Evolution why does it happen

Such individuals were more successful and had more offspring. In later generations, more genetic changes occurred, moving the nose farther back on the head. Other body parts of early whales also changed. Front legs became flippers. Back legs disappeared. Their bodies became more streamlined, and they developed tail flukes to better propel themselves through water. Darwin also described a form of natural selection that depends on an organism's success at attracting a mate — a process known as sexual selection.

The colorful plumage of peacocks and the antlers of male deer are both examples of traits that evolved under this type of selection. But Darwin wasn't the first or only scientist to develop a theory of evolution. Around the same time as Darwin, British biologist Alfred Russel Wallace independently came up with the theory of evolution by natural selection, while French biologist Jean-Baptiste Lamarck proposed that an organism could pass on traits to its offspring, though he was wrong about some of the details.

Like Darwin, Lamarck believed that organisms adapted to their environments and passed on those adaptations. He thought organisms did this by changing their behavior and, therefore, their bodies — like an athlete working out and getting buff — and that those changes were passed on to offspring. For example, Lamarck thought that giraffes originally had shorter necks but that, as trees around them grew taller, they stretched their necks to reach the tasty leaves and their offspring gradually evolved longer and longer necks.

Lamarck also believed that life was somehow driven to evolve through the generations from simple to more complex forms, according to Understanding Evolution , an educational resource from the University of California Museum of Paleontology. Though Darwin wasn't sure of the mechanism by which traits were passed on, he did not believe that evolution necessarily moved toward greater complexity, according to Understanding Evolution; rather, he believed that complexity arose through natural selection.

A Darwinian view of giraffe evolution, according to Quanta , would be that giraffes had natural variation in their neck lengths, and that those with longer necks were better able to survive and reproduce in environments full of tall trees, so that subsequent generations had more and more long-necked giraffes. The main difference between the Lamarckian and Darwinian ideas of giraffe evolution is that there's nothing in theDarwinian explanation about giraffes stretching their necks and passing on an acquired characteristic.

Darwin didn't know anything about genetics, Pobiner said. That came later, with the discovery of how genes encode different biological or behavioral traits, and how genes are passed down from parents to offspring.

The incorporation of genetics into Darwin's theory is known as "modern evolutionary synthesis. The physical and behavioral changes that make natural selection possible happen at the level of DNA and genes within the gametes, the sperm or egg cells through which parents pass on genetic material to their offspring. Such changes are called mutations. Mutations can be caused by random errors in DNA replication or repair, or by chemical or radiation damage.

Usually, mutations are either harmful or neutral, but in rare instances, a mutation might prove beneficial to the organism. I wrote a previous book about emerging diseases, like Ebola, and much work has been done on that at Porton Down.

The particular bug is called shigella flexneri , and was isolated from a British soldier named Ernest Cable in during WWI, when he died in a hospital in France from dysentery, which killed a lot of soldiers.

This specimen was tucked away at Porton Down until about a decade ago when one tube of the samples was pulled out, thawed, regrown in a laboratory by a team of scientists led by one Kate Baker, and its genome examined. Lo and behold!

One of the things they found out about this sample of bacteria was that it was already resistant to penicillin. But here was a bug killing a British soldier back in that already had resistance to that anti-bacterial substance, penicillium.

Horizontal gene transfer is essentially sideways heredity. It can even go from one kingdom of life into another, sideways, across great barriers. That was thought to be undoable. In fact, genes can go sideways across vast species boundaries. For instance, a gene for resistance to one kind of antibiotic in one form of bacteria, like staphylococcus, can move sideways into another, completely different form of bacteria, say, E.

This can happen not just in bacteria but also in animals, plants, and higher organisms, generally as a result of infection or parasitism. One example is a form of transposon.

Big, complicated word. But scientists have discovered that these things can also jump from one creature to another, and even from one species to another. One transposon has been given the name space invaders. It seems able to pass, for instance, from a reptile into an insect or from a possum into a rat, by way of something called a kissing bug, an insect that, when sucking blood, sucks in some of this transposon.

The transposon then moves from one species to another and becomes part of the heritable genome of that new species. There are two phases in classic Darwinian evolution. First, there is the arising of variations from one creature to another or one individual population to another. That was thought to occur incrementally, in very slow stages, by mutations in the genome.

Once there are variations among individuals, natural selection, the survival of the fittest, acts upon those variations. But horizontal gene transfer has revealed that nature does sometimes make leaps, whereby huge lumps of DNA can appear in an individual or population quite suddenly and then natural selection acts on them.

That can be a very important mechanism in the evolution of new species. We now realize, because we can sequence genomes, that we have great populations of bacteria living within us. Genes are the portions of an organism's DNA that carry the code responsible for building that organism in a very specific way.

Genes -- and, thus, the traits they code for -- are passed from parent to offspring. From generation to generation, well-understood molecular mechanisms reshuffle, duplicate, and alter genes in a way that produces genetic variation.

This variation is the raw material for evolution. Learn More Evolution Since Darwin. Sexual reproduction allows an organism to combine half of its genes with half of another individual's genes, which means new combinations of genes are produced every generation.

In addition, when eggs and sperm are produced, genetic material is shuffled and recombined in ways that produce new combinations of genes. Sexual reproduction thus increases genetic variation, which increases the raw material on which natural selection operates. Genetic variation within a species -- also known as genetic diversity -- increases a species' opportunity for change over successive generations.

Learn More The Advantage of Sex. Evolution is not a random process. The genetic variation on which natural selection acts may occur randomly, but natural selection itself is not random at all. The survival and reproductive success of an individual is directly related to the ways its inherited traits function in the context of its local environment. Whether or not an individual survives and reproduces depends on whether it has genes that produce traits that are well adapted to its environment.

Learn More Life's Grand Design. Evolution and "survival of the fittest" are not the same thing. Evolution refers to the cumulative changes in a population or species through time. Natural selection is the process of slow accumulation of helpful mutations over generations, causing organisms to become better adapted to their environments. Organisms that are less well-adapted will have more trouble surviving than will organisms whose mutations have helped them adapt.

When mutations produce significant changes in an organism, they can lead to the development of a new species [ 1 ].

The classical way to demonstrate that species change over time is through the fossil record. Fossils show how primitive life formed and, if we find enough fossils, it is possible to observe how an organism has evolved over time. However, if the fossil record is incomplete, it is impossible to see all of these changes or to figure out how the organism evolved. To solve this problem, it would be easier to study evolution directly, by examining all of the genetic changes that happen in each generation.

To do this, researchers must first find an ideal organism to study. Bacteria are an obvious choice, because they are easy to cultivate in the laboratory and they reproduce quickly. Escherichia coli is a widely studied bacterium that inhabits the digestive system of humans and other warm-blooded animals Figure 2.

Just to compare, if we wanted to do an evolutionary experiment with humans, we would need to wait, on average, 26 years to have a new generation, while E. Additionally, E. In the sections that follow, we will describe some of the experiments scientists have performed, using E.

By studying E. To study this, they devised an experiment in which E. The result? The survival of E. When scientists studied the genome of these evolved bacteria, they found mutations that disabled a gene that plays a role in the production of cardiolipin, a molecule that makes the outer covering of the bacterial cell sturdy and rigid.

Further investigations proved that turning off the production of cardiolipin helped to keep the covering fluid and flexible after freezing, which increased survival [ 3 ]. The most ambitious study of evolution was carried out by a scientist named Richard Lenski. Lenski kept E. In this culture medium, the bacteria grew rapidly but depleted the nutrients just as quickly.

He repeated this process again and again.