In this series, we explore the genetic evidence that indicates humans became a separate species as a substantial population, rather than descending uniquely from an ancestral pair.
Evolution as gradual change at the population level
One of the most common misunderstandings about evolution is failing to appreciate that evolution happens to populations, not individuals. But, you might interject, populations are made up of individuals – so how does that work? The answer is that yes, genetic variation enters a population through mutations in individuals – but that only when such variation accumulates within a population and shifts its average characteristics do we observe its effects.
Perhaps an analogy will help here – one that I have used before, that of language change over time. Languages are like populations – they have a large number of individuals who speak it. While each individual has their own particular quirks (favorite phrases, word preferences, and perhaps even chronic spelling mistakes) any one person cannot, on the whole, cause significant change to their language within their lifetimes. Additionally, any person who does change radically from their language group will effectively be placing themselves outside it – if their changes are large enough to hinder their intelligibility to others. So, language evolution is not an individual affair. Yet languages do change over time, and individuals do contribute to that change. Someone might invent a new word that catches on, for example. Others might be part of the “catching on” – hearing a new word, or new phrase, and repeating it to others. Over time (perhaps generations) a language will slowly adopt new words, new spellings, and new rules of grammar (such as split infinitives in English – to boldly split what no man has split before – but I digress). Yet, such adoptions are gradual. They enter the language as rarities, slowly become more common, and eventually become the “normal” way of doing things (much to the chagrin of English teachers). Thankfully, such changes typically take longer than one generation, sparing those of us who cringe at the novelties of our day. If the English words “there, their and they’re” ever officially collapse into one word determined solely by context or the use of apostrophes to form plurals becomes standard, I’m thankful I won’t be here to see it.
Just as a language has many speakers, a species has many members. They must be genetically compatible – i.e. speak the same language – but they are not all genetically identical – they have their own particular variation within an acceptable range to be in the group. In other words, though populations are a unit (they interbreed) they have genetic variation.
In the terminology of genetics we can understand this in terms of genes and alleles. While all members of the population have the same genes, they do not all have exactly the same version of any given gene. Different versions of a gene are called alleles, and they arise through mutations – that is, copying errors when chromosomes are replicated. For humans, we may have up to two different alleles for any gene – the allele we inherit from our mother, and the allele we inherit from our father. If have two different alleles, we are said to be heterozygous for that particular gene. If we have two identical alleles for a gene, we are homozygous for that gene. While any individual can have up to only two alleles, populations as a whole can have hundreds or even thousands of alleles.
In any given generation, the vast majority of the alleles present in a population were inherited from the previous generation – just like a language group learning from their parents, and picking up the language as a whole, but also some of their particular linguistic quirks. Each generation also can contribute its own novelty to the population in the form of new alleles arising through mutation – just like teenagers coming up with new words or phrases. These new alleles, are rare of course – they are only held by one individual at the beginning. Over many, many generations, however, such new alleles can become more common within a population if they are passed down, progressively, to more and more offspring. In time, the new allele might become the most common one within a population. Many generations later, it might be the only allele present for that gene in the entire population. Combined with the actions of other new alleles of many other genes, over time the average characteristics of the population can change. While it’s challenging to imagine this for genes and alleles, it’s simple to illustrate with language – for example, the change we see in linguistic trajectory towards present-day English in a verse from John’s gospel: John 1:29, West Saxon Gospels, c. 990.
Anothir day Joon say Jhesu comynge to hym, and he seide, Lo! the lomb of God; lo! he that doith awei the synnes of the world. (Wycliffe Bible, 1395)
The nexte daye Iohn sawe Iesus commyge vnto him and sayde: beholde the lambe of God which taketh awaye the synne of the worlde. (Tyndale New Testament, 1525)
The next day Iohn seeth Iesus coming vnto him, and saith, Behold the Lambe of God, which taketh away the sinne of the world. (KJV, 1611)
The next day John seeth Jesus coming unto him, and saith, Behold the Lamb of God, which taketh away the sin of the world. (KJV, Cambridge Edition)
The next day John saw Jesus coming toward him and said, “Look, the Lamb of God, who takes away the sin of the world!” (NIV, 2011)
Note well that these “forms”, as we see them here, themselves have many gradations between them, of course. These selections are useful for our illustrative purposes, however. Note that the overall shift to the “modern” text is the cumulative result of many individual changes. To return to our analogy, if every word is a gene, we see shifts in alleles over time as follows:
|cwæð||→ seide → sayde → saith → said|
|synne||→ synnes → synne → sinne → sin|
|to hym cumende||→ comynge to hym → commyge vnto him → coming vnto him → coming unto him|
Though the changes in each word contribute to the overall transformation, each word has only a relatively minor effect on its own. Yet the combined actions of changes in many words, over time, can change West Saxon to modern English. There are not, however, large jumps at any point along the way – each generation of speakers could easily understand their parents and their children – but over time, the shifts are large enough that West Saxon and present-day English are not even close to being the same language.
In the next post in this series, we’ll consider how changes in alleles can lead to new species – much like change over time can lead to new languages.