5.1 Essential ideas
5.1.1 Evidence for evolution
Evolution is the change in heritable characteristics of a species over time. Isolated populations, after a long period of evolution, gradually diverge into a new species. In general, however, macroevolution – the evolution of gene pools into separate species or classes of organisms – can be distinguished from microevolution – the change in gene frequencies within a species. Changes in gene frequencies can be observed in one’s lifetime, while speciation tends to occur over timeframes that are too long to study directly. There is overwhelming evidence that both types of evolution have occurred and are in progress right now. Evidence comes from:
The fossil record
Fossils are the remains of animals and plants preserved in rock layers. The majority are formed when dead organisms are buried in layers of sediment, and their bodies are mineralised. Intact organisms may also be trapped in fossilised tree resins such as amber. Fossils are often incomplete, as soft bodies are not well preserved.
It is estimated that the fossil record represents less than 5% of the species present in each geological era. However, it provides two very important lines of evidence for evolution. Firstly, it proves that many species have become extinct. Secondly, it shows succession in the forms and shapes of extant species.
Figure 5.1.1b – Succession in fossils
The classic ‘horse series’ of fossilised bones demonstrates the transitional phases in the evolution of weight-bearing middle toes in the modern horse.
Comparative anatomy of extant species provides evidence for evolution in the form of homologous structures. A homology is a structure that, while anatomically similar, is used for different functions by different organisms. An example is the mammalian forelimb.
The existence of homologies suggests that at one time in evolutionary history, an ancestor common to the extant group possessed a prototype of the shared characteristic which was subsequently modified by adaptive radiation.
A very interesting type of homologous structure is the one that may have served a function in the evolutionary past, but in the modern species has a different function or no function at all. Such structures are known as vestigial structures. The pelvis and femur of a whale are examples.
Figure 5.1.1d – Vestigial structures
Vestigial structures are evidence that there were land-dwelling ancestors of modern whales. The fossil record provides further evidence.
Agriculturalists and breeders have been directing evolution for centuries, by selectively mating animals and plants with desirable traits. Over many generations, the offspring of these pairings produce populations in which the desirable traits predominate. Selective breeding has led to considerable diversity of phenotypes within a single species. As we will see in the next section (see Page 5.1.2), selective breeding is often called artificial selection.
Continuous species variation
Very compelling evidence for evolution comes from studying the diversity of populations in a group of species across a geographical range. It was this line of evidence that first convinced Charles Darwin of evolution. Here are some interesting trends observed from biogeography:
- Different species inhabiting geographically continuous locations are anatomically homologous, even when the environments of those locations are different. This is consistent with a pattern of divergent evolution.
- When the entire gene pool is taken into account, the variation shown by different populations is continuous. This is consistent with the pattern of gradual divergence.
- In similar environments, species inhabiting geographically remote locations may share behavioural or structural characteristics even if these are anatomically different from each other. This is consistent with a pattern of convergent evolution.
Figure 5.1.1g – Flying falanger and flying squirrel
Both Petaurus (left) and Glaucomys (right) have evolved the ability to glide between trees – the first is a marsupial and the second is a placental mammal.
Food for thought
Evolutionary timescales are difficult to imagine. One thousand generations of human life is equal to approximately 25#000 years, while 1000 generations of fast-growing bacteria such as E. coli is equal to about 50 hours!
Examine the structures in Fig 5.1.1c. How are each of the adaptations related to how that animal moves from one place to another?
Evolutionary history is difficult to reconstruct because experiments establishing past causes or events are impossible to perform. However, scientific methods of establishing chronologies in evolution (molecular methods, etc) are reliable and prove beyond a reasonable doubt, that when certain events have occurred. In what ways do these methods compare to those used by historians to recreate the past?
Did you know?
There are plenty of ‘useless’ structures in the human body. Scientists are still debating the function of the human appendix. Could it be a vestigial organ? Can you think of others?