L05_eco_evo.pdf

Principles in Ecology PCB 4043 Eco-Evo Prof. Fahimipour Topics: Evolution, selection, drift [email protected] D: 437 DW | B: 206 Sanson  Key Concepts Evolution can be viewed as genetic change over time or as a process of descent with modification. Natural selectio...

Principles in Ecology PCB 4043 Eco-Evo Prof. Fahimipour Topics: Evolution, selection, drift [email protected] D: 437 DW | B: 206 Sanson  Key Concepts Evolution can be viewed as genetic change over time or as a process of descent with modification. Natural selection, genetic drift, and gene flow can cause allele frequencies in a population to change over time. Natural selection is the mechanism for adaptive evolution. Long-term patterns of evolution are shaped by large-scale processes like speciation, extinction, and adaptive radiation. Ecological interactions and evolution exert a profound influence on one another.  Trophy Hunting and Inadvertent Evolution: A Case Study Bighorn sheep populations have been reduced by 90% by hunting, habitat loss, and introduction of domestic cattle. Trophy hunting removes the largest and strongest males—the ones that would sire many healthy offspring. The average size of males and their horns decreased over 30 years of study. Figure 6.2 Trophy Hunting Decreases Ram Body and Horn Size  Trophy Hunting and Inadvertent Evolution: A Case Study This is also being observed in other species: – By targeting older, larger fish, commercial cod fishing has selected for genes that result in maturation at earlier ages and smaller size; small fish produce fewer eggs. – African elephants are poached for ivory; the proportion of the population that have tusks is decreasing. The unintended effects of human harvesting on these animals illustrate how populations can change, or evolve, over time.  Introduction Humans have a large impact on the environment— pollution, land use change, climate change, etc. We are just beginning to realize that we also cause evolutionary change and the consequences of this. Ecology and evolution are strongly interconnected.  What Is Evolution? Genes: made of DNA; specify (encode) protein structure; can have two or more forms called alleles. Genotype: genetic makeup of an individual; represented by letters. – If a gene has two alleles, A and a, genotype could be AA, Aa, or aa. One allele is inherited from the mother, one from the father.  What Is Evolution? Evolution: change in allele frequencies (proportions) in a population over time. – Example: if the frequency of `a` is 0.4 or 40%, the frequency of `A` is 0.6 or 60%. – If the frequency of `a` changed to 71%, the population would have evolved at that gene.  What Is Evolution? Evolution can be defined more broadly as descent with modification. As a population accumulates differences over time and a new species forms, it is different from its ancestors. But the new species has many of the same characteristics as its ancestors and resembles them.  What Is Evolution? Charles Darwin used the phrase “descent with modification;” populations change over time through natural selection: – Individuals with certain heritable traits survive and reproduce more successfully than other individuals. – I f t w o p o p u l a t i o n s e x p e r i e n c e d i ff e re n t environmental conditions, different characteristics may be favored and the populations will diverge genetically over time (and vice versa in similar environments) – Natural selection can be responsible for the modification part of “descent with modification.”  What Is Evolution? Natural selection acts as a sorting process. – Individuals with favored traits have more offspring, and their alleles will increase in frequency in the population. The population will evolve, but individuals do not evolve.  Focus on… Summarize how mutations contribute to the process of evolution. Compare the effects of stabilizing selection and disruptive selection on the temporal changes in the genetic structure of a population. Evaluate how random events can affect populations through time via genetic drift. Describe the role of gene flow among populations in terms of homogenizing genetic structure as well as enhancing evolutionary change.  Mechanisms of Evolution Key processes of evolutionary change: mutation, natural selection, genetic drift, and gene flow. – Mutation: source of new alleles. – Natural selection, genetic drift, gene flow: mechanisms that cause allele frequencies to change over time.  Mechanisms of Evolution Mutation Phenotype: Observable characteristics that are influenced by the genotype. Individuals differ from one another in part because they have different alleles of genes that influence phenotype. Different alleles arise by mutation—a change in DNA. – Mutations result from copy errors during cell division, mechanical damage, exposure to chemicals (mutagens) or high-energy radiation.  Mechanisms of Evolution Formation of new alleles is critical – without mutation all members of a population would have identical genotypes and evolution could not occur. Recombination: Offspring have combinations of alleles that differ from their parents, producing different genotypes within a population. Horizontal gene transfer (microbes and plants) Mutation provides the raw material on which evolution is based; recombination rearranges the raw material into new combinations.  Mechanisms of Evolution Mutations are actually very rare. – In a generation, a mutation occurs in every 10,000 to 1,000,000 copies of a gene. – Generally, background mutation rate is a weak agent of allele frequency change. – In some cases, mutation rates are frequent enough to affect allele frequencies, as in development of antibiotic resistance in bacteria.  Mechanisms of Evolution Natural Selection Three types of natural selection: 1. Directional selection: Individuals at one phenotypic extreme are favored. – Example: In medium ground finches, drought favored large beak size for cracking hard seeds.  Mechanisms of Evolution 2. Stabilizing selection: Individuals with an intermediate phenotype are favored. – Example: Parasitic wasps select for small gall size of Eurosta flies; while birds select for large gall size. – As a result, larvae in galls of intermediate size have an advantage. Figure 6.6 Three Types of Natural Selection (Part 2)  Mechanisms of Evolution 3. Disruptive selection: Individuals at both phenotypic extremes are favored. – Example: African seedcrackers (birds) have two food sources— hard seeds that require large beaks to crack, and smaller, softer seeds that smaller beaks are more suited to. Figure 6.6 Three Types of Natural Selection (Part 3)  Mechanisms of Evolution Natural selection can result in populations in which all individuals have the favored allele: – Andean geese have evolved a type of hemoglobin with a very high affinity for O2, an advantage at high altitudes. – The allele frequency for this trait is 100% (it has reached fixation).  