This course presents the principles of evolution, ecology, and behavior for students beginning their study of biology and of the environment. It discusses major ideas and results in a manner accessible to all Yale College undergraduates. Recent advances have energized these fields with results that have implications well beyond their boundaries: ideas, mechanisms, and processes that should form part of the toolkit of all biologists and educated citizens.
The lecture presents an overview of evolutionary biology and its two major components, microevolution and macroevolution. The idea of evolution goes back before Darwin, although Darwin thought of natural selection. Evolution is driven by natural selection, the correlation between organism traits and reproductive success, as well as random drift. The history of life goes back approximately 3.7 billion years to a common ancestor, and is marked with key events that affect all life.
Genetic transmission is the mechanism that drives evolution. DNA encodes all the information necessary to make an organism. Every organism's DNA is made of the same basic parts, arranged in different orders. DNA is divided into chromosomes, or groups of genes, which code for proteins. Asexually reproducing organisms reproduce using mitosis, while sexually reproducing organisms reproduce using meiosis. Both these mechanisms involve duplication of DNA, which then gets passed to offspring. RNA is a key component in the duplication of DNA.
Adaptive Evolution is driven by natural selection. Natural selection is not "survival of the fittest," but rather "reproduction of the fittest." Evolution can occur at many different speeds based on the strength of the selection driving it. These types of selection can result in directional, stabilizing, and disruptive outcomes. They can be driven by frequency-dependent selection and sexual selection, in addition to more standard types of selection.
Neutral evolution occurs when genes do not experience natural selection because they have no effect on reproductive success. Neutrality arises when mutations in an organism's genotype cause no change in its phenotype, or when changes in the genotype bring about changes in the phenotype that do not affect reproductive success. Because neutral genes do not change in any particular direction over time and simply "drift," thanks in part to the randomness of meiosis, they can be used as a sort of molecular clock to determine common ancestors or places in the phylogenetic tree of life.
Genetics controls evolution. There are four major genetic systems, which are combinations of sexual/asexual and haploid/diploid. In all genetic systems, adaptive genetic change tends to start out slow, accelerate in the middle, and occur slowly at the end. Asexual haploids can change the fastest, while sexual diploids usually change the slowest. Gene frequencies in large populations only change if the population undergoes selection.
Mutations are the origin of genetic diversity. Mutations introduce new traits, while selection eliminates most of the reproductively unsuccessful traits. Sexual recombination of alleles can also account for much of the genetic diversity in sexual species. In some instances, population size can affect diversity and rates of evolution and fixation, but in other cases population size does not matter.
Development is responsible for the complexity of multicellular organisms. It helps to map the genotype into the phenotype expressed by the organism. Development is responsible for ancient patterns among related organisms, and many structures important to development shared by many life forms have changed little over hundreds of millions of years. Development is expressed combinatorially, allowing a relatively small amount of genetic information to be expressed in many different ways.
Reaction norms depict the range of phenotypes a single genotype can produce, depending on the environment. Reaction norms must fit within an organism's phylogenetic constraints. They can differ for different individuals within a population, but some traits differ very little based on the environment; some do not differ at all.
There are several explanations for the evolution of sex and its continued prevalence. One is facilitating the spread of helpful mutations while hastening the removal of harmful ones. Another is expediting resistance against pathogens. Sex does have several costs compared to asex, such as only giving half your genome to offspring, having to find mates, and the risk of predation and STDs. Overall, the benefits outweigh the costs and sex has a firm hold on the majority of the recent branches of the tree of life.
Genomic conflict arises when the interests of various genomic elements, such as chromosomes and cytoplasmic organelles, are not aligned. These conflicts arise in two situations: either when one unit is contained within another, as a mitochondrion is contained within a cell, or when inheritance is asymmetrical. Genomic conflict can thus occur within a cell, within an organism, or between two organisms, such as a mother and developing fetus. There have been several steps taken to avoid these conflicts in sexual species, including the fairness of meiosis and the uniparental inheritance of cytoplasmic genomes.
