In social species like humans, this contribution can be indirect — for example, helping to raise your sibling's kids increases your own fitness. Longer survival may or may not increase fitness, because it depends on the timing and frequency of reproduction.
Fitness can also be measured as the overall reproductive rate of a given population. And most importantly: fitness is relative to the environment — an individual that has high fitness in one environment might have low fitness in a different environment. In an environment with infinite resources and minimal threats, most individuals would probably have the same fitness. Everyone would happily pass on their alleles, and the only cause of evolution would be random events.
But we do not live in a world with infinite resources. Individuals need to compete for limited resources in their environment, and whoever is best able to acquire those resources is more likely to survive and reproduce.
If there is a specific phenotype that helps individuals acquire and use resources more efficiently, individuals with that phenotype are likely to reproduce more than individuals with other phenotypes, and if it is heritable, that helpful phenotype will be more common in future generations. This change in frequency of a phenotype and change in allele frequencies for the underlying genes due to a fitness advantage is evolution by natural selection, otherwise known as adaptation.
One example of an adaptation in humans is the increased oxygen-carrying capacity of red blood cells in populations that live at high elevations.
This phenotype is heritable and evolved increased in frequency in direct response to natural selection, because individuals with higher oxygen saturation have higher fitness at high altitude. Whether or not a phenotype is an adaptation depends on a whether it confers a fitness advantage in its environment, and b whether its function evolved because of that fitness advantage.
There are cases where the function of a phenotype confers a fitness advantage, but the trait did not evolve as a result of natural selection for that function. Some traits affect fitness through multiple functions, but they are only considered to be an adaptation for the function that they originally evolved with. One example is the structure of feathers in birds. Evidence in the fossil record indicates that the earliest feathers to evolve were not used for flying, and it is now thought that feathers may be an adaptation to survive colder temperatures, because they provide insulation.
So although feathers are used for flight, and flight provides a fitness advantage in many environments, feathers are not an adaptation for flight. Traits like this whose current function differs from an ancestral function are known as exaptations.
Adaptation is a natural consequence of variation and differences in fitness, but it doesn't yield perfection. Phenotypic evolution is constrained by physical and genetic limitations. Some mutations increase fitness in one way but decrease fitness in a different way; this is an evolutionary trade-off.
For example, when predators are around, Trinidadian guppies are under selection to put more energy towards reproduction, but also under selection for fast-start swimming to evade predators. Increased reproductive capacity in females makes that fast-start more difficult, so there is a direct trade-off between those traits. Sexual selection, which is caused by preferences in mate choice, can also lead to trade-offs.
For example, in widowbirds, females have a preference for males with long tails, but for the males, making a longer tail comes at a cost — more energy. So why do the females care about a long tail? Why not choose a partner who is really good at acquiring resources?
The long tail is a signal that the male has been successful enough to have energy to spare on tail length, and will probably have successful offspring as well. Another important note on adaptation. Phenotypes are not created as a result of a specific need, but they can evolve as a result of that need. Random mutations create new phenotypes. A phenotype will persist in the population if it has no effect on fitness and remains in existence by random chance.
A phenotype will increase in frequency in a population if it increases the number of offspring that individuals contribute to the next generation. This distinction between evolution and creation is especially important in the context of the origin of life. Evolution does not attempt to explain the origin of life. It explains how life has changed after it originated. It also explains the origin of new species, most famously described by Darwin in On the Origin of Species. This population then had a much greater probability of avoiding extinction after a rapid and severe perturbation.
Journal Reference : G. Bell, A. Science , ; : DOI: ScienceDaily, 23 June McGill University. Evolution to the rescue: Species may adapt quickly to rapid environmental change, yeast study shows. Retrieved November 10, from www. Evolution is a gradual change to the DNA of a species over many generations.
It can occur by natural selection , when certain traits created by genetic mutations help an organism survive or reproduce. Such mutations are thus more likely to be passed on to the next generation, so they increase in frequency in a population.
Gradually, these mutations and their associated traits become more common among the whole group. By looking at global studies of our DNA, we can see evidence that natural selection has recently made changes and continues to do so. Though modern healthcare frees us from many causes of death, in countries without access to good healthcare, populations are continuing to evolve.
Survivors of infectious disease outbreaks drive natural selection by giving their genetic resistance to offspring. Our DNA shows evidence for recent selection for resistance of killer diseases like Lassa fever and malaria.
Selection in response to malaria is still ongoing in regions where the disease remains common. Humans are also adapting to their environment. Mutations allowing humans to live at high altitudes have become more common in populations in Tibet , Ethiopia , and the Andes. Throughout the group's existence, individual dinosaur species were evolving and going extinct.
Some species diverged and gave rise to other species, while others disappeared. A mass extinction event at the end of the Cretaceous period, 65 million years ago, ended the reign of dinosaurs on Earth.
Recently, many scientists have come to the conclusion that, while dinosaurs may have disappeared, one dinosaur lineage had evolved into birds long before the extinction event that wiped out the other dinosaurs -- and so, in a sense, dinosaurs are still around today.
Learn More History of Life. The oldest known hominid, or humanlike species, has been dated at 4. Another species, which is yet to be confirmed as a hominid, has been dated at 6 million years old. Scientists estimate that the hominid lineage diverged from the ape lineage 5 to 8 million years ago. Homo sapiens , the species to which we belong, has existed for about , years.
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