bio final 8D.
There are three important elements to an organism's fitness.
1. An individual's fitness is measured relative to other genotypes or phenotypes in the population 2. Fitness depends on the specific environment in which the organism lives 3. Fitness depends on an organism's reproductive success compared with other organisms in the population.
Fitness depends on the specific environment in which the organism lives.
A sand-colored mouse living in a beach habitat will be more fit than a chocolate-colored mouse. But that same sand-colored mouse will practically call out to potential predators if it lives in the darker brush away from the beach. An organism's fitness, although genetically based, is not fixed in stone and unchanging—it can change over time and across habitats
TAKE-HOME MESSAGE 8.15
Acting on traits for which populations show a large range of phenotypes, natural selection can change populations in several ways. These include directional selection, in which the average value for the trait increases or decreases; stabilizing selection, in which the average value of a trait remains the same while extreme versions are selected against; and disruptive selection, in which individuals with extreme phenotypes have the highest fitness.
TAKE-HOME MESSAGE 8.12
Adaptation, which refers both to the process by which organisms become better matched to their environment and to the specific traits that make an organism more fit, occurs as a result of natural selection.
example of disruptive selection
Among some species of fish—the coho salmon, for instance—only the largest males acquire good territories. They generally enjoy relatively high reproductive success. While the intermediate-size fish regularly get run out of the good territories, some tiny males are able to sneak in and fertilize eggs before the territory owner detects their presence. Consequently, we see an increase in the frequency of small and large fish, with a reduction in the frequency of medium-size fish
TAKE-HOME MESSAGE 8.14
Animal breeders and farmers are making use of natural selection when they modify their animals and crops through selective breeding, because the three conditions for natural selection are satisfied. Since the differential reproductive success is determined by humans rather than by nature, this type of natural selection is also called artificial selection.
artificial selection
Breeding organisms with specific traits in order to produce offspring with identical traits.
Some traits fall into clear categories, others range continuously.
Certain traits are easily categorized—blue eyes and brown eyes, or white tiger fur and orange tiger fur, for example. Other traits, such as height, are influenced by many genes and environmental factors, so a continuous range of phenotypes occurs
factors that prevent populations from progressing inevitably toward perfection:
Environments change quickly. Natural selection may be too slow to adapt the organisms in a population to such a constantly moving target. Variation is needed as the raw material of selection—remember, it is the first necessary condition for natural selection to occur. If a mutation creating a new, "perfect" version of a gene never arises, the individuals within a population will never be perfectly adapted. There may be different alleles for a trait, each causing individuals to have the same fitness. In this case, each allele represents an equally fit "solution" to the environmental challenges.
Often, structures are enhanced or elaborated on by natural selection because they enhance fitness by serving some other purpose.
Experiments using models of insects demonstrated that, as expected, a small percentage of a wing, in the form of a nub or "almost wing," confers no benefit at all when it comes to flying. (The nubs don't even help flies keep their orientation during a "controlled fall.") The incipient wings do help the insects address a completely different problem, though. They allow much more efficient temperature control, so that an insect can gain heat from the environment when the insect is cold and dissipate heat when the insect is hot.
In practice, plant and animal breeders understood natural selection before Darwin did; they just didn't know that they understood it.
Farmers bred crops for maximum yield, and dog, horse, and pigeon fanciers selectively bred the animals with their favorite traits to produce more and more of the offspring with more and more exaggerated versions of those traits.
example of fitness: Suppose there are two fruit flies. One fly carries the genes for a version of a trait that allows it to survive a long time without food. The other has the genes for a different version of the trait that allows it to survive only a short while without food. Which fly has the greater fitness?
If the environment is one in which there are long periods of time without food, such as in the experiment described at the beginning of the chapter, the fly that can live a long time without food is likely to produce more offspring than the other fly, and so over the course of its life it has greater fitness.
