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In most living animals, egg cells are much larger than sperm cells
.
Taking humans as an example, the size of a single egg is 10 million times that of sperm
.
In a new study, Northwestern University researchers solved the mystery: competition and natural selection caused this strange size difference
.
Researchers used mathematical models to propose that in the very early stages of evolution, primitive species reproduced through external fertilization
.
In this model, larger germ cells or gametes have a competitive advantage because they can provide more nutrients to potential zygotes
.
However, smaller gametes require fewer resources to make, which reduces the pressure on parents
.
The lead author of the study, Daniel Abrams, Professor of Engineering Science and Applied Mathematics at the McCormick School of Engineering, said: "Organisms need to use the most supplies to produce the largest gametes, or they need to use the least resources to produce the smallest gametes
.
Based on sex With reasonable assumptions of reproduction and natural selection, we believe that this difference in body size is almost inevitable
.
"
The research was published in the online edition of The Journal of Theoretical Biology
.
The researchers’ model starts with isogamy, which is one of the sexual reproduction methods.
In this state, all gametes are roughly the same size, and there are no different sexes
.
Then, the team developed and applied a simple mathematical model to illustrate how isogamy is transformed into anisogamy
.
In this state of anisogamy, the gametes either become very small or quite large
.
In this model, anisogamy is produced by competition in an environment with limited resources
.
If gametes are more advantageous in size than their neighbors, then they are more likely to survive, which leads to an "arms race" in favor of larger gametes
.
However, if the organism itself does not need more and more resources, it cannot produce a large number of germ cells
.
However, they can save resources by producing a large number of tiny gametes
.
"In the early evolution of sexual reproduction, gametes were symmetrical
.
But that's where the symmetry was broken.
We eventually discovered that some organisms specialize in large gametes and others specialize in small gametes
.
"
According to the author, a mystery that has not yet been solved is why some species that practice homosexual reproduction still exist
.
For example, some species of algae and fungi reproduce asexually or symmetrically mate
.
"Since Charles Darwin, there have been many different theories about the origin of heterozygous reproduction
.
" "The problems in evolutionary biology are difficult to test because we can only study today's species
.
We cannot see them for billions of years.
The way it was before
.
Using mathematical models can generate new insights and understandings
.
"
references:
“A dynamical model for the origin of anisogamy” by Joseph D.
Johnson, Nathan L.
White, Alain Kangabire and Daniel M.
Abrams, 18 March 2021, Journal of Theoretical Biology .
DOI: 10.
1016/j.