In a species, there are usually equal numbers of males and females and occasionally not. Furthermore, parents bias investment to male or female offspring. Discuss the relation of these phenomena.
The phenomena you mentioned can be explained by principles of evolutionary biology and parental investment theory.
In many species, there is an equal ratio of males to females. This is known as a 1:1 sex ratio. The reason for this is an evolutionary strategy called "Fisher's principle." According to this principle, in a population where the sex ratio deviates from 1:1, individuals who belong to the less common sex will have a higher reproductive success. As a result, natural selection favors individuals of the rarer sex, causing the sex ratio to equalize.
However, there are cases where the sex ratio is not equal. This occurs because individuals have different strategies for investing in their offspring, known as parental investment. Parental investment refers to the time, energy, and resources that parents put into raising their offspring.
In species where parental investment is generally equal between males and females, such as in birds and many mammal species, the sex ratio tends to be close to 1:1. Both parents contribute equally to raising the offspring, so there is no advantage to producing more males or females.
On the other hand, some species exhibit biased parental investment towards one sex. This is often seen in species where one sex has a higher reproductive potential or where the reproductive success of one sex depends on access to limited resources. For example, in some species of mammals, females have a higher investment because they have internal fertilization, pregnancy, and lactation.
In such cases, it becomes advantageous for parents to bias their investment towards producing offspring of the sex that has higher reproductive potential. This leads to a deviation from the equal sex ratio. For instance, if a species produces more male offspring, then male-biased parental investment would be favored, as it maximizes the chances of passing on the genes of the parent whose reproductive success depends on producing males.
In summary, the relationship between the equal sex ratio and biased parental investment depends on the reproductive strategies and ecological factors of a species. Generally, species with equal parental investment tend to have a 1:1 sex ratio, while species with biased parental investment towards one sex may deviate from equal sex ratios to optimize their reproductive success.