A3.1 Diversity of Organisms
Definition of Species
While there are many proposed definitions for the word species, the one that conventionally agreed upon is:
“Organisms with shared traits that are capable of interbreeding to produce a fertile offspring.”
Variation & the morphological species concept
A core aspect of existence is variation. Variation within and between species, which serves as a basis for natural selection.
Their first method of distinguishing between organisms was through their morphological traits. Morphology is the classification of species based on their outer form and inner structure.
The Morphological species concept:
States that species are an unchanging group of organisms with clear differences in their external and internal structure.
This definition is limited, as it does NOT consider the genetic materials of organisms.
Example
Lions and tigers challenge the morphological species concept by showing that appearance alone doesn't always define species boundaries.
The Biological Species Concept
States that species are a group of organisms capable of interbreeding to produced a fertile offspring.
This definition is overall conclusive, however there are some aspects that limits this definition, that will be explored in the HL section.
Binomial Nomenclature
The binomial naming system is a universal method used by biologists to classify organisms based on shared traits, DNA, and common ancestry. Each organism is assigned a unique binomial name consisting of two parts: the Genus (which is capitalized) and the species (written in lowercase). This standardized system ensures consistent identification and communication across the global scientific community.
Genetic Diversity vs Unity
Genetic unity: Refers to how all individuals in a species has the same gene on the same locus on a chromosome
Genetic Diversity: Refers to how that gene has Single Nucleotide Polymorphisms (Different base sequences that code for that same gene)
Another aspect of diversity is the chromosome number.
However, Chromosome number ≠ Complexity of Organism.
Exceptions to the applying the biological species concept
The biological species concept, which defines species as groups of organisms that can interbreed and produce fertile offspring, has several important limitations:
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Asexual organisms – Many organisms, such as bacteria, reproduce asexually and do not interbreed at all. This makes the concept inapplicable, since there's no mating or offspring involved to test reproductive isolation.
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Horizontal gene transfer – In microorganisms like bacteria, genes can be transferred across species boundaries, allowing traits to spread without reproduction. This blurs the line between species and challenges the idea of reproductive isolation.
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Geographical separation and gradual divergence – Populations of the same species may become geographically isolated and slowly diverge over time. It becomes difficult to determine exactly when they become separate species, especially if they could still interbreed under the right conditions.
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Migration and overlapping ranges – When previously isolated species come into contact due to migration or environmental changes, the ability to interbreed (and produce fertile offspring) can reemerge, revealing the fluidity of species boundaries.