Natural selection - Wikipedia
It is common to contrast these with natural selection, but I think it's helpful to remember that First, concepts such as group selection and cultural selection. for such tails from their mother, resulting in a genetic correlation between the two. The theory of evolution by natural selection was proposed roughly inheritance to form the modern evolutionary synthesis, in which the connection between the. Natural selection affects innate behaviors such as reflexes, modal action patterns and general behavior traits. The difference between reflexes.
Repeated branching events, in which new species split off from a common ancestor, produce a multi-level "tree" that links all living organisms. Darwin's sketch above illustrates his idea, showing how one species can branch into two over time, and how this process can repeat multiple times in the "family tree" of a group of related species.
Natural selection Importantly, Darwin didn't just propose that organisms evolved. If that had been the beginning and end of his theory, he wouldn't be in as many textbooks as he is today! Instead, Darwin also proposed a mechanism for evolution: This mechanism was elegant and logical, and it explained how populations could evolve undergo descent with modification in such a way that they became better suited to their environments over time. Darwin's concept of natural selection was based on several key observations: Traits are often heritable.
In living organisms, many characteristics are inherited, or passed from parent to offspring. Darwin knew this was the case, even though he did not know that traits were inherited via genes. More offspring are produced than can survive. Organisms are capable of producing more offspring than their environments can support.
Thus, there is competition for limited resources in each generation. Offspring vary in their heritable traits. The offspring in any generation will be slightly different from one another in their traits color, size, shape, etc.
Based on these simple observations, Darwin concluded the following: In a population, some individuals will have inherited traits that help them survive and reproduce given the conditions of the environment, such as the predators and food sources present. The individuals with the helpful traits will leave more offspring in the next generation than their peers, since the traits make them more effective at surviving and reproducing.
Because the helpful traits are heritable, and because organisms with these traits leave more offspring, the traits will tend to become more common present in a larger fraction of the population in the next generation. Over generations, the population will become adapted to its environment as individuals with traits helpful in that environment have consistently greater reproductive success than their peers. Darwin's model of evolution by natural selection allowed him to explain the patterns he had seen during his travels.
If groups of finches had been isolated on separate islands for many generations, however, each group would have been exposed to a different environment in which different heritable traits might have been favored, such as different sizes and shapes of beaks for using different food sources. These factors could have led to the formation of distinct species on each island.
Wait, how would that work? Let's consider a simplified example to see how natural selection, operating on isolated populations of finches in different environments, could have led to a change in beak shape. If one island had plants that made large seeds, but few other food sources, birds with larger, tougher beaks than average might have been more likely to survive and reproduce there.
That's because the big-beaked birds would have been more able to crack open the seeds and eat the contents, and thus less likely to starve. If another island had many insect species but few other food sources, birds with thinner, sharper beaks than average might have been more likely to survive and reproduce there.
That's because the sharp-beaked birds would have been better able to catch insects as prey, and thus less likely to starve. Over many generations, these patterns of different survival and reproduction based on beak shape a heritable trait could have caused a shift in the average beak shape of each population. Specifically, the population on the first island might have shifted towards a larger, tougher beak on average, while the population on the second island might have shifted towards a thinner, sharper beak on average.
Eventually, the two populations of finches might have looked different enough from one another due to this change, and, potentially, other similar changes to be classified as different species. How natural selection can work To make natural selection more concrete, let's consider a simplified, hypothetical example. In this example, a group of mice with heritable variation in fur color black vs.
This environment features hawks, which like to eat mice and can see the tan ones more easily than the black ones against the black rock. Because the hawks can see and catch the tan mice more easily, a relatively large fraction of the tan mice are eaten, while a much smaller fraction of the black mice are eaten.
If we look at the ratio of black mice to tan mice in the surviving "not-eaten" group, it will be higher than in the starting population. Hawk outline traced from " Black and white line art drawing of Swainson hawk bird in flight ," by Kerris Paul public domain. So, the increased fraction of black mice in the surviving group means an increased fraction of black baby mice in the next generation.
After several generations of selection, the population might be made up almost entirely of black mice.
What is the difference between natural selection and evolution? | MyTutor
This change in the heritable features of the population is an example of evolution. What genes and alleles are we assuming here? You don't actually need to think in terms of genes and alleles to get this concept, as long as you accept that a tan mouse is more likely to leave tan offspring than a black mouse, and vice versa. However, if the question of inheritance pattern is bothering you, here is one way you can think about it: If we see only black and tan mice in the population, then a simple explanation is that the fur color trait is controlled by a single gene whose two alleles have a complete dominance relationship.
Let's say, for the sake of argument, that tan is dominant T and black is recessive t. This means that a tan mouse could be either Tt or TT, while a black mouse must be tt.
In the extreme case where all tan mice are eaten by predators before reproductive age, the only mice who will leave any offspring are black tt mice, who will mate with one another and produce more black tt offspring. In reality, selection probably would not be that strong.
Natural selection and evolution
Some tan mice would make it to mating season, and when they mated with the black mice, some tan baby mice would be born along with black baby mice. However, the more tan mice that got siphoned out of the gene pool by the predators, the higher the fraction of black-furred baby mice we'd expect to see in the next generation.
The genetics would be a little less obvious if we flipped the dominance relationships making black dominant to tanbut the same principle would hold: You can learn more evolution at the level of alleles and genes in the population genetics tutorial. Key points about natural selection When I was first learning about natural selection, I had some questions and misconceptions!
Here are explanations about some potentially confusing points, which may help you get a better sense of how, when, and why natural selection takes place.
