Evolution And Adaptive Radiation: The Basics

Adaptive Radiation:

• Evolution is a process of change in all forms of life over generations.
• Evolutionary biology is the study of how evolution occurs.
• Biological Evolution: Any genetic change in a population that is inherited over several generations. The changes may be small or large, noticeable or not so noticeable. Changes should occur at genetic level of a population and passed on from one generation to the next. It is also defined as changes in genetic characteristics of a population over time. Alleles are passed on and changes are noticed in phenotypes of a population.

Macroevolution:

• Life forms are connected and can be traced back to one common ancestor.
• Production of new adaptive types.
• Occurs due to population fragmentation and genetic divergence.
• Operates above species level.
• Results in splitting of population of species into several subtypes.
• Evolution on a scale of separated gene pools.
• Changes that occur at or above the level of species.
• Changes occurring on geological time scale.
• Macro evolutionary changes: Changes that are significant enough and lead to the formation of entirely new species.
• Evolved organisms could not be able to mate with their ancestors.
• Found in fossil records.
• Evolution is not random but it is the sum of long periods of micro evolution.
• Evolution is adaptive movements, never exclusively sequential, always involve divergent and repeated taking of one or more of the characters important in the trend.
• Macroevolution is holistic (whole and interdependent of its parts) qualitatively identical while quantitatively different.
• Eg: Natural selection.

Microevolution:

• Changes in allele frequencies that occur over time within a population.
• Change in gene frequencies due to mutation, selection, gene flow, genetic drift.
• Small scale change.
• Gradual evolutionary changes build upon small genetic variation that is passed on from generation to generation.
• Changes would not result in newer organisms being considered as different species.
• Observable instances of evolution are examples of microevolution.
• Eg: Progressive replacement of light colored moths by dark colored moths in industrial region in England. This is called industrial melanism.
• Bacterial strains having antibiotic resistance.

Mega Evolution:

• Abrupt transformations from one biological system to another.
• Leads to convergent evolution.
• Eg: Passing of life from water into land. Transition from invertebrates to vertebrates.

Process of natural selection is based on 3 conditions:

1. Every individual is supplied with hereditary material in the form of genes, received from parents passed onto offsprings.
2. Organisms tend to produce more offsprings than the environment can support.
3. Variations among offsprings either due to introduction of new genes via random changes called mutations or reshuffling of existing genes during reproduction.

If the above 3 conditions hold true natural selection occurs.

• Individuals will not have equal chances of reproductive success.
• Random genetic drift regulates evolution.
• Mutated genes neutrally sort among populations and survive across generations by chance alone.
• Natural selection is not a random process.
• Natural selection acts on traits necessary for survival.

Adaptive radiation:

• Evolutionary pattern whereby a single ancestral form or species diversifies or speciates into several or many related forms or species. Also termed as adaptive divergence or divergent evolution.
• Mechanism of this process is not specified.
• Adaptive radiation term coined by Professor Osborn.
• Causes of adaptive radiation: Need for food,safety,better breeding grounds, migration to new habitats, due to absence of enemies, presence of unoccupied environmental niches, isolation etc.
• Adaptive radiation leads to new species, genera, subclasses, families, orders, classes.
• It also leads to macroevolution and mega evolution.

Types of adaptive radiation:

1. Local Adaptive Branching: Development of diversification in a largely distributed population due to microhabitat preference. Eg: 2 distinct types of rhinos survive in Africa. One with square mouth and hypsodont dentition, the grazing type. It is white in color and lives in open country. Second one browsing type having pointed mouth, brachydont dentition, black color and lives in wooded areas.

2. Continental Adaptive Radiation: Evolutionary lines occurring in particular classes living in a continent. Eg: Marsupial mammals in Australia.

3. Contemporaneous Radiation: Adaptive radiations in 3 great zoogeographic divisions of the world Arctogaea, Neogaea and Notogaea.

Arctogaea: All of Northern hemisphere and Africa. Placental mammals evolved in 5 different lines cursorial, arboreal, volant, aquatic and subterranean.

Neogaea includes South America.

Notogaea: Australia: Metatherians, protherians have undergone adaptive radiation in all lines like that of placental mammals.