What Is Natural Selection In Biology

What Does ‘survival Of The Fittest’ Mean

In terms of evolution, an animal that is ‘fit’ is one that is adapted to its environment. This concept is at the core of natural selection, although the term ‘survival of the fittest’ has often been misunderstood and may be best avoided.

There is also a degree of randomness to evolution, so the best-adapted animal won’t always be the one to survive.

Adrian explains, ‘If you’re going to get hit by a rock or something, it’s just bad luck. But on average and over time, the ones that survive are the ones that are fittest – the ones that have the best adaptations.’

Peppered moths are difficult to see when they perch on tree bark. Those that blend in best are less likely to be preyed on, so have advantage for survival.

Other Types Of Natural Selection

Natural selection can come in an infinite variety of forms. Every organism will be more or less successful depending on what genes it carries and how those genes interact with the environment. Genes can cause novel ways of processing nutrients, allow for different structures to be formed, and allow for old structures to be repurposed. Completely different organisms occupying the same niche are often found to have similar structures. These structures were not obtained from a common ancestor but from the forces of natural selection alone. Natural selection is the primary driving force behind all the different forms and functions of life on Earth.

How Does Natural Selection Occur

Individuals who are well suited to their environments are more likely to survive and reproduce.

It follows that individuals who are not well suited to an environment will not be able to survive and reproduce, and their frequency will dwindle. Over time, the population becomes fitter and more adapted to its environment.

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Social And Psychological Theory

The social implications of the theory of evolution by natural selection also became the source of continuing controversy. Friedrich Engels, a German political philosopher and co-originator of the ideology of communism, wrote in 1872 that “Darwin did not know what a bitter satire he wrote on mankind, and especially on his countrymen, when he showed that free competition, the struggle for existence, which the economists celebrate as the highest historical achievement, is the normal state of the animal kingdom.” Herbert Spencer and the eugenics advocate Francis Galton‘s interpretation of natural selection as necessarily progressive, leading to supposed advances in intelligence and civilisation, became a justification for colonialism, eugenics, and social Darwinism. For example, in 1940, Konrad Lorenz, in writings that he subsequently disowned, used the theory as a justification for policies of the Nazi state. He wrote “… selection for toughness, heroism, and social utility … must be accomplished by some human institution, if mankind, in default of selective factors, is not to be ruined by domestication-induced degeneracy. The racial idea as the basis of our state has already accomplished much in this respect.” Others have developed ideas that human societies and culture evolve by mechanisms analogous to those that apply to evolution of species.

What Happens When A Population Becomes Too Small

In contrast, a small population with slight variation might not survive such changes. It may be that there simply isnt an individual that possesses a phenotype that suits the new conditions.

Chance can also affect which alleles get passed onto the next generation in a small population over time, some alleles may be lost or favoured randomly. Genetic drift is the gradual change in allele frequencies in a small population due to chance rather than natural selection. This is why smaller populations are much more vulnerable to sudden changes in their environment. The smaller a population becomes, the smaller its chances of survival.

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But not all characteristics of an animal are adaptations.

Adaptations for one purpose can be co-opted for another. For instance, feathers were an adaptation for thermoregulation – their use for flight only came later. This means that feathers are an exaptation for flight, rather than an adaptation.

Adaptations can also become outdated, such as the tough exterior of the calabash fruit . This gourd is generally thought to have evolved to avoid being eaten by Gomphotheres, a family of elephant-like animals. But these animals went extinct around 10,000 years ago, so the fruit’s adaptation no longer has a survival benefit.

The large, spherical calabash fruit has an extremely tough exterior. But this adaptation is now outdated © Wendy Cutler via Flickr

Selection for adaptation is not the only cause of evolution. Species change can also be caused by neutral mutations that have no detriment or benefit to an individual, genetic drift or gene flow.

Gray Vs Green Treefrogs

In a wooded area, gray treefrogs and green treefrogs share a common habitat the bark of the tree- and an ecological niche. Snakes and birds are the predators of the treefrogs. On the bark of the tree, gray treefrogs blend better than green treefrogs. So, green treefrogs are more visible for predators to find and likely to be eaten. Over time gray treefrogs produce more offspring that are less likely to be eaten. In this case, natural selection has favored treefrogs that live in habitats in which they are more camouflaged. Besides, there are many ways natural selection shapes organisms.

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Is A Human Lifetime Long Enough For Somatic Cells To Evolve Complex Adaptations

One of the arguments against applying an adaptationist framework to cancer cells is that the scope of resulting adaptation may be limited because the human lifetime is too short for complex cell adaptations to evolve de novo . In this section, we examine the validity of this claim by reviewing the literature on the pace of evolutionary change in neoplasms. The rate of evolutionary change depends critically on parameters such as mutation rate, population size, and population turnover rate . Considering these factors suggests that somatic cell evolution can be much more rapid than evolution among multicellular organisms. Many cancer cells have the capacity to divide daily, evolve over a period of decades, and comprise populations numbering in the billions to trillions . Based solely on population size and generation time, there are more reproductive events among the cells within one host individual than there have been among individuals in the entire history of the human species. Each such reproductive event is an opportunity for mutation and selective reproduction and, thus, for adaptive evolution.

