Speciation, Extinction, and Biodiversity

speciation

Speciation is the evolution of two species from one species because of divergent natural selection in response to changes in environmental conditions. It usually takes thousands to millions of years to take place.

The most common mechanism of speciation takes place in two phases: geographic isolation and reproductive isolation.

Geographic Isolation

It occurs when groups of the same population of a species become physically separated for long periods. For example, part of a population may migrate in search of food and then begin living in another area with different environmental conditions (Figure). Populations also may become separated by a physical barrier (such as a mountain range, stream, lake, or road), by a change such as a volcanic eruption or earthquake, or when a few individuals are carried to a   new area by wind or water.

                                                                                                         

                                                                                                     

Figure: How geographic isolation can lead to reproductive isolation, divergence, and speciation.

Reproductive Isolation

It occurs when mutation and natural selection operate independently in two geographically isolated populations with same ancestral genetic heritage and change the allele frequencies in different ways. If this process, called divergence, continues long enough, members of the geographically and reproductively isolated populations may become so different in external forms, structures and genetic makeup that they cannot interbreed—or if they do, they cannot produce live, fertile offspring. Then one species has become two, and speciation has occurred through divergent evolution.

                                                                               

It is now believed that the ostrich, the rhea, and the emu have a common ancestor but evolved separately, and believed to be examples of divergent evolution.

For some rapidly reproducing organisms, this type of speciation may occur within hundreds of years. However, for most species such speciation takes from thousands to millions of years. Given this time scale, it is difficult to observe and document the appearance of a new species. 

                          

                                            

Biological Evolution

Biological evolution refers to the change in inherited characteristics of a population from generation to generation, i.e., a genetic change in a population of organisms that occurs over time. It can result in new species—populations that can no longer reproduce with members of the original species.

It is a one-way process: when a new species evolves, it cannot evolve backward into its parents.

Processes That Lead to Evolution

Four processes lead to evolution: mutation, natural selection, migration, and genetic drift.

Mutation: A change in the DNA of an organism, i.e., a change in the inherited characteristics. A mutation in reproductive cells may be passed on to the next generation, where it may result in birth defects or genetic disease. (DNA: A gene—a distinct piece of DNA that determines the characteristics an individual displays). When DNA is changed the DNA is said to have undergone mutation.

In extreme cases, cells or offspring with mutation cannot survive. In less extreme cases, individuals with mutation are so different from their parents that they cannot reproduce with the normal offspring. In milder cases, the mutation can simply add variability of the inherited characteristics. So the change is not always for better: Mutation can result in a new species whether or not that species is better adapted than its parental species to the environment.

Natural Selection

Natural selection is the process that determines which individuals within a species will reproduce more effectively and therefore results in changes in the characteristics within a species.

                                              

Migration

The migration of one population of a species into a habitat previously occupied by another population can lead to changes in gene frequency. Sometimes reproductive structures like seeds of flowering plants are blown by the wind and moved by animals.

Genetic Drift

It refers to changes in the frequency of a gene in a population as a result of chance rather than the other three processes of evolution. Chance may determine which individuals become isolated in a small group from a larger population and thus which genetic characteristics are most common in that isolated population. Genetic drift can occur in any small population. It can be a problem for a rare or endangered species: (1) characteristics that are less adapted to existing environmental conditions may dominate, making the chance of survival of the species less likely; (2) genetic variability of the species is greatly reduced, meaning that its ability to adapt to future changes in the environment through natural selection has been reduced.  

Terms related to Species Habitat

Exotic species: A species introduced into a new geographic area. (Invasive species, a foreign species, not native, usually cause economic or environmental damage).

Endemic species: A species that is native to a particular area and not native elsewhere. For example, Monterey pine is endemic to a portion of the California coast and exotic in New Zealand.

Cosmopolitan species: A species with a broad distribution, occurring all over the world wherever the environment is appropriate. (house mouse).

