Biology by Karl Irvin Baguio (the ebook reader TXT) 📕

leaves a population and establishes a new one in a geographically isolated region. For example, when a small population of fish is placed in a lake, the fish population will evolve into one that is different from the original. Fitness of a population is not considered in genetic drift, nor does genetic drift occur in a very large population.

 

Natural selection

 

Another mechanism for evolution is natural selection, which occurs when populations of organisms are subjected to the environment. The fittest creatures are more likely to survive and pass their genes to their offspring, producing a population that is better adapted to the environment. The genes of less-fit individuals are less likely to be passed on to the next generation. The important selective force in natural selection is the environment.

 

Environmental fitness may be expressed in several ways. For example, it may involve an individual’s ability to avoid predators, it may imply a greater resistance to disease, it may enhance ability to obtain food, or it may mean resistance to drought. Fitness may also be measured as enhanced reproductive ability, such as the ability to attract a mate. Better-adapted individuals produce relatively more offspring and pass on their genes more successfully than less-adapted individuals.

 

Several types of natural selection appear to affect populations. One type, stabilizing selection, occurs when the environment selects against organisms of a population with extreme versions of a trait. Another type of natural selection is disruptive selection. Here, the environment favors extreme types in a population at the expense of intermediate forms, thereby splitting the population into two or more subpopulations. A third type of natural selection is directional selection. In this case, the environment selects for an extreme characteristic. The development of antibiotic-resistant bacteria in the modern era is an example of directional selection.

Species development

 

A species is a group of individuals that share a number of features and are able to interbreed with one another, producing fertile (non-sterile) offspring. (When individuals of one species mate with individuals of a different species, any offspring are usually sterile.) A species is also defined as a population whose members share a common gene pool.

The evolution of a species is speciation, which can occur when a population is isolated by geographic barriers, such as occurred in the isolation of Australia, New Zealand, and the Galapagos Islands. The variety of life forms found in Australia but nowhere else is an example of speciation by geographic barriers.

Speciation can also occur when reproductive barriers develop. For example, when members of a population develop anatomical barriers that make mating with other members of the population difficult, a new species can develop. The timing of sexual activity is another example of a reproductive barrier. Spatial difference, such as one species inhabiting treetops while another species lives at ground level, is another reason why species develop.

 

Gradual versus rapid change

 

Darwin’s theory included the observation that evolutionary changes take place slowly. In many cases, the fossil record shows that a species changed gradually over time. The theory that evolution occurs gradually is known as gradualism.

 

In contrast to gradualism is the theory of punctuated equilibrium, which is a point of discussion among scientists. According to the theory of punctuated equilibrium, some species have long, stable periods of existence interrupted by relatively brief periods of rapid change.

 

Both groups of scientists agree that natural selection is the single most important factor in evolutionary changes in species. Whether the change is slow and gradual or punctuated and rapid, one thing is certain: Organisms have evolved over time.

 

Theory of Evolution

 

In his book The Origin of Species, Darwin presented evidence for his “descent with modification” theory, which has come down to us as the theory of evolution, although Darwin avoided the term “evolution.” Essentially, Darwin suggested that random variations take place in living things and that the environment selects those individuals better able to survive and reproduce. The method of selecting individuals is known as natural selection. The selected individuals are more likely to pass on their traits to their offspring, and the population continues to evolve.

 

Two essential points underlie natural selection. First, the genetic variations that take place in living things are random variations. Second, the genetic variations are small and cause little effect relative to a given population. Over time, these small genetic variations lead to the gradual development of a species rather than the sudden development of a species. Darwin proposed that variations appear without direction and without design. He assumed that among inherited traits, some traits were better than others. If an inherited trait provided an advantage over another, it would provide a reproductive advantage to the bearer of the trait. Thus, if long-necked giraffes could reach food better than short-necked giraffes, the long-necked giraffes would survive, reproduce, and yield a population consisting  of more long-necked giraffes.

 

As the central concept of Darwin’s theory of evolution, natural selection implies that the fittest survive and spread their traits through a population. This concept is called the survival of the fittest. The fitness implied is reproductive fitness; that is, the ability to survive in the environment and propagate the species. Natural selection serves as a sieve to remove the unfit from a population and allow the fittest to reproduce and continue the population. Today, scientists know that other factors also influence evolution.

Chapter 13: The Origin and Evolution of Life

  Origin of Cells

 

The appearance of the first cells marked the origin of life on Earth. However, before cells could form, the organic molecules must have united with one another to form more complex molecules called polymers. Examples of polymers are polysaccharides and proteins.

 

In the 1950s, Sidney Fox placed amino acids in primitive Earth conditions and showed that amino acids would unite to form polymers called proteinoids. The proteinoids were apparently able to act as enzymes and catalyze organic reactions.

