Evidence for evolution
The theory of evolution is supported by instances of direct observation, the existence of homologies and fossils, and certain biogeographical patterns.
- Evidence for large-scale evolution (macroevolution) comes from anatomy and embryology, molecular biology, biogeography, and fossils.
- Similar anatomy found in different species may be homologous (shared due to ancestry) or analogous (shared due to similar selective pressures).
- Molecular similarities provide evidence for the shared ancestry of life. DNA sequence comparisons can show how different species are related.
- Biogeography, the study of the geographical distribution of organisms, provides information about how and when species may have evolved.
- Fossils provide evidence of long-term evolutionary changes, documenting the past existence of species that are now extinct.
We can sometimes directly see small-scale evolution, or microevolution, taking place (for example, in the case of drug-resistant bacteria or pesticide-resistant insects). However, many of the most fascinating evolutionary events – such as the divergence, or splitting, of plant and animal lineages from a common ancestor – happened far in the past. Not only that, but they occurred over very long time periods, not on the days-to-weeks timescales of bacterial and viral evolution. This large-scale evolution is sometimes called macroevolution.
We can't directly observe evolutionary events that happened in the past. However, we often want to understand them. For instance, we may want to know whether two present-day species are closely related. Or we may have a group of species, and want to understand the evolutionary relationships among them. How can we answer these kinds of questions?
Evidence for evolution: Tracing evolutionary histories
In this article, we'll look at several types of information biologists use to trace and reconstruct evolutionary histories of organisms across long timescales.
- Anatomy and embryology. Anatomical features shared between organisms (including ones that are visible only during embryonic development) can indicate a shared evolutionary ancestry.
- Molecular biology. Similarities and differences between the "same" gene in different organisms (that is, a pair of homologous genes) can help us determine how closely related the organisms are.
- Biogeography. The geographical distribution of species can help us reconstruct their evolutionary histories.
- Fossils. The fossil record is not a complete record of evolutionary history, but it confirms the existence of now-extinct species and sometimes captures potential "in-between" forms on the path to present-day species.
Let's take a closer look at these strategies for reconstructing evolutionary histories over long time periods.
Evidence for evolution: Anatomy and embryology
Darwin thought of evolution as "descent with modification," a process in which species change and give rise to new species over many generations. He proposed that the evolutionary history of life forms a branching tree with many levels, in which all species can be traced back to an ancient common ancestor.
Branching diagram that appeared in Charles Darwin's On the origin of species, illustrating the idea that new species form from pre-existing species in a branching process that occurs over extended periods of time.
In this tree model, more closely related groups of species have more recent common ancestors, and each group will tend to share features that were present in its last common ancestor. We can use this idea to "work backwards" and figure out how organisms are related based on their shared features.
If two or more species share a unique physical feature, such as a complex bone structure or a body plan, they may all have inherited this feature from a common ancestor. Physical features shared due to evolutionary history (a common ancestor) are said to be homologous.
To give one classic example, the forelimbs of whales, humans, and birds look quite different on the outside. That's because they're adapted to function in different environments. However, if you look at the bone structure of the forelimbs, you'll find that the organization of the bones is remarkably similar across species. It's unlikely that such similar structures would have evolved independently in each species, and more likely that the basic layout of bones was already present in a common ancestor of whales, humans, and birds.
The similar bone arrangement of the human, bird, and whale forelimb is a structural homology. Structural homologies indicate a shared common ancestor.
Some homologous structures can be seen only in embryos. For instance, did you know that you once had a tail and gill slits? All vertebrate embryos, from humans to chickens to fish, share these features during early development. Of course, the developmental patterns of these species become increasingly different later on (which is why your embryonic tail is now your tailbone, and your gill slits have turned into your jaw and inner ear). However, the shared embryonic features are still homologous structures, and they reflect that the developmental patterns of vertebrates are variations on an ancestral program.
The small leg-like structures of some snakes species, like the Boa constrictor , are vestigial structures. These remnant features serve no present purpose in snakes, but did serve a purpose in the snakes' tetrapod ancestor (which walked on four limbs).
Vestigial structures are reduced or non-functional versions of features, ones that 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 are homologous to useful structures found in other organisms, and they can provide insights an organism's ancestry. For instance, the tiny vestigial legs found in some snakes, like the boa constrictor at right, reflect that snakes had a four-legged ancestor.
To make things a little more interesting and complicated, not all physical features that look alike are marks of common ancestry. Instead, some physical similarities are analogous: they evolved independently in different organisms because the organisms lived in similar environments or experienced similar selective pressures. This process is called convergent evolution. (To converge means to come together, like two lines meeting at a point.)
For example, two distantly related species that live in the Arctic, the arctic fox and the ptarmigan (a bird), both undergo seasonal changes of color from dark to snowy white. This shared feature doesn’t reflect common ancestry – i.e., it's unlikely that the last common ancestor of the fox and ptarmigan changed color with the seasons. Instead, this feature was favored separately in both species due to similar selective pressures. That is, the genetically determined ability to switch to light coloration in winter helped both foxes and ptarmigans survive and reproduce in a place with snowy winters and sharp-eyed predators.
Arctic fox and ptarmigan. Both are white-colored and shown in snowy winter landscapes.
Determining relationships from similar features
In general, biologists don't draw conclusions about how species are related on the basis of any single feature they think is homologous. Instead, they study a large collection of features (often, both physical features and DNA sequences) and draw conclusions about relatedness based on these features as a group. We will explore this idea further when we examine phylogenetic trees.
