Genetic Variation and Patterns of Evolution Evidence of Evolution Genetic Variation and Patterns of Evolution

Elements of Natural Selection All species have genetic variation. Since organisms generally produce more offspring than can be supported by the environment, individuals within a species frequently complete with each other for survival. The environment itself presents many challenges for an organism's survival. Survival of the fittest occurs The traits of the organisms best suited to a certain habitat tend to become more frequent in a population over time.

Variation and Gene Pools A species is a group of similar organisms that can interbreed and produce fertile offspring. A population is a group of individuals of the same species that interbreed. A gene pool consists of all genes, including all the different alleles, that are present in a population. Populations, not individual organisms, can evolve over time. Copyright Pearson Prentice Hall

directional selection stabilizing selection disruptive selection Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution. Natural selection can affect the distributions of phenotypes in any of three ways: directional selection stabilizing selection disruptive selection Copyright Pearson Prentice Hall

Directional Selection   When individuals at one end of the curve have higher fitness than individuals in the middle or at the other end, directional selection takes place. Copyright Pearson Prentice Hall

Stabilizing Selection   When individuals near the center of the curve have higher fitness than individuals at either end of the curve, stabilizing selection takes place. Copyright Pearson Prentice Hall

Disruptive Selection  When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle, disruptive selection takes place. Copyright Pearson Prentice Hall

Consider that a flying beetle has been introduced to a new tropical island. Identify the type of natural selection at work in each example. – stabilizing, directional, or disruptive Only the beetles with the darkest bodies thrive in the new environment. The smallest beetles cannot compete for food. The largest beetles are easy prey for birds. The beetles with the smallest wings a thrive in rotten tree trunks. The beetles with the largest wings thrive along the coastline.

Variation and Gene Pools A gene pool consists of all genes, including all the different alleles, that are present in a population. The relative frequency of an allele is the number of times the allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur. Relative frequency is often expressed as a percentage, and it is not related to whether an allele is dominant or recessive.

Variation and Gene Pools Gene Pool for Fur Color in Mice Copyright Pearson Prentice Hall When scientists determine whether a population is evolving, they may look at the sum of the population’s alleles, or its gene pool. This diagram shows the gene pool for fur color in a population of mice. 

Variation and Gene Pools In genetic terms, evolution is any change in the relative frequency of alleles in a population. The two main sources of genetic variation are mutations and the genetic shuffling that results from sexual reproduction. Copyright Pearson Prentice Hall

What is genetic drift? A random change in allele frequency Genetic drift may occur when a small group of individuals colonizes a new habitat. Individuals may carry alleles in different relative frequencies than did the larger population from which they came.

Genetic Drift Genetic Drift Copyright Pearson Prentice Hall In small populations, individuals that carry a particular allele may have more descendants than other individuals. Over time, a series of chance occurrences of this type can cause an allele to become more common in a population. This model demonstrates how two small groups from a large, diverse population could produce new populations that differ from the original group.

Genetic Drift Copyright Pearson Prentice Hall

Genetic Drift Copyright Pearson Prentice Hall

Population A Population B Descendants Copyright Pearson Prentice Hall Population A Population B When allele frequencies change due to migration of a small subgroup of a population it is known as the founder effect.

Evolution Versus Genetic Equilibrium The Hardy-Weinberg principle states that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change. When allele frequencies remain constant it is called genetic equilibrium. Copyright Pearson Prentice Hall

Evolution Versus Genetic Equilibrium Five conditions are required to maintain genetic equilibrium from generation to generation: there must be random mating, the population must be very large, there can be no movement into or out of the population, there can be no mutations, and there can be no natural selection. Evolution Versus Genetic Equilibrium Copyright Pearson Prentice Hall

The Process of Speciation Copyright Pearson Prentice Hall Photo credit: ©MURRAY, PATTI/Animals Animals Enterprises

Speciation is the formation of new species. Natural selection and chance events can change the relative frequencies of alleles in a population and lead to speciation. Speciation is the formation of new species. A species is a group of organisms that breed with one another and produce fertile offspring. Copyright Pearson Prentice Hall

What factors are involved in the formation of new species? Isolating Mechanisms What factors are involved in the formation of new species? The gene pools of two populations must become separated for them to become new species. Copyright Pearson Prentice Hall