Why Is Natural Selection So Difficult To Understand

Two obvious hypotheses present themselves for why misunderstandings of natural selection are so widespread. The first is that understanding the mechanism of natural selection requires an acceptance of the historical fact of evolution, the latter being rejected by a large fraction of the population. While an improved understanding of the process probably would help to increase overall acceptance of evolution, surveys indicate that rates of acceptance already are much higher than levels of understanding. And, whereas levels of understanding and acceptance may be positively correlated among teachers , the two parameters seem to be at most only very weakly related in students . Teachers notwithstanding, it appears that a majority on both sides of the evolution-creation debate do not understand the process of natural selection or its role in evolution .

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How Does Natural Selection Work

In natural selection, genetic mutations that are beneficial to an individual’s survival are passed on through reproduction. This results in a new generation of organisms that are more likely to survive to reproduce.

For example, evolving long necks has enabled giraffes to feed on leaves that others can’t reach, giving them a competitive advantage. Thanks to a better food source, those with longer necks were able to survive to reproduce and so pass on the characteristic to the succeeding generation. Those with shorter necks and access to less food would be less likely to survive to pass on their genes.

The evolution of a long neck is an adaptation that helps giraffes survive in their environment © FluffyCreature via Flickr

Adrian explains, ‘If you took 1,000 giraffes and measured their necks, they’re all going to be slightly different from one another. Those differences are at least in part determined by their genes.

‘The ones with longer necks may leave proportionally more offspring, because they have fed better and have maybe been better in competing for mates because they are stronger. Then, if you were to measure the necks of the next generation, they’re also going to vary, but the average will have shifted slightly towards the longer ones. The process carries on generation after generation.’

Selection: Survival And Reproduction

Environments cannot support unlimited populations. Because resources are limited, more organisms are born than can survive: some individuals will be more successful at finding food, mating or avoiding predators and will have a better chance to thrive, reproduce, and pass on, their DNA. Small variations can influence whether or not an individual lives and reproduces. Differences in color, for instance, aid some individuals in camouflaging themselves from predators. Sharper eyes and claws help an eagle catch its dinner. And brighter coloration improves a male peacock’s chances of attracting a mate.

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History Of Evolutionary Biology

In the 1800s, many people were trying determine why there were so many different kinds of plants and animals in the world. Charles Darwin wondered about the diversity of animals he saw while in the Galapagos Islands. This led him to develop the theory of natural selection, which is the best explanation we have for the diversity of life. Alfred Russell Wallace also hypothesized that the environment could help to shape the diversity of life by favoring certain traits over others. Wallace noticed that insects in the jungles of Africa and South America were very well adapted to unique environments. These two men, working independently of one another, developed the same basic explanation for the diversity of life: natural selection. These principles are supported by current scientific research.

Mechanisms Of Natural Selection:

1.Variation

o Within a population, we see a wide variety of traits and differences among the same species.

o All of the traits and phenotypes are due to differences in DNA.

• These differences in DNA arise due to MUTATION. A mutation is any change in an organisms DNA it can be negative, positive, or neutral.

• Mutation is caused by spontaneous switching of nucleotide bases in the DNA.
• Mutation can also be caused by chromosome abnormalities i.e. missing parts, moved sections, or inability to separate during cell division.

See 2-2 notes for more information.

Variation and Sexual reproduction:

• Sexually reproductive animals have the benefit of a greater genetic diversity.

• An offspring gets one set of chromosomes from the mother and another from the father.

• During meiosis there are many mechanisms which increase the variation in each sex cell as well. This makes billions of combinations possible for each sperm and egg.
• Without variation, populations could not survive changing environments. Natural selection selects the best adapted to survive leading others to die off. If they all have the same DNA, the population could not survive the same change.

More on meiosis in unit 3!

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Type : Directional Selection

Directional selection is a type of natural selection that occurs when one side of the spectrum of a certain trait is favored over the other. For instance, if the smallest organisms get eaten, and larger organisms are totally protected, the population will tend to get much larger. If the opposite is true, the population will decrease in size over time.

It is also using directional selection artificially that humans can create miniature breeds of animals, which look like tiny copies of their larger counterparts. However, artificial selection only focuses on a single trait. This allows many negative traits to become present in the population, which would have naturally been selected against.

