Wednesday, 12 July 2017

Zebra Finches Show How Evolution Works

Zebra finch, Taeniopygia guttata.
Source: Wikipedia
Courtship song preferences in female zebra finches are shaped by developmental auditory experience | Proceedings of the Royal Society of London B: Biological Sciences

A paper published recently in Proceedings of the Royal Society B by researcher from McGill University, California, USA, illustrates a couple of interesting aspects to evolution and how species diverge.

The team found that zebra finches (Taeniopygia guttata) reared in the absence of their fathers do no react in the same way to male courtship songs as those raised by both parents.

Now, this in itself might not be particularly important apart from one thing - female sex selection. Females normally select the best singers as their mates and there is growing evidence that the best singers are also the healthiest and fittest males. Females who don't have the ability to discriminate lose out in the competition for the best mates. This, after all, is probably one of the main drivers for the evolution of female sex selection.

In the wild, females would rarely be raised without a father/tutor. That said, it could mean that the environment that birds are raised in will influence song preferences. We already know that male birds can shift their songs to make them easier to hear in noisy, urban environments. Our data could mean that the ability of females to detect those songs may be affected by what they hear when they are young.

Professor Sarah Woolley. Co-author.
McGill University Department of Biology. “
It also illustrates that the ability to recognise and react to song is not wholly genetic but is at least partly dependent on auditory experience. The authors did not identify exactly how this experience results in change in perception or behaviour so it remains unclear whether this a neurophysiological change in the developing bird or what might be thought of as memetic inheritance but that makes no difference to the significance of this finding. The important fact is that the ability to select a suitable mate is dependant on the environment and to an extent, the reproductive success of the male. If he doesn't survive, his offspring may be less fitted for reproductive success themselves.

But why would female sex selection and male sexual attractiveness matter in the first place?

The reason for this can be found in the way birds like finches (and several other families) speciate in evolutionary history. As isolated populations diverge, each will tend to evolve specialism such as the choice of food and the equipment to get it. Finches, just as with Darwin's Galapagos finches evolve a beak suitable for their food. It may be short and fat for cracking tough seeds of long and thin for extracting seeds from cones for example. As you would expect, each population follows and evolutionary pathway dependent on it's own local environment.

The problem arises when these isolated population come back into contact, maybe because of climate change - intervening desert reverting to savannah, for example. Each population will be suited to it's own source of food, but what of any hybrids? Hybrids might have a beak unsuitable for either food so they will be at a distinct disadvantage with reduced reproductive vigour; the effort of their parents in rearing them will have been wasted.

This creates conditions in which anything which acts as a barrier to hybridisation will give an advantage to carriers over individuals without these barrier mechanisms.

Barriers to hybridisation come in two type - pre-zygotic and post-zygotic. Pre-zygotic barriers are those that prevent fertilisation in the first place; post-zygotic barriers are those that prevent a viable offspring developing even if mating and fertilisation actually takes place. When has happened in the zebra finch is that female sex selection is the primary barrier to hybridisation as zebra and other related species of finch diverged from a common ancestor. In this case, male courtship song and plumage have evolved because these were most likely to be zebra finch males and then later, the best zebra finch males.

Taeniopygia guttata x Poephila acuticauda


Taeniopygia guttata x Stagonopleura guttata
The zebra finch genome was the second bird genome to be fully analysed after the domestic chicken. The only other member of the Taeniopygia is the T. bichenovii which doesn't appear yet to have had its genome analysed, however, I would be willing to bet a small wager on it being found to have the same number of chromsomes and differences only in specific loci. In other words, there will be no post-zygotic barriers to hybridisation. These finches should be able to hybridise if their populations overlapped, but they will not do so because of these pre-zygotic barriers.

I say this with a fair degree of confidence because bird breeders have produced fertile hybrids between zebra finches and other finches from related genera. Hybridisation in captivity is possible between T. guttata, the Bengalese finch, Lonchura striata, the long-tailed finch (Poephila acuticauda), the diamond firetail, Stagonopleura guttata and probably several others, so it is likely that even closely related genera have the same genetic arrangement.

Exactly the same situation exists with the various European finches, many of which which can be hybridised in captivity but in the wild rarely interbreed due to pre-zygotic barriers. Consequently, a characteristic of these finches is sexual dimorphism (usually but not always), female sex selection and male mating song and courtship rituals - all resulting in the large number of very beautiful songbirds which are such a feature of the British countryside. In all of these finches, over their evolutionary history, there have been major climate change leading to populations becoming isolated for long enough to diversify, then a re-mixing of these divergent evolution giving pressure for pre-zygotic barriers.

The same applies to ducks where again sexual dimorphism and female sex selection are the norm.

Whenever you see sexual dimorphism, especially when associated with mating tituals and female sex selection, think pre-zygotic barriers. As sure as eggs is fertile eggs, there will have been a time in the evolution of that species when a period of isolation and diversification was followed by the two divergent populations coming together again.

I suggest it would be difficult if not impossible for an intelligent (sic) design advocate to explain why an intelligent designer would design so many related species with genomes arranged so similarly that they can interbreed and then have to design elaborate barriers to hybridisation to prevent it happening. But, I daresay simple denialism or abandoning the pretence that ID is science not religion and chanting 'God did it!', will get them by.

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