Isolating Mechanisms As new species evolve, populations become reproductively isolated from each other. When the members of two populations cannot interbreed and produce fertile offspring, reproductive isolation has occurred. Copyright Pearson Prentice Hall

Isolating Mechanisms   Behavioral isolation occurs when two populations are capable of interbreeding but have differences in courtship rituals or other reproductive strategies that involve behavior. Copyright Pearson Prentice Hall

Kaibab Abert Isolating Mechanisms Geographic isolation occurs when two populations are separated by geographic barriers such as rivers or mountains. Kaibab Abert Copyright Pearson Prentice Hall

Isolating Mechanisms    Temporal isolation occurs when two or more species reproduce at different times. Copyright Pearson Prentice Hall

Speciation in Darwin's Finches Founders Arrive  A few finches—species A—travel from South America to one of the Galápagos Islands. There, they survive and reproduce. Copyright Pearson Prentice Hall Speciation in the Galápagos finches occurred by founding of new populations, geographic isolation, gene pool changes, reproductive isolation, and ecological competition. Small groups of finches moved from one island to another, became reproductively isolated, and evolved into new species.

Speciation in Darwin's Finches Geographic Isolation Some birds from species A cross to a second island. The two populations no longer share a gene pool. Copyright Pearson Prentice Hall Speciation in the Galápagos finches occurred by founding of new populations, geographic isolation, gene pool changes, reproductive isolation, and ecological competition. Small groups of finches moved from one island to another, became reproductively isolated, and evolved into new species.

Speciation in Darwin's Finches Changes in the Gene Pool Seed sizes on the second island favor birds with large beaks. The population on the second island evolves into population B, with larger beaks. Copyright Pearson Prentice Hall Speciation in the Galápagos finches occurred by founding of new populations, geographic isolation, gene pool changes, reproductive isolation, and ecological competition. Small groups of finches moved from one island to another, became reproductively isolated, and evolved into new species.

Speciation in Darwin's Finches Reproductive Isolation If population B birds cross back to the first island, they will not mate with birds from population A. Populations A and B are separate species. Copyright Pearson Prentice Hall

Patterns of Evolution Macroevolution refers to large-scale evolutionary patterns and processes that occur over long periods of time. Copyright Pearson Prentice Hall

Six important topics in macroevolution are: extinction adaptive radiation convergent evolution coevolution punctuated equilibrium changes in developmental genes Copyright Pearson Prentice Hall

Extinction More than 99% of all species that have ever lived are now extinct. Copyright Pearson Prentice Hall

Adaptive Radiation Adaptive radiation is the process by which a single species or a small group of species evolves into several different forms that live in different ways. For example, in the adaptive radiation of Darwin's finches, more than a dozen species evolved from a single species. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall

Convergent Evolution Different organisms undergo adaptive radiation in different places or at different times but in similar environments. The process by which unrelated organisms come to resemble one another is called convergent evolution. Results in analogous structures. Copyright Pearson Prentice Hall

Coevolution Sometimes organisms that are closely connected to one another by ecological interactions evolve together. The process by which two species evolve in response to changes in each other over time is called coevolution. Copyright Pearson Prentice Hall

Punctuated Equilibrium Darwin felt that biological change was slow and steady, an idea known as gradualism. Copyright Pearson Prentice Hall Biologists have considered two different explanations for the rate of evolution, as illustrated in these diagrams. Gradualism involves a slow, steady change in a particular line of descent. Punctuated equilibrium involves stable periods interrupted by rapid changes involving many different lines of descent.

Punctuated equilibrium is a pattern of evolution in which long stable periods are interrupted by brief periods of more rapid change. Copyright Pearson Prentice Hall Biologists have considered two different explanations for the rate of evolution, as illustrated in these diagrams. Gradualism involves a slow, steady change in a particular line of descent. Punctuated equilibrium involves stable periods interrupted by rapid changes involving many different lines of descent.

Developmental Genes and Body Plans Hox Genes are the master control genes of body layout. Evolution of Wings in Insects Copyright Pearson Prentice Hall Some ancient insects, such as the mayfly nymph (top), had winglike structures on many body segments. Modern insects have only four wings or two wings. Changes in the expression of developmental genes may explain how these differences evolved.