Natural selection depends on the environment Natural selection doesn't favor traits that are somehow inherently superior. Instead, it favors traits that are beneficial that is, help an organism survive and reproduce more effectively than its peers in a specific environment.
Traits that are helpful in one environment might actually be harmful in another. Example For example, in the simplified scenario above, the black mice don't become more common over generations because they are inherently "better" or "more evolved" than tan mice.The Evolution of Populations: Natural Selection, Genetic Drift, and Gene Flow
Instead, they become more common because they have a heritable feature that makes them better able to survive and reproduce in a specific setting, one that happens to include black rocks. In a setting with light-colored rocks, the helpful and harmful traits would be reversed. Natural selection acts on existing heritable variation Natural selection needs some starting material, and that starting material is heritable variation. For natural selection to act on a feature, there must already be variation differences among individuals for that feature.
Also, the differences have to be heritable, determined by the organisms' genes. Example For instance, in the mouse example, there was heritable variation in fur color: If all the mice had been tan, the population would have had no way to adapt to its new environment by natural selection and might instead have been wiped out.
- What is the difference between natural selection and evolution?
- Natural selection
What if there had been non-heritable variation e. This might have affected the survival of the individuals, but it wouldn't have changed the composition of the population over generations because the babies of the dyed tan mice would have inherited the tan-mouse gene variants present in their parents, not black-mouse ones.
Heritable variation comes from random mutations The original source of the new gene variants that produce new heritable traits, such as fur colors, is random mutation changes in DNA sequence. Random mutations that are passed on to offspring typically occur in the germline, or sperm and egg cell lineage, of organisms.
Sexual reproduction "mixes and matches" gene variants to make more variation. Do organisms mutate on purpose? An important point here is that mutation and genetic variation are random, not directed.
That is, a mouse can't intentionally mutate to make itself or its offspring a different color.
Instead, if there by chance happens to be a mutation that changes mouse fur color, the variation produced by that mutation may be acted on by natural selection.
Natural selection and the evolution of species Let's take a step back and consider how natural selection fits in with Darwin's broader vision of evolution, one in which all living things share a common ancestor and are descended from that ancestor in a huge, branching tree.
What is happening at each of those branch points? In the example of Darwin's finches, we saw that groups in a single population may become isolated from one another by geographical barriers, such as ocean surrounding islands, or by other mechanisms. Once isolated, the groups can no longer interbreed and are exposed to different environments. In each environment, natural selection is likely to favor different traits and other evolutionary forces, such as random drift, may also operate separately on the groups.
Over many generations, differences in heritable traits can accumulate between the groups, to the extent that they are considered separate species. Based on various lines of evidencescientists think that this type of process has repeated many, many times during the history of life on Earth. A second synthesis was brought about at the end of the 20th century by advances in molecular geneticscreating the field of evolutionary developmental biology "evo-devo"which seeks to explain the evolution of form in terms of the genetic regulatory programs which control the development of the embryo at molecular level.
Natural selection is here understood to act on embryonic development to change the morphology of the adult body. However, natural selection is "blind" in the sense that changes in phenotype can give a reproductive advantage regardless of whether or not the trait is heritable.
Following Darwin's primary usage, the term is used to refer both to the evolutionary consequence of blind selection and to its mechanisms. A dark melanic morph of the peppered moth largely replaced the formerly usual light morph both shown here.
Since the moths are subject to predation by birds hunting by sight, the colour change offers better camouflage against the changed background, suggesting natural selection at work. Genetic variation Natural variation occurs among the individuals of any population of organisms.
Some differences may improve an individual's chances of surviving and reproducing such that its lifetime reproductive rate is increased, which means that it leaves more offspring.
Darwin, evolution, & natural selection
If the traits that give these individuals a reproductive advantage are also heritablethat is, passed from parent to offspring, then there will be differential reproduction, that is, a slightly higher proportion of fast rabbits or efficient algae in the next generation. Even if the reproductive advantage is very slight, over many generations any advantageous heritable trait becomes dominant in the population. In this way the natural environment of an organism "selects for" traits that confer a reproductive advantage, causing evolutionary change, as Darwin described.
Artificial selection is purposive where natural selection is not, though biologists often use teleological language to describe it. This gave dark-coloured moths a better chance of surviving to produce dark-coloured offspring, and in just fifty years from the first dark moth being caught, nearly all of the moths in industrial Manchester were dark.
The balance was reversed by the effect of the Clean Air Actand the dark moths became rare again, demonstrating the influence of natural selection on peppered moth evolution. Fitness biology The concept of fitness is central to natural selection. In broad terms, individuals that are more "fit" have better potential for survival, as in the well-known phrase " survival of the fittest ", but the precise meaning of the term is much more subtle.
Modern evolutionary theory defines fitness not by how long an organism lives, but by how successful it is at reproducing. If an organism lives half as long as others of its species, but has twice as many offspring surviving to adulthood, its genes become more common in the adult population of the next generation.
Though natural selection acts on individuals, the effects of chance mean that fitness can only really be defined "on average" for the individuals within a population. The fitness of a particular genotype corresponds to the average effect on all individuals with that genotype. A mathematical example of "survival of the fittest" is given by Haldane in his "The Cost of Natural Selection" paper .
Haldane called this process "substitution" or more commonly in biology, this is called "fixation". This is correctly described by the differential survival and reproduction of individuals due to differences in phenotype.
On the other hand, "improvement in fitness" is not dependent on the differential survival and reproduction of individuals due to differences in phenotype, it is dependent on the absolute survival of the particular variant.
The probability of a beneficial mutation occurring on some member of a population depends on the total number of replications of that variant. The mathematics of "improvement in fitness was described by Kleinman. Fixation or substitution is not required for this "improvement in fitness".