Editing Sexual differentiation (section)

== Flexibility ==
The most intensively studied species, such as [[Drosophila melanogaster|fruit flies]], [[nematode]]s, and mice, reveal that evolutionarily, sex determination/differentiation systems are not wholly conserved and have evolved over time.<ref name=":02"/> Beyond the presence or absence of chromosomes or social/environmental factors, sexual differentiation can be regulated in part by complex systems like the ratio of genes on X chromosomes and autosomes, protein production and transcription, and specific mRNA splicing.<ref name=":02" />

Differentiation pathways can be altered at many stages of the process. [[Sex reversal]], where the development of a sexual phenotype is redirected during embryonic development, happens in the initiation phase of gonadal sex differentiation. Even in species where there is a well-documented master regulator gene, its effects can be overridden by a downstream gene.<ref>{{Cite journal|last=Capel|first=Blanche|date=2017-08-14|title=Vertebrate sex determination: evolutionary plasticity of a fundamental switch|journal=Nature Reviews Genetics|volume=18|issue=11|pages=675–689|doi=10.1038/nrg.2017.60|pmid=28804140|s2cid=4313871|issn=1471-0056}}</ref>

Furthermore, hermaphrodites serve as examples of the flexibility of sexual differentiation systems. [[Sequential hermaphroditism|Sequential hermaphrodites]] are organisms that possess reproductive capabilities of one sex, and then that sex changes.<ref>{{Cite journal|last=Warner|first=Robert R.|date=1975|title=The Adaptive Significance of Sequential Hermaphroditism in Animals|journal=The American Naturalist|volume=109|issue=965|pages=61–82|doi=10.1086/282974|jstor=2459637|s2cid=84279130|issn=0003-0147}}</ref> Differentiated gonadal tissue of the organism's former sex degenerates, and new sex gonadal tissue grows and differentiates.<ref name=":13"/> Organisms that have the physiological capability to reproduce as a male and as a female at the same time are known as [[simultaneous hermaphrodite]]s. Some simultaneous hermaphroditic organisms, like certain species of [[Gobiidae|goby]], have distinctive male and female phases of reproduction and can flip back and forth, or "sex reverse", between the two.<ref>{{Cite journal|last=St. Mary|first=Colette M.|date=1996-02-01|title=Sex allocation in a simultaneous hermaphrodite, the zebra goby Lythrypnus zebra: insights gained through a comparison with its sympatric congener, Lythrypnus dalli|journal=Environmental Biology of Fishes|language=en|volume=45|issue=2|pages=177–190|doi=10.1007/BF00005232|bibcode=1996EnvBF..45..177S |s2cid=1769706|issn=1573-5133}}</ref>

=== Socially-determined ===
In some species, such as sequentially hermaphroditic [[Amphiprioninae|clownfish]], changes in social environment can lead to sexual differentiation or sex reversal, i.e. differentiation in the opposite direction.<ref name=":13"/> In clownfish, females are larger than males. In social groups, there is typically one large female, multiple smaller males, and undifferentiated juveniles. If the female is removed from the group, the largest male changes sex, i.e. the former gonad tissue degenerates and new gonad tissue grows. Furthermore, the pathway of differentiation is activated in the largest juvenile, which becomes male.<ref name=":13" />

=== Alternative morphs ===
{{Further|Polymorphism (biology)}}Sexual differentiation in a species does not have to produce one recognizable female and one recognizable male type. In some species alternative morphs, or [[morphotype]]s, within one sex exist, such as flanged (larger than females, with large flap-like cheek-pads) and unflanged (about the same size as females, with no cheek-pads) male [[orangutan]]s,<ref>{{Cite journal|last1=Knott|first1=Cheryl Denise|last2=Emery Thompson|first2=Melissa|last3=Stumpf|first3=Rebecca M.|last4=McIntyre|first4=Matthew H.|date=2010-01-07|title=Female reproductive strategies in orangutans, evidence for female choice and counterstrategies to infanticide in a species with frequent sexual coercion|journal=Proceedings of the Royal Society B: Biological Sciences|volume=277|issue=1678|pages=105–113|doi=10.1098/rspb.2009.1552|issn=0962-8452|pmc=2842634|pmid=19812079}}</ref> and sometimes differences between male morphs can be more noticeable than differences between a male and a female within such species.<ref>{{Cite journal|last1=Taborsky|first1=Michael|last2=Schütz|first2=Dolores|last3=Goffinet|first3=Olivier|last4=Doorn|first4=G. Sander van|date=2018-05-01|title=Alternative male morphs solve sperm performance/longevity trade-off in opposite directions|journal=Science Advances|language=en|volume=4|issue=5|pages=eaap8563|doi=10.1126/sciadv.aap8563|pmid=29806019|pmc=5966226|bibcode=2018SciA....4.8563T|issn=2375-2548}}</ref> Furthermore, [[sexual selection]] can be involved in the development of different types of males with alternative reproductive strategies, such as sneaker and territorial males in [[dung beetle]]s<ref>{{Citation|last=Partridge|first=Charlyn|title=Sneak Copulation as an Alternative Mating Strategy |date=2017|encyclopedia=Encyclopedia of Evolutionary Psychological Science|pages=1–3|editor-last=Shackelford|editor-first=Todd K.|publisher=Springer International Publishing|language=en|doi=10.1007/978-3-319-16999-6_3610-1|isbn=978-3-319-16999-6|editor2-last=Weekes-Shackelford|editor2-first=Viviana A.}}</ref> or harem males and pair-bonding males in the Nigerian [[Cichlid|cichlid fish]] ''P. pulcher.''<ref name=":72"/><ref>{{Cite book|last=Oliveira, Rui F. (Rui Filipe Nunes Pais de)|title=Alternative reproductive tactics an integrative approach|date=2008|publisher=Cambridge University Press|isbn=978-0-521-83243-4|pages=1–21|oclc=850824972}}</ref> Sometimes alternative morphs are produced by genetic differences, and in other cases, the environment can be involved, demonstrating some degree of [[phenotypic plasticity]].<ref>{{Cite journal|last1=Gotthard|first1=Karl|last2=Berger|first2=David|last3=Bergman|first3=Martin|last4=Merilaita|first4=Sami|date=2009-10-01|title=The evolution of alternative morphs: density-dependent determination of larval colour dimorphism in a butterfly|journal=Biological Journal of the Linnean Society|language=en|volume=98|issue=2|pages=256–266|doi=10.1111/j.1095-8312.2009.01290.x|issn=0024-4066|doi-access=free}}</ref>