of Development Organisms are shaped by their environment. Change is goal-directed -- organisms have an internal drive to become more and more complex. Use and disuse of parts causes change that is passed on from generation to generation.

Charles Darwin ( ) and Alfred Russell Wallace ( ) Darwin and Wallace, working separately, devised a model for organic change that was based on their observations of living and fossil organisms over many years. Natural Selection was the first evidence-based mechanism for evolution that was proposed.

Darwin and Wallace observed that change did not happen equally to all members of a population, as Lamarck and others had thought.

Instead, Darwin and Wallace proposed that selection happens each generation. Some individuals have traits that help them survive and reproduce, while others have traits that put them at a disadvantage. Over generations, the number of individuals with favorable traits increases while those with unfavorable traits decrease.

Natural Selection Process by which individuals with inherited characteristics well-suited to the environment leave more offspring 4 Principles of Natural Selection Variation Heritability Overproduction Reproductive Advantage

Natural Selection Variation Within any population, individuals differ from one another in many ways.

Natural Selection Heritability Some differences between individuals are inheritable, and can be passed from parent to offspring. X

Natural Selection Overproduction In most populations, more young are born than can survive. Many of the young will die. A few will live long enough to reproduce.

Natural Selection Reproductive Advantage While random accidents do happen, an individual’s survival depends mainly on the individual’s traits. Those that are better suited to their environment may pass their traits to the next generation.

Evidence for Evolution Darwin spent over 20 years collecting evidence and reading research by others before he wrote On the Origin of Species. He found many features of organisms that suggested they were related by ancestry.

Fossils Fossils showed that species existed in the past that no longer exist today, and species exist today that did not exist in the past. In many groups of organisms, fossils demonstrated change from one form to another over long periods of time.

Homologous Structures Four-limbed vertebrate animals all have the same bones in the forelimbs, but the bones are shaped differently. In modern terms, this suggests a shared set of genes (for the bones) arising from common ancestry. Common structure – different function

Vestigial Structures Vestigial organs are remnants of structures that may have had important functions in an ancestral species, but have no clear function in some of the modern descendents.

Embryological Similarities LemurPigHuman Similar structures and similar developmental processes in embryos demonstrate a shared set of genes between groups of organisms, suggesting common ancestry. Note: Humans do not become fish or grow gills during development. We do share several embryological features with other vertebrate animals.

Biochemical and Genetic Analysis Darwin inferred shared ancestry from shared traits. Modern analysis of DNA demonstrates shared genes for many traits between related species, suggesting descent from a common ancestor. Mice and humans both have the gene for the protein Cytochrome c. Small differences show changes over time as the two groups diverged.

Evolution is the change in inherited traits in a population over generations. Fossils, molecular data, and other evidence shows that the genetics of a population do change over time. Natural Selection, one mechanism for change, is well-supported by evidence. Recap

Evidence for Selection In order to construct his theory, Darwin had to gather and examine evidence to discover whether selection happens in nature.

Natural Selection Selection favors organisms that are best adapted to the environment. Biological “fitness” is about survival, and those organisms with traits that help them survive are more likely to reproduce.

Artificial Selection Darwin knew from his studies of pigeon breeding that selection can change organisms, sometimes rapidly. Human selection created many breeds of dogs from wild dogs or wolves as humans chose to breed those animals with traits that the humans wanted.

Selection acts on variations Studies have shown that guppies living in ponds with few predators are more brightly colored than those living with many predators. In both populations there are guppies with genes for bright colors. Why are dull colors favored in ponds with many predators? Why are bright colors favored in ponds with few predators?

Selection does not cause new variations. Studies on Anole lizards showed that long, fast-moving legs were favored where trees had thicker branches, while shorter, more agile legs were favored where trees were flimsier. If lizards from places where trees had thin branches were moved to a place where trees had thick branches, those with the longest legs survived better. Each generation had more individuals with longer legs because the genes for longer legs were already in the population.