Science: Explaining evolution 1 – Species

Here is the first in a series of science blogs exploring evolution.

Explaining evolution 1 – Species

Evolution is an explanation to answer the questions: ‘Why have some species of plants and animals died out?’ and, ‘How do new species of plants and animals develop?’ Yet evolution is not an easy concept for students of science to fully understand which is probably due to a combination of factors:

  • Evolution is not immediately noticeable – we cannot see it, smell it, touch it or hear it as we often do in other branches of science.
  • Evidence to support the concept of evolution requires an understanding of biology, chemistry, ecology, geology and geography.
  • The measurement of this evidence involves physics and chemistry.
  • Evolution also requires an appreciation of vast time scales – far greater than a human life span.
  • Evolution is often a controversial subject in opposition to many religious explanations of life.

Before trying to answer: ‘Why have some species of plants and animals died out?’ and, ‘How do new species of plants and animals develop?’ we need to determine what is a species.  A common definition of a species is that only members of the same species can interbreed to produce viable and fertile offspring.  Successful interbreeding can only take place if the chromosomes (which contain genetic information) of each parent are of a similar enough structure to combine together to form viable zygotes which can develop into fertile offspring.  The important concept here is ‘fertile offspring’, since some closely related species can breed but only produce infertile offspring (e.g. an infertile mule is the offspring of a jack (male donkey) and a mare (female horse)).  Different dogs can successfully interbreed because all dogs are of the same species, although different varieties can have widely differing characteristics.

Close members of the bird family (e.g. finches) can breed together to give hybrids. However, most hybrids are either unviable and/or infertile in a natural environment.  Breeding between different species often happens in an artificial human-controlled environment but not often in the natural environment because of social grouping  (see blog no. 6 Explaining evolution – Survival of the fittest).

Recent DNA analysis of different species allows identification and confirmation of an organism into a species and family.  This technique was not available to pioneering scientists in the 19th century (see the next blog no. 2 Explaining evolution – Early observation).

John Roach


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