Genetic relatedness between different species (e.g. humans, chimps)

98% of the same genes like... We both have a gene that codes for eye formation, we both have a gene that codes for hair thickness, we both have a gene that codes for the formation of nipples, etc. Of course, for each gene we share in common, there are variants for that gene, known as alleles, that we don't share in common. For example, we both have a gene for eye color, but we have variations of eye color - blue, green, brown - whereas they only have brown.

Hope that helps.

It does help - thanks!

So here's my next question (or a better articulation of my original question): When making cross-species comparisons, how do scientists determine whether a gene is the "same gene" as opposed to just a really similar gene?  Or put another way how do they determine if two proteins are the "same" protein or just really similar proteins?

When examining structures that are similar-but-different, like a human eye and a squid's eye for example, won't a squid's eye sometimes have a protein that is similar in both structure and function to a protein that is found in the human eye, yet is a qualitatively different protein to the one found in the human eye?  Or is this just not the case?  Can every protein that makes up a squid's eye be classified as either clearly the same as some human eye protein or clearly different from every human eye protein?

Perhaps the answer is too technical for someone with a limited life science background to understand, but I'd like to try.

Two ways, location on the chromosome and base-pair comparison. This fact only came out once the whole human genome and ape genome were sequenced. Prior to that, we only could say that we belong to the same family biological without genetic proof. A squid's eye, although similar in the sense that it allows the organism to see works a lot differently than the way a mammal's eye creates an image. In biology, this is called analogous structures, which are different than homologies (existence of shared ancestry between a pair of structures or genes in different species). Sort of like butterflies have wings, so do birds, but do the two share the same 'flying' gene, no.

Given your interest in this stuff, I would definitely recommend watching this video:

https://biology-forums.com/index.php?article=1126.0

Awesome, thanks.

Do you know of something I can read to get into the really nitty-gritty of this?  I mean, I've heard "% DNA shared" comparisons for even humans and yeast.  Since yeast has a totally different number of chromosomes it seems difficult bordering on impossible to find the same genes within the two genomes but I guess they just use computer programs to analyze the entire genome for similar base pair sequences?

Also I guess I'm confused about how new genes appear in the course of evolution.  I was taught (back in high school) that mutations + natural selection = evolution, and by mutations I always assumed we were talking about changes in just one or two base pairs (which may or may not affect the amino acid that the mutated codon codes for, since some codons code for the same amino acids and some code for different ones).  

So if two populations become reproductively isolated, say Chimps and Bonobos, through mutations and selection/genetic drift their base pair sequences will start to diverge.  But won't this process BEGIN as the two populations drifting apart in terms of allelic frequencies for the same gene?  At what point would it jump to being differences in the GENES that they have?  Like, how many amino acids have to be altered before you have a "new gene"?

Both humans and yeast share a gene that codes for the protein myosin. This is one of many examples, you just have to search for them. Remember, yeasts are single-celled organisms, but they have many housekeeping genes that are the same as the genes in humans, such as those that enable energy to be derived from the breakdown of sugars. The two species might look completely different, so they might not share phenotypic genes in common (obviously), but they do share biochemical genes in common.


Yes, that's the driving force, mutation and natural selection, precisely.


Great question. Two isolated species become distinct species when they are no longer able to produce viable offspring. This is referred to as speciation (a lot more to it by the way).

This idea was published in Wallace’s Darwinism in 1889, several years after Darwin had died. The Wallace Effect (http://en.wikipedia.org/wiki/Wallace_effect) proposes that natural selection contributes to reproductive isolation by encouraging diverging populations to stop mating. Each population will have adaptations which increase its fitness in a local environment; matings between individuals of the two different populations will jumble up these adapatations with the result that the offspring cannot compete with individuals from either population; so the mixed offspring have decreased fitness. Now if an individual of one population were to breed with one from the other, they will have less successful young than if they mate within their own population. So natural selection acts to favour either behavioural or morphological mechanisms to prevent mixing, over time the two populations become reproductively isolated and form two species.