A moral theory needs to be compatible with what is known in other fields of study.
What is known in the field of biology is that humans are the result of billions of years of evolution. What desirism tells us about morality must be compatible with that fact.
What desirism tells us about morality is that a moral system has very few requirements. Those requirements are easy to meet, and they are all true of the human organism.
Morality requires a creature capable of intentional action - of basing its behavior on goals (ends) with action plans designed to realize those ends. Humans use a system of beliefs and desires where desires identify the ends of human action and beliefs are used in the creation of action-plans for realizing those ends.
A system of morality requires that the ends be malleable. They must be capable of being changed. More specifically, agents need the capacity to acquire new desires or, at least, to have existing desires modified by interaction with one's environment.
Humans have a reward system, where desires that produce rewards for the agent aversions that avoid punishment (in the biological sense) are reinforced.
Once these conditions are met, a creature has the ability to adopt an action plan for reaching its ends by manipulating another's interaction with the environment so as to acquire useful desires.
In a sufficiently large community, there are desires and aversions that people generally have many and strong reasons to promote.
Humans are not the only creatures that meet these criteria. Any animal group where members can alter the desires of others using reward and punishment has a crude morality.
Evolution and the Content of Moral Claims
Some people argue that the content of our moral attitudes can be grounded in our evolved dispositions. For example, they argue that since evolution supports certain types of altruism and altruism is morally good, then evolution makes us morally good.
A similar story is told about parental affection for their children and other forms of sacrifice.
This type of view is deeply confused.
Without questioning the fact that we evolved some dispositions towards altruism, what makes altruism a virtue? Evolution supports a great many things - not all of them qualify as virtues. Our capacity to rape can be found in our biological heritage. Our disposition towards tribalism and tribal warfare as well as our capacity for prejudice and bigotry are no less compatible with our evolution as our altruism. What makes altruism a virtue?
An evolutionary explanation of altruism is not sufficient. We need an evolutionary defense of the proposition that altruism is a virtue.
Here, the argument may state that altruism is a virtue, while these other dispositions equally compatible with our evolution are not, because we evolved a tendency to feel a particular way about altruism.
This ad hoc answer faces a number of challenges.
First, is it true? Again, without questioning that we evolved some form of altruism, defending that claim does not defend the conclusion that we evolved a particular attitude towards altruism. We have evolved to have an appendix. However, our appendix works just fine without an additional pro-attitude towards having an appendix.
Second, evolved altruism in specific, and evolved morality in general, leaves no role for praise or condemnation. Desirism explains praise and condemnation as tools for molding malleable desires. Evolved morality seems to suggest that we praise or condemn people for their genetic makeup, or that we evolved dispositions to praise and condemn that serve no purpose since morality is grounded in our genes.
Third, if it is true that we have an evolved attitude of approval, this would not imply that what we approve of is good. If this implication was valid, it would imply that rape and tribal violence would also be virtues if we adopted similar attitudes towards them. Perhaps, like lions, we could evolve a sense of approval over the killing of our step children so we only spend resources on our own children. Perhaps, like certain insects, we could come to approve of killing and eating our mates. Perhaps we could come to have a sense of approval over enslaving those with dark skin, imprisoning women in breeding pens, and discarding those who do not produce children. Nothing in the theory rules out these possibilities.
Desirism notes that we can still evaluate evolved dispositions by their tendency to objectively satisfy or thwart other desires. Then, by noting the degree to which they are malleable or can be augmented or countered by other desires that are malleable, use social tools to sculpture the character of its members accordingly.
On this matter, it should be noted that desirism does not deny that there are some evolved dispositions such as parental affection. It simply asserts that, insofar as it is genetic, it is no more worthy of moral praise or condemnation than the capacity to feel pain or the desire for sex. Morality enters the picture in the fact that these desires are malleable, and can be strengthened, weakened, or redirected using praise and condemnation. Parental affection can be strengthened by praising good parents and condemning poor parents. The desire for sex can be redirected by adding a strong aversion to acting without the consent of others intimately involved.
Fourth, attempting to account for the content of moral propositions by appeals to evolution plays havoc with the logic of moral claims. "I have evolved a disposition to kill people like you and feel good about doing so; therefore, you deserve to die," would have to be considered a valid form of moral argument. If evolved dispositions determine moral facts and genetic tests show that people have an evolved sense of satisfaction at killing homosexuals, then homosexuals deserve to die.
Fifth, moral attitudes simply change too rapidly to have a direct basis in genetics. We cannot explain the abolition of slavery or the shift from the divine right of kings to governments get their just power from the consent of the governed in genetic terms. These shifts in attitude were not inherited. They were learned.
This is not to say that evolutionary theory has nothing to say about morality. We evolved malleable brains. We evolved the capacity to make plans and to execute intentional actions. The methods by which interactions with nature alter our desires has been subject to evolutionary pressures.
Evolution has a lot to say about the capacity to use social tools to promote desires people generally have many and strong reasons to promote. However, it has nothing to say about the content of moral claims. Evolution may make us altruistic, but it cannot make altruism a virtue - not directly, at any rate.