 

More recent evidence indicates that RNA molecules have the ability to direct the synthesis of new RNA molecules as well as DNA molecules. Because DNA provides the genetic code for protein synthesis, it is conceivable that DNA may have formed in the primitive Earth environment as a consequence of RNA activity. Then DNA activity could have led to protein synthesis .

 

For a cell to come into being, some sort of enclosing membrane is required to hold together the organic materials of the cytoplasm. A generation ago, scientists believed that membranous droplets formed spontaneously. These membranous droplets, called protocells, were presumed to be the first cells. Modern scientists believe, however, that protocells do not carry any genetic information and lack the internal organization of cells. Thus the protocell perspective is not widely accepted. Several groups of scientists are currently investigating the synthesis of polypeptides and short nucleic acids on the surface of clay. The origin of the first cells remains a mystery.

 

Ancient Life

 

Earth came into existence about 4.6 billion years ago, and about 3.8 billion years ago, the chemical composition of Earth’s surface began to change. Scientists estimate that at about 3.5 billion years ago, the first cells were in existence.

 

Scientists believe that the first cells lived within the organic environment of Earth and used organic foods for their energy. The type of chemistry in those first cells was somewhat similar to fermentation, which uses organic molecules such as glucose. The energy yield, although minimal, is enough to sustain simple living things (see Chapter 6). However, organic material would soon have been used up if this were the sole source of nutrition, so a new process had to develop.

 

The evolution of a pigment system that could capture energy from sunlight and store it in chemical bonds was an essential evolutionary breakthrough. The organisms that possess these pigments are commonly referred to as cyanobacteria (at one time, they were called blue-green algae). These single-celled organisms produce carbohydrates by the process of photosynthesis. In doing so, they produce oxygen as a waste product (see Chapter 5). For about 1 billion years, photosynthesis provided oxygen to the atmosphere, which gradually changed until it became oxygen-rich, as it is today.

 

Another group of organisms that was present at the same time as the cyanobacteria was a group of bacteria called archae. Archae differ from other species of bacteria (known as kingdom eubacteria and representative of domain Bacteria) in that archae have a different ribosomal structure, different cell membrane composition, and different cell wall composition. The archae have been traced to a period of about 3 billion years ago. They are able to multiply at the very high temperatures that were present on Earth then, and their nutritional requirements reflect the composition of primitive Earth.

 

First Eukaryotes

 

The cyanobacteria and archae of primitive Earth are also referred to as prokaryotes (together with the eubacteria). Prokaryotes are discussed in Chapter 16. Approximately 1.5 billion years ago, in an oxygen-containing atmosphere, the first eukaryotes came into being. Eukaryotes have a nucleus, a nuclear membrane, a number of organelles, a ribosomal structure different from that of prokaryotes, cell division by mitosis, and other features that distinguish them from prokaryotes .

 

No one is certain how eukaryotes came into being. The endosymbiotic theory suggests that bacteria were engulfed by larger cells. The bacterial cells remained in the cell, assumed some of the chemical reactions for these cells, and became the mitochondria of these cells. The cells then reproduced and flourished, becoming animal cells.

 

An extension of the endosymbiotic theory refers to plants. In this case, pigmented bacteria, such as the cyanobacteria, were engulfed by larger cells. The cyanobacteria remained in the cells and became the chloroplasts of these cells. Photosynthesis occurs in the chloroplasts of plant cells and some protists.

 

Life on Land

 

For billions of years, the only life present on Earth existed in the nutrient-rich environments of the oceans, lakes, and rivers. About 600 million years ago, the Paleozoic Era began. Scientists believe that living things first came to occupy land during this era. They also believe that during a subdivision of the Paleozoic Era called the Cambrian Period, the main groups of marine invertebrates in existence today evolved. A so-called “Cambrian explosion” occurred. The appearance of multicellular organisms is notable in the Cambrian Period, when evolution by natural selection led to a vast array of organisms filling every conceivable niche on Earth. Many organisms that arose at that time have since become extinct.

 

After the Cambrian Period came the Ordovician Period. In the Ordovician Period, wormlike animals with stiff rods along their backs came into being. These organisms, now called chordates, include reptiles, amphibians, birds, and mammals. A backbone composed of vertebrae was first developed during the next period, the Silurian Period. During the Devonian Period that followed, bony fish developed. The first terrestrial plants also evolved at about this time.

 

The next most recent era after the Paleozoic Era is the Mesozoic Era, which began about 250 million years ago. During this era, reptiles such as the dinosaurs evolved and became the predominant life form on Earth. A mammal-like animal also evolved during this period. After the dinosaurs died out at the end of the Mesozoic Era, the mammal-like animal evolved into the modern mammals. Birds first appeared during the Mesozoic Era, and lush greenery spread over Earth.

 

During the Mesozoic Era, the continents existed as one landmass called Pangaea. Toward the end of the era, the landmass broke into smaller pieces, and the pieces