Life history covers three main classes of traits in organisms: age and size at maturity, number and size of offspring, and lifespan and reproductive investment. Organisms must make tradeoffs among these traits that typically cause them to come to evolutionary equilibrium at intermediate values. Life history traits are evolutionary solutions to the ecological problems of the risk of mortality and the acquisition of food, and they are expressed in reaction norms that determine the particular traits that an organism will exhibit when its genes encounter a specific environment during development.
Sex allocation is an organism's decision on how much of its reproductive investment should be distributed to male and female functions and/or offspring. Under most conditions, the optimal ratio is 50:50, but that can change under certain circumstances. Sex allocation determines what sexes sequential hermaphrodites should be at each part of their life as well as how simultaneous hermaphrodites should behave. Some species have more control over the sexes of their offspring than others, and adjust the sex ratios of their offspring depending on the environment and conditions.
Sexual selection is a component of natural selection in which mating success is traded for survival. Natural selection is not necessarily survival of the fittest, but reproduction of the fittest. Sexual dimorphism is a product of sexual selection. In intersexual selection, a sex chooses a mate. In intrasexual selection, individuals of one sex compete among themselves for access to mates. Often honest, costly signals are used to help the sex that chooses make decisions.
Speciation is the process through which species diverge from each other and/or from a common ancestor. There are several definitions of species, most of which focus on reproductive isolation and/or phylogenetic similarities. This can cause some controversy. Speciation can result from geographical separation or ecological specialization. There are stages of speciation in which organisms cluster first into distinct populations before finally becoming different species.
The Tree of Life must be discovered through rigorous analysis. Genetic information is crucial because appearances can be deceiving, and species that look similar can prove to be genetically very dissimilar and not share recent common ancestors. Two criteria, used to determine what the "correct" Tree is, are simplicity and whether the tree maximizes the probability of observing what we actually see.
We can use methods of genetic analysis to connect phylogenic information to geographical histories. Human migration has left genetic traces on every continent, and allows us to trace our roots back to Africa. Molecular genetic methods allow us to determine whether or not trait states were ancestral, which can have profound implications for fundamental biological ideas.
The history of life and evolution has been characterized by several key events. These events can be grouped as new hierarchal levels of selection coming into play, as biological units coming together in symbiosis and specialization, or in a number of other ways. Other important events are situations of conflict resolution or information transmission, from the genetic to the cultural level.
Geology and climate have shaped the development of life tremendously. This has occurred in the form of processes such as the oxygenation of the atmosphere, mass extinctions, tectonic drift, and disasters such as floods and volcanic eruptions. Life, particularly bacteria, has also been able to impact the geological makeup of the planet through metabolic processes.
The fossil record holds a lot of evolutionary information that can't be seen on shorter time scales, although the more recent fossil record is more complete. Among other things, the fossil record demonstrates that extinctions can open up ecological space for new speciation and radiation, and that life forms tend to begin small and evolve to be bigger over time.
Coevolution happens at many levels, not just the level of species. Organelles such as mitochondria and chloroplasts serve as good intracellular examples. Other living things make up a crucial component of an organism's environment. Coevolution can occur in helpful ways (symbiosis) and in harmful ways (parasitism). Many factors can influence coevolution, such the frequency and degree of interaction.
Evolution plays an important though underutilized role in medicine. Evolution guides how our bodies respond to various treatments, how pathogens will respond to treatments, and how pathogens' responses will change over time. Pathogens oftentimes will evolve to an intermediate level of virulence where they become strong enough to infect a host and reproduce, but not so strong as to kill the host before it can spread the pathogen.
There is a distinct possibility that humans are currently part way through an evolutionary transition between individuals and groups. The conflict between these two units of selection and levels of organization, between biology and culture, may explain some of the tensions in modern human life. Examples of selfishness and altruism exemplify how these types of selection act on humans.
While there are many differences between modern science and philosophy, there are still a number of lessons in modes of thought that scientists can take from philosophy. Scientists' ideas about the nature of science have evolved over time, leading to new ideas about falsifiability, creativity, revolutions, and the boundaries and limits of what can be accomplished by different types of science.
