Natural Selection Learning Targets

• Explain the processes of artificial and natural selection. • Describe the principle of descent with modification and common descent. On the Origin of Species

• In 1859, Darwin published the results of his work, On the Origin of Species. • In his book he proposed a mechanism for evolution that he called natural selection. • He presented evidence that evolution has been occurring for millions of years – and continues in all living things. Inherited Variation and Artificial Selection

• Darwin observed that individual members of species are similar in many ways but also vary from one another in important ways. • For example, all humans share certain traits: two eyes, two arms, two legs, same organs, etc. • But there are also many variations in human traits: eye color, hair color, skin color, straight hair/curly hair, height, size, strength, speed, intelligence, etc. Inherited Variation and Artificial Selection Some People Have Faces That Look Like a Dog’s Ear. Rainbow Trout Variations Inherited Variation and Artificial Selection

• Observations from his travels and conversations with plant and animal breeders, convinced Darwin that variation in traits occurred in nature and on farms. • Some plants in a species bear larger fruit than others. • Some cows give more milk than others. Inherited Variation and Artificial Selection

• From breeders, Darwin also learned that some of these variations could be passed from parents to offspring. • He noted that plant and animal breeders used heritable variation (genetic variation) to improve crops and livestock. • They would select for breeding only the largest hogs, the fastest horses, or the cows that produced the most milk. Inherited Variation and Artificial Selection

• Darwin called this process artificial selection. • In artificial selection, nature provided the variation, and humans selected those variations that they found useful. • Artificial selection has produced many diverse domestic animals and crop plants. Artificial Selection Examples Artificial Selection Examples Artificial Selection in Guppies Natural Selection

• Darwin was convinced that a process like artificial selection also worked in nature. Darwin’s Theory of Natural Selection Natural Selection – Overproduction

• Most organisms produce more offspring than can be sustained by the environment. • Overproduction forces competition among organisms for resources such as food, living space, etc., it creates a struggle for survival. • In a sense, through overproduction, nature sets up competition and guarantees that there will be winners and losers. Natural Selection – Fitness

• A key factor in the struggle for existence is how well suited an organism is to its environment. • Darwin called the ability of an individual to survive and reproduce in its specific environment fitness. Natural Selection – Variation

• Individuals in a population show variation in traits such as size, coloration, body structures, behaviors, speed and ability to obtain food or avoid predators. • These variations can be inherited. Natural Selection – Selection • Some organisms have traits that increase their fitness making it more likely that they will survive and reproduce. • Darwin called inherited traits that increase an organism’s chances for survival adaptations. • Some organisms have traits that decrease their fitness making it less likely that they will survive and reproduce. • Darwin called this process survival of the fittest. Natural Selection – Selection

• Because individuals with higher levels of fitness survive and reproduce more successfully, their traits become more common in future generations. • Because individuals with lower levels of fitness survive and reproduce less successfully, their traits become less common in future generations. Natural Selection vs. Artificial Selection

• Because of its similarities to artificial selection, Darwin referred to the survival of the fittest as natural selection. • In both artificial and natural selection, only certain individuals of a population reproduce. • In artificial selection, humans decide which individuals will reproduce based on the traits desired among the offspring. • In natural selection, the traits selected – and therefore increasing over time – are those that contribute to an organism’s fitness in its environment. • Natural selection takes place without human inference. Natural Selection – Summary

• Over time, natural selection results in changes in the inherited characteristics of a population. These changes increase a specie’s fitness in its environment.

Natural Selection in Peppered Moths http://www6.district125.k12.il.us/~nfischer/Moth/default.htm Descent With Modification

• Darwin proposed that over long periods of time, natural selection produces organisms that have different structures, establish different niches (roles in their environment), or occupy different habitats. • As a result, species today look different from their ancestors. • Each living species has descended, with changes, from other species over time. • He referred to this principle as descent with modification. Descent With Modification

• Descent with modification implies that all living organisms are related to one another. • Look back in time, and you will find common ancestors shared by tigers, panthers and cheetahs. • Look farther back, and you will find ancestors that these felines share with horses, dogs, and bats. • Farther back still are the common ancestors of mammals, birds, alligators and fishes. Descent With Modification

• If we look back far enough, the logic concludes, we could find the common ancestors of all living things. • This is the principle known as common descent. • According to this principle, all species – living and extinct – were derived from common ancestors. • Therefore a single “tree of life” links all living things. The Importance of Variation • Variation is critical to a population’s ability to adapt to environmental changes. • Without variation, populations would have more difficulty adapting to environmental changes and be less likely to survive long term. • Without variation, all individuals would respond to environmental changes in the same way. If that response is an inability to adapt, the entire population dies. • Because of variation, certain individuals with favorable traits can survive environmental changes when others cannot.