Biology revision 2012 - patterns of evolution

·         Speciation through niche differentiation  

In a heterogeneous environment (one that is not the same everywhere), a population exist within a diverse collection of microhabitats. Some organisms prefer to occupy one particular type of microhabitat. Some organisms become so dependent on the resources offered by their particular microhabitat that they never meet up with their counterparts in different microhabitats. individual groups have remained genetically isolated for so long that they have become reproductively isolated. They have become new species. Gene flow is limited to organisms that share a similar microhabitat preference.  

 ·         Instant speciation by polyploidy

Polyploidy may result in the formation of a new species without isolation from the parent species. This event occurring during meiosis produces sudden reproductive isolation for the new group. Because the sex determining mechanism is distributed animals are rarely able to achieve new species status this way they are effectively sterile. Many plants on the other hand are able to reproduce vegetatively or carry out self-pollination. This ability to reproduce on their own enables such polyploid plants to produce a breeding population.                                                                                 

o   Speciation by allopolyploidy                                                                                                                

This type of polyploidy usually arises from the doubling of chromosomes in a hybrid between two different species. The doubling often makes the hybrid fertile.

·         Stage 1: moving into new environments                                    

 A single population will move into new regions of their environment when they are subjected to intense competition (interspecific or intraspecific). They are competing for identical resources in the habitat. Theoretically any individual has access to all members of the opposite sex for mating purposes.

·         Stage2: geographical isolation                                                   

Isolation due to formation of physical barriers. These barriers may cut off those parts of the population that are at the extremes of the species range and gene flow is prevented or rare.

·         Stage 3: different selection pressures                                            

The isolated populations are subjected to quite different selection pressures hence traits that suit each particular environment are favoured. the population becomes a subspecies as allele frequencies change

·         Stage 4: reproductive isolation                                                        

The separated populations (isolated subspecies) will often undergo changes in their genetic makeup as well as their behaviour patterns. These ensure that the gene pool of each population remains isolated and ‘undiluted’ by genes from other populations, even if the two populations should be able remix. The gene flow does not occur. 

o   Temporal isolation

Individuals from different species do not mate because they are active during different times of the day, or in different seasons.  

o   Gamete isolation

The gametes from different species are often incompatible, so even if they meet they do not survive. For animals where fertilisation is internal, the sperm may not survive in the reproductive tract of another species. If the sperm does survive and reach the ovum, the chemical differences in the gametes prevent fertilisation.

o   Behavioural (ethological) isolation

Behavioural isolation operates through differences in species courtship behaviours. Mates of the same species are attracted with distinctive, usually ritualised, dances, vocalisations, and body language. The courtship behaviours of one species will be unrecognised and ignored by individuals of another species.

o   Mechanical (morphological) isolation

Structural differences/incompatibility in the anatomy of reproductive organs prevents sperm transfer between individuals of different species.

o   Hybrid sterility

Even if two species mate and produce hybrid offspring , the species are still reproductively isolated if the hybrids are sterile (genes cannot flow from on species gene pool to the other). One cause of this sterility is the failure of meiosis to produce normal gametes in the hybrid. This can occur if the chromosomes of the two parents are different in number or structure.

o   Hybrid in viability 

Mating between individuals of two species may produce a zygote, but genetic incompatibility may stop development of the zygote. Fertilised eggs often fail to divide because of miss-matched chromosome numbers from each gamete. if the hybrid zygote completes embryonic development it will not survive for long

o   Hybrid breakdown

The first generation may be fertile, but the second generation are infertile or in viable                                                                                            

Not all similarities between species are inherited from a common ancestor. Species from different evolutionary branches may come to resemble each other if they have similar ecological roles and natural selection has shaped similar adaptations. This is called convergent evolution or convergence. Similarity of form due to convergence is called analogy.

·         Convergence

 Although similarities in the body form and function can arise because of common ancestry it may also be a result of convergent evolution. Selection pressures in a particular environment may bring about similar adaptations in unrelated species.

·         Analogous structures

 Analogous structures are those that have the same function and often the same basic external appearance, but quite different origins.

·         Punctuated equilibrium

 Long periods of evolutionary stasis are punctuated by rapid periods of speciation. These burst of evolutionary activity are usually stimulated by major environmental change.

·         gradualism

species evolve and diverge regularly and gradually at a relatively slow, constant rate.

Isolating mechanisms are barriers to successful interbreeding between species.  Prezygotic isolating mechanisms act before fertilization occurs, preventing species ever mating, whereas postzygotic barriers take effect after fertilization. Geographic barriers are not regarded as reproductive isolating mechanisms because they are not part of the species’ biology. Ecological isolating mechanisms are those that isolate gene pools on the basis of ecological preferences e.g. habitat selection. Ecological isolation involves a component of the species biology.

·         Geographical isolation                                                                                                        

Geographic isolation describes the isolation of a species population (gene pool) by some kind of physical barrier e.g. mountain range. Geographical isolation is a frequent first step in the subsequent reproductive isolation for a species.

·         Ecological (habitat) isolation                                                                                                 

Ecological isolation describes the existence of a prezygotic reproductive barrier between two species (or sub-species) as a result of them occupying or breeding in different habitats within the same general geographical area. Ecological isolation includes small scale differences (e.g., ground or tree dwelling) and broad differences (e.g. desert vs. grasslands). Ecological isolation often follows geographical isolation.