Evidence for evolution: Molecular biology
Like structural homologies, similarities between biological molecules can reflect shared evolutionary ancestry. At the most basic level, all living organisms share:
- The same genetic material (DNA)
- The same, or highly similar, genetic codes
- The same basic process of gene expression (transcription and translation)
These shared features suggest that all living things are descended from a common ancestor, and that this ancestor had DNA as its genetic material, used the genetic code, and expressed its genes by transcription and translation. Present-day organisms all share these features because they were "inherited" from the ancestor (and because any big changes in this basic machinery would have broken the basic functionality of cells).
Although they're great for establishing the common origins of life, features like having DNA or carrying out transcription and translation are not so useful for figuring out how related particular organisms are. If we want 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 genes found in different species (often called homologous or orthologous genes) to figure out how those species are evolutionarily related to one another.
The basic idea behind this approach is that two species have the "same" gene because they inherited it from a common ancestor. For instance, humans, cows, chickens, and chimpanzees all have a gene that encodes the hormone insulin, because this gene was already present in their last common ancestor.
In general, the more DNA differences in homologous genes between two species, the more distantly the species are related. For instance, human and chimpanzee insulin genes are much more similar (about 98% identical) than human and chicken insulin genes (about 64% identical), reflecting that humans and chimpanzees are more closely related than humans and chickens.
Evidence for evolution: Biogeography
The geographic distribution of organisms on Earth follows patterns that are best explained by evolution, in combination with the movement of tectonic plates over geological time. For example, broad groupings of organisms that had already evolved before the breakup of the supercontinent Pangaea (about 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. For instance, there are unique groups of plants and animals on northern and southern continents that can be traced to the split of Pangaea into two supercontinents (Laurasia in the north, Gondwana in the south).
Marsupial mammals on Australia likely evolved from a common ancestor. Because Australia's has remained isolated for an extended period time, these mammals have diversified into a variety of niches (without being outcompeted by placental mammals).
The evolution of unique species on islands is another example of how evolution and geography intersect. For instance, most of the mammal species in Australia are marsupials (carry young in a pouch), while most mammal species elsewhere in the world are placental (nourish young through a placenta). Australia’s marsupial species are very diverse and fill a wide range of ecological roles. Because Australia was isolated by water for millions of years, these species were able to evolve without competition from (or exchange with) mammal species elsewhere in the world.
The marsupials of Australia, Darwin's finches in the Galápagos, and many species on the Hawaiian Islands are unique to their island settings, but have distant relationships to ancestral species on mainlands. This combination of features reflects the processes by which island species evolve. They often arise from mainland ancestors – for example, when a landmass breaks off or a few individuals are blown off course during a storm – and diverge (become increasingly different) as they adapt in isolation to the island environment.
Evidence for evolution: Fossil record
Fossils are the preserved remains of previously living organisms or their traces, dating from the distant past. The fossil record is not, alas, complete or unbroken: most organisms never fossilize, and even the organisms that do fossilize are rarely found by humans. Still, the fossils we have been lucky enough to find offer unique insights into evolution over long timescales.
Earth's rocks form layers on top of each other over very long time periods. These layers, called strata, form a convenient timeline for dating embedded fossils. Strata that are closer to the surface represent more recent time periods, whereas deeper strata represent older time periods.
To interpret fossils accurately, we need to know how old they are. Fossils are often contained in rocks that build up in layers called strata, and the strata provide a sort of timeline, with layers near the top being newer and layers near the bottom being older. Fossils found in different strata at the same site can be ordered by their positions, and "reference" strata with unique features can be used to compare the ages of fossils across locations. In addition, scientists can roughly date fossils using radiometric dating, a process that measures the radioactive decay of certain elements.
Fossils document the existence of now-extinct species, showing that different organisms have lived on Earth during different periods of the planet's history. They can also help scientists reconstruct the evolutionary histories of present-day species. For instance, some of the best-studied fossils are of the horse lineage. Using these fossils, scientists have been able to reconstruct a large, branching "family tree" for horses and their now-extinct relatives. Changes in the lineage leading to modern-day horses, such as the reduction of toed feet to hooves, may reflect adaptation to changes in the environment.
Fossil skeletons of horse relatives dating from various time periods.
From most recent to least recent:
Equus - recent, single toe
Pliohippus - late Miocene, single toe
Merychippus - middle Miocene, three toes but with the lateral toes more reduced
Mesohippus - late Eocene, three toes
Biologists use multiple types of evidence to trace evolutionary changes that occur over long time periods. For example:
- Homologous physical features shared between species can provide evidence for common ancestry (but we have to be sure they are really homologous, and not the result of convergent evolution).
- Similarities and differences among biological molecules (e.g., in the DNA sequence of genes) can be used to determine species' relatedness.
- Biogeographical patterns provide clues about how species, both alive and extinct, are related to each other.
- The fossil record, though incomplete, provides valuable information about what species existed at particular times in Earth’s history.
Want to join the conversation?
- I understand that a fossil not being found doesn't mean the organism didn't exist, simply that it hasn't been found yet (or never fossilized). But several fossils that do exist don't seem to fit in with the theory of evolution. What about pollen from Cambrian or older sediments the Romairma formation in South America or in the Hakatai Shale in the Grand Canyon in North America? Flowering plants are highly advanced and wouldn't have existed in the Cambrian or Precambrian epochs.(7 votes)
- There are multiple possible explanations for those examples you cite. The fossils or their age could have been misidentified. It is also possible that our current understanding of the sequence of events is incorrect. Hopefully future scientific inquiry will decrease apparent contradictions.(3 votes)
- the possible explanations for the examples you may cite not the fossils or the age you could also have been misidentified. and it can be possible that the current understanding of the process of events is incorrect.(1 vote)
- The identified age is counted as evidence for evolution. In the pictures showing the evolution of horses, there are dates shown. Why do you think those are more accurate then evidence against evolution.(2 votes)