OLD ways...

When classifying organisms, many possibilities in placing organisms with the same characteristics can occur. This was done based on observation of physical appearance of the organisms. For instance:

• similar species are grouped together into the same genus (plural, genera)

• similar genera are grouped together into families

• similar families are grouped together in orders 

• similar orders are grouped together in classes

This allows us to predict group of organisms based on physical appearance. 

• Some grouping may be incorrect as physical observations cannot always distinguish homologous and analogous structures. Evidence from cladistics has shown that some groups based on structure does not correspond with the evolutionary origins of a group or species

  • Homologous structure: similar structure shown in two different species, evolved from the same common ancestor

  • Analogous structure: similar structure shown in two different species, evolved from the different ancestors. The structure evolved as two ancestors were naturally selected in the same environment (theory of evolution) 

    • Bats and eagles are not from the same group (mammals and birds respectively) even though they both have the same structure: wings. 

• Morphology (form and structure) of organisms can lead to mistakes in classification, due to misinterpreting whether structures are analogous or homologous. Base or amino acid sequences are more accurate ways of determining members of a clade because they represent true homology

More accurate ways...

• Using molecular biology, we can find similarities in organisms. 

• Molecular biology can be analysed by checking the degree of similarities by comparing:

  • chromosomes

  • DNA sequence

    • This can be done by the cell's DNA, mitochondial DNA and/or chloroplast DNA

  • other nucleic acid sequence such as mRNA, rRNA etc

  • protein sequence (amino acids)

• Molecular method provides more accuracy

  • The use of base and amino acid sequences has made the study of phylogenetic trees more accurate. 

• Reclassification has led to:

  • some groups merging with others

  • some groups being divided 

  • some species being transferred from one group to another.

•Life cycles vary between species in reproductive behaviour and lifespan.

•Species can be classified according to how rapidly they reproduce and the degree to which they give parental care

•Species that are characterized by periods of rapid growth followed by decline, tend to inhabit unpredictable, rapidly changing environments and are termed opportunistic species or r-strategist or r-species.

•In contrast, slow-growing organisms tend to be limited by the carrying capacity

of an environment (K) and so are known as K-strategists or K-species.

•They inhabit stable environments and have lower reproductive rates but better competitive ability.

•K-species have long life spans, large body size and develop slowly.

•K-species produce very few, often exceptionally large, offspring that mature slowly and receive a great deal of parental support.

•Elephants and whales are good examples. As a result of the low birth rate, K-species are vulnerable to high death rates and extinction

•r- and K-species have reproductive strategies that are better adapted to pioneer and climax communities respectively

•In predictable environments – where the availability of resources does not fluctuate –

•there is little advantage to rapid growth.

•Instead, evolution favours species that can maximize the use of natural resources and that produce few young that have a high probability of survival.

•In contrast, disturbed habitats with rapidly changing conditions favour species that can respond rapidly, develop quickly and have early reproduction.

•This leads to a high rate of productivity. Such colonizer species often have a high dispersal ability to reach areas of disturbance.

Understanding the extent of human impacts on species 

•Ecological studies are vital in providing information on species’ classifications, niche requirements and life cycles.

•This information can then be used to help understand the extent of human impacts on them.

•Human impacts on the life cycles of plants and animals include temperature changes from climate change that affect the life cycles of plants, which in turn affect the life cycles of animals.

•The life cycles of many species are synchronized with those of others and with the seasons.

•Human impacts contribute to climate change, which in turn may disrupt

plant and animal life cycles.