Figure 1: The "drunkard's walk" as an explanatory metaphor for patterns of increasing complexity in evolution A drunken man leaving a bar at the end of the night starts with the locked door to his back and is equally likely to stagger to the left or to the right. Because he cannot move back through the door, it is inevitable that he will eventually fall into the gutter despite not having made a conscious decision to move in that direction.
Evolutionary change likewise does not progress towards a goal or final destination. A second important point is that evolution and natural selection are not equivalent terms. Natural selection is one force that can drive and influence evolutionary change, but other mechanisms can be equally important.
Trait changes among the members of a population are not always a result of selective processes. For instance, the appearance and accumulation of a deleterious trait e. Similarly, alleles that have no effect on traits under selection may undergo mutations that do not influence the fitness of the organism carrying them.
Proponents of the neutral theory of molecular evolution argue that many, if not most, of the genetic differences between species are selectively neutral. What follows is an overview of the variety of forces, including natural selection, that can drive or otherwise influence evolutionary change. One can distinguish between two general classes of evolutionary change: microevolution change below the level of the species and macroevolution change above the level of the species.
Population ecologists, conservation biologists, and behavioral ecologists are most directly concerned with microevolutionary processes. These include shifts in the values and frequencies of particular traits among members of populations, often due to ecological processes such as the movement of organisms and changing environmental conditions as well as interactions with members of different species e.
These processes can, but do not necessarily, lead to the formation of new species over time but instead result in fluctuating frequencies of traits within populations tracking ever-changing selective pressures Thompson Since some microevolutionary processes may occur over just a few generations, they can often be observed in nature or in the laboratory.
An appropriate illustration of microevolution in action is the well-documented tendency for insects to rapidly develop resistance to pesticides Gassmann et al.
For example, during summer in Southern France, pesticides are applied to control Culex mosquitoes from the Mediterranean coast to about 20 km inland. Certain mosquito genes confer resistance to the pesticides but are costly in the absence of pesticides Figure 2 ; frequencies of the pesticide-resistance gene increase during summers in areas where spraying is common, but do not increase in areas where spraying is not practiced. Lenormand et al. Coastal frequencies of Ace.
Instead, studies of macroevolution tend to rely on inferences from fossil evidence, phylogenetic reconstruction, and extrapolation from microevolutionary patterns. Often the focus of macroevolutionary studies is on speciation: the process by which groups of previously-interbreeding organisms become unable or unwilling to successfully mate with each other and produce fertile offspring.
Ecologists may be interested in macroevolution as a means to make inferences regarding present-day ecological questions. Scientists interested in modeling the effects of present-day climate change, for instance, can couple prehistoric climatological data with fossil-derived patterns of speciation and extinction to understand how contemporary animal and plant species are faring today and how they will fare in the future.
For example, many marine invertebrates e. As anthropogenic CO 2 accumulates in the atmosphere, a significant fraction of it dissolves into the ocean, releasing free hydrogen ions in the process and thus decreasing oceanic pH.
Among other things, this ocean acidification reduces the amount of carbonate available to shell-making marine invertebrates that rely on it for their calcium-carbonate shells, making it difficult for them to make and maintain their shells. By combining oceanic pH data from hundreds of millions of years ago with fossil records of foramifera shell-making marine invertebrates , Zachos et al.
From these data, one can model current patterns of ocean acidification and begin to predict its effects on present-day and future marine animals e.
Orr et al. Evolution describes changes to the inherited traits of organisms across generations. Evolutionary change is not directed towards a goal, nor is it solely dependent on natural selection to shape its path. Ecology, as with any biological discipline, is rooted in evolutionary concepts and best understood in its terms. Dobzhansky, T. Biology, Molecular and Organic. American Zoologist 4 , — Hewitt, G. The genetic legacy of the Quaternary ice ages.
Nature , — Gassmann A. Evolutionary analysis of herbivorous insects in natural and agricultural environments. Pest Management Science 65 , —81 Gould, S. New York, NY: W. Norton and Company, Lenormand, T. Tracking the evolution of insecticide resistance in the mosquito Culez pipiens. Orr, J. Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms.
A person may be related to blood relatives, such as cousins, aunts, and uncles, because she shares with them one or more common ancestors, such as a grandparent, or great-grandparent. But those cousins, aunts, and uncles are not her ancestors. In the same way, humans and other living primates are related, but none of these living relatives is a human ancestor.
Learn More All in the Family. Members of one species do not normally interbreed with members of other species in nature. Sometimes, members of different species, such as lions and tigers, can interbreed if kept together in captivity.
But in nature, geographic isolation and differences in behavior, such as choice of habitat, keep these sorts of closely related animal species apart.
Similarly, closely related species of plants can sometimes be hybridized by horticulturists, but these hybrids are rarely found in nature. A species, then, is defined by science as a group of interbreeding or potentially interbreeding populations that is reproductively isolated from other such groups. Learn More An Origin of Species.
Genes are the portions of an organism's DNA that carry the code responsible for building that organism in a very specific way. Genes -- and, thus, the traits they code for -- are passed from parent to offspring. From generation to generation, well-understood molecular mechanisms reshuffle, duplicate, and alter genes in a way that produces genetic variation.
This variation is the raw material for evolution. Learn More Evolution Since Darwin. Sexual reproduction allows an organism to combine half of its genes with half of another individual's genes, which means new combinations of genes are produced every generation.
In addition, when eggs and sperm are produced, genetic material is shuffled and recombined in ways that produce new combinations of genes. Sexual reproduction thus increases genetic variation, which increases the raw material on which natural selection operates. Genetic variation within a species -- also known as genetic diversity -- increases a species' opportunity for change over successive generations.
Learn More The Advantage of Sex. Evolution is not a random process. The genetic variation on which natural selection acts may occur randomly, but natural selection itself is not random at all. The survival and reproductive success of an individual is directly related to the ways its inherited traits function in the context of its local environment.
Parallel evolution usually occurs between unrelated species that do not occupy the same or similar niches in a given habitat. SparkTeach Teacher's Handbook. Summary Types of Evolution. Divergent Evolution When people hear the word "evolution," they most commonly think of divergent evolution, the evolutionary pattern in which two species gradually become increasingly different.
Convergent Evolution Convergent evolution causes difficulties in fields of study such as comparative anatomy. Parallel Evolution Parallel evolution occurs when two species evolve independently of each other, maintaining the same level of similarity.
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