Types Of Natural Selection

As animals diversify and fill different niches, the pressures exerted on them can change in many ways. The functional requirements to be a bird are much different than those required to be a fish. Their food is different, the medium they exist in is different, and they must obtain oxygen in a different way. Natural selection thus selects for very different looking animals to fill the different niches of the ecosystem.

Regardless of the trait, natural selection tends to do one of three things to a population. It can keep the trait the same, stabilizing selection, move the trait in one direction, directional selection, or select for the extreme values of the trait, diversifying selection. In addition to being classified by the effects it causes, natural selection can also be classified by the relationships of the organisms causing the natural selection, and sometimes the selection can be made by abiotic factors.

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It Acts On An Organisms Physical Characteristics

The theory of natural selection has to do with an organisms physical body. Humans, for example, are believed by scientists and many others, to have descended from primates. Our coccyx, otherwise known as the tailbone, is a remnant we have from our days as a primate, and is our vestigial tail.

When primates descended from the trees and began walking upright, a tail was no longer needed for them to balance and get around. Humans have thus evolved to have no tail. The theory goes that, those early ancestors of homo sapiens who stopped developing tails were the ones to survive and reproduce. In this way, they were part of natural selection.

The Basis And Basics Of Natural Selection

Though rudimentary forms of the idea had been presented earlier , it was in On the Origin of Species by Means of Natural Selection that Darwin provided the first detailed exposition of the process and implications of natural selection. According to Mayr , Darwin’s extensive discussion of natural selection can be distilled to five facts and three associated inferences. These are depicted in Fig. .

Fig. 1

The basis of natural selection as presented by Darwin , based on the summary by Mayr

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Are We Still Evolving

For millennia, the world was viewed as static. The ideas that mountains could rise, and climate and organisms could change didn’t exist. Earth was thought to exist in an optimal form.

But natural selection relies on the fact that the world is constantly changing. Evolution occurs automatically for survival and for millions of years it has been playing catch-up with our dynamic world.

Poaching and habitat loss have had huge impacts on the now critically endangered saiga antelope . Natural selection stands little chance in cases like this. ©Andrey Giljov via Wikimedia Commons

‘Organisms are either adapted enough to survive and reproduce, or they are sub-optimal and the population shrinks. It may even shrink to zero, and that means extinction,’ states Adrian.

Scientists have been able to predict natural selection over short terms. But it is almost impossible to accurately determine its effects in the future due to unpredictable fluctuations of the environment.

Natural selection implies that if organisms are surviving, they are adapted. But as the environment changes, we may find that what was once an adaptation may no longer be useful.

Although it is possible for evolution to occur quickly, the more rapidly the planet changes, the harder it is for evolution to keep pace and the more serious the risk of a massive rise in extinctions becomes.

The Theory Was Popularized By Charles Darwin

The theory of natural selection is associated with Charles Darwin. Darwin was an English scholar who focused his work in geology, biology and the natural world.

He famously traveled to the Galápagos Islands in 1832 and lived there for five years.

During that time, he watched the natural world, while collecting specimens and studying them. Darwin went on to collect data about the natural world for twenty more years and in 1859, his groundbreaking book, On the Origin of Species, was published. This shared his ideas about evolution with the world.

Darwin was the first person to state that all the species on Earth have descended from common ancestors. As such, he is seen as the godfather of evolution, of which his theory of natural selection is a cornerstone.

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Events And Absolutes Versus Processes And Probabilities

A proper understanding of natural selection recognizes it as a process that occurs within populations over the course of many generations. It does so through cumulative, statistical effects on the proportion of traits differing in their consequences for reproductive success. This contrasts with two major errors that are commonly incorporated into naïve conceptions of the process:

Do Cancer Cells Evolve Novel Traits

One of the classic conceptual challenges to adaptationist thinking that has reappeared in cancer biology is the question of novelty, or the distinction between new adaptations versus repurposed exaptations that originally served a different function . Many traits of cancer are novel for the cell types in which the cancer arises but are normally expressed in other cell types or at other stages of development, from the same normal genome. In neoplasms, genes and traits that originally functioned in multicellularity are often hijacked, or repurposed for competition among cells. For example, the seemingly novel cancer phenotypes of rapid proliferation, motility, and tissue invasion often result from activation of functions that are normally expressed only by embryonic stem cells and are built into the normal human genome for that context .

Another dramatic example of a complex cancer cell behavior that appears novel in its repurposed context of cancer progression is cell cannibalism, or consuming and digesting other cells . This cell behavior is normal only for lymphocytes and is presumably enabled by normal human genes that are abnormally expressed in cancer cells.

Many other traits that evolve in cancers also are novel only in their abnormal context, such as shifting to a glycolytic metabolism in the presence of plentiful oxygen . Normal cells can shift to a glycolytic metabolism under oxygen deprivation, but constitutive expression is novel to cancer cells .

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