How Organisms Evolve Chapters 14-15 The Theory of Evolution Evolution is the process of change in the inherited traits of a population of organisms from one generation to the next. The inherited traits that are the basis of evolution come from the genes that are passed on to offspring during reproduction. Evolution can be traced through the fossil record. The Theory of Evolution There are two major mechanisms driving evolution: Natural selection is a process causing inheritable traits that are helpful for survival and reproduction become more common in a population, and harmful traits to become more rare. A comparison of skeletal structures shows how the whale has evolved from smaller land animals. large ground finch small tree finch woodpecker finch
Evolution by Natural Selection leaves behind evidence of ancestors. Human tail Evolution by Natural Selection leaves behind evidence of ancestors. Snake legs Ostrich wings Human ear muscles Cave salamander eyes Vestigial structures are remnants from ancestors that no longer serve a purpose. Dog thumbs Natural Selection on short time scales Anole lizard adapts shorter legs over 14 years when introduced to islands with shorter vegetation than their natural habitat. Natural Selection leads to Convergent Evolution Isolated animals in Australia evolved similar adaptations to mainland animals. Independent evolution of traits for a common purpose or niche is convergent evolution.
The Theory of Evolution An individual does not evolve. A population, however, can change its phenotype over time. Evolution is a change in the genetic make-up of a population over generations. A population is a group that includes all the members of a species living in a given area. Understanding Evolution To understand evolution, we must first understand under what circumstances a population will not evolve. The Hardy-Weinberg Principle describes the conditions that cause an equilibrium population. A population that is not evolving, in which allele frequencies remain the same. The Hardy-Weinberg Principle Mutations cause Evolution There must be no mutation. There must be no gene flow between populations. No movement of alleles into or out of the population. The population must be very large. All mating must be random. There must be no natural selection. All genotypes must reproduce with equal success. Violation of any of these conditions will cause a population to evolve. Mutations are changes in the DNA sequence of a gene. Most mutations arise during DNA replication. Inherited mutations are rare but important. Mutations occur in roughly 1 out of every 100,000 gametes produced. ~1 gene out of 60,000 alleles in a human. Mutations are the source of new alleles.
Mutations cause Evolution Mutations are not goal directed. Mutations arise randomly, not because of an environmental need. Antibiotic resistance in bacteria occurs spontaneously in only a few individuals, not because the bacteria were exposed to the antibiotic. Gene Flow accelerates Evolution Gene flow is the movement of alleles between populations. New alleles enter populations when emigrating individuals interbreed with the existing population. In baboon troops, juvenile males migrate away from their troop to find a new troop to join, bringing their gene pool with them. Gene Flow accelerates Evolution Flowering plants use pollen and seeds to exchange genes. Pollen contains sperm cells which may be carried long distances by wind or animal pollinators. When the pollen reaches flowers of a new population, its alleles are integrated into that new population. The size of a population can both hinder and accelerate evolution. Events often occur that allow only a small subset of the population reproduce, limiting the number of alleles available. These chance events that change allele frequencies cause genetic drift. Genetic drift describes a change in allele frequencies that occurs entirely from chance.
Genetic drift has a much larger affect on small populations than larger ones. Population bottlenecks are an example of genetic drift. In a population bottleneck, a population size is drastically reduced such that only a few individuals. Only the alleles of the survivors contribute to the genetic diversity of the next generation. Population bottlenecks are an example of genetic drift. The Northern Elephant Seal was hunted to almost extinction in the 1800s. By the 1890s, only 20 individuals survived. Though now there are about 30,000 individuals in the wild, genetic analysis shows they are nearly identical. Population bottlenecks are an example of genetic drift. The cheetah was once found on 5 continents. It is now an endangered species. Loss of habitat, conflict with Cheetahs humans, as well as its own loss of genetic variation have threatened its survival. The genetic inbreeding in cheetahs has led to low survivorship, poor sperm quality, and greater susceptibility to disease.
Population bottlenecks are an example of genetic drift. Isolated colonies founded by a small number of organisms can lead to founder effects. The fig wasp carries fertilized eggs to a particular fig variety where its offspring hatch, mate, and search for a new fig to lay eggs. Each type of fig has its own genetically unique fig wasp species. Population bottlenecks are an example of genetic drift. Isolated colonies founded by a small number of organisms can lead to founder effects. One couple among the original 200 Amish to immigrate to Lancaster, PA had a defective allele giving their children short limbs and extra fingers. Because the founding population was rather small, this syndrome is now found in 14% of this Amish population. Mating enhances Evolution Mating within a population is almost never random. Non-random mating has large effects on the distribution of different genotypes within a population. In animals, individuals have preference for particular traits. Mating display of a male frigatebird. Mating enhances Evolution Non-random mating leads to sexual selection. Male mandrill baboon displaying his teeth and rear end as a show of aggression to gain female favor.
Mating enhances Evolution Non-random mating leads to sexual selection. Mating enhances Evolution Non-random mating leads to sexual selection. Males fighting for attention from picky females leads to sexual dimorphism. Male fringehead fish competing for female attention. Mating enhances Evolution Mating within a population is almost never random. Sometimes the pressure to assert dominance for female favor has its drawbacks. Natural Selection leads to Evolution Not all genotypes are equally beneficial. Natural selection is the process by which favorable traits become more common in a population. The long neck of the giraffe allows it to exert dominance over other males, but also makes it rather difficult to drink. The long neck of the giraffe is beneficial for accessing hard-toreach leaves for food.
Natural Selection leads to Evolution Principles of Natural Selection: It does not cause genetic changes in individuals. Though natural selection acts on individuals, it is populations that are changed by evolution. Evolution leads to unequal success at reproduction among organisms bearing different alleles. Evolution is not progressive; it does not make organisms better. Natural Selection leads to Evolution Natural selection is often termed survival of the fittest. Fitness is the reproductive success of an organism. This jewel cichlid has high fitness, judging from the babies seen. Sexual Selection vs Natural Selection Sexual Selection vs Natural Selection Satin bowerbirds Why do females prefer elaborate ornaments? Honesty Advertisement: the trait is directly advertising a good gene or a good parent. Why do females prefer elaborate ornaments? Runaway Sexual Selection: males have the trait, females have the preference.
Sexual Selection vs Natural Selection Why do females prefer elaborate ornaments? Handicap Hypothesis: With this handicap, I m still here! long-tailed widowbird Satin bowerbirds Natural Selection leads to Evolution Natural selection is often termed survival of the fittest. Not all genotypes are equally beneficial. Antibiotic resistance evolves by natural selection. Before the introduction of penicillin (World War II), very few resistant bacteria were known. The resistance came from a mutated digestive enzyme, capable of breaking down penicillin. After a few generations, the frequency of the penicillindestroying allele is 100%. Antibiotic resistance evolves by Natural Selection. Methicillin-resistant Staphylococcus aureus (MRSA) is resistant to beta-lactam antibiotics including the penicillins and cephalosporins. S. aureus is a common bacteria that infects skin, blood, and respiratory systems. MRSA causes the same diseases as normal S. aureus but cannot be retarded by common antibiotics. Our introduction of antibiotics into the environment (food, water, soil) has selected for resistant strains.