Mutations can be harmful, neutral, or sometimes helpful, resulting in a new, advantageous trait. When mutations occur in germ cells eggs and sperm , they can be passed on to offspring. If the environment changes rapidly, some species may not be able to adapt fast enough through natural selection. Through studying the fossil record, we know that many of the organisms that once lived on Earth are now extinct.
Dinosaurs are one example. An invasive species , a disease organism, a catastrophic environmental change, or a highly successful predator can all contribute to the extinction of species. Today, human actions such as overhunting and the destruction of habitats are the main cause of extinctions. Extinctions seem to be occurring at a much faster rate today than they did in the past, as shown in the fossil record.
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Text on this page is printable and can be used according to our Terms of Service. Any interactives on this page can only be played while you are visiting our website. You cannot download interactives. The theory of natural selection was explored by 19th-century naturalist Charles Darwin. Natural selection explains how genetic traits of a species may change over time. 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. Adrian explains, 'If you took 1, 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. 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. An adaptation is a physical or behavioural characteristic that helps an organism to survive in its environment. Max Barclay, Senior Curator in Charge of Coleoptera, explains how bombardier beetles get themselves out of trouble.
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 Crescentia cujete.
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, years ago, so the fruit's adaptation no longer has a survival benefit. The large, spherical calabash fruit has an extremely tough exterior. 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.
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 Biston betularia 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.
Darwin collected many animal specimens during the voyage of HMS Beagle The birds sit within the same taxonomic family and have a diverse array of beak sizes and shapes. These correspond to both their differing primary food sources and divergence due to isolation on different islands. The green warbler-finch Certhidea olivacea , for example, has a sharp, slender beak which is perfect for feeding on small insects.
In comparison, the large ground finch Geospiza magnirostris has a short, stocky beak to crack seeds and nuts. Darwin's finches are often thought of as inspiring a 'eureka moment', but it was actually mockingbirds that impacted Darwin's thoughts on evolution.
But on nearby Floreana Island he saw that the mockingbirds were considerably different. Darwin realised that differences between species of mockingbird on the islands were greater than between those he'd seen across the continent. He began contemplating while aboard HMS Beagle, but it took several years before he came up with his theory of evolution by natural selection.
The finches - once they had been identified as different species by the British ornithologist John Gould - became one useful example among the many other animals he saw. The finches are of scientific interest today. Both species' beaks have been seen to shrink over time, but followed different patterns. Darwin thought that natural selection progressed slowly and only occurred over a long period of time. Figure 1: A phylogenetic "tree of life" constructed by computer analysis of cyochrome c molecules in the organisms shown; there are as many different trees of life as there are methods of analysis for constructing them.
However, by the 19th Century, a number of natural historians were beginning to think of evolutionary change as an explanation for patterns observed in nature. The following ideas were part of the intellectual climate of Darwin's time. This habit sustained for long, has had the result in all members of its race that the forelegs have grown longer than the hind legs and that its neck has become so stretched, that the giraffe, without standing on its hind legs, lifts its head to a height of six meters.
In essence, this says that the necks of Giraffes became long as a result of continually stretching to reach high foliage. Larmarck was incorrect in the hypothesized mechanism, of course, but his example makes clear that naturalists were thinking about the possibility of evolutionary change in the early 's. Darwin's Theory. Species populations of interbreeding organisms change over time and space. The representatives of species living today differ from those that lived in the recent past, and populations in different geographic regions today differ slightly in form or behavior.
These differences extend into the fossil record, which provides ample support for this claim. All organisms share common ancestors with other organisms. Over time, populations may divide into different species, which share a common ancestral population. Far enough back in time, any pair of organisms shares a common ancestor. For example, humans shared a common ancestor with chimpanzees about eight million years ago, with whales about 60 million years ago, and with kangaroos over million years ago.
Shared ancestry explains the similarities of organisms that are classified together: their similarities reflect the inheritance of traits from a common ancestor.
Since then, biologists and paleontologists have documented a broad spectrum of slow to rapid rates of evolutionary change within lineages. T he primary mechanism of change over time is natural selection, elaborated below. This mechanism causes changes in the properties traits of organisms within lineages from generation to generation.
This process is natural selection. The traits that confer an advantage to those individuals who leave more offspring are called adaptations. In order for natural selection to operate on a trait, the trait must possess heritable variation and must confer an advantage in the competition for resources. If one of these requirements does not occur, then the trait does not experience natural selection. Natural selection operates by comparative advantage, not an absolute standard of design.
Natural selection can only work on existing variation within a population. Such variations arise by mutation, a change in some part of the genetic code for a trait. Mutations arise by chance and without foresight for the potential advantage or disadvantage of the mutation. In other words, variations do not arise because they are needed. Let's look at an example to help make natural selection clear. Industrial melanism is a phenomenon that affected over 70 species of moths in England.
It has been best studied in the peppered moth, Biston betularia. Prior to , the typical moth of the species had a light pattern see Figure 2.
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