Natural selection and phenotypic variation essay

Natural selection theory

Only two individuals in the second generation reproduce and, by chance, these individuals are homozygous dominant for brown coat color. The founder effect occurs when a portion of the population i. Nonrandom Mating and Environmental Variance Population structure can be altered by nonrandom mating the preference of certain individuals for mates as well as the environment. Open image in new window Fig. Genetic variation is essential for natural selection because natural selection can only increase or decrease frequency of alleles that already exist in the population. The dots show the locations of alternative genotypes, placed by their average phenotypes in two dimensions. However, the proper measure should relate the average phenotype of a genotype, , to the expected fitness, here ignoring variation in fitness, Frank, To build those complex adaptations takes a long time and many changes.

Matching the environment by plasticity or learning To survive a novel environmental challenge, a phenotype must be near the nonzero part of the new fitness landscape.

Box 4: Vertebrate immunity Invading pathogens present a vast diversity of foreign molecules that must be recognized.

limitations of natural selection

Faster evolutionary response may also be achieved by enhanced phenotypic variability through learning or plasticity. The important issue is that fitness falls off from its peak by the pattern of a normal distribution.

Natural selection essay examples

Abstract A central question in evolutionary biology is how gene flow and natural selection shape geographic patterns of genotypic and phenotypic variation. The red dot shows the average phenotype for a genotype relatively far from the new peak. Population Genetics Genetic Variation Genetic variation is a measure of the variation that exists in the genetic makeup of individuals within population. Sources of Genetic Variation Gene duplication, mutation, or other processes can produce new genes and alleles and increase genetic variation. Introduction A single genotype produces different phenotypes. Those previous examples showed the reaction norm and fitness both varying across a single dimension. Having presented the theory of evolution by natural selection as a syllogism in previous meetings, we introduced this exercise to our students in the form of a challenge: the theoretical ideas outlined in class would be subject to experimental scrutiny, and we would determine whether the data support or reject the process of evolution by natural selection. Temperature-dependent sex determination: The sex of the American alligator Alligator mississippiensis is determined by the temperature at which the eggs are incubated. The example genotype can come close to the new peak by modulating expression to produce an average phenotype of. Here, I highlight a few key points in relation to this article. Assortative mating in the American Robin: The American Robin may practice assortative mating on plumage color, a melanin based trait, and mate with other robins who have the most similar shade of color. She emphasized three important points. Geographic separation between populations can lead to differences in the phenotypic variation between those populations. Less-successful types do not pass on as many traits because they have fewer descendants.

For instance, some beaks were designed for crushing seed shells, whereas others for catching insects. The sum is then equivalently written as 2 This equation shows how one averages the fitness, f xfor each phenotypic value, x, over the reaction norm,to obtain the expected fitness of a genotype.

Over time, failure to reproduce at a rate equal to or greater than the mortality rate leads to extinction.

natural selection argumentative essay

A population may survive by having a variety of genotypes that produce different phenotypes, increasing the chance that at least one of the phenotypes will be close to a new fitness peak. Strong competition can be thought of as a form of extreme challenge.

That is hard to do. We teach the theory of evolution by natural selection in the form of a syllogism a logical argument consisting of a major premise, minor premise, and conclusion; for further discussion, see Haldane ; Lande and Arnold : If trait variation within a population is due, in part, to differences among individuals in their genetic constitution [heritability], and If trait variation within a population is associated with the variation in survival or reproduction in the current environment [selection], then Over generational time, there will be an increase in the frequency of individuals having those trait values that confer a relative increase in reproductive success [evolution].

Natural selection examples

Genetic variation is caused by: mutation random fertilization crossing over or recombination between chromatids of homologous chromosomes during meiosis The last three of these factors reshuffle alleles within a population, giving offspring combinations which differ from their parents and from others. Figure 1 The reaction norm smooths the fitness landscape. Thinking about evolution by natural selection in this manner illustrates how characteristics of populations can change through time. The theory of natural selection maintains that some individuals in a population have traits that enable to survive and produce more offspring, while other individuals have traits that are detrimental and may cause them to die before reproducing. Can you make a specific prediction about the magnitude of this evolutionary change? That clonal expansion can be thought of as survival and reproduction of those genotypes that land on the fitness surface imposed by the unpredictable invader. Strong competition can be thought of as a form of extreme challenge. The baseline truncates phenotypes with low vigor, setting their fitnesses to zero. Nonrandom Mating and Environmental Variance Population structure can be altered by nonrandom mating the preference of certain individuals for mates as well as the environment. In this case, we assume the center of the fitness distribution is at a phenotypic value of zero to give a fixed point for comparison — any value to center fitness could be used. That genetic diversity by itself could not cover the huge space of possible challenges. This slope was the estimated heritability of our trait Fig. Averaging over the reaction norm, p, flattens and smooths the fitness function, f.

This is especially true in large populations spread over large geographic distances where not all individuals will have equal access to one another. The smoothed fitness surface is sufficiently broad that, for any location of the fitness peak after environmental challenge, the fitness surface touches at least one of the genotypes.

Natural selection and phenotypic variation essay

None of the phenotypes for the narrow reaction norm are above the truncation point, so all die. The genetic structure of the survivors becomes the genetic structure of the entire population, which may be very different from the pre-disaster population. A species with a great deal of genetic variability is more likely to survive as a species in a changing environment than a species with limited variability. Some mutations are unfavorable or harmful and are quickly eliminated from the population by natural selection. Some organisms are faster, more colorful, bigger, or smarter than others in their species. Improvement occurs if there is genetic variation for the average phenotype, , of the reaction norm. In so doing, they were favoring the one morph type over another, which created uneven reproductive rates that favored an increase in one type of moth over the others. Genetic variation covers widely separated locations in the phenotype space. Reaction norms cover the phenotype space around each genotype. Thus, nonheritable variation would seem to be irrelevant for evolutionary change, which instead depends on the genetic component of variation. Evolvability has developed into a large subject of its own e. He tagged each moth with a touch of paint and then set traps for their recapture. The developmental flexibility to match environments may arise by phenotypic plasticity or learning.
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EVOLUTION OF PHENOTYPIC VARIANCE