It is tempting to see evolution as a grand progressive ladder with Homo sapiens emerging at the top. But evolution produces a tree, not a ladder — and we are just one of many twigs on the tree. Populations, not individual organisms, evolve. Changes in an individual over the course of its lifetime may be developmental e. New gene variants i. We have many examples of slow and steady evolution — for example, the gradual evolution of whales from their land-dwelling, mammalian ancestors, as documented in the fossil record.
But we also know of many cases in which evolution has occurred rapidly. For example, we have a detailed fossil record showing how some species of single-celled organism, called foraminiferans, evolved new body shapes in the blink of a geological eye, as shown here. And since humans often cause major changes in the environment, we are frequently the instigators of evolution in other organisms. Here are just a few examples of human-caused evolution for you to explore: — Several species have evolved in response to climate change.
Genetic drift occurs because, due to chance, the individuals that reproduce may not exactly represent the genetic makeup of the whole population. For example, in one generation of a population of captive mice, brown-furred individuals may reproduce more than white-furred individuals, causing the gene version that codes for brown fur to increase in the population — not because it improves survival, just because of chance.
The same process occurs in large populations: some individuals may get lucky and leave many copies of their genes in the next generation, while others may be unlucky and leave few copies. However, in large populations, the changes in gene frequency from generation to generation tend to be small, while in smaller populations, those shifts may be much larger.
Whether its impact is large or small, genetic drift occurs all the time, in all populations. To learn more about genetic drift , visit Evolution To learn more about population size as it relates to genetic drift , visit this advanced article. We have invented medical treatments, agricultural practices, and economic structures that significantly alter the challenges to reproduction and survival faced by modern humans.
So, for example, because we can now treat diabetes with insulin, the gene versions that contribute to juvenile diabetes are no longer strongly selected against in developed countries.
However, this is not the case. Humans still face challenges to survival and reproduction, just not the same ones that we did 20, years ago. The direction, but not the fact of our evolution has changed. For example, modern humans living in densely populated areas face greater risks of epidemic diseases than did our hunter-gatherer ancestors who did not come into close contact with so many people on a daily basis — and this situation favors the spread of gene versions that protect against these diseases.
Scientists have uncovered many such cases of recent human evolution. That definition of a species might seem cut and dried — and for many organisms e. For example, many bacteria reproduce mainly asexually. How can the biological species concept be applied to them? Many plants and some animals form hybrids in nature, even if they largely mate within their own groups. Should groups that occasionally hybridize in selected areas be considered the same species or separate species?
The concept of a species is a fuzzy one because humans invented the concept to help get a grasp on the diversity of the natural world. It is difficult to apply because the term species reflects our attempts to give discrete names to different parts of the tree of life — which is not discrete at all, but a continuous web of life, connected from its roots to its leaves.
To learn more about the biological species concept , visit Evolution To learn about other species concepts , visit this side trip. Natural selection naturally results from genetic variation in a population and the fact that some of those variants may be able to leave more offspring in the next generation than other variants. Instead, resistance evolves because random mutation happens to generate some individuals that are better able to survive the antibiotic, and these individuals can reproduce more than other, leaving behind more resistant bacteria.
If a population happens to have genetic variation that allows some individuals to survive a challenge better than others or reproduce more than others, then those individuals will have more offspring in the next generation, and the population will evolve. If that genetic variation is not in the population, the population may survive anyway but not evolve via natural selection or it may die out.
Of course, some species may possess traits that allow them to thrive under conditions of environmental change caused by humans and so may be selected for, but others may not and so may go extinct. For example, as climate change causes the Arctic sea ice to thin and break up earlier and earlier, polar bears are finding it more difficult to obtain food.
However, this impression is incorrect. Natural selection has no foresight or intentions. Theoretically, in fact, a trait that is advantageous to the individual e. The more fertile offspring an organism leaves in the next generation, the fitter it is.
For example, a puny male bird with bright tail feathers might leave behind more offspring than a stronger, duller male, and a spindly plant with big seed pods may leave behind more offspring than a larger specimen — meaning that the puny bird and the spindly plant have higher evolutionary fitness than their stronger, larger counterparts.
To learn more about evolutionary fitness , visit Evolution In most populations, organisms with many different genetic variations survive, reproduce, and leave offspring carrying their genes in the next generation. This is apparent in the populations around us: for example, a plant may not have the genes to flourish in a drought, or a predator may not be quite fast enough to catch her prey every time she is hungry. For example, living things are made up of traits resulting from a complicated set of trade-offs — changing one feature for the better may mean changing another for the worse e.
And of course, because organisms have arisen through complex evolutionary histories not a design process , their future evolution is often constrained by traits they have already evolved.
For example, even if it were advantageous for an insect to grow in some way other than molting, this switch simply could not happen because molting is embedded in the genetic makeup of insects at many levels.
To learn more about the limitations of natural selection , visit our module on misconceptions about natural selection and adaptation.
Some may be the chance results of history. There is nothing special about the relationship between GGC and glycine. Others traits may be by-products of another characteristic. For example, the color of blood is not adaptive. To read more about explanations for traits that are not adaptive , visit our module on misconceptions about natural selection and adaptation. To learn more about what traits are adaptations , visit another page in the same module. Organisms that share a more recent branching point i.
For example, on the tree here, taxon A is adjacent to B and more distant from C and D. However, taxon A is equally closely related to taxa B, C, and D. Similarly, in the tree below, taxon B is adjacent to taxon A, but taxon B is actually more closely related to taxon D. The following phylogenies are all equivalent. Even though each phylogeny below has a different order of taxa at the tips of the tree, each portrays the same pattern of branching. These are value judgments that have no place in science.
One form of a trait may be ancestral to another more derived form, but to say that one is primitive and the other advanced implies that evolution entails progress — which is not the case.
For more details , visit our misconception on this topic. For example, on the tree below, taxon D may be more or less specialized than taxa A, B, and C. Unfortunately, students may assume that all traits follow this simple model, and that is not the case.
Both quantitative e. In terms of evolution, this misconception can be problematic when students are learning about Hardy-Weinberg equilibrium and population genetics. Students may need frequent reminders that traits may be influenced by more than one locus and that these loci may not involve simple dominance.
Because students may not have made connections between Mendelian genetics and the molecular structure of DNA , they may not realize that many different alleles may be present at a locus and so may assume that all traits are influenced by only two alleles. This misconception may be reinforced by the fact that students usually focus on diploid genetic systems and by the use of upper and lowercase letters to represent alleles. The use of subscripts to denote different alleles at a locus as well as frequent reminders that loci may have more than two alleles can help correct this misconception.
First, many scientific investigations do not involve experiments or direct observation. Explanation: The fossil record contains many 'gaps'. David Drayer. No one knows why or if gaps truly exists. Explanation: Darwinian Evolution is a naturalistic theory based on the principles of uniform process. Related questions Why is the fossil record an incomplete history of life?
How does the fossil record corroborate phylogenetic history? How does the fossil record support the theory of continental drift?
Why are there gaps in the fossil record? How many mass extinctions are recorded in the fossil record? How does the fossil record support Darwin's theories? Why does the presence of extinct and transitional forms in the fossil record support the pattern What conclusion did George Cuvier come to after his inspection of the fossil record? Whereas previous remnants were dated to million years ago, this new evidence is 2.
They were discovered in Print Email Share. Boy or Girl? You Need a Chickadee Brain. Living Well. View all the latest top news in the environmental sciences, or browse the topics below:. Keyword: Search.
0コメント