Pemilihan semula jadi: Perbezaan antara semakan

Kandungan dihapus Kandungan ditambah
KLITE789 (bincang | sumb.)
KLITE789 (bincang | sumb.)
Baris 17:
 
Inilah hubungan antara pemilihan semula jadi dan genetik, seperti yang diterangka oleh [[sintesis evolusi moden]]. Walaupun [[teori evolusi]] yang lengkap juga perlu menerangkan bagaimana variasi genetik timbul pada mulanya (seperti melalui [[mutasi]] dan [[pembiakan seks]]) serta merangkumi mekanisme evolusi yang lain (seperti [[aliran gen]]), namun pemilihan semula jadi masih diakui sebagai mekanisme asas evolusi.
 
===Tatanama dan kegunaan===
Istilah ''pemilihan semula jadi'' ini apabila lain konteksnya, maka berlainan sedikit pengertiannya. Namun begitu, takrifan yang paling kerap digunakan adalah tindakannya pada sifat-sifat yang boleh diwarisi, kerana sifat-sifat inilah yang memainkan peranan langsung dalam evolusi. Namun begitu, pemilihan semula jadi adalah "buta" kerana perubahan dalam fenotip (baik sifat fizikal mahupun perilaku) boleh memberikan kelebihan membiak tanpa mengira sama ada sifat itu boleh diwarisi (sifat bukan terwaris boleh terjadi kesan faktor persekitaran atau pengalaman hidup organisme itu).
 
Semenjak pertama kali digunakan oleh Darwin,<ref name="origin" /> istilah ini sering dipakai untuk memaksudkan kesan evolusi daripada pemilihan membuta dan juga mekanismenya.<ref name=fisher>Fisher RA (1930) ''[[The Genetical Theory of Natural Selection]]'' Clarendon Press, Oxford</ref><ref name="nomenclature1">Hasil kerja yang menggunakan atau menghuraikan kegunaan ini:<br>{{cite book| author=Endler JA | year=1986| title=Natural Selection in the Wild| publisher=[[Princeton University Press]]| location=[[Princeton, New Jersey|Princeton]], [[New Jersey]]| isbn=0-691-00057-3}}<br>{{cite book| author=Williams GC| year=1966| title=[[Adaptation and Natural Selection]]| publisher=Oxford University Press}}</ref> Sesekalinya ada baiknya kita terang-terangan membezakan mekanisme dan kesannya; di mana pentingnya pembezaan ini, ahli sains mentakrifkan "pemilihan semula jadi" khususnya sebagai "mekanisme-mekanisme yang menyumbang kepada pemilihan individu yang membiak", tanpa mengira sama ada asas pemilihan itu boleh diwarisi atau tidak. Ini juga dikenali sebagai "pemilihan semula jadi fenotip".<ref name="nomenclature2">Works employing or describing this usage:<br>Lande R & Arnold SJ (1983) The measurement of selection on correlated characters. [[Evolution (jurnal)|''Evolution'']] 37:1210-26<br>[[Douglas J. Futuyma|Futuyma DJ]] (2005) ''Evolution''. Sinauer Associates, Inc., [[Sunderland, Massachusetts|Sunderland]], [[Massachusetts]]. ISBN 0-87893-187-2<br>Haldane, J.B.S. 1953. The measurement of natural selection. Proceedings of the 9th International Congress of Genetics. 1: 480-487</ref>
 
Sifat-sifat yang meningkatkan keberhasilan membiak suatu organisme dikatakan terpilih untuk peningkatan tersebut, manakala yang mengurangkan keberhasilan itu juga terpilih untuk pengurangan itu. Pemilihan sifat juga boleh menyebabkan terpilihnya sifat-sifat lain yang sekaitan dengan sifat itu tadi tanpa mempengaruhi kelebihan membiak secara langsung. Ini mungkin terjadi sebagai hasil [[pleiotropi]] atau [[rangkaian gen]].<ref>[[Elliott Sober|Sober E]] (1984; 1993) ''The Nature of Selection: Evolutionary Theory in Philosophical Focus'' [[University of Chicago Press]] ISBN 0-226-76748-5</ref>
 
===Ketegapan===
Konsep [[Ketegapan (biologi)|ketegapan]] adalah mustahak kepada proses pemilihan semula jadi. Umumnya, individu yang lebih "tegap" lebih berpotensi untuk bermandiri, seperti yang diterangkan oleh ungkapan "[[siapa kuat dia menang]]" (''survival of the fittest''). Teori evolusi moden mentakrifkan ketegapan bukan mengikut tempoh hayat organisme itu, sebaliknya berasaskan betapa berhasilnya pembiakannya. Jika suatu hidupan cuma hidup separuh usia berbanding hidupan lain yang sama spesies, tetapi berganda anaknya yang mencapai usia dewasa, maka gen-gennya semakin kerap ditemui dalam populasi dewasa generasi seterusnya.
<!--
Walaupun pemilihan semula jadi bertindak pada individu, namun kesan ke effects of chance mean that fitness can only really be defined "on average" for the individuals within a population. The fitness of a particular genotype corresponds to the average effect on all individuals with that genotype. Very low-fitness genotypes cause their bearers to have few or no offspring on average; examples include many human [[genetic disorder]]s like [[cystic fibrosis]].
 