Mechanisms of Evolution Genetic Drift Occurs when chance events determine which alleles are passed to the next generation. Four effects on small populations: – 1. Allele frequencies fluctuate at random; some may disappear, others may become fixed. – 2. Genetic variation of the population is reduced. – 3. Frequency of harmful alleles can increase if they have only mildly deleterious effects. – 4. Chance events may lead to allele fixation in one population and loss from another population. Figure 6.7 Genetic Drift Causes Allele Frequencies to Fluctuate at Random  Mechanisms of Evolution 2 and 3 can have dire consequences: – Loss of genetic variation reduces the ability of the population to respond to changing environmental conditions. – Increase of harmful alleles can reduce survival and reproduction. – These effects are important for species that are near extinction.  Mechanisms of Evolution Gene Flow Alleles move between populations via movement of individuals or gametes. Gene flow has two effects: – 1. Populations become more similar. – 2. New alleles can be introduced into a population (acts similarly to mutation).  Mechanisms of Evolution In the 1960s, new alleles that provide insecticide resistance arose by mutation in mosquitoes in Africa or Asia. Mosquitos with the new alleles were blown by winds or transported by humans to new locations. The allele frequency increased rapidly in populations exposed to insecticides. Figure 6.9 Gene Flow: Introducing Alleles for Insecticide Resistance  Adaptive Evolution Adaptations: Features of organisms that improve their ability to survive and reproduce. – Includes morphological and physiological features such as enzymes that function at high temperatures. Natural selection is not a random process. By consistently favoring individuals with certain alleles, natural selection causes adaptive evolution—traits that confer advantages tend to increase in frequency over time. Figure 6.10 Some Striking Adaptations  Adaptive Evolution Example of adaptive evolution: – Soapberry bugs pierce fruits with a needle-like beak. – Feeding is most efficient if beak size matches fruit size. – In populations with different food sources, Carroll and Boyd (1992) predicted that beak size would evolve to adapt to fruits of introduced tree species. Figure 6.11 Adaptive Evolution in Soapberry Bugs  Adaptive Evolution There are many examples of rapid adaptive evolution: – Antibiotic resistance in bacteria – Insecticide resistance in insects – Drab coloration in guppies, making them harder for predators to see – Increased beak size in ground finches – Hurricanes can exert strong selection pressure for traits that enhance the ability of anole lizards to cling to trees.  Adaptive Evolution Natural selection does not result in a perfect match between organisms and their environments. Environments are constantly changing, and there are constraints on evolution: – Lack of genetic variation – Evolutionary history – Ecological trade-offs  Adaptive Evolution Lack of genetic variation: If there is no beneficial allele, adaptive evolution at that gene cannot occur. – Example: Initially, mosquito populations lacked alleles for pesticide resistance so the pesticides were effective. Advantageous alleles arise by chance, not “on demand.”  Adaptive Evolution Evolutionary history: Natural selection works on traits that already exist. Organisms have certain characteristics and lack others because of their ancestry. – Example: Dolphins evolved from terrestrial mammals; they have lungs and cannot “breathe” underwater.  Adaptive Evolution Ecological trade-offs: The ability to perform one function may reduce the ability to perform another function. Adaptations are the product of compromises in the abilities of organisms to perform different and sometimes conflicting functions. Figure 6.14 Trade-Off between Reproduction and Survival ­  Joint Effects of Ecology and Evolution Ecological interactions can cause evolutionary change. Evolution can result from a range of ecological interactions, including predation, competition, herbivory, parasitism, and mutualism. Speciation is often caused by ecological factors. – Examples: Directional selection on soapberry bugs and genetic drift in greater prairie chickens.  Joint Effects of Ecology and Evolution Evolution can alter ecological interactions. – If a predator evolves a new way to capture prey, the prey species may go extinct, decline, migrate to other areas, or evolve new ways to cope with the more efficient predator. – Similar changes can occur among species that compete for resources.  Joint Effects of Ecology and Evolution Evolutionary changes over long time scales can have a profound effect on ecological interactions. – Adaptive radiation of land plants altered all aspects of life on land: from soil stability to food sources available to other organisms to nutrient cycling.  Joint Effects of Ecology and Evolution Evolution can also occur over short time periods. Reciprocal feedback between ecological and evolutionary factors can also occur over short periods of time. Feedback effects can operate at multiple levels in an ecosystem. – In a field experiment, evolutionary changes in life span and flowering time in evening primrose populations led to consistent changes in the abundance of a moth that eats the seeds of this plant. Figure 6.22 Rapid Feedback Effects Can Occur between Ecological and Evolutionary Factors Figure 6.23 Feedback of Food Plant Evolution on Insect Abundance  Connections in Nature: The Human Impact on Evolution Many human actions can alter the course of evolution. Pollutants and introduction of invasive species change aspects of the environment and alter selection pressures. Habitat fragmentation leaves isolated patches, which can affect evolutionary processes. Human actions can alter the mechanisms of evolution: natural selection, genetic drift, gene flow. Figure 6.25 Evolutionary Effects of Habitat Fragmentation on a Hypothetical Species  Connections in Nature: The Human Impact on Evolution Habitat fragmentation, pollution, overharvesting, and introductions of invasive species are among the main reasons why Earth is undergoing a biodiversity crisis. The extinction rate today is 100 to 1,000 times higher than the background extinction rate seen in the fossil record. Climate change is likely to become a leading cause of extinctions in the coming decades.