This lecture provides an overview of the physical aspects of earth's biomes. Temperature, water, latitude, and altitude all come into play. Regions with similar levels of these climatic features tend to have similar life-forms living there. These same climatic features can also affect weather patterns, which in turn affect life by altering habitats and ecosystems. On a large enough scale, such as El Niño, these weather patterns can affect life all over the earth.
Every species on earth has an environmental range in which it can live. Usually it flourishes in the central portion of this range. Organisms contain a host of adaptations that allow them to manipulate their environments to remain within their preferred range. Plants and animals differ in the nature of these adaptations, which include the control of water, temperature, pH, and ion concentration.
The growth of populations is held in check by several factors. These can include predators, food and other resources, and density. Population density affects growth rate by determining how likely is it that an organism will interact with a member of its own species compared to an organism of a different species. Population growth studies rely on the mathematics of logs and exponents.
Competition among species, or interspecific competition, can have an even greater effect on selection than competition within species (intraspecific competition). This is often the case in lower density populations. Different species can have positive, neutral, or negative effects on each other's fitness, and the effect species 1 has on species 2 is not necessarily the same that 2 has on 1. The effects that cohabiting species have on each other shapes evolution the same way that selective pressures from within a species or the physical environment shapes it.
The idea of ecological communities has changed tremendously over the past forty years. The classical view stated that there were so many different species because evolution packed them tightly into the available niches. The modern view emphasizes the idea of trophic cascades, or top-down control in food chains. This emphasized the importance of predation in ecology, although it downplayed the significance of food webs, which showed the interrelated nature of ecosystems better than simple food chains.
Geography is very important in ecology. Two major systems have been designed to model this, island biogeography and metapopulations. The idea of metapopulations is more recent, and has emerged as the dominant theory. Metapopulations are populations in multiple neighboring areas. The population of a species in any individual area may go extinct, but the metapopulation still survives. The theory of metapopulations has gained momentum in recent years because of its applications to epidemiology, the study of diseases.
The movement of matter and energy around the planet is very important, and its study draws on geology, and meterology in addition to chemistry. Energy tends to flow upwards from plantlike producers to herbivores to carnivores before being decomposed by detritovores and cycling back into energy usable by producers, in addition to the photosynthesis or chemosynthesis used by producers to produce energy. Like energy, compounds vital to life such as carbon, nitrogen, and phosphorous flow around the planet in cycles.
One can look at biodiversity from several perspectives. An ecological point of view tries to determine how necessary diversity is for an ecosystem to function. An economic point of view tries to capture the value of the "services" nature provides for mankind. An evolutionary point of view shows how artificial the human "right" to dominance is. Finally, a personal point of view captures the emotional basis for the values that humans place on biodiversity.
There are several ways to examine the behaviors of organisms when they forage or hunt for food or mates. These behaviors become more complex in higher organisms, such as primates and whales, which can hunt in groups. Foragers and hunters have been shown to examine the marginal cost and marginal benefit of continuing an action and then adjust their behaviors accordingly. They are also able to handle risk by hoarding resources.
The economic concept of game theory can be readily applied to evolution and behavior. By analyzing encounters between organisms as a mathematical "game," important information such as fitness payoffs and the proportions of "strategies" played by each group within a population can be inferred. While oftentimes these games are too simplified to apply directly to actual examples in nature, they are still useful models that help convey important concepts.
Mating systems and parental care vary tremendously from species to species. Every species differs in how it protects its young from predators and provides its young with food, if it does so at all. The physical environment as well as behavioral dynamics in intraspecies relationships all influence parental care. Often the mating system, which sex is dominant in mating, and whether fertilization is external or internal will determine much of the process of parental care.
Breeding strategies differ both among males and females of the same species as well as among different species. The difference in breeding strategies among members of the same species can usually be linked to frequency dependence. If the species is at evolutionary equilibrium, the relative fitnesses of these different strategies will be identical. Differing strategies have been found at the level of the gamete as well as at the level of different organisms and species.
Originally, altruism and self-sacrifice were thought to be incompatible with natural selection, even by Darwin. Now we have several explanations for how altruism can increase an individual's fitness. One is kin selection, or the idea that helping relatives can help increase one's genes in the population. Another involves ecological constraints and punishments. Here, individuals contribute to the group and wait their turn to reproduce.