Question

Which of the following statements describe natural extinction?
(i) Extinctions abetted by human activities
(ii) Slow replacement of existing species
(iii) Also known as background extinction
(iv) A small population is most likely to be extinct

A. (i) and (iii)
B. (i), (ii), and (iii)
C. (ii), (iii), (iv)
D. (i), (ii), (iii) and (iv)

Answer 1

Hint: A species is extinct when the last existing member dies. Extinction therefore becomes a certainty when there aren't any surviving individuals that may reproduce and make a brand new generation.

Complete answer:
To answer this question we need to discuss extinction and its causes.
Extinction is the termination of an organism or of a bunch of kinds (taxon), usually a species. A species may become functionally extinct when only a couple of people survive, which cannot reproduce because of poor health, age, sparse distribution over an oversized range, a scarcity of people of both sexes (in sexually reproducing species), or other reasons.

Causes of extinction:
Human activities: Humans can cause extinction of a species through overharvesting, pollution, habitat destruction, introduction of invasive species (such as new predators and food competitors), overhunting, and other influences. Explosive, unsustainable human increment is an important reason behind the extinction crisis.
Genetics and demographic phenomena: If adaptation increasing population fitness is slower than environmental degradation plus the buildup of slightly deleterious mutations, then a population will go extinct.Smaller populations have fewer beneficial mutations entering the population each generation, slowing adaptation.
Genetic pollution: Extinction can threaten species evolved to specific ecologies through the method of genetic pollution—i.e., uncontrolled hybridization, introgression genetic swamping which results in homogenization or out-competition from the introduced (or hybrid) species.
Habitat destruction: This can be currently the most anthropogenic reason for species extinctions. The most explanation for habitat degradation worldwide is agriculture, with conurbation, logging, mining, and a few fishing practices close behind. The degradation of a species' habitat may alter the fitness landscape to such an extent that the species is not any longer able to survive and becomes extinct. This could occur by direct effects, like the environment becoming toxic, or indirectly, by limiting a species' ability to compete effectively for diminished resources or against new competitor species.
Predation, competition, and disease: Within the natural course of events, species become extinct for a variety of reasons, including but not limited to the extinction of a necessary host, prey or pollinator, inter-species competition, inability to pander to evolving diseases, and changing environmental conditions (particularly sudden changes) which might act to introduce novel predators, or to get rid of prey.
Co-extinction: Co-extinction refers to the loss of a species thanks to the extinction of another; for instance, the extinction of parasitic insects following the loss of their hosts. Co-extinction can even occur when a species loses its pollinator, or to predators in an exceedingly organic phenomenon who lose their prey. Co-extinction is particularly common when a keystone species goes extinct. Models suggest that co-extinction is the most typical kind of biodiversity loss.

Therefore, the correct option is c, (ii), (iii), (iv).

Note: Recent developments: Extinction is a crucial research topic within the field of zoology, and biology normally, and has also become a section of concern outside the scientific community. A variety of organizations, like the Worldwide Fund for Nature, are created to preserve species from extinction. Governments have attempted, through enacting laws, to avoid habitat destruction, agricultural over-harvesting, and pollution. While many human-caused extinctions are accidental, humans have also engaged within the deliberate destruction of some species, like dangerous viruses, and also the destruction of other problematic species has been suggested. Other species were deliberately driven to extinction, or nearly so, because of poaching or because they were "undesirable", or to push for other human agendas.