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An introduction to evolution The definition Biological evolution, simply put, is descent with inherited modification. This definition encompasses everything from small-scale evolution (for example, changes in the frequency of different gene versions in a population from one gener...

An introduction to evolution The definition Biological evolution, simply put, is descent with inherited modification. This definition encompasses everything from small-scale evolution (for example, changes in the frequency of different gene versions in a population from one generation to the next) to large-scale evolution (for example, the descent of different species from a shared ancestor over many generations). Evolution helps us to understand the living world around us, as well as its history. All life on Earth shares a common ancestor, just as you and your cousins share a common grandmother. Through the process of descent with modification, this common ancestor gave rise to the diverse species that we see documented in the fossil record and around us today.helps us to understand the living world around us, as well as its history. PHYLOGENETIC TREE A phylogenetic tree is a hypothesis about evolutionary relationships.The process of evolution produces a pattern of relationships between species. As lineages evolve and split and modifications are inherited, their evolutionary paths diverge. This produces a branching pattern of evolutionary relationships This tree, like all phylogenetic trees, is a hypothesis about the relationships among organismsFor example, evidence discovered in the last 50 years suggests that birds are dinosaurs, which required adjustment to several “vertebrate twigs. The root of the tree represents the ancestral lineage, and the tips of the branches represent the descendants of that ancestor. As you move from the root to the tips, you are moving forward in time. Building the tree To build a phylogenetic tree such as the one shown below, biologists collect data about the characters of each organism they are interested in. Characters are heritable traits that can be compared across organisms, such as physical characteristics (morphology), genetic sequences, and behavioral traits. A shared character is one that two lineages have in common, and a derived character is one that evolved in the lineage leading up to a clade and that sets members of that clade apart from other individuals Shared derived characters can be used to group organisms into clades. For example, amphibians, turtles, lizards, snakes, crocodiles, birds and mammals all have, or historically had, four limbs. If you look at a modern snake you might not see obvious limbs, but fossils show that ancient snakes did have limbs, and some modern snakes actually do retain rudimentary limbs. Four limbs is a shared derived character inherited from a common ancestor that helps set apart this particular clade of vertebrates Homologies and Analogies We use homologous characters — characters in different organisms that are similar because they were inherited from a common ancestor that also had that character. An example of homologous characters is the four limbs of tetrapods. Birds, bats, mice, and crocodiles all have four limbs. Sharks and bony fish do not. The ancestor of tetrapods evolved four limbs, and its descendents have inherited that feature — so the presence of four limbs is a homology Not all characters are homologies. For example, birds and bats both have wings, while mice and crocodiles do not. Does that mean that birds and bats are more closely related to one another than to mice and crocodiles? No. When we examine bird wings and bat wings closely, we see that there are some major differences. Bat wings consist of flaps of skin stretched between the bones of the fingers and arm. Bird wings consist of feathers extending all along the arm. These structural dissimilarities suggest that bird wings and bat wings were not inherited from a common ancestor with wings.Bird and bat wings are analogous — that is, they have separate evolutionary origins, but are superficially similar because they have both experienced natural selection that shaped them to play a key role in flight. Analogies are the result of convergent evolution.Interestingly, though bird and bat wings are analogous as wings, as forelimbs they are homologous. Birds and bats did not inherit wings from a common ancestor with wings, but they did inherit forelimbs from a common ancestor with forelimbs. MECHANISM OF EVOLUTION It takes place due to factors 1)descent with modification 2)natural selection 3)mutation 4)genetic drift 5)migration DESCENT WITH MODIFICATION Evolution occurs when there is a change in the heritable information passed from one generation to the next. Typically, we think of biological evolution as changes in gene frequency within a population over time – if, say, birds with genes that produce wide beaks went from being rare to being common over multiple generations. But biological evolution also includes changes in DNA that does not code for genes and changes in heritable information not encoded in DNA at all. In all of these cases, the modifications are heritable and can be passed on to the next generation — which is what really matters in evolution: long term change NATURAL SELECTION Natural selection is one of the basic mechanisms of evolution, along with mutation, migration, and genetic drift. Natural selection will only happen when there is : Variation : Differential reproduction : Heritability examples  Behavior can also be shaped by natural selection. Behaviors such as birds’ mating rituals, bees’ wiggle dance, and humans’ capacity to learn language have genetic components In some cases, we can directly observe natural selection occurring. Very convincing data show that the shape of finches’ beaks on the Galapagos Islands has tracked weather patterns: after droughts, the finch population has deeper, stronger beaks that let them eat tougher seeds. MUTATIONS Mutations are changes in the information