Since fitness is an averaged quantity, it is also possible that a favorable mutation arises in an individual that does not survive to adulthood for unrelated reasons. Fitness also depends crucially upon the environment. Conditions like [[sickle-cell anemia]] may have low fitness in the general human population, but because the [[sickle-cell trait]] confers immunity from malaria, it has high fitness value in populations which have high malaria infection rates.
 
===Types of selection===
Natural selection can act on any phenotypic trait, and selective pressure can be produced by any aspect of the environment, including [[sexual selection]] and [[Competition#Biology and ecology|competition]] with members of the same species. However, this does not imply that natural selection is always directional and results in adaptive evolution; natural selection often results in the maintenance of the status quo by eliminating less fit variants.
 
The [[unit of selection]] can be the individual or it can be another level within the hierarchy of biological organisation, such as genes, [[cell (biology)|cells]], and [[kin selection|kin groups]]. There is still debate about whether natural selection acts at the level of [[group selection|groups or species]] to produce adaptations that benefit a larger, non-kin group. Selection at a different level such as the gene can result in an increase in fitness for that gene, while at the same time reducing the fitness of the individuals carrying that gene, in a process called [[intragenomic conflict]]. Overall, the combined effect of all selection pressures at various levels determines the overall fitness of an individual, and hence the outcome of natural selection.
 
[[Image:Life cycle of a sexually reproducing organism.svg|thumb|right|The life cycle of a sexually reproducing organism. Various components of natural selection are indicated for each life stage.<ref>Modified from Christiansen FB (1984) The definition and measurement of fitness. In: ''[[Evolutionary ecology]]'' (ed. Shorrocks B) pp65-79. [[Blackwell Publishing|Blackwell Scientific]], [[Oxford]] by adding survival selection in the reproductive phase</ref>]]
Natural selection occurs at every life stage of an individual. An individual organism must survive until adulthood before it can reproduce, and selection of those that reach this stage is called ''viability selection''. In many species, adults must compete with each other for mates via sexual selection, and success in this competition determines who will parent the next generation. When individuals can reproduce more than once, a longer survival in the reproductive phase increases the number of offspring, called ''survival selection''.
 
The [[fecundity]] of both females and males (for example, giant [[spermatozoon|sperm]] in certain species of ''[[Drosophila]]'')<ref>Pitnick S & Markow TA (1994) Large-male advantage associated with the costs of sperm production in ''Drosophila hydei'', a species with giant sperm. [[PNAS|''Proc Natl Acad Sci USA'']] 91:9277-81; Pitnick S (1996) Investment in testes and the cost of making long sperm in ''Drosophila''. [[American Naturalist|''Am Nat'']] 148:57-80</ref> can be limited via "fecundity selection". The viability of produced [[gamete]]s can differ, while intragenomic conflicts such as meiotic drive between the [[haploid]] gametes can result in gametic or "genic selection". Finally, the union of some combinations of eggs and sperm might be more compatible than others; this is termed ''compatibility selection''.
 
===Sexual selection===
{{main|Sexual selection}}
It is useful to distinguish between "[[ecological selection]]" and "sexual selection". Ecological selection covers any mechanism of selection as a result of the environment (including relatives, e.g. [[kin selection]], [[Competition (biology)|competition]], and [[Infanticide (zoology)|infanticide]]), while "sexual selection" refers specifically to competition for mates.<ref>{{cite book| author=Andersson, M| year=1995| title=Sexual Selection| publisher=Princeton University Press| location=Princeton, New Jersey| isbn=0-691-00057-3}}</ref>
 
Sexual selection can be ''intrasexual'', as in cases of competition among individuals of the same sex in a population, or ''intersexual'', as in cases where one sex controls reproductive access by choosing among a population of available mates. Most commonly, intrasexual selection involves male–male competition and intersexual selection involves female choice of suitable males, due to the generally greater investment of resources for a female than a male in a single offspring. However, some species exhibit sex-role reversed behavior in which it is males that are most selective in mate choice; the best-known examples of this pattern occur in some fishes of the family ''[[Syngnathidae]]'', though likely examples have also been found in amphibian and bird species.<ref name="Eens">Eens M, Pinxten R. (2000). Sex-role reversal in vertebrates: behavioural and endocrinological accounts. ''Behav Processes'' 51(1-3):135-147. PMID 11074317</ref>
 