Fitness depends on an organism's reproductive success compared with other organisms in the population
If you carry an allele that gives you the trait of surviving for 200 years, but that also causes you to be sterile, your fitness is zero; On the other hand, if you inherit an allele that gives you a trait that causes you to die at half the age of everyone else, but also causes you to have twice as many offspring as the average individual, your fitness is increased. It is reproductive success that is all-important in determining whether particular traits increase in frequency in a population.
example of directional selection
Milk production in cows is an example. There is a lot of variation in milk production from cow to cow. As you might expect, farmers select for breeding those cows with the highest milk production and have done so for many decades. The result: between 1920 and 1945, average milk production increased by about 50% in the United States. Those at the other end of the range, the cows that produce the smallest amounts of milk, have reduced fitness, since the farmers do not allow them to reproduce, limiting the number of "less milk" alleles in the next generation.
TAKE-HOME MESSAGE 8.17
Natural selection can change allele frequencies for genes involved in complex physiological processes and behaviors. Sometimes a trait that has been selected for one function is later modified to serve a completely different function.
TAKE-HOME MESSAGE 8.13
Natural selection does not lead to organisms perfectly adapted to their environment, because (1) environments can change more quickly than natural selection can adapt organisms; (2) mutation does not produce all possible alleles; and (3) there is not always a single, optimum adaptation for a given environment.
Evolution in general, and natural selection specifically, does not guide organisms toward "betterness" or perfection.
Natural selection is simply a process by which, in each generation, the alleles that cause organisms to have the traits that make them most fit in that environment tend to increase in frequency. If the environment changes, the alleles that are most favored may change, too. What's fit in one time and place may not be fit in another.
the word fitness is not defined by an organism's ability to survive or its physical strength or its health.
Rather, fitness has everything to do with an organism's reproductive success.
Disruptive Selection
Selection that, for a given trait, increases fitness at both extremes of the phenotype distribution and reduces fitness at middle values.
Directional Selection
Selection that, for a given trait, increases fitness at one extreme of the phenotype and reduces fitness at the other, leading to an increase or decrease in the mean value of the trait.
Stabilizing Selection
Selection that, for a given trait, produces the greatest fitness at the intermediate point of the phenotypic range. said to take place when individuals with intermediate phenotypes are the most fit.
example of stabilizing selection
The death rate among babies, for example, is lowest between 7 and 8 pounds, and is higher for both lighter and heavier babies
adaptation
The process by which, as a result of natural selection, a population's organisms become better matched to their environment; also, a specific feature, such as the quills of a porcupine, that makes an organism more fit.
"Survival of the fittest."
This is perhaps one of the most famous but most misunderstood phrases. Taken literally, it seems to be circular: those organisms that survive must be the fittest. But this is true only if being fit is defined as the ability to survive. As we'll see, in evolution,
An individual's fitness is measured relative to other genotypes or phenotypes in the population
Those traits that confer the highest fitness will generally increase in frequency in a population, and their increase will always come at the expense of alternative traits that confer lower fitness.
Natural selection can cause the evolution of complex traits and behaviors.
We have seen that natural selection can change allele frequencies and modify the frequency with which simple traits, such as fur color or turkey-breast size or plant height, appear in a population. But what about complex traits, such as behaviors, that involve numerous physiological and neurological systems? The short answer is yes.
the organisms that possess traits that allow them to better exploit the environment in which they live will tend to produce more offspring than the organisms with alternative traits.
With passing generations, a population will be made up of more and more of these fitter organisms. And, as a consequence, populations of organisms will tend to be increasingly well matched or adapted to their environment.
fitness
is a measure of the relative amount of reproduction of an individual with a particular phenotype compared with the reproductive output of individuals of the same species with alternative phenotypes.
"Survival of the fittest" is a misleading phrase because
it is the individuals with the greatest reproductive output that are the most fit in any population
why? example
the beak size of Galápagos finches. closely monitored the average size of the finches' beaks. They found that the average beak size within a population fluctuated according to the food supply. During dry years—when the finches had to eat large, hard seeds—birds with bigger, stronger beaks were more successful and multiplied. During wet years, smaller-beaked birds were more successful, because there was a surplus of small, soft seeds. The ever-changing "average" finch beak illustrates that adaptation does not simply march toward some optimal endpoint
the stranger the new environment
the less likely it is that the transplanted organism will survive.
Artificial selection is considered
the three conditions are satisfied for natural selection, however it is a special case because the differential reproductive success is being determined by humans rather than by nature
Natural selection does not lead to perfect organisms.
we might logically conclude that, eventually, fitness will reach a maximum, and all organisms in all populations will be perfectly adapted to their environment. But this never happens.
