Natural selection results in adaptation

The evolutionary agents we have just discussed influence the frequencies of alleles and genotypes in populations. As we saw in the previous chapter, major perturbations, such as colliding continents, volcanic eruptions, and meteorite impacts, also have periodically altered the survival and reproductive rates of organisms. All of these agents dramatically affect the

Stigma Style

Stamen

An insect visiting a thrum flower picks up pollen on its head and body. When it then visits a pin flower,

Stigma Style

Stamen

Structure Thrum Flower

Anther Stamen

Stigma

Style

An insect visiting a pin flower picks up pollen on its proboscis and head. When it then visits a thrum flower, it deposits pollen on the short stigma.

Anther Stamen

Stigma

Style

An insect visiting a pin flower picks up pollen on its proboscis and head. When it then visits a thrum flower, it deposits pollen on the short stigma.

course of life's evolution on Earth, but none of them result in adaptations. For adaptation to occur, individuals that differ in heritable traits must survive and reproduce with different degrees of success. When some individuals contribute more offspring to the next generation than others, allele frequencies in the population change in a way that adapts individuals to the environments that influenced their success. This process is known as natural selection.

The reproductive contribution of a phenotype to subsequent generations relative to the contributions of other phe-notypes is called its fitness. The word "relative" is critical: The absolute number of offspring produced by an individual does not influence the genetic structure of a population. Changes in absolute numbers of offspring are responsible for increases and decreases in the size of a population, but only the relative success of different phenotypes within a population leads to changes in allele frequencies—that is, to evolution. To contribute genes to subsequent generations, individuals must survive to reproductive age and produce offspring. The relative contribution of individuals of a particular phenotype is determined by the probability that those individuals survive multiplied by the average number of offspring they produce over their lifetimes. In other words, the fitness of a phenotype is determined by the average rates of survival and reproduction of individuals with that phenotype.

Was this article helpful?

0 0
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Post a comment