High school biology
|Evolution||The process by which modern organisms have descended from ancient organisms over time|
|Common ancestor||An ancestor shared by two or more descendant species|
|Fossil||Preserved remains of ancient organisms|
|Homologous structure||Structure that are similar in different species due to common ancestry|
|Vestigial structure||Structure that is non-functional, or reduced in function|
|Analogous structure||Structure that evolved independently in different organisms because the organisms lived in similar environments or experienced similar selective pressures|
|Embryology||The study of embryos and their development|
|Biogeography||The study of where organisms live currently, and where their ancestors lived in the past|
Evidence of evolution
Scientists who study evolution may want to know whether two present-day species are closely related. Evidence for evolution can be structural, genetic, or biogeographical.
Structural evidence for evolution
Observing anatomical features shared between organisms (including ones that are visible only during development) can indicate that they share a common ancestor.
Structural evidence can be compared between extant (currently living) organisms and the fossils of extinct organisms.
If two or more species share a unique physical trait they may all have inherited this trait from a common ancestor. Traits that are shared due to common ancestry are homologous structures.
For example, the forelimbs of whales, humans, and birds look different on the outside because they're adapted to function in different environments. However, if you look at the bone structure of the forelimbs, the organization of the bones is similar across species.
Embryology is important to understanding a species' evolution, since some homologous structures can be seen only in embryo development. For example, all vertebrate embryos, from humans to chickens to fish, have a tail during early development, even if that tail does not appear in the fully developed organism.
Vestigial structures serve little or no present purpose for an organism. The human tail, which is reduced to the tailbone during development, is one example. Vestigial structures can provide insights into an organism's ancestry. For instance, the tiny vestigial leg bones found in some snakes reflect that snakes had a four-legged ancestor.
While similar structure can indicate relatedness, not all structures that look alike are due to common ancestry.
Analogous structures evolved independently in different organisms because the organisms lived in similar environments or experienced similar selective pressure.
For example, the leg of a cat and the leg of a praying mantis are analogous. Both legs are used for walking, but they have separate evolutionary origins. On the outside, they appear similar because they have both experienced similar selection pressures that optimized them for walking. However, the actual structures that make up the leg are quite different, suggesting that the limbs are not due to a common ancestor.
DNA evidence for evolution
At the most basic level, all living organisms share the same genetic material (DNA), similar genetic codes, and the same basic process of gene expression (transcription and translation).
In order to determine which organisms in a group are most closely related, we need to use different types of molecular features, such as the nucleotide sequences of genes.
Biologists often compare the sequences of related (or homologous) genes. If two species have the "same" gene, it is because they inherited it from a common ancestor.
In general, the more DNA differences in homologous genes between two species, the more distantly the species are related.
Reading DNA gels
Segments of DNA can be analyzed using gel electrophoresis, in which fragments of DNA are separated by size.
Fragments are represented by horizontal bands. Bands that are similar in size between samples will be on the same horizontal line and indicate that DNA sequence is shared. The more fragments two samples share, the more related they are to one another.
Biogeographical evidence for evolution
The notion of biogeography is what first indicated to Charles Darwin that species evolve from common ancestors. Patterns of distribution of fossils and living species may tell us how modern organisms evolved.
For example, broad groupings of organisms that had already evolved before the breakup of the supercontinent Pangaea (about 200 million years ago) tend to be distributed worldwide. In contrast, broad groupings that evolved after the breakup tend to appear uniquely in smaller regions of Earth.
Environment cannot always account for either similarity or dissimilarity. Closely related species can evolve different traits under different environmental pressures. Likewise, very distantly related species can evolve similar traits if they have similar environmental pressures.
Common mistakes and misconceptions
- Evolution is not "just" a theory. In science, a "theory" addresses a broader question and is supported by a large amount of data from multiple sources. Evolution is a well-supported and accepted scientific theory that is supported by the evidence listed above.
- Biologists do not draw conclusions about how species are related on the basis of structure or biogeographical evidence alone. Instead, they study both physical features and DNA sequences, and draw conclusions about relatedness based on these features as a group.
- Not all species left fossils behind. Some people believe that all living organisms leave behind fossil evidence. Unfortunately, fossilization is fairly rare, because it requires many different conditions to occur over time in a specific order. Because these conditions do not occur all the time, we do not have fossils for all of the extinct organisms.Because many species that existed on earth were not fossilized, this has left gaps in our fossil record. However, that doesn’t mean these organisms didn’t exist, and the fossil record we do have contains many transitional fossils, all of which support evolution!
Want to join the conversation?
- when did whales have hands?(7 votes)
- The 'hand' is the whale's flipper. The image above shows how they are similar in bone structures, but their function is different depending upon the environment.(5 votes)
- are we descendants of whales(2 votes)
- Humans and whales likely have a common ancestor, but I don't think all mammals just came from whales that went on land because I remember seeing that whales actually evolved from land dwelling wolf-like mammals.(6 votes)
- why the animals can change?(1 vote)
- Animals have changed over thousands and millions of years and this has been adapted to the environment in which they live. Therefore, many genetic changes and deviations have occurred over the ages with different traits in individuals, whether of the same species or of different kinds, I hope to be able to help you and answer your question :-) .(6 votes)
- what causes natural selection?(3 votes)
- As you probably know, Natural Selection is the process by which organisms in a population that have traits suited better to their environment than other organisms within the population, survive + reproduce more efficiently and successfully.
So when you asked what causes natural selection, it would be the change in environment that would hinder survival and reproduction for organisms with disadvantageous traits in a population and facilitate survival and reproduction for organisms with advantageous traits in a population. This forms the basis of natural selection and therefore its cause as well.
There might be other factors that cause natural selection, (I'm still new to all this) but changes in the environment is definitely a factor, contributing to the cause of Natural Selection.
Hope that helped!(1 vote)
- what is the cause of natural selection(2 votes)
- the cause of natural selection, i think, is life! if you think about it, animals eat and get eaten in life. the ones with traits that allow them to perform better, survive, and pass on their genes. hope this helps!(2 votes)