R raster / rasterize creating GeoTIFF with Null Value "grid lines"

I am attempting the seemingly simple task of converting a .csv with lat, lon and population values into a .tif (raster).

Here's my R code:

geoName <- read.csv("/pathToData/geoName.csv")
geoName_raster <- raster(xmn=min(geoName$longitude), xmx=max(geoName$longitude), ymn=min(geoName$latitude), ymx=max(geoName$latitude), res=0.0027, crs="+proj=longlat +datum=WGS84")
geoName_rasterize <- rasterize(geoName[, c('longitude', 'latitude')], geoName_raster, geoName[, 'population'], fun=sum)
writeRaster(geoName_rasterize, "/pathToData/geoName_rasterize.tif")

The .csv is formatted like this:

longitude,latitude,population
-73.274688427539,19.817098050328,100.001
-72.697905721098,19.82226569178,200.002
-73.153838375345,19.799009402678,300.003
-72.71163913536,19.873934311831,400.004
-72.887420723946,19.863602053517,500.005
-73.288421351258,19.850685573779,400.004
-72.752838665811,19.79384121785,300.003
-72.939605787255,19.871351380997,200.002
-73.126372968453,19.713707358991,100.001

The output has bands of null values which appear to be influenced by the res value: the bands are further apart as res approaches 0.

• The .tif with narrow bands below is with no res specified

• The .tif with wide bands is with res = 0.0027 (which is the approximate width in degrees of a Bing tile at z17 which is the resolution of the population data set)

(Apologies for linking out to the images; I don't have enough reputation points to post them directly.)

I've read a few different R documentation pages outlining resolution, but I don't really understand its function.

My questions: what is res doing and how do I replace these bands with the data which belongs in those pixels?

Be gentle: I'm very new to R.