Ubiquitous species: A species that are found almost anywhere.  (humans, E. coli).

Habitat:  Place or type of place where an organism or population of organisms lives.

Biodiversity, Kinds of Biological Diversity

Biodiversity 

One important renewable resource is biological diversity, or biodiversity: the different forms of life and life-sustaining processes that can best survive the variety of conditions currently found on Earth.  At the most general level it refers to the variety to be found among living organisms throughout the Earth (  "the variability among living organisms from all sources, including, 'inter alia', terrestrial, marine, and other aquatic ecosystems, and the ecological complexes of which they are part : this includes diversity within species, between species and of ecosystems"). It is an important quality of landscape beauty.

The biodiversity found on Earth today consists of many millions of distinct biological species, which is the product of nearly 3.5 billion years of evolution. Biodiversity has developed through biological evolution. Understanding biological evolution is the key to understanding biodiversity

                                 

                             Fig.: Some of the biodiversity of coral reefs

Kinds of Biological Diversity

Kinds of biodiversity include the following:                

1.    Genetic diversity: the total number of genetic characteristics, sometimes of a

      specific species, subspecies, or group of species.

2.    Habitat (Ecological) diversity: the diversity of habitats in a given unit area.

3.    Species diversity: which has three aspects:

(a)  Species richness—the total number of species.

(b)  Species evenness—the relative abundance of species.

(c)  Species dominance—the most abundant species.

Extinction and Causes of extinction

Extinction occurs when the last individual of a species dies. Ultimate fate of all species. It is an irreversible process. Once a species is extinct it cannot reappear. Extinction has increased rapidly since the industrial revolution. Two types:   

Local extinction occurs when a species disappears from a part of its range but persists elsewhere. 

Global extinction means that a species becomes extinct everywhere.

Causes of extinction

1. Population risk
2. Environmental risk
3. Natural catastrophe
4. Genetic risk
5. Human actions

Population Risk

Random variations in population rates can cause a species in low abundance to become extinct. For example, if in one year most whales were unsuccessful in finding a mate, then birth could be dangerously low. It is a risk especially to species that consist of only a single population in one habitat, can occur without any change in the environment.

Environmental Risk

It involves variation in the physical or biological environment, including variations in predator, prey, symbiotic, or competitor species. In some cases, species are sufficiently rare and isolated that such normal variations (e.g. late snow & freeze) can lead to their extinction.

Natural Catastrophe

Fires, major storms, earthquakes, and floods are catastrophes on land. Changes in currents, upwellings, and tsunami are ocean catastrophes. These catastrophes can cause many species to become extinct.

Genetic Risk

Detrimental change in genetic characteristics is called genetic risk. Genetic changes can occur in small populations from reduced genetic variation, genetic drift, and mutation. In a small population, only some of the possible inherited characteristics will be found. The species is vulnerable to extinction because it lacks variety.

Human Actions

Humans have had a significant impact on the extinction of many kinds of species. Wherever humans have modified the environment for their purposes (farming, forestry, cities, hunting, introducing exotic sp), species are typically displaced from the area. If large areas are modified, entire species may be displaced. Other human actions that cause extinction include:

v  Intentional hunting or harvesting

v  Disruption or elimination of habitat

v  Introduction of new parasites, predators, or competitors of a species (Exotic Species)

v  Pollution of the environment.

Although extinction is a natural process humans have become a major force in the premature extinction of species. The IUCN estimates that 75% of the extinction of birds and mammals since 1600 were caused by human beings. Hunting caused 42% & 33% of the extinctions of birds and mammals, respectively. During this century human expansion may cause the premature extinction of up to a quarter of the Earth’s current species.

       

                                                                      

Fig: The Passenger Pigeon, one of hundreds                              Fig: The Bali Tiger was declared extinct in

       of sp. of extinct birds, was hunted to extinction                          1973 due to hunting & habitat loss.

       over the course of a few decades