• Organisms overproduce

• Traits are inherited but they may be in new combinations

• There is competition for resources

• Best adapted individuals in a species population survive to reproduce and produce viable offspring

• fitness - a measure of reproductive success

• relative fitness - the fitness of a genotype relative to that of the most favoured genotype. I.e. a measure of the proportionate contribution of offspring to the next generation by one genotype, compared with a standard genotype, which is the most favoured

• In a population of constant size, the relative fitness of the most favoured genotype is 1.0

• Differences in relative fitness result in selection that changes genotype and gene frequency

Natural selection causes change

The environment in which an organism exists provides numerous selection pressures which will lead to change in an organisms form, physiology and/or behaviour.

For example, a mobile prey species will be affected by its predator. If the predator always catches the slower individuals, the average running speed in the prey population will increase. Then the slower predators will not be able to chase down and catch prey. Thus, increasing the average running speed in the predator population. Obviously, this process could not continue forever or all predators and prey would already be travelling at ever increasing speeds, many exceeding the speed of light !

This doesn't happen because of physical limitations of body plan and physiology. Also, importantly, no population is responding to a single selection pressure and there will be a balanced response to each that it is subjected to.

Change is change, not focused directional change

An organism living, surviving and reproducing successfully in an environment is well adapted to that environment. However, if the environment changes, different members of the population will be better adapted and evolution will take place. If the environmental change is cyclical, e.g. a rainfall cycle, when the cycle returns to the same point similar adaptations will be selected again. But, the genotypes and gene pool will not be the same because of recombination and other selection pressures acting on the population.

The biological species concept provides one definition of what is a species. The concept refers to freely interbreeding populations that share a gene pool and produce viable offspring. Members of two different species do not produce fertile offspring or, if they do, fertility is low and the next generation is sterile.

Species are reproductively isolated from each other. There are many biological incompatibilities that can cause one population to be reproductively isolated from another, thus maintaining these populations as separate species. The  properties of organisms that prevent interbreeding are called reproductive isolating mechanisms.

The classic example of two species which when crossed produce infertile offspring is the horse (Equus caballus) and the donkey (E. asinus) producing a mule or hinny. This cross has been performed a multitude of times over thousands of years therefore it is not surprising that a few fertile offspring have been obtained.

In nature, lions (Panthera leo) and tigers (P. tigris) are reproductively separated geographically. However, hybrids do occur in captivity. Ligers (male lion X female tiger) are invariably infertile but tigons (male tiger X female lion) can be fertile. 

There are several ring species, the most famous example is the herring gull  (Larus argentatus). In the British Isles, these are white. They breed with the herring gulls of eastern America, which are also white. American herring gulls breed with those of Alaska, and Alaskan ones breed with those of Siberia. But as you go to Alaska and Siberia, you find that herring gulls are getting smaller, and picking up some black markings. And when the series of populations is followed all the way back to Britain, they have become Lesser Black-Backed Gulls (Larus fuscus).

So, the situation is that there is a big circle/ring around the north pole. As you travel this circle, you find a series of gull populations, each of which interbreeds with the populations to each side. There is a complete series of intermediate subspecies of one species or the other. In the British Isles, the two ends of the circle are functionally two different biological species of bird. They do not interbreed. They nest in different sites and have different appearances.

                     Larus fuscus

An understanding of the process of speciation is often gained by studying geographic variation and, perhaps, the pattern seen in ring species is the most informative. Such variation shows us that intraspecific variation can be sufficient to result in the formation of two species. The processes (different selection pressures, founder effect, restricted gene flow) which lead to differentiation among populations (microevolution) also may result in speciation (macroevolution). 

What is important here is not that speciation is an outcome of evolution but that evolution occurs without speciation. This is exemplified by:

• industrial melanism (e.g. Biston betularia)

• the acquisition of insecticide resistance by pestiferous insects

• the development of antibiotic resistance

From the USA

Evolution that involves relatively small changes in the gene pool of a particular population from one generation to the next without any new species being formed. However, the descendant population is not genetically identical with preceding population.

Industrial melanism, insecticide and antibiotic resistance are examples.

This is the classic case of evolution where one species becomes two or more. In the case of ring species, like the gulls, the intermediates are still alive which is not normally the case. Other examples involve adaptive radiation such as the tortoises (above) and finches on the Galapagos Islands or the Amazon parrots (left) in the Caribbean.

This is a direct result of divergent evolution. It describes two related lineages that have made similar evolutionary changes after they diverged from a common ancestor. Marsupials in Australia have paralleled the evolution of placental mammals in other parts of the world. There are marsupials that resemble placental wolves, cats, squirrels, moles, and mice. The placentals and their corresponding marsupials share a very distant relative, and after separation they evolved independently but along parallel lines because of their adaptation to similar ways of life.

Convergent evolution

This is when completely unrelated species share similar traits. These similarities arise because each has become independently adapted to similar environmental conditions, not because they share a common ancestor. For example, sharks (fish), porpoises (mammals), and penguins (birds) have similar shaped bodies and peripheral fins. These common traits arose as a direct result of adaptations to aquatic life and not by direct inheritance from a common ancestor/

Also, consider new world Cactaceae and old world succulent Euphorbiaceae, also growing in dry habitats, or South and Central American toucans (e.g. at left) and African and Asian hornbills (above). 

Similar environments, similar selection pressures, different starting materials but similar evolutionary solutions = convergent evolution. 

This is the evolution of one species in response to new adaptations that appear in other species. It should be mutual, i.e. a change in either interacting species should lead to change in the other.

Plants and their pollinators are good examples as are herbivores and their food plants.  

1. How do ring species help our understanding of evolution?
2. How do organisms become adapted to their environment?
3. How is hybridisation between species prevented?
4. What is a new species?
5. What are the flaws in the classical version of industrial melanism in Biston betularia?
6. Why are new species more common on islands?
7. How are convergent and parallel evolution similar and how do they differ? Is this distinction worth recognizing? Explain your answer.