How to easily defeat any argument for God

[Belacqua]

But if good is linked to wellbeing, then natural selection would've ensured that over time we (as a species, not individually) would be motivated enough to do what's good, even if in a tribalistic manner whereby we are selective of who we generally do good to.

Well, it's all in that "if," isn't it? 

Natural selection selects for survival. Not truth, not goodness, not sophistication, not wonderfulness. 

To say that what causes us to survive is necessarily good is to beg the question. I guess you could make an argument that "good" only and always means "that which helps us survive." But that's a big job.

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Just to show that what we desire isn't simple, this is from Peirce's Theory of Signs, by T.L. Short:

Most remarkable of all, we have learned to take pleasure in signs and
sign interpretation for their own sakes, independently of any practical
purpose. Practical discourse has been made poetry and story and history;
diagrams have been made mathematics and scientific theory and pictorial
art; auditory signals, music. Truth and beauty have become human purposes.
Immense wealth and power have been devoted to science and art,
sometimes knowingly at the expense of life. Our capacity to depict unactualized
possibilities, fearful or attractive, and to define nonbiological
purposes and rules of behavior subordinate thereto, has transformed
structures that came into being initially as means to biological survival.
Thus the influence of religion and morality on politics, for example. In
Aristotle’s words, the polis originated to secure life, but continued for the
sake of a good life. Once again, whence that judgment ‘good’?

It is often supposed that pleasure and avoidance of pain are the ultimate
purposes for which these other purposes are adopted as means. But
that is to overlook a fact we have been at pains to establish: that a purpose
formed is independent of the conditions that explain its formation.
Aging Don Juans pursue pleasant ends beyond the point at which they
cease to be pleasurable. Dogs chase rabbits even when they are fed well at
home. We want to know the truth even when it is painful – quite beyond
any practical purpose knowledge of the truth might serve. Besides, pleasure
and pain have turned out to be highly malleable. We learn to take
pleasure in things – caviar, alcohol, hard work – initially unpleasant. Art
appreciation is taught. We have a moral duty to learn to take pleasure
in doing our duty and in exercising self-restraint. But most importantly,
our capacity to diagram and symbolize means that we can formulate possible
purposes independently of any motive to adopt them. Sometimes,
we then adopt them, arbitrarily or for reasons not well considered.
Human culture and irrational purpose would each seem to be an ultimate
emancipation of purpose from biology. However, there is one further
step to take. Purposes adopted irrationally tend not to endure. By
contrast, some possibilities seem fated to become purposes and, once
adopted, to persist and to gradually strengthen their hold. Among the
latter are those that dominate human cultures: not only the arts but certain
genres of art especially, not only religion but certain forms of religion
especially, not only politics but certain political ideals especially seem to
have staying power and a power to spread from one nation to another.
In Peirce’s words (in which ‘idea’ stands indifferently for a content of
consciousness and a general type or possibility):

it is the idea which will create its defenders, and render them powerful. (1.217)
every general idea has more or less power of working itself out into fact; some
more so, some less so. (2.149)
Again, ideas are not all mere creations of this or that mind, but on the contrary have a
power of finding or creating their vehicles, and having found them, of conferring
upon them the ability to transform the face of the earth. (1.217)
This last passage is particularly telling. Just as life, the first form that purpose
took, transformed the physical surface of the Earth, so also human
culture, the last embodiment of purpose with which we are acquainted,
has transformed the biosphere, making grasslands into pastures, rivers
into electrical power, forests into libraries, and stones into cathedrals.
Peirce’s conception of final causation turns out to be more Platonic
than Aristotelian: it attributes a power to the type itself, independently of
that type’s being the nature of any existing individual or being otherwise
embodied.

[T]he idea does not belong to the soul; it is the soul that belongs to the idea. The
soul does for the idea just what the cellulose does for the beauty of the rose; that
is to say, it affords it opportunity. (1.216)

Platonism of this sort is presupposed in Peirce’s theory of the normative
sciences (chapter 3, section 1). Aesthetics discovers ultimate ends by their
appeal to our unqualified admiration. Ultimate ends, then, are those
possibilities that create in us a desire for them, causing us to adopt them
as our purposes. As ethics establishes the rules that must be followed in
order to attain the ends that are ultimate, we are here given a Platonic
path to ethics, in place of the Aristotelian foundation denied.