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Pregnancy Timeline by SemestersDevelopmental TimelineFertilizationFirst TrimesterSecond TrimesterThird TrimesterFirst Thin Layer of Skin AppearsEnd of Embryonic PeriodEnd of Embryonic PeriodFemale Reproductive SystemBeginning Cerebral HemispheresA Four Chambered HeartFirst Detectable Brain WavesThe Appearance of SomitesBasic Brain Structure in PlaceHeartbeat can be detectedHeartbeat can be detectedFinger and toe prints appearFinger and toe prints appearFetal sexual organs visibleBrown fat surrounds lymphatic systemBone marrow starts making blood cellsBone marrow starts making blood cellsInner Ear Bones HardenSensory brain waves begin to activateSensory brain waves begin to activateFetal liver is producing blood cellsBrain convolutions beginBrain convolutions beginImmune system beginningWhite fat begins to be madeHead may position into pelvisWhite fat begins to be madePeriod of rapid brain growthFull TermHead may position into pelvisImmune system beginningLungs begin to produce surfactant
CLICK ON weeks 0 - 40 and follow along every 2 weeks of fetal development


Why does nature rely on sex for reproduction?

Why is sex so popular among plants and animals, when cloning is a more common strategy?

As recently as the 1970s, John Maynard Smith questioned why sex is so prevalent when it is a more costly reproductive strategy than asexual reproduction. Mathematically, he showed how asexual females make more grandchildren than sexual females - and should therefore be the logical choice for creating more offsping.

Approximately half the offspring of sexually reproducing females must be sons, who can't physically bear them grandchildren. While asexual females don't make sons, so they make twice as many daughters than sexual females. Maynard Smith called his result the "two-fold cost of males."

So, it seems asexual reproduction should increase in frequency every generation — and outcompete sexual reproducers, driving them extinct.

Now, investigators studying the snail Potamopyrgus antipodarum, found proof of Maynard's theory. The snail has two kinds of females: one is asexual and only producing daughters, the other is sexual and producing sons and daughters. All coexist with each other in lakes and streams in New Zealand.

The recent work appears in Evolution Letters.

When researchers collected both types of female snails and allowed them to reproduce in big outdoor tanks, the asexual females did increase in numbers of offspring more than sexual snails. The increase was consistent with the "two-fold cost of sex" hypothesis.

"This study provides the first direct estimate of the cost of sex, and the results validate Maynard Smith's foundational theory in evolutionary biology. Our experimental confirmation ... also justifies a continued hunt for the selective forces that favor sex, because sex is indeed costly in P. antipodarum."

Amanda Gibson PhD, Department of Biology, Indiana University, Bloomington, Indiana; Department of Biology, Emory University, Atlanta, Georgia and lead author of the Evolution Letters study.

Indeed, there are many human animals who would agree there is at least a "two-fold cost of sex" without disavowing their own lifestyle. But, no one can deny the variety of individuals within a species is certainly increased by sexual reproduction. In sexual reproduction, different mutations are continually reshuffled from one generation to the next, increasing genetic diversity. This diversity allows species to adapt to unique environments and survive. The cost, however, can be high for offspring genetically unequipped.

Most eukaryotes (any living thing with membranes surrounding a nucleus of organelles containing genetic material) reproduce sexually, more evidence of evolutionary success with sexual reproduction. And, on average, a sexually-reproducing population will leave more offspring than a similar asexually-reproducing population. The evidence? Multicellular organisms exclusively depending on asexual reproduction — are very rare.

Over four decades ago, John Maynard Smith showed that a mutation causing asexual reproduction should rapidly spread in a dioecious sexual population. His reasoning was that the per-capita birth rate of an asexual population would exceed that of a sexual population, because asexual females do not invest in sons. Hence, there is a cost of sexual reproduction that Maynard Smith called the “cost of males.” Assuming all else is otherwise equal among sexual and asexual females, the cost is expected to be two-fold in outcrossing populations with separate sexes and equal sex ratios. Maynard Smith's model led to one of the most interesting questions in evolutionary biology: why is there sex? There are, however, no direct estimates of the proposed cost of sex. Here, we measured the increase in frequency of asexual snails in natural, mixed population of sexual and asexual snails in large outdoor mesocosms. We then extended Maynard Smith's model to predict the change in frequency of asexuals for any cost of sex and for any initial frequency of asexuals. Consistent with the “all-else equal” assumption, we found that the increase in frequency of asexual snails closely matched that predicted under a two-fold cost.
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May 12, 2017   Fetal Timeline   Maternal Timeline   News   News Archive   

Potamopyrgus antipodarum, the New Zealand fresh water mud snail.
Recently found infesting Washington state lakes and streams.
Image Credit:
Landcare Research New Zealand


Phospholid by Wikipedia