JSFiddle - React, Tailwind, and code Playground

HTML

<div class="mock-table">
    <div class="entryRow">
        <div class="mock-td nested-label-column">
            <label class="entryFieldLabel" for="Mutations_0__GeneSpecies">Gene donor</label>
        </div>
        <div class="mock-td nested-field-column">
            <select class="SpeciesList SpeciesListGene entryField" data-val="False" data-val-dropdown="False" data-val-number="False" id="SpeciesListGene_8449" name="Mutations[0].GeneSpecies.Value">
                <option value="2">Black rat</option>
                <option value="61">Cat</option>
                <option value="4">Cattle</option>
                
            </select>
            <button class="addNewSpecies flatButtonSecondary" type="button" value="A_8449" id="GeneSpeciesList_8449">add other species</button>
        </div>
    </div>
</div>

CSS

.dialog-label-column, .nested-label-column {
    width: calc(50% + 2em);
}
.dialog-field-column, .nested-field-column {
    width: calc(50% - 2em);
}
.entryRow {
    overflow: hidden;
    display: table-row;
    width: 100%;
}
div.mock-td {
    display: table-cell;
    vertical-align: top;
    padding-bottom: 0.7em;
}
div.mock-table {
    display: table;
}