contained in genetic material. For most of life, this means a change in the sequence of DNA, the hereditary material of life Mutations are random :Mutations can be beneficial, neutral, or harmful for the organism, but mutations do not “try” to supply what the organism “needs.” Not all mutations matter to evolution Since all cells in our body contain DNA, there are lots of places for mutations to occur; however, not all mutations matter for evolution. Somatic mutations occur in non-reproductive cells and so won’t be passed onto offspring For example, the yellow color on half of a petal on this red tulip was caused by a somatic mutation. The seeds of the tulip do not carry the mutation. Cancer is also caused by somatic mutations that cause a particular cell lineage (e.g., in the breast or brain) to multiply out of control. Such mutations affect the individual carrying them but are not passed directly on to offspring. GENETIC VARIATION Without genetic variation, some key mechanisms of evolutionary change like natural selection and genetic drift cannot operate There are three primary sources of new genetic variation: 1. Mutations are changes in the information contained in genetic material. (For most of life, this means a change in the sequence of DNA.) A single mutation  can have a large effect, but in many cases, evolutionary change is based on the accumulation of many mutations with small effects. 2. Gene flow is any movement of genetic material from one population to another (e.g., through migration) and is an important source of genetic variation. 3. Sex can introduce new gene combinations into a population. This genetic shuffling is another important source of genetic variation GENE FLOW Gene flow — also called migration — is any movement of individuals, and/or the genetic material they carry, from one population to another.The genetic variation in modern human populations has been critically shaped by gene flow. For example, by sequencing ancient DNA, researchers have reconstructed the entire Neanderthal genome – and they’ve found that many snippets of these archaic sequences live on in modern humansFurthermore, this ancient gene flow seems to affect who we are today. Neanderthal gene versions have been linked to immune functions, metabolic functions (e.g., affecting one’s risk of developing diabetes), and even skin color. EVOLUTIONARY FITNESS Evolutionary biologists use the word fitness to describe how good a particular genotype is at leaving offspring in the next generation relative to other genotypes. In evolution, fitness is about success at surviving and reproducing, not about exercise and strength Of course, fitness is a relative thing. A genotype’s fitness depends on the environment in which the organism lives.The fittest individual is not necessarily the strongest, fastest, or biggest. A genotype’s fitness includes its ability to survive, find a mate, produce offspring — and ultimately leave its genes in the next generationWe tend to think of natural selection acting on survival ability — but, as the concept of fitness shows, that’s only half the story. When natural selection acts on mate-finding and reproductive behavior, biologists call it sexual selection SEXUAL SELECTION Sexual selection is a “special case” of natural selection. Sexual selection acts on an organism’s ability to obtain (often by any means necessary!) or successfully copulate with a mate.Sexual selection is even powerful enough to produce features that are harmful to the individual’s survival. For example, extravagant and colorful tail feathers or fins are likely to attract predators as well as interested members of the opposite sex. EXAMPLE The venomous female redback spider – also known as the Australian black widow – poses a danger to humans … and to male redback spiders, which are often eaten by their mates. Males seem to go out of their way to make this happen, flipping themselves over and presenting their abdomens to the female while mating. This behavior might at first seem like one that selection would act against. After all, how could risking one’s life be adaptive? Remember that evolutionary fitness is about getting genes into the next generation, not just survival.Male redback spiders deliver their sperm to females using specialized mouthparts. If the female is hungry, she will eat the male during the mating process. In the wild, this happens about 65% of the time. Females often mate with more than one male and can store sperm (sometimes for years!) to use later. Females produce multiple egg sacs throughout their lives, each of which can contain hundreds of eggs. Different eggs in a single egg sac may be fertilized by sperm from different fathers. MICROEVOLUTION Microevolution is evolution on a small scale — within a single population. That means narrowing our focus to one branch of the tree of life. There are a few basic ways in which microevolutionary change happens. Mutation, migration, genetic drift, and natural selection are all processes that can directly affect gene frequencies in a population. Population : For animals, it’s fairly easy to decide what a population is. It is a group of organisms that interbreed with each other — that is, they all share a gene poolBiologists who study evolution at this level define evolution as a change in gene frequency within a population. SPECIATION A species is often defined as a group of individuals that actually or potentially interbreed in nature. In this sense, a species is the biggest gene pool possible under natural conditions.That definition of a species might seem cut and dried, but it is not — in nature, there are lots of places where it is difficult to apply this definition. For example, many bacteria reproduce mainly asexually. The bacteria reproduces asexually, by binary fission. The definition of a species as a group of interbreeding individuals cannot be easily applied to organisms that reproduce only or mainly asexually. Some more examples are hooded crow , oak trees speciation is a lineage-splitting event that produces two or more separate species