Some features that are confined to one sex only of a particular species can be explained by selection exercised by the other sex in the choice of a mate, for example, the extravagant plumage of some male birds. Similarly, aggression between members of the same sex is sometimes associated with very distinctive features, such as the antlers of [[stag]]s, which are used in combat with other stags. More generally, intrasexual selection is often associated with [[sexual dimorphism]], including differences in body size between males and females of a species.<ref name="Barlow">Barlow GW. (2005). How Do We Decide that a Species is Sex-Role Reversed? ''[[The Quarterly Review of Biology]]'' 80(1):28–35. PMID 15884733</ref>
 
==Examples of natural selection==
 
[[Image:Antibiotic resistance.svg|thumb|upright|Resistance to antibiotics is increased though the survival of individuals which are immune to the effects of the antibiotic, whose offspring then inherit the resistance, creating a new population of resistant bacteria.]]
A well-known example of natural selection in action is the development of [[antibiotic resistance]] in [[microorganism]]s. Since the discovery of [[penicillin]] in 1928 by [[Alexander Fleming]], [[antibiotic]]s have been used to fight [[bacteria]]l diseases. Natural populations of bacteria contain, among their vast numbers of individual members, considerable variation in their genetic material, primarily as the result of mutations. When exposed to antibiotics, most bacteria die quickly, but some may have mutations that make them slightly less susceptible. If the exposure to antibiotics is short, these individuals will survive the treatment. This selective elimination of maladapted individuals from a population is natural selection.
 
These surviving bacteria will then reproduce again, producing the next generation. Due to the elimination of the maladapted individuals in the past generation, this population contains more bacteria that have some resistance against the antibiotic. At the same time, new mutations occur, contributing new genetic variation to the existing genetic variation. Spontaneous mutations are very rare, and advantageous mutations are even rarer. However, populations of bacteria are large enough that a few individuals will have beneficial mutations. If a new mutation reduces their susceptibility to an antibiotic, these individuals are more likely to survive when next confronted with that antibiotic.
 
Given enough time, and repeated exposure to the antibiotic, a population of antibiotic-resistant bacteria will emerge. This new changed population of antibiotic-resistant bacteria is optimally adapted to the context it evolved in. At the same time, it is not necessarily optimally adapted any more to the old antibiotic free environment. The end result of natural selection is two populations that are both optimally adapted to their specific environment, while both perform substandard in the other environment.
 
The widespread use and misuse of antibiotics has resulted in increased microbial resistance to antibiotics in clinical use, to the point that the [[Methicillin-resistant Staphylococcus aureus|methicillin-resistant ''Staphylococcus aureus'']] (MRSA) has been described as a "superbug" because of the threat it poses to health and its relative invulnerability to existing drugs.<ref>{{cite web|url=http://www.inboxrobot.com/news/MRSASuperbug|title=MRSA Superbug News|accessdate=2006-05-06}}</ref> Response strategies typically include the use of different, stronger antibiotics; however, new [[strain (biology)|strains]] of MRSA have recently emerged that are resistant even to these drugs.<ref name=Schito_2006>{{cite journal | author=Schito GC | title=The importance of the development of antibiotic resistance in ''Staphylococcus aureus'' | journal=Clin Microbiol Infect | year=2006|pages=3–8|volume=12 Suppl 1|pmid=16445718 | doi=10.1111/j.1469-0691.2006.01343.x }} [http://www.blackwell-synergy.com/doi/abs/10.1111/j.1469-0691.2006.01343.x]</ref>
 
This is an example of what is known as an [[evolutionary arms race]], in which bacteria continue to develop strains that are less susceptible to antibiotics, while medical researchers continue to develop new antibiotics that can kill them. A similar situation occurs with [[pesticide resistance]] in plants and insects. Arms races are not necessarily induced by man; a well-documented example involves the elaboration of the [[RNA interference]] pathway in plants as means of [[innate immunity]] against [[virus]]es.<ref name="Lucy">Lucy A, Guo H, Li W, Ding S (2000). "Suppression of post-transcriptional gene silencing by a plant viral protein localized in the nucleus". ''[[EMBO Journal|EMBO J]]'' '''19''' (7): 1672–80. PMID 10747034.</ref>-->
 
==Asas genetik